Acta Metallurgica Sinica
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ISSN 0412-1961
CN 21-1139/TG
Started in 1956
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, Volume 47 Issue 3
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STUDY OF Mn AND P SOLUTE DISTRIBUTIONS AND THEIR EFFECT ON THE TENSILE BEHAVIOR IN ULTRA LOW CARBON BAKE HARDENING STEELS
WANG Hua SHI Wen HE Yanlin FU Renyu LI Lin
Acta Metall Sin, 2011,
47
(3): 263-268. DOI:
10.3724/SP.J.1037.2010.00693
Abstract
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With the increasing requirement of vehicle weight reduction and energy conservation from automobile industry, the investigation and development of high strength steel sheet has been stressed extensively. Bake hardening steel, as a new kind of automotive steel, exhibits low strength and good formability before drawing, after which increases obviously in the yield strength during baking process, and is then widely used in the outer plate of modern cars. Mn and P are often added to sheet steel to increase the strength, and their distributions have significant effect on drawability, bake hardening property and surface quality of bake hardening steels. In this paper, the distributions of Mn and P and their effect on tensile behavior in bake hardening steels were studied. For investigation, two kinds of bake hardening steels (BH–Mn and BH–P steels) were heated to 800 ℃, held for 2 min and cooled by water quenching. Three dimensional atom probe (3DAP) technique, internal friction experiments and tensile tests were carried out to analysis the effect of Mn and P distribution patterns on the interstitial atom distribution and Cottrell atmosphere in the matrix, so as to obtain the influence of solute istributions on tensile behavior. The results indicate that P segregates mainly in bake hardening steel, and part of P segregates together with C, which strongly pin the dislocations and is the main reason that induces the yield point elongation during tensile process. In BH–Mn steel, Mn hardly segregates in the matrix and C segregates very little, so the strength of BH–Mn steel is lower than that of BH–P steel, whereas the plasticity is better than BH–P steel. The segregation of P together with C and its pinning of dislocations will influence Snoek–Ke–Koster internal friction, and mkes the disappearance of Snoek–Ke–Koster peak.
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APT AND EXTRACTION REPLICA CHARACTERIZATION OF Cu–RICH CLUSTERS PRECIPITATED IN PRESSURE VESSEL MODEL STEELS
CHU Dafeng XU Gang WANG Wei PENG Jianchao WANG Jun’an ZHOU Bangxin
Acta Metall Sin, 2011,
47
(3): 269-274. DOI:
10.3724/SP.J.1037.2010.00476
Abstract
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(1717KB) (
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The precipitation of Cu–rich clusters in reactor pressure vessel (RPV) model steel was investigated by means of atom probe tomography (APT), extraction replica (ER) and HRTEM. RPV model steel was prepared by vacuum induction furnace melting with higher content of Cu (0.6%, mass fraction). The ingot (about 50 kg of weight) was forged and hot–rolled to 4 mm in thickness and then cut to specimens of 40 mm×30 mm. Those specimens were further heat treated by 880 ℃/0.5 h water quenching and 660 ℃/10 h tempering, and finally aged at 370 ℃ for different time from 1000 to 6000 h. 4% nitric acid alcohol solution was used as an etchant to extract the precipitates of Cu–rich clusters from α–Fe matrix. The results obtained by APT analysis show that the number density of Cu–rich clusters reaches 3.1×10
23
m
−3
in the specimen aged at 370 ℃ for 4500 h, and the Cu content in the clusters increases rapidly during their growth from 1 nm to 5 nm. The segregation of Ni and Mn elements within and around the Cu–rich clusters was detected. The results obtained by ER, EDS and HRTEM analyses show that the majority of Cu–rich clusters are Cu–Fe (Ni, Mn) alloys with 10%—80% Cu (atomic fraction)but they are a single phaswith R or fcc crystal structure.
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CONVECTION EFFECTS AND BANDING STRUCTURE FORMATION MECHANISM DURING DIRECTIONAL SOLIDIFICATION OF PERITECTIC ALLOYS I. Experimental Result
LUO Liangshun ZHANG Yumin SU Yanqing WANG Xin GUO Jingjie FU Hengzhi
Acta Metall Sin, 2011,
47
(3): 275-283. DOI:
10.3724/SP.J.1037.2010.00485
Abstract
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Systematic directional solidification experiments were conducted to investigate the convection effects on the banding structure evolution and macrosegregation in nonfaceted–non–faceted model Fe–Ni alloy using conventional resistance heating and induction heating Bridgman directional solidification methods in this paper. It was found that convection can induce severe axial and radial macrosegregation in the directionally solidified samples, and make the microstructures complex and mae the steady state difficult to achieve. Axiamacrosegregation was reflectein finite samples olidified from the beginning to the enof soidification with the transiion from primary phase to peritectic phase. The primary–peritectic transition depended on the alloy composition and convection strengh. Radial macrosegregation reflected in the solute concentration poor in the center and rich in edge, and a primary–peritectic transition also exist in the lateral directional from the sample to the edge.
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CONVECTION EFFECTS AND BANDING STRUCTURE FORMATION MECHANISM DURING DIRECTIONAL SOLIDIFICATION OF PERITECTIC ALLOYS II. Theoretical Analysis
LUO Liangshun FU Hengzhi ZHANG Yumin LI Xinzhong SU Yanqing GUO Jingjie
Acta Metall Sin, 2011,
47
(3): 284-290. DOI:
10.3724/SP.J.1037.2010.00486
Abstract
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(2193KB) (
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The axial macrosegregation and relevant microstructure evolution of Fe–Ni peritectic alloys directionally solidified under different convection strength were discussed using pure diffusion model, boundary layer convection model and the Scheil equation. Comparing with the experiments of Fe–Ni alloys conducted in resistance heating and induction heatinBridgman system, it was found that the predictions of boundary layer convection model areed well with the experimental results of induction heating drectional solidification when the convection strength parameters lies in the region 1.7<λ≤2, indicating that the model can be used to predict the axial macrosegregation and microstructures evolution of peritectic alloys.
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MATHEMATICAL MODEL AND THEORETICAL RESEARCH OF UNIT ROLLING PRESSURE DISTRIBUTION IN RECTANGULAR GROOVE DURING RHEO–ROLLING OF SEMISOLID ALLOY
HUANG Hongqian CAO Furong GUAN Renguo ZHAO Zhanyong XING Zhenhuan
Acta Metall Sin, 2011,
47
(3): 291-297. DOI:
10.3724/SP.J.1037.2010.00590
Abstract
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Semisolid metal forming (SSF) is recognized as a new forming technology, which has been paid more and more attention by the researchers all over the world. The semisolid rolling process (rheo–rolling) combines the fabrication of semisolid slurry with continuously rolling as an important neoteric means to achieve near–shape forming. Compared with the traditional rolling process, this technology has the features of short machining process, low energy consumption, low cost equipment and high yield. However, one of the difficulties of using this technology to produce strip is that the solid and liquid phases prone to separation with each other during rheo–rolling deformation, especially when the semisolid slurry has a low solid fraction. The phenomenon causes macrosegregation and reduces the quality of the strip, as a result, limiting its industrial application. Using rectangular groove roller may solve this problem. In this paper, the mathematical model of unit rolling pressure distribution in rectangular groove during rheo–rolling was established. AZ91D magnesium alloy was taken as an example, and the effects of rolling strip thickness, width and roller radius on unit pressure were calculated and studied. he calculation results reveal that the smaller the thickness of rheo–rolled strip is, the narrower the width is, the bigger the peak unit rolling pressure is and the larger the average unit rolling pressure is. However, when the strip thickness decreases, the peak unit rolling pressure moves towards the exit while when width decreases, the peak unit rolling pressure moves towards the entrance. The larger the roller radius is, the bigger the peak unit rolling pressure is, the larger the average unit olling pressure is, and the peak value deviates towards the exit.
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EFFECT OF HOT PRESSING TEMPERATURE ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF SiC PARTICLE REINFORCED ALUMINUM MATRIX COMPOSITES
JIN Peng XIAO Bolu WANG Quanzhao MA Zongyi LIU Yue LI Shu
Acta Metall Sin, 2011,
47
(3): 298-304. DOI:
10.3724/SP.J.1037.2010.00413
Abstract
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The effects of hot pressing temperature on microstructures and tensile properties of 15% (volume fraction) SiC
p
/2009Al composites were investigated in this paper. The relative density of the composites increased rapidly with increasing the hot pressing temperature up to 580 ℃ and decreased with further increasing the temperatures. TEM observations revealed that the interface bonding was quite weak with the interface crack when the hot pressing temperature was below 560 ℃. When the composites were hot pressed at 580 and 600 ℃, the interface was clean and had a good interface bonding. The MgAl
2
O
4
and Al
4
C
3
formed at the interfaces when the hot pressing temperature was above 620 ℃. Tensile tests indicated that the composite fabricated at 580 ℃ exhibited the optimum strengtand ductility. Fractography revealed that for the composite fabricated at the hot pessing temperture below 560 ℃, the fracture mechanism was mainly the interfacial debonding. For the compositfabrcated at 580 and 600 ℃, the fracture mechanism of the composite was the matrix ductile fracture and the SiC particle fracture, When the hot pressing temperature was above 620 ℃, the interface fractured along MgAl
2
O
4
and Al
4
C
3
, and the fracture mechanism of the composite was the matrix ductile fracture, the interface crack and the particle fracture.
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HIGH SPEED AND HIGH MAGNIFICATION IN SITU OBSERVATION OF RESIDUAL LIQUID METAL DURING LASER WELDING PROCESS
WEN Peng Shinozaki Kenji Yamamoto Motomichi
Acta Metall Sin, 2011,
47
(3): 305-310. DOI:
10.3724/SP.J.1037.2010.00591
Abstract
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(2454KB) (
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Weld solidification cracking is resulted from the low ductility during solidification progress of weld metal, which is considered to be determined by the distribution of residual liquid metal at solidification front. Thus, the in situ observation of residual liquid metal during welding can give significant information of solidification progress and weld solidification cracking. Few reports have been found so far in this aspect though many in situ observation researches have been carried out in welding field, for example, the observation of droplet transfer, weld pool shape, weld plasma and phase transformation etc.. In this paper, solidification behavior of residual liquid metal at the trailing edge of the weld molten pool was in situ observed directly during laser welding by using high speed camera and high magnification optical lens. The precipitation of columnar grain from the molten pool and the solidification of the residual liquid metal at the trailing edge of weld molten pool were recorded in detail by using the high speed video system. Meanwhile, the solidifying structures in solidification process were frozen and reserved to room temperature by using the liquid Sn quenching method during laser welding. The quantity and morphology of residual liquid metal in quenched samples were observed and compared with the in situ observation results. It was found that the in situ observed residual liquid metal was the initial part of the coexistence range of liquid and solid during solidification progress. The in situ observed existence range of residual liquid metal has correspondence with the solidification cracking susceptibility in accordance with the fan–shaped hot cracking test results for three different austenite stainless steels. Consequently, this research provides experimental basis for the relationship between residual liquid metal and solidification cracking susceptibility. It shows the possibility to develop an in situ observation method to directly evaluate solidification cracking susceptiility of different materials during welding process.
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HARDENING BEHAVIOR OF THE AS–CAST Al–Mg–Sc–Zr ALLOY
DU Gang YANG Wen YAN Desheng RONG Lijian
Acta Metall Sin, 2011,
47
(3): 311-316. DOI:
10.3724/SP.J.1037.2010.00645
Abstract
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(2040KB) (
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Addition of minor Sc and Zr to the Al–Mg alloys can effectively improve the recrystallization temperature and strength of the alloys due to the formation of L12 structured Al
3
(Sc, Zr) precipitates. To optimize the mechanical properties of the Al–Mg–Sc–Zr alloys, it is important to understand the precipitation behavior of the Al
3
(Sc, Zr) precipitates and the corresponding strengthening effect during the annealing of Al–Mg–Sc–Zr alloys. In this work, isothermal annealing at the temperatures between 250 and 450 ℃ was conducted to study the precipitation behavior of Al
3
(Sc, Zr) particles in the as–cast Al–6Mg–0.2Sc–0.15Zr alloy (mass fraction, %). The results show that dramatic precipitation hardening occurred with the formation of the coherent Al
3
(Sc, Zr) precipitates during the annealing. Precipitation rocess of the Al
3
(Sc, Zr) accelerates with the increase of the annealing temperature. After the sample was annealed at 300 ℃ for 24 h, the Al
3
(Sc, Zr) particles were in a size of 5 nm ad the yield strength of the as–cast alloy can be increased by 90 MPa. Severe coarsening of Al
3
(Sc, Zr) occurred when the annealing temperature is bove 400 ℃. After annealed at 450 ℃ for 24 h, average particle size of Al
3
(Sc, Zr) particles was found increase to 30 nm, wth the corresponding precipitation strengthening effect to be about 30 MPa.
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SUPERPLASTICITY OF A MO-9SI-8B-3HF MULTIPHASE REFRACTORY ALLOY PREPARED BY MECHANICAL ALLOYING AND HOT PRESSING SINTERING
No Author
Acta Metall Sin, 2011,
47
(3): 317-320.
Abstract
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821
)
The demand for materials suitable for elevated temperature applications in the aircraft and aerospace industry beyond the realm of Ni-base superalloys has generated significant research interest in refractory metals and alloys, intermetallics and ceramics. The lack of damage tolerance in many intermetallics and most ceramics has guided research in the direction of multiphase refractory alloys which contained a matrix phase that is capable of providing damage tolerance, and a significant volume fraction of second phase that enhances the creep resistance of the alloy. Mutiphase Mo-Si-B alloys comprised of a α-Mo (Mo solid solution) and the intermetallic phases Mo3Si and Mo5SiB2 appear to offer favorable combinations of mechanical properties and oxidation resistance. It is widely accepted that the ideal microstructure of Mo-Si-B alloys would possess a continuous α-Mo matrix with embedded, homogeneously distributed intermetallic particles. For optimum toughening, microstructures containing a continuous matrix of the toughening phase are preferred over those in which the toughening phase occurs in the form of discrete particles. One purpose of this paper is to show that mechanical alloying (MA) in the solid state may be used to fabricate Mo-Si-B alloys with a continuous α-Mo matrix. One of the major drawbacks for hindering applications of Mo-Si-B alloys is poor formability even at elevated temperauture. Superplasticity which has the advantages of precision forming to the final net shape is a viable method to overcome this problem. The main purpose of this paper is to report the extensive plasticity or superplasticity of Mo-9Si-8B-3Hf alloy. A Mo-9Si-8B-3Hf multiphase refractory alloy was prepared by mechanical alloying (MA)and hot pressing sintering. Influence of milling time on density was studied. The tensile properties of this alloy at elevated temperature were evaluated by tensile tests in vacuum. Analysis of XRD reveals that this alloy consists of α-Mo (Mo solid solution), intermetallics Mo3Si and Mo5SiB2. The average grain sizes of each phase are all about 3μm and the grains shapes are nearly equiaxed. This alloy displayed extensive plasticity or superplasticity at temperatures ranging from 1400 ℃to 1560 ℃with strain rate of 3×10-4 s-1.
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EXTRACT THE PLASTIC PROPERTIES OF METALS USING REVERSE ANALYSIS OF NANOINDENTATION TEST
MA Yong YAO Xiaohong TIAN Linhai ZHANG Xiangyu SHU Xuefeng TANG Bin
Acta Metall Sin, 2011,
47
(3): 321-326. DOI:
10.3724/SP.J.1037.2010.00583
Abstract
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(1653KB) (
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)
Using traditional methods to evaluate mechanical properties of bulk materials is not applicable for metal surface studying and metals with very small volume. Nanoindentation testing at very low load is a new successful technique for study of mechanical properties on small scales or near surfaces. However, so far there is not a robust approach to determine plastic properties of metal materials using nanoindentation test. The aim of this paper is to present a method for determining the plastic properties, e.g. the true plastic stress–true plastic strain relation of metals combining nanonindentation test and finite element simulation. This methodology contains three main parts. Firstly, considering the special case of metals without strain hardening, the representative stress
ε
r
is determined by varying assumed representative stress over a wide range until the reverse and forward loading curves are consistent. Then, also by comparing the reverse and forward loading curves, the representative strain "r is obtained, but with different values of strain hardening exponent
n
, which are in the range of 0—0.6. Secondly, a series of simulations are performed for 124 combinations of each parameter (
E
,
σ
y
,
n
,
ν
) expressing the elastic–plastic behaviors of the universal engineering metals. Fom the computational results, a dimensinless function ∏
u
is constructed, and then the strain hardening exponent idetermined. At last, substituting the strain hardening exponent n into the power law constitution, the yield stress
σ
y
of metals is acquired. The examination of 5 kinds of metals from the forward analysis metal materials indicates that the dimensionless function ∏
u
has generality and the strain hardening exponent has stability and uniqueness. The accuracy of this method is also examined by comparing the elasto–plastic properties of practical metal AISI 304 steel obtained from nanoindentation test and finite element simulation with the tensile test results. In order to make the reverse analysis results get higher precision, in the practical applcation of this technique, the test error of nanoindentation should be maximally reduced.
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PREPARATION AND PROPERTIES OF HIGH TEMPERATURE SELF LUBRICATING COATINGS BY INDUCTION SINTERING
XIAO Ling DING Chunhua SUN Yanhua YU Lie
Acta Metall Sin, 2011,
47
(3): 327-332. DOI:
10.3724/SP.J.1037.2010.00585
Abstract
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(3650KB) (
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)
NiCr–based self–lubricating PM304 coatings have been used successfully in many high–temperature, high–speed applications such as air cycle machines, bleed air turbo compressors and turbo expanders. However, coarse PM304 coatings are not fully dense, containing a few percentages of pores and cracks which are harmful not only to the coating’s tribological properties but also to the coating’s overall mechanical strength. Furthermore, fine IS304 was prepared by the methods of high energy ball milling and induction sintering. The size oself–lubricating phase was refinedwith Ag particles of about 5 μm and BaF
2
/CaF
2
of about 1 μm. The results show that temperature and microstructure have a great effect on wear properties of IS304 and PM304 coatings. At the temperature of 20—340 ℃, the friction coefficients of two types of coatings (IS304 and PM304) are higher, and the microscopic brittle fracture is the dominant wear mechanism. With the increase of the temperature, the coefficient of the coatings decreases rapidly due to the formation of self–lubricating film on the worn surfaces which consist of Ag and fluoride. At the temperature of 340—700 ℃, since the self lubricating phase is fine and uniformly distributed in the IS304 coatings, it makes the formation of an intact lubricating film as well as Ag film fine and well distributed. Thus, the friction coefficient of IS304 is lower than that of PM304. With a further increase at the temperature of 700—800 ℃, there is the plastic flow on the surfces of the softened self–lubricating film under the influence of the friction force. Consequently, the friction coefficients of the coatings increase obviously. Comparing the PM304 with IS34 coatings, as the size of Ag film is large and he plastic flow of Ag film is much more severe than that of fluoride film, the friction coefficient of PM304 is higher than that of IS304.
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MAGNETISM OF MONOLITHIC AND PARTIALLY CRYSTALLIZED AMORPHOUS Al–Ni–Y ALLOYS
GONG Jing YANG Hongwang YANG Baijun WANG Ruichun LI Rongde WANG Jianqiang
Acta Metall Sin, 2011,
47
(3): 333-336. DOI:
10.3724/SP.J.1037.2010.00406
Abstract
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(466KB) (
728
)
Al
90
Ni
2
Y
8
and Al
84
Ni
8
Y
8
alloy ribbons were produced by melt–spinning, the structural characterization of the as–quenched samples was performed by XRD, the thermal stability of the as–quenched alloys was characterized using a differential scanning calorimeter (DSC), the magnetism of both Al
90
Ni
2
Y
8
and Al
84
Ni
8
Y
8
alloys fully amorphous and partially crystallized were investigated using a superconducting quantum interference device (SQUID). The results show that the magnetism of thamorphous Al
90
Ni
2
Y
8
and Al
84
Ni
8
Y
8
alloys are diamagnetism, and the alloys are magnetized more easily with higher Ni content. When the magnetic field reaches 0.5 T, the specific magnetizations of Al
90
Ni
2
Y
8
and Al
84
Ni
8
Y
8
alloy are −0.083 and −0.091 Am
2
/kg, and the magnetisabilities are −1.66×10
−5
and −1.2×10
−5
, respectively. The magnetism of alloys remains unchanged after partially crystallized, but the absolute value of specific magnetization is correspondingly increased. After partially crystallization, the absolute value of specific magnetization of the Al
90
Ni
2
Y
8
alloy increases from 0.083 Am
2
/kg to 0.231 Am
2
/kg, and that of the Al84Ni8Y8 alloy increases from 0.091 Am
2
/kg to 0.163 Am
2
/kg corresponding to a magnetic field of 0.5 T, and also the absolute value of magnetisability reaches to 4.62×10
−5
and 3.26×10
−5
, respectiely, which is attributed to the Ni and Y elemental build–up around the nanometer sized pure Al crystals after partially crystallizatin.
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STRUCTURE AND MAGNETIC BEHAVIOR OF Zn
1−x
Co
x
O CRYSTAL POWDERS PREPARED BY SOL–GEL TECHNIQUE
GAO Qian SUN Benzhe QI Yang QI Lianzhong
Acta Metall Sin, 2011,
47
(3): 337-343. DOI:
10.3724/SP.J.1037.2010.00541
Abstract
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(1195KB) (
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)
ZnO–based diluted magnetic semiconductors (DMSs) have been considered as one of the promising candidates for fabricating DMSs due to their initial prediction in theory of having the Curie temperature greater than room temperature, its high solubility for transition metals, and its superior semiconductor properties. Recently, Co substituted ZnO DMSs were reported frequently to show ferromagnetic properties at room temperature. However, various subsequent studies do not seem to converge on the origin of room–temperature ferromagnetism. Among the controversy, Co–cluster, Co–oxides and substitutions of Co for Zn are typical viewpoints. Meanwhile, it is not completely understood how the fabricating process influence the magnetic properties of Co–doped ZnO DMS. For this purpose, relationships among Co–doping concentration, sintered temperature, microstructure and magnetic properties of ZnO need to be invesigated particularly. In this paper, Zn
1−x
Co
x
O (
x
=0.01,0.02, 0.03, 0.04 and 0.05, atomic fraction) nanocrystal powders were prepared by sol–gel technique. The crystal structure, lattice parameters, orphology and composition were characterized and analyzed by XRD, TEM, SEM and EDS, respectively. The magnetic properties were examined at room temperature sing a vibrating sample magnetometer (VSM). It can be found that all the samples are composed of the particles with hexagonal wurtzite structure and the sizes of the particles are about 100 nm. For all synthesized Zn
1−x
Co
x
O samples, the lattice constants are smaller than those of un–doping ZnO crystals under the condition of the same sintered temperature. It indicates Co
2+
ions have substituted Zn
2+
sites. All the samples exhibit room–temperature ferromagnetic characteristics, and the ferromagnetism is their intrinsic attribute, in which the Zn
0.98
Co
0.02
O sample sintered at 500 and 700 ℃ has the highest coercivity (
H
c
) of 334.02 Gs and the highest remanent magnetization ratio (
M
r
/
M
s
) of 0.1813, respectively, and the Zn
0.96
Co
0.04
O sample sintered at 950 ℃ has the maximum magnetic energy product (
BH
) of 1.764×10
−4
J/kg ad saturation magnetization (Ms) of 0.5583 Am
2
/kg. he magnetic behaviors of tese samples vary not only with the cocetration of Co, but also with the size of crystal grains and te concentration of oxygen vacacies. It can be found that the moderate Co content and the lager size of crystal grains are in favour of increasing room–temperature ferromagnetic characteristics. Meawhile, as far as the same concentration of Co is concened, the smaller volume of the cecontributes to increase room–temperature ferromagnetism.
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MAGNETOCALORIC EFFECT IN MnFeP
0.63
Ge
0.12
Si
0.25
B
x
(
x
=0, 0.01, 0.02, 0.03) COMPOUNDS
WANG Dongmei SONG Lin WANG Yaohui ZHANG Wei Bilige Tegus O
Acta Metall Sin, 2011,
47
(3): 344-348. DOI:
10.3724/SP.J.1037.2010.00566
Abstract
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(877KB) (
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)
Structural and magnetocaloric properties of the MnFeP
0.63
Ge
0.12
Si
0.25
B
x
(x=0,0.01, 0.02, 0.03, atomic fraction) compounds have been inestigated. XRD result shows that the MnFeP
0.63
Ge
0.12
Si
0.25
B
x
(x=0, 0.01, 0.02, 0.03) compounds mainly consist of the Fe
2
P–typhxagonal structure phase with space group
P
¯62
m
. It is fond that the lattice paraeter
a
increases and the lattice parameter
c
slighly decreases with increasing x, and the unit celvolume does not change. The agnetic measurements show that the Curie temperature increases from 300 K to 347 K, and thermal hysteresis are 20, 17, 11 and 6 K for x= 0 to 0.03. The aximm values of the magnetic entropy chage are 11.1, 11.26 and 11.8 J/(kg·K), respectively, or 0—1.5 T fields.
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RESEARCH ON THE SELECTIVELY DISSOLUTION CHARACTERISTICS OF Cu–Mn ALLOYS AND FABRICATION OF NANOPOROUS COATINGS BY LASER CLADDING HYBRID ELECTROCHEMICALLY DEALLOYING
GU Yu DONG Changsheng MA Mingxing ZHONG Minlin LIU Wenjin
Acta Metall Sin, 2011,
47
(3): 349-353. DOI:
10.3724/SP.J.1037.2010.00421
Abstract
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(1307KB) (
983
)
Nanoporous metals have attracted considerable attention for a wide range of applications in catalysis, sensing and bio–detection due to their large surface–to–volume ratios and excellent thermal and electrical conductivities. It has been proven that dealloying is the most effective method to yield nanoporous metals. Recently, a number of nanoporous metals, including Au, Ag and Pt have been synthesized by dealloying. Most of the research focused on etching the more active components from the precursors in different electrolyte owing to their higher reactivity, such as Ag, Al, Zr and Mn. In this paper selective dissolution of Cu or Mn components from Cu–Mn alloys was reported, and nanoporous coatings were fabricated by a two–step process involving high power laser cladding of a homogeneous Cu40Mn60 alloy coatings followed by selectively electrochemical dealloying. Cu–Mn alloy coatings with fine shape, low dilute ratio and refined microstructure were fabricated on mild steel by means of laser processingThe second dendrite arm spacing (SDAS) decreased with the increasing of lser remelting speed. The SDAS can be refined to be 1.6 μm with the laser remelting speed 83 mm/s. Polarization curve indicates that Cu–Mn alloy shows selective dissolution characteristics for different electrolytes. Nanoporous Cu and nanoporous Mn were fabricated with optimal electrochemically dealloying parameters in 0.1 mol/L HCl and 0.1 mol/L KNO
3
solution respectively. Nanoporous Cu with pore size ranes from 30 nm to 50 nm, while the surface morphology of the porous Mn was a ribbon–like structurwith ultrahigh roughness factor up to 900.
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EFFCTS OF pH AND Cl
−
CONCENTRATION ON THE CORROSION BEHAVIOR OF COPPER IN BORIC ACID BUFFER SOLUTION
WANG Changgang DONG Junhua KE Wei CHEN Nan
Acta Metall Sin, 2011,
47
(3): 354-360. DOI:
10.3724/SP.J.1037.2010.00440
Abstract
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(1038KB) (
1019
)
The strategy for disposal of high–level radioactive waste in china is to enclose the spent nuclear fuel in sealed metal canisters which are embedded in bentonite clay hundreds meters deep in the bed–rock. The choice of container material depends largely on the redox conditions and the aqueous environment of the repository. One of the choices for the fabrication of waste canisters is copper, for it is thermodynamically stable under the saline, anoxic conditions over the large majority of the container lifetime. For this advantage, some other countries (Canada, Sweden) have selected copper as the material of nuclear waste container. However, in the early aerobic phase of the geological disposal the corrosion of copper could take place, and the corrosion behavior of copper would be influenced by the complex chemical conditions of groundwater markedly. Regularly, in atmosphere environment the semiconductor passive film which is constructed by Cu
2
O would generate on the surface of copper. On the one hand, the Cu
2
O film could protect copper from corrosion. On the other hand, the formation of Cu
2
O film is necessary to maintain the propagation of crack during the stress corrosion cracking. Therefore, the study of the effect of water chemistry conditions on the stability of Cu
2
O passive film makes great sense for the general corrosion and stress corrosion cracking of copper. For copper and copper alloy, Cl
−
is a highly aggressive ion. Lots of cases of failure and corrosion behavior of copper and copper alloy in Cl
−
environment have been studied. The immersion of Cl
−
would affect the semconductor properties of Cu
2
O, therefore, we focused on the effect of pH and Cl
−
concentration on the corrosion behavior and semiconductor properties of Cu
2
O in this paper. The polarization behavior of Cu electrodes and the stability of passive film Cu
2
O in boric acid buffer solution have been investigated respectively by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and semiconductor capacitance method (Mott–Schottky method). The results showed that the destruction and dissolutioof the passive film Cu2O happened in the conditions of low pH and high Cl
−
concentration. The semconductor character of passive film changed from p–type into n–type in the solution with high Cl
−
concentration, so that Cl
−
is more likely to enter the passivfilm and take complex reaction wth Cu
+
, which could destroy thpassive film and accelerate corrosion. The passive film Cu
2
O is more steady in the condition of high pH and low Cl
−
concentration.
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EFFECT OF COLD DEFORMATION ON THE CORROSION BEHAVIOUR OF Mn–CONTAINING ALUMINIUM ALLOY TUBE
WANG Guan LIN Xiaoqun
Acta Metall Sin, 2011,
47
(3): 361-366. DOI:
10.3724/SP.J.1037.2010.00636
Abstract
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(1468KB) (
1005
)
Aluminium alloys are extensively employed as heat transfer tube materials in heat exchangers. Previous work has focused on the corrosion behaviour of tube materials in brazed heat exchangers. However, very little attention has been devoted to corrosion of tubes in mechanically–expanded heat exchangers despite the observation of some instances of early corrosion failure in bent region of aluminium alloy tube. The effects of cold deformation on the corrosion behaviour of aluminium tube materials have been studied in 0.6 mol/L NaCl and SWAAT solutions by SEM, TEM, potentiodynamic polarizations and immersion test. The results show that high Mn content (0.22%, mass fraction) can experience preferential corrosion and early failure in the bent region, but not for one with a lower Mn content (0.08%). SEM/TEM observations of the microstructure of the alloys show that each alloy has one main type of coarse intermetallic particle. However, TEM observations show that there is a distinct difference in particle morphology between the bent and straight regions of the high Mn alloy tube, the ent region has more nano–scale Mn–rich particles than the straight region, and no such effects are observed on the low Mn alloy. The microelectrochemical polarisation measurements show that the straight region of high Mn has highest pitting potential, but cold deformation can decrease the pitting potential of bent region of high Mn tube, but no such effects on low Mn tube. The immersion test shows that the bent region of high Mn alloy has highest attack; this is associated with precipitation of 20—100 nm Mn–rich particles, which cause increased anodic reactivity as they provide further pit initiation sites and cause solute depletion in the matrix. In addition, the Mn–rich particles are aso sites for enhanced cthodic reactivity. The relationship between the microstructure and electrochemicaproperties of tube materials before and after cold deformation is established. The results indicate Mn can improve the corrosion resistance of aluminium alloy, but the mechanical cold deformation will weaken the effect.
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RESEACH ON LUBRICATION IN HOT EXTRUSION OF G3 CORROSION RESISTANT Ni–BASED ALLOY TUBE I. Establishment and Application of Glass Lubricating Film Thickness Model
WANG Baoshun LIN Ben ZHANG Maicang DONG Jianxin
Acta Metall Sin, 2011,
47
(3): 367-373. DOI:
10.3724/SP.J.1037.2010.00511
Abstract
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(1127KB) (
947
)
In the hot extrusion of steel, adequate lubrication can be obtained by placing a glass pad made of compacted glass powder in front of the billet. During the extrusion, the glass pad progressively melts in thin layer, allowing lubrication to be maintained over the entire stroke. However, the thickness of glass pad in a single operation is markedly depends on the process parameters and glass properties. Therefore, the mechanism of glass lubrication film forming and film thickness calculation model has been investigated during hot extrusion of G3 nickel–based alloy tube. Furthermore, combined with FEM code, the operation variables and viscosity of glass lubricant have been systematically researched in the hot extrusion of G3 alloy tube which is conducted on the 6000 t horizontal extrusion press. The results showed that the processing parameters can be optimized using the thickness formulation of lubrication film and glass pad during a single press operation. Moreover, under the prctically technical parameters of hot extrusion G3 alloy tube, the requirements of glass viscosity are s follows: softening temperature about 720 ℃; the viscosity coefficient ranges −0.05 ℃
−1
to −0.04 ℃
−1
when temperature is between 720 and 800 ; the viscosity ranges 25 Pa·s to 200 Pa·s uring the hot working temperature 1100—1200 ℃.
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RESEACH ON LUBRICATION IN HOT EXTRUSION OF G3 CORROSION RESISTANT Ni–BASED ALLOY TUBE II. Calculation and Application of Glass Lubricant Viscosity–Composition
LIN Ben WANG Baoshun ZHANG Maicang DONG Jianxin
Acta Metall Sin, 2011,
47
(3): 374-379. DOI:
10.3724/SP.J.1037.2010.00512
Abstract
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(635KB) (
1327
)
Based on the classical first–order mixture method which describes the relationship between the property value and component, a modified mixture method for predicting the viscosity property of borosilicate glass lubricant used in the hot extrusion of G3 alloy was established. Then, the viscosity properties of 10 different borosilicate glasses were predicted using this method. The viscosity data predicted by this calculation method coincided well with the experimental data, which showed that the calculaton model was of strong reliability. Therefore, according to this model, the glass composition could be reversely designed, which will assure the viscosity of glass to meet the requirements of glass lubricated hot extrusion of G3 alloy which has beeraised in the first article. It provides theoetical guidance for the design and preparation of glass lubrcant in the glass lbricated hot extrusion of G3 alloy.
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STUDY ON LATERAL GROWTH RATE OF PERITECTIC REACTION PRODUCTS
CHANG Guowei JIN Guangcan CHEN Shuying LI Qingchun YUE Xudong
Acta Metall Sin, 2011,
47
(3): 380-384. DOI:
10.3724/SP.J.1037.2010.00409
Abstract
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(805KB) (
872
)
Aiming at the problem which the experimental results of lateral growth rates of austenite (γ–phase) in the peritectic reaction of Fe–C alloy is much larger than the calculation based on Bosze and Trivedi
[2]
models, solute distribution in front of the solid/liquid interface in the peritectic reaction was analyzed in detail in this article, and the expression of solute distribution while the cylindrical solid phase gew in melt was also put forward. On the basis of the growth rate formula for the coarse interface deduced by Jackson
[9]
, the advisable epression was obtained for calculating ateral growth rate of the peritectic reaction products, and which was applied to the peritectic reaction progress of Fe–C nd Fe–Ni alloys, respectively. The results showed that the calculation lateral growth rates of the peritectic reaction products (γ–phase) were coincide with the experimental results made by Shibata et al
[4]
and Mcdonald et al
[5]
through the observation of the peritectic reaction of Fe–C and Fe–Ni alloys using a confocl scanning laser microscope (CSLM).
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