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Study on Stability of Residual Stress Induced by Laser Shock Processing in Titanium Alloy Thin-Components
Weifeng HE, Xiang LI, Xiangfan NIE, Yinghong LI, Sihai LUO
Acta Metall Sin    2018, 54 (3): 411-418.   doi:10.11900/0412.1961.2017.00135
Accepted: 14 September 2017

Abstract78)   HTML2)    PDF (2445KB)(520)      

Because the compressor thin-blades of aero-engine often fractured in service, laser shock processing was suggested to be applied as a surface strengthening technology. Aim at the problem of compressive residual stress relaxation in laser-peened compressor thin-blades, TC11 titanium alloy thin-components were treated by laser shock processing and then conducted in axial tensile-tensile fatigue test and thermal insulation in vacuum. X-ray diffraction tests were carried out to obtain the relaxation rules of residual stress under fatigue loading and thermal stress loading. In addition, the relaxation mechanisms of residual stress were indicated. Experiment results demonstrate that surface compressive residual stress relaxes by 53%, and 95% of stress relaxation occurs in the previous 5 fatigue cycles under the fatigue loading (maximum stress σmax=500 MPa, stress ratio R=0.1). The surface relaxation degree and severely-relaxed depth increase with fatigue loading, and the relaxation mechanism is that plastic deformation of local area material results in residual stress redistribution. Surface compressive residual stress relaxes by 3%, 29% and 48% respectively after thermal insulation for 120 min under the constant temperature of 200 ℃, 300 ℃ and 400 ℃. Surface compressive residual stress relaxes by 18% and 58% respectively after thermal insulation for 120 min under the altering temperature of 200 ℃+400 ℃ and 300 ℃+400 ℃. The relaxation all occurs in the previous 60 min. There is a similar trend with temperature in the aspect of severely-relaxed depth. The relaxation mechanism under thermal stress loading is that dislocations and grain-boundaries are activated to move and annihilated, and then plastic deformation recovery occurs. Due to the distinction of relaxation mechanisms, there is an obvious superimposed effect under the combined action of fatigue loading and thermal stress loading.

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In Vitro Corrosion Resistance of Ta2N Nanocrystalline Coating in Simulated Body Fluids
Jiang XU, Xike BAO, Shuyun JIANG
Acta Metall Sin    2018, 54 (3): 443-456.   doi:10.11900/0412.1961.2017.00246
Accepted: 19 December 2017

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Due to its combination of outstanding characteristics, such as superior biocompatibility, excellent mechanical properties as well as good corrosion resistance, Ti-6Al-4V alloy has gained much attention as one of the most popular load-bearing biomedical metals in the area of orthopedic and dental. Unfortunately, Ti-6Al-4V alloy suffers from the localized corrosion damage in human body ?uids containing high chloride ion concentrations, which leads to the release of metal ions into the human body. The released ions (e.g., Al and V) are found to not only cause allergic and toxic reactions but also exhibit potential negative effects on osteoblast behavior. To improve the corrosion resistance of Ti-6Al-4V alloy in simulated body ?uids, a 40 μm thick Ta2N nanocrystalline coating with an average grain size of 12.8 nm was engineered onto a Ti-6Al-4V substrate using a double cathode glow discharge technique. The hardness and elastic modulus of the Ta2N coating were determined to be (32.1±1.6) GPa and (294.8±4.2) GPa, respectively, and the adhesion strength of the coating deposited on Ti-6Al-4V substrate was found to be 56 N. There is no evidence of crack formation within the coating under loads ranging from 0.49 N to 9.8 N, implying that the Ta2N nanocrystalline coating has a high contact damage resistance. Moreover, the corrosion resistance of the Ta2N nanocrystalline coating is significantly greater than that of Ti-6Al-4V alloy when tested in naturally aerated Ringer's solution at 37 ℃. This is due to that the passive film developed on the coating has superior compactness compared with that formed on the uncoated Ti-6Al-4V alloy. XPS analysis indicated that at a low polarized potential, the passive film consisted of TaOxNy, which would be converted to Ta2O5 at a higher polarized potential. The analysis of Mott-Schottky curves suggested that the passive film formed on the coating exhibits n-type semiconductor properties and, as such, the density and diffusivity of carrier for the coating was considerably lower than that for the uncoated Ti-6Al-4V alloy.

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A First-Principles Study on Basal/Prismatic Reorientation-Induced Twinning Path and Alloying Effect in Hexagonal Metals
Gang ZHOU, Lihua YE, Hao WANG, Dongsheng XU, Changgong MENG, Rui YANG
Acta Metall Sin    2018, 54 (4): 603-612.   doi:10.11900/0412.1961.2017.00252
Accepted: 22 August 2017

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In hexagonal metals and alloys, deformation twinning plays an important role, because it is closely relevant to the mechanical behaviors. Recent studies have proposed a new twinning mode via direct lattice reorientation, which results in the basal/prismatic boundary, however, some important details remain unanswered, e.g., the twinning path and alloying effect. In this work, first principles calculations were employed to systematically study the reorientation process from basal to prismatic orientation in hexagonal metals and corresponding alloying effect. The result indicates that different activation energies are required to reorient in various hexagonal metals, and among them, the energy in Mg is the lowest and Os is the highest. Shear and shuffle components compose the reorientation process, where the shuffle component always contributes a significant part of the activation energy in Mg, whereas in Ti with sufficient shear strain, subsequent transition becomes energy-downhill. The pure shear was effected by alloying elements in Mg alloys, but pure shuffle in Ti alloys. Under certain shear or shuffle, subsequent activation energy has a complex dependence on alloying elements.

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Rapid Solidification of Ti-6Al-4V Alloy Micro-Droplets Under Free Fall Condition
Bin ZHAI, Kai ZHOU, Peng Lü, Haipeng WANG
Acta Metall Sin    2018, 54 (5): 824-830.   doi:10.11900/0412.1961.2017.00312
Accepted: 27 November 2017

Abstract109)   HTML2)    PDF (7656KB)(548)      

Especially in the past decades, Ti-6Al-4V alloy has received much attention, not only due to its high melting temperature, good corrosion resistance, low density and high hardness, but also because of the diverse and complicated microstructures formed under different conditions. This makes Ti-6Al-4V a potential candidate in both aerospace industries and fundamental research. It is well known that the solidified microstructures of alloy have a great influence on their mechanical properties. Therefore, it is crucial to investigate the mechanical properties of Ti-6Al-4V solidified under different conditions, in particular in the undercooling conditions. However, it is noted that most research on the solidification of Ti-6Al-4V alloy was carried out under equilibrium condition. With respect to Ti-6Al-4V alloy solidified under substantial undercooling conditions, few studies could be found. Thus, it is interesting to study two points: (1) the feature of the microstructure of Ti-6Al-4V alloy solidified under highly undercooled conditions and large cooling rate, (2) the influence of undercooling and cooling rate on the mechanical property of Ti-6Al-4V alloy. To address these two problems, Ti-6Al-4V alloy was rapidly solidified in a drop tube. The main results are summarized as follows. The microstructure of the Ti-6Al-4V alloy solidified under free fall condition displays "lamellar α+βα dendrites→basket-weave α'+β→ needle-like α'→ needle-like α'+ anomalous β " transformation with decreasing the droplets diameter. And the needle-like α' phase in the original boundaries of equiaxed β grains is transformed into a continuous distribution and anomalous structure of β phase when the droplet size is less than about 400 μm. The microhardness of this alloy ranges from 506 kg/mm2 to 785 kg/mm2 when the droplet diameter decreases from 1420 μm to 88 μm, which is much higher than that of the master alloy. For "lamellar structure of α+β phases", "needle-like α' phase" and "needle-like α' phase+ anomalous β phase", the microhardness increases with the decrease of droplet diameter. But for 'basket-weave' microstructure, the microhardness diminishes with the decrease of droplet diameter.

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Preparation and Properties of Biological Functional Magnesium Coating on Ti6Al4V Substrate
Xiaoming YU, Lili TAN, Zongyuan LIU, Ke YANG, Zhonglin ZHU, Yangde LI
Acta Metall Sin    2018, 54 (6): 943-949.   doi:10.11900/0412.1961.2017.00285
Accepted: 09 November 2017

Abstract52)   HTML3)    PDF (4451KB)(456)      

Currently, metallic biomaterials used in orthopedics are normally bioinert which is hard to integrate with the bone tissue inducing aseptic loosening and easy to get infection, which is the main reason of implantation failure. Mg base metals are considered to be a new generation of revolutionary metallic biomaterials due to its similar density and mechanical properties with natural bone, good biocompatibility, degradability in the body as well as the biological functional ability to promote new bone tissue formation. In addition, the degradation of Mg may increase the local pH which can inhibit the growth of bacteria. In this work, pure Mg coating was deposited on Ti6Al4V substrate by arc ion plating. The effects of different working pressures on the surface quality and properties of Mg coating were investigated. The degradation, antibacterial and biosafety properties were studyied. The results showed that the pure Mg coating can be deposited on the surface of Ti6Al4V substrate and the coating was uniform and smooth. The immersion test in vitro showed that the degradation was very fast because of galvanic corrosion, and the whole process was finished in about one week. The results of antimicrobial experiments showed that the Mg coating can kill staphylococcus aureus and showed good antibacterial function. The results of cytotoxicity test showed that Mg coating promoted rabbit bone marrow mesenchymal stem cells (rBMSCs) growth and proliferation.

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Influence of Layer Thickness on Microstructure and Mechanical Properties of Selective Laser Melted Ti-5Al-2.5Sn Alloy
Piao GAO, Kaiwen WEI, Hanchen YU, Jingjing YANG, Zemin WANG, Xiaoyan ZENG
Acta Metall Sin    2018, 54 (7): 999-1009.   doi:10.11900/0412.1961.2017.00384
Accepted: 02 January 2018

Abstract100)   HTML4)    PDF (6969KB)(584)      

As an additive manufacturing technology, selective laser melting (SLM) process can solve the manufacturing difficulty of Ti-5Al-2.5Sn (TA7) easily. But the low building efficiency of SLM retards its wide applications in aviation, petrochemical and other fields. In order to solve the above problem, the influence of layer thickness on relative density, microstructure and mechanical properties of SLMed TA7 samples were studied in this work. The results show that when the laser power and hatching space are constant, the relative density gradually increases with the decrease of the laser volume energy density under the layer thicknesses less than or equal to 40 μm, whereas first increases and then declines with the decrease of the laser volume energy density under the layer thicknesses larger than 40 μm. At the same time, with the increase of layer thickness and the decrease of scanning velocity, the cooling rate gradually decreases during the SLM processing, when the cooling rate is lower than 6.8×107 K/s, the microstructure will gradually transform from acicular martensite α' to massive αm. Through the optimization of SLM parameters, the dense TA7 bulk specimens with higher microhardnesses, yield strengths and ultimate strengths in comparison to the as-cast and deformed TA7 alloys can be obtained under all layer thicknesses (20~60 μm). While when the layer thicknesses are not larger than 40 μm, the ductility of the SLMed TA7 is also superior to that of the as-cast TA7 and comparable to that of the deformed TA7. Finally, the optimal layer thickness and combination of SLM process parameters are successfully determined to balance the building efficiency, metallurgical quality and mechanical properties of the TA7 alloy parts.

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Kinetics of (Ti, V, Mo)C Precipitated in γ /α Matrix of Ti-V-Mo Complex Microalloyed Steel
Ke ZHANG, Xinjun SUN, Mingya ZHANG, Zhaodong LI, Xiaoyu YE, Zhenghai ZHU, Zhenyi HUANG, Qilong YONG
Acta Metall Sin    2018, 54 (8): 1122-1130.   doi:10.11900/0412.1961.2018.00011
Accepted: 04 June 2018

Abstract76)   HTML0)    PDF (852KB)(567)      

In recent years, in order to develop the higher strength steel, the idea of increasing the strength of the hot rolled ferritic steel via complex Ti microalloyed technology has been widely accepted and applied, such as Ti-Nb, Ti-Mo, Ti-Nb-Mo and Ti-V-Mo. It is important to know the thermodynamics and kinetics of complex Ti contained precipitates for controlling the precipitation behavior of carbides and improving the mechanical properties of complex Ti microalloyed steels. In this work, according to the classical nulceation and growth kinetics theory and the solubility products of various carbides in austenite/ferrite (γ /α) matrix, the precipitation-time-temperature (PTT) curve, nucleation-time (NrT) curve and the nucleation parameters of (Ti, V, Mo)C carbides in γ /α matrix of Ti-V-Mo complex microalloyed steel were obtained through the theoretical calculation. Moreover, the effects of deformation stored energy and the amount of strain-induced precipitation in γ matrix on the precipitation kinetics of (Ti, V, Mo)C were discussed. The results showed that the PTT diagrams of (Ti, V, Mo)C in γ /α matrix showed "C" shape curve, while the NrT curves showed inverse "C" shape curve. The nose temperature of (Ti, V, Mo)C in γ matrix is about 1020~1050 ℃. Increasing the deformation stored energy of γ matrix moves the PPT curve to the upper left. In addition, the NrT curve of (Ti, V, Mo)C precipitated in α matrix moves towards to the lower right by properly increasing the amount of strain-induced precipitation in γ matrix. The maximum nucleation rate temperature of (Ti, V, Mo)C in ferrite is around 630~650 ℃ from the theoretical calculation, which agrees well with the result of experimental observation.

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Effect of {332}<113> Twins Combined with Isothermal ω-Phase on Mechanical Properties in Ti-15Mo Alloy with Different Oxygen Contents
Xiaohua MIN, Li XIANG, Mingjia LI, Kai YAO, Satoshi EMURA, Congqian CHENG, Koichi TSUCHIYA
Acta Metall Sin    2018, 54 (9): 1262-1272.   doi:10.11900/0412.1961.2018.00022
Accepted: 13 April 2018

Abstract81)   HTML8)    PDF (5221KB)(461)      

β-type alloys have a wide application prospect in aerospace, biomedical and marine engineering and other fields, owing to their high specific strength, good corrosion resistance and low elastic modulus. Their yield strength and uniform elongation are affected by the second phase precipitation, plastic deformation mode and interstitial element, especially the oxygen element. In this work, the effect of tensile pre-deformation induced {332}<113> twins combined with isothermal ω-phase after subsequent ageing on the mechanical properties of β-type Ti-15Mo alloy with different oxygen contents from 0.1% to 0.5% (mass fraction) was examined by OM, XRD, TEM and DSC, Vickers hardness tester and tensile testing machine. The results indicated that with increasing the oxygen content, the formation of mechanical twins and isothermal ω-phase in the alloy was suppressed, and the effect of pre-deformation induced twins on the precipitation of isothermal ω-phase was negligible. After pre-deformation combined with subsequent ageing, the alloy with low oxygen content had the relatively high yield strength and large uniform elongation, but it with high oxygen content exhibited the brittle fracture. A good combination of strength with ductility in the alloy with low oxygen content was contributed to the twinning and dislocation slip coupled deformation. The high yield strength was mainly dominated by the dislocation slip, and the large uniform elongation was due to the static and dynamic grain refinement effects, which were caused by the pre-deformation induced twins and subsequent twinning deformation, respectively. Through utilizing the alloying element of oxygen effectively, and changing the plastic deformation mode and phase precipitation behavior based on the reasonable process of pre-deformation and heat treatment, the combination of strength and ductility can be controlled in a large range for the β-type titanium alloys.

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Research on the Microstructure and Properties of In Situ (TiB2-TiB)/Cu Composites
Jianqiang REN, Shuhua LIANG, Yihui JIANG, Xiang DU
Acta Metall Sin    2019, 55 (1): 126-132.   doi:10.11900/0412.1961.2017.00532
Accepted: 08 May 2018

Abstract88)   HTML0)    PDF (5229KB)(484)      

Copper matrix composites have attracted a lot of interest regarding their application as electrical materials. However, the development of copper matrix composites has suffered setbacks because of a trade-off between electrical conductivity and strength. In this work, TiB2 particles and TiB whiskers hybrid reinforced copper matrix composites were in situ fabricated by mechanical alloying and hot pressing. The microstructures of hot-pressed composites were characterized by XRD, OM, SEM and TEM. The mechanism of in situ reaction during hot pressing process and the influence of microstructures on physical properties of hot-pressed composites were analyzed. The Cu and Ti raw powders were firstly reacted at 800 ℃ by forming Cu3Ti transient phase. Then, the Cu-Ti liquid micro-zone was formed at 850 ℃, which is higher than the melting point of Cu3Ti phase. With the increasing of temperature further, TiB2 particles and TiB whiskers were formed in the liquid micro-zone by the diffusion of B atoms from copper matrix. When the reinforcing phase is consisted of mainly TiB whiskers, the hardness of composites is relatively high. But the composites reinforced mainly by TiB2 particles have a higher electrical conductivity. The combined properties of hybrid reinforced copper matrix composites were optimized due to the combination action of TiB2 particles and TiB whisker. For the case of 3%(TiB2-TiB)/Cu composites, the hardness and the electrical conductivity are 86.6 HB and 70.4% IACS, respectively.

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Effect of Cold Rotary-Swaging Deformation on Microstructure and Tensile Properties of TB9 Titanium Alloy
Dechun REN, Huhu SU, Huibo ZHANG, Jian WANG, Wei JIN, Rui YANG
Acta Metall Sin    2019, 55 (4): 480-488.   doi:10.11900/0412.1961.2018.00241
Accepted: 04 July 2018

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TB9 titanium alloy has been widely used for aerospace due to it's superior low stiffness, corrosion resistance and workability. It has been reported that cold deformation can improve the comprehensive mechanical properties of titanium alloys. At the same time, the cold rotary-swaging deformation facilitates the production of small batches and the acquisition of special shape and size bars. However, current studies on the microstructure and properties of cold rotary-swaged titanium alloys are not systematic. So, the effects of cold deformation rate on the microstructure, texture evolution and mechanical property of TB9 alloy during cold rotary-swaging were investigated using OM, EBSD, XRD, TEM and tensile test. The results showed that the grain size of TB9 titanium was refined with the increase in diameter reduction. Meanwhile, with the deformation increases, the grains rotation along the swaging axis occurs, forming a preferred orientation, the textures change from initial {001}<110> and {001}<100> to α-fiber and γ-fiber {001}<110>, {112}<110> and {111}<110>. All of grains refinement, texture components and substructures contributed to the enhancement of strength after cold rotary-swaging. And the ductile kept on a high level after 70% cold working, which means the TB9 titanium has a great cold deformation ability.

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Effect of Pulsed Magnetic Field on the Microstructure of TC4 Titanium Alloy and Its Mechanism
Qingdong XU, Kejian LI, Zhipeng CAI, Yao WU
Acta Metall Sin    2019, 55 (4): 489-495.   doi:10.11900/0412.1961.2018.00257
Accepted: 16 January 2019

Abstract96)   HTML1)    PDF (5361KB)(429)      

In this work, the effect of pulsed magnetic treatment (PMT) on the microstructure of TC4 titanium alloy was investigated. TC4 titanium alloy is widely used in the manufacture of the blade of aviation engine. The microstructure of TC4 titanium alloy determines its property. PMT is a novel method used to modify the microstructures of alloys and has been explored in several papers recently. PMT has many advantages in the aspect of efficiency, energy-saving, non-deformation, etc. Therefore, the effect of PMT on the microstructures of TC4 titanium alloy was explored in this work. The variation of the dislocation density and the grain boundary angle of TC4 titanium alloy was observed after PMT. In the experiment, the magnetic induction density is 2 T, the pulse frequency is 5 Hz and the pulse number is 100. According to XRD tests, the dislocation density in TC4 alloy after PMT increased about 10.9%. KAM maps in EBSD test were used for evaluating the same area's dislocation density of the TC4 alloy before and after PMT. The dislocation distribution of TC4 titanium alloy changes notably: the in-grain dislocation density became more homogeneous and some local high-density areas disappeared, the distribution of dislocation near grain boundaries caused the angles of the grain boundaries altered and the fraction of low-angle grain boundaries decreased while the fraction of Σ11 grain boundaries (CSL grain boundary) increased. The motivation mechanism of the dislocation in TC4 titanium alloy under PMT was speculated based on the experimental results and some previous researches. The PMT may change the energy state of the electrons in pinning area of dislocations, which accelerates the electrons transformation from singlet state to triplet state and then increases the mobility of the vacancy or impurity atoms so that the dislocation de-pinning could occur under the original stress field and thus leads to dislocation movement and transformation of microstructure.

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Microstructure and Tensile Property of TC4 Alloy Produced via Electron Beam Rapid Manufacturing
Zheng LIU,Jianrong LIU,Zibo ZHAO,Lei WANG,Qingjiang WANG,Rui YANG
Acta Metall Sin    2019, 55 (6): 692-700.   doi:10.11900/0412.1961.2019.00007
Accepted: 04 April 2019

Abstract98)   HTML2)    PDF (10313KB)(479)      

Electron beam rapid manufacturing (EBRM) is one of the 3D printing technologies. The main attractions of EBRM technology are its high efficiency and economy in fabricating large, complex near net shape components dielessly and only needing limited machining. In general, the microstructure and texture of titanium alloy can play a significant role in determining its mechanical behaviors. In the present work, the microstructure, texture and tensile property of TC4 alloy produced by electron beam rapid manufacturing (EBRM) are investigated. Results show that the microstructure is comprised of columnar prior β grains that orient parallel to the building direction. The width of the columnar β grains increased rapidly at the initial several build layers, and the subsequent increase rate of the width of the columnar β grains tends to slow down. Fine α lamellae with gradient size are observed inside the columnar prior β grains, which occur because the alloy experiences different complex thermal histories during the EBRM-produced process. The size of α lamellae tends to decrease with the increase of build layers. The XRD result shows that the TC4 alloy has a typical α phase texture, (the c-axes are either concentrated at about 45° or are perpendicular to the building direction). At the same time, the <$10\bar{1}0$> poles are relative to random distribution. For the tensile samples along the electron beam scanning direction, the yield strengths do not show significant change with the increase of build layers, but the tensile strengths increase. The ductility of the alloy also has an upward trend, despite of a slightly decreasing ductility in the top sample. The tensile samples at the bottom of the alloy (10 mm and 20 mm away from the substrate) have similar work hardening exponents, which are lower than the top sample. The top sample shows the highest work hardening exponent. This difference in the tensile properties can be highly attributed to the gradient microstructure. The alloy also presents obvious anisotropy in tensile strength. The tensile sample along the 45° direction has a higher strength than the sample along the X direction, while the tensile sample along the Z direction shows the lowest strength. This anisotropic strength is strongly associated with the α phase texture. When the loading direction is 45° to the building direction, most of the c-axes of α phase are about parallel to the loading direction, showing a "hard" orientation, leading to a higher strength than other oriented samples. Conversely, when the loading direction is along the building direction, most of the α phase present a "soft" orientation, resulting in lower strength compared to the tensile samples along the 45° or the X direction.

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Influence of Alloying Elements Partitioning Behaviors on the Microstructure and Mechanical Propertiesin α+β Titanium Alloy
Sensen HUANG,Yingjie MA,Shilin ZHANG,Min QI,Jiafeng LEI,Yaping ZONG,Rui YANG
Acta Metall Sin    2019, 55 (6): 741-750.   doi:10.11900/0412.1961.2018.00460
Accepted: 21 February 2019

Abstract139)   HTML2)    PDF (22782KB)(521)      

During the thermal treatments of α+β titanium alloys in (α+β) phase field, alloying element partitioning effect takes place accompanying with the α?β transformation, which results in the segregation of α stabilizing elements (Al, O) and β stabilizing elements (V, Mo, etc.) into the corresponding phases respectively. The element partitioning effect will further affect the microstructure characteristics (phase constitution, microstructure size), plastic deformation modes and the final mechanical properties of the alloy. In this work, the influences of solution temperature and cooling rate on the element partitioning behavior during solution process of Ti-6Al-4V alloy in (α+β) phase field were investigated. The element concentrations in primary α phase (αp) and β transformed region (βt) were characterized by EPMA technique. The microstructural variation of βt with respect to solution temperature was analyzed. It was found that βt showed an obvious increase of Al content and decrease of V content with the increasing of solution temperature, while the αp exhibited less noticeable change, which led to the reduction of concentration difference between the two phases. Under the same solution temperature, the microstructures and element distributions at different cooling rates (water quenching, air cooling, furnace cooling) were exhibited. The slow cooling processing especially furnace cooling would induce higher volume fraction of αp phase and more pronounced element partitioning. The microstructural characteristics of βt cooled from different solution temperatures were further analyzed. During the water or air cooling process, the transformations of β→matensite/αs happened, and the sizes of martensite or αs were postulated to be dependent on the element concentration of β phase. The properties of local microstructure (αp, βt) were further measured by nanoindentation. It indicates that the intrinsically anisotropic character of the hexagonal crystal structure (hcp) of the αp phase has decisive consequences for the properties, while the elastic modulus and hardness of βt calculated by nanoindentation are mainly dominated by the width of αs lamellas. On the basis of the above results, the relationship between solution temperature, element concentration of local microstructure, microstructure size and mechanical properties of local microstructure was finally discussed.

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Study on Interface of Linear Friction Welded Joint Between TC11 and TC17 Titanium Alloy
Suigeng DU,Man GAO,Wanting XU,Xifeng WANG
Acta Metall Sin    2019, 55 (7): 885-892.   doi:10.11900/0412.1961.2018.00512
Accepted: 29 April 2019

Abstract89)   HTML2)    PDF (22210KB)(367)      

As a solid-state welding technology, linear friction welding has unique advantages in machining dissimilar titanium alloy blade disc. However, there still lacks sufficient support in basic applied research, and the mechanism of interface formation is still under study. In this work, the microstructure of the welded joint between TC11 and TC17 titanium alloys was analyzed by OM, SEM and TEM, respectively. The results showed that common grains and common grain boundaries are formed at the weld interface. In the common grain, a phase boundary is formed in the weld interface. Elements diffusion is observed on both sides of the common grain boundary and the phase boundary in the common grain. Under the action of rejection, adsorption and towing of solute elements in the formation of common grains and common grain boundary, the observed diffusion distance of elements in the phase boundary of the common grain is longer than the one in the common grain boundary. The composition change at the phase boundary of the weld zone is greater than the one inside the phase. A large number of small needle-like α phases are formed at the weld interface that has a large number of deformed twins. The structure of the interface in common grains consists of two interfaces (recrystallization growth interfaces of both sides) and two growth regions (ordered and disordered). The dynamic recrystallization also has an ordered and disordered crystallization process similar to that of solidification crystallization.

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Crystal Plasticity Finite Element Method Investigation of the High Temperature Deformation Consistency in Dual-Phase Titanium Alloy
Xuexiong LI,Dongsheng XU,Rui YANG
Acta Metall Sin    2019, 55 (7): 928-938.   doi:10.11900/0412.1961.2018.00380
Accepted: 25 April 2019

Abstract111)   HTML3)    PDF (13091KB)(443)      

Based on the rate-dependent crystal plasticity constitutive model considering all slip systems, a series of dual-phase polycrystalline models were established using 3D Voronoi tessellation to investigate the high temperature plastic deformation of Ti-6Al-4V alloy with different microstructure features. The spatial distributions and evolution of stress and strain in various grains and phases were calculated in detail, and a new method was proposed to evaluate quantitatively the deformation consistency in the alloy with two phases. Simulations show that grain boundary region responds preferentially in the early stage of deformation. The encircling structure formed between β and α grains can enhance the differences in the local strain distribution. Increasing the aspect ratio of grains and the fractions of heterogeneous phase interface can reduce the local compatibility of deformation. The stress frequency statistics of both α and β phases show a double peak form, with α phase higher in average strain, and β phase higher in stress distribution. Increasing of the volume fractions of α phase may reduce the tensile yield strength, and cause the stress consistency coefficient to decrease, while the strain consistency coefficient decreases first and then increases. As initial α-basal texture intensity increases, both tensile yield strength and stress consistency coefficient increase, while the strain consistency coefficient decreases first and then increases.

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Microstructure, Texture and Mechanical Property ofTA32 Titanium Alloy Thick Plate
CHENG Chao,CHEN Zhiyong,QIN Xushan,LIU Jianrong,WANG Qingjiang
Acta Metall Sin    2020, 56 (2): 193-202.   doi:10.11900/0412.1961.2019.00226
Accepted: 12 September 2019

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TA32 alloy is a new near α titanium alloy designed by optimizing the alloy elements ratio based on a series of elements Ti-Al-Sn-Zr-Mo-Si-Nb-Ta which has less β-stabilizing elements. This alloy has an excellent match of heat resistance and heat stability at 550 ℃, and good short-term mechanical properties at 600~650 ℃. TA32 titanium alloy thick plate can be applied to the key components in high temperature service of the hypersonic vehicle. Due to the low deformation degree of thick plate during rolling process, the heterogeneity of microstructure, texture and mechanical properties of the thick plate increases. In order to provide theoretical basis and experimental basis for the subsequent optimization of mechanical properties of TA32 titanium alloy thick plate, the microstructure, texture and mechanical properties of this alloy with a thickness of 60 mm are investigated in this work. Results show that the microstructure of the as-received material is mainly composed of lamellar α grains with few retained thin β layers, and the microstructure difference is not obvious from the surface to the center along the thickness direction of the plate no matter of the RD (rolling direction)-ND (normal direction) plane or the TD (transverse direction)-ND plane. Moreover, the rolling streamline can be obviously observed on the two planes. The morphology of α grains of the alloys presents either straight or wavy depending on their orientations with respect to the principal rolling directions. XRD results show that the as-received material has a typical T-type texture with c-axis of α phase approximately parallel to TD. At the same time, the <$10\bar{1}0$> poles are parallel to RD while <$10\bar{1}1$> poles present random distribution. As the c-axis gradually deviates from the TD of the surface to the center along the thickness direction of the plate, the Schmidt factors gradually increase, which is one of the main reasons for the gradual decrease of tensile strength; and the decrease of fraction of intragranular substructure from the surface to the center along the thickness direction is another important factor. The tensile properties have no obvious difference along the TD and RD at the same thickness position of the as-received material, but slightly worse along the ND. In addition, the influences of microstructure and texture on tensile properties are further clarified by adding two sets of heat treatment experiments (920 ℃, 30 min, AC+600 ℃, 5 h, AC; 950 ℃, 30 min, AC+600 ℃, 5 h, AC). The results show that the texture is the main factor affecting the tensile strength of TA32 titanium alloy plate at different positions under the condition of no obvious difference in microstructure. After double annealing, microstructure difference is the main factor affecting tensile strength.

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Numerical Simulation of Stress Evolution of Thin-Wall Titanium Parts Fabricated by Selective Laser Melting
KE Linda,YIN Jie,ZHU Haihong,PENG Gangyong,SUN Jingli,CHEN Changpeng,WANG Guoqing,LI Zhongquan,ZENG Xiaoyan
Acta Metall Sin    2020, 56 (3): 374-384.   doi:10.11900/0412.1961.2019.00198
Accepted: 18 November 2019

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Selective laser melting (SLM) is a very promising additive manufacturing (AM) technology for fabrication of thin-walled parts due to its high forming accuracy with complex shape. The higher temperature gradient in rapid heating and cooling process is prone to produce larger thermal stress, which will induce warpage deformation of SLMed parts. However, most of the current SLM stress studies focus on the residual stress, and only a few reports on the transient stress in the thermal cycle during SLM. In this work, a thermal-mechanical coupled transient dynamic finite element model was established to study the effects of laser scan rate and layer thickness on stress evolution during SLM processing. The results show that under the action of thermal cycle, the internal stress evolution in SLM of titanium alloy thin-walled parts presents a thermal stress cycle. Under the relief annealing of the thermal stress cycle, the peak thermal stress increases first and then decreases in the heating stage, and stabilizes and approaches the value of residual stress in the cooling stage. The residual stress of SLMed thin-walled parts is less than the transient peak stress during heating. After several thermal cycles with stress relief annealing effect, the peak thermal stress of SLM thin-walled parts can be reduced by more than 30%.

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