The effect of addition of Ru and change of Cr content on solidification behaviors are systematically investigated in a Re-free Ni-based single crystal superalloy with a fixed mass fraction additions of Ru and Cr, with the use of DTA, OM, SEM and EPMA. The results show that the addition of Ru can increase the eutectic fraction, facilitate the formation of Ru-rich phase which would increase largely at the time of Ru and Cr addition synchronously. Ru addition can promotes W segregate to the dendrite arm, and inhibits Mo to segregate to the interdendritic region. The increasement of Cr addition can decrease the segregation ratio of W, but have little effect on Mo. The combination of Ru and Cr influences the solidification microstructures and segregation behaviors corporately, and thus influences heat treatment processes and the microstructure stability.
Hot-corrosion directionally solidified Nickel base superalloy DZ444 is generally used as the candidate material for blade of gas turbine, which required excellent alloy microstructural stability. As one of the constitutional phase of the alloy, primary MC carbides are often thermally unstable, and its degradation reactions can happen when the alloys are in services or thermally exposed in high temperature circumstances. There existed several different kinds of MC decompostion reactions in some traditional Ni-based superalloys. To figure out the thermal stability of primary MC carbide in the DZ444 alloy and better understand its degradation mechanism, some related discussions to the thermal stability and degeneration process of primary MC carbide and its effects on the microstructure were made. In this work, microstructures of DZ444 alloy after long-term exposure up to 1×104 h at 800, 850 and 900 ℃ have been observed by OM, SEM and TEM. The results show that the thermal stability of MC was low. As long-term exposure proceeds, MC decompostion became more and more serious. Firstly, a typical sandwich microstructure (SM) gradually formed and thickened in the MC/g interface; secondly, h phase precipitated in the SM/MC interface; lastly, h-M6C and h-M23C6 locally precipitated inside the h phase. Finally, SM structure, h phase, h-M6C and h-M23C6 successively formed in MC degeneration areas at three stages of its decomposition process. Basically, MC decompostion process could be described with such raction formula as follows: MC+g→SM-M23C6+SM-M6C+SM-g'→SM-M23C6+SM-M6C+SM-g'+h→SM-M23C6+SM-M6C+SM-g'+h+h-M6C+h-M23C6. Generally, the type of secondary carbide from MC degeneration was M23C6, and, with the increase of long-term exposure temperature and time, the amount of secondary M6C carbide slightly increased. Besides, MC degeneration might result in the precipitation of transgranular M23C6 carbide and s phase in the vicinity of MC degeneration areas, and the coarsening of grain boundary (GB).
Nb-Si base alloys have attracted considerable attentions as the potential high temperature structural materials working in the service temperature range of 1200~1400 ℃ because of their high melting points (>1750 ℃), moderate densities (6.6~7.2 g/cm3) and excellent high temperature strength. However, the mismatching between room temperature fracture toughness and high temperature strength has limited their practical applications. Directional solidification (DS) and alloying have been proved to be the effective methods to overcome this critical issue. The DS processes used to prepare Nb-Si base alloys included Czochralski directional solidification in a copper crucible, electron beam directional solidification, optical floating zone melting, integrally directional solidification and electromagnetic cold crucible directional solidification (ECCDS). The previous studies focused on the effect of process parameters on microstructure and mechanical properties in the steady-state growth region (SSGR). However, the microstructure in the SSGR was controlled by the solid-liquid interface, and the solid-liquid interface was controlled by process parameters. Therefore, the study about the effect of process parameters on solid-liquid interface was very important. In this work, the master alloy with the nominal composition of Nb-22Ti-16Si-3Cr-3Al-2Hf (atomic fraction, %) was prepared by vaccum non-consumable arc-melting first, and then induction skull melting. The DS experiments were performed in the ECCDS device equipped with a square water cooled copper crucible (internal dimension: 26 mm×26 mm×120 mm) and a Ga-In alloy pool. There were three processing parameters in ECCDS including heating power of power supply, withdrawal rate and holding time. The DS ingots were prepared according to the orthogonal test (L9 (33)). Instability degree was defined as the ratio of the height of solid-liquid interface to the width of the DS ingot. The results showed that there were three macroscopic morphologies of solid-liquid interfaces; the increase of holding time, decrease of withdrawal rate and elevation of heating power were conducive to keeping the solid-liquid interface macroscopic morphology planar. With the increase of withdrawal rate, primary dendrite arm spacing (d1) and secondary dendrite arm spacing (d2) decreased gradually; with the increase of heating power, d1 and d2 increased gradually; with the increase of holding time, d1 and d2 increased first and then decreased. The higher withdrawal rate, lower heating power and less holding time were beneficial to refining the d1 and d2.
The Al-Si-Cu-Mg cast aluminum alloys have high mechanical properties and good cast performance. Due to their excellent comprehensive properties, the Al-Si-Cu-Mg cast aluminum alloys have wide application, and have become one of the most important structural materials applied in the equipment manufacturing industry. Actually, many key components in practical engineering application are often subjected to the alternating load, and thus the fatigue failure has become an important factor which concerns the safety and economy for those structures used in various engineering fields. Although some researches for the fatigue behavior of aluminum alloys have been performed, mainly focus on the regularity understanding. Especially, the influences of rare earth elements and heat-treat condition on the low-cycle fatigue behavior of aluminum alloys have not been comprehensively revealed. Obviously, the investigation concerning the microstructure and fatigue property of the Al-Si-Cu-Mg cast aluminum alloys can not only provide the theoretical basis for the development of new type cast aluminum alloys but also the reliable theoretical foundation for the safety design and reasonable use of these alloys. In order to determine the influence of rare earth element Sc on the low-cycle fatigue behavior of casting Al-9.0%Si-4.0%Cu-0.4%Mg alloy with T6 treated state, the cyclic stress response behavior, fatigue life behavior and cyclic deformation mechanism of the Al-9.0%Si-4.0%Cu-0.4%Mg(-0.3%Sc) cast aluminum alloys with T6 treated states under low-cycle fatigue loading condition were investigated. The results show that at the low total strain amplitude, the Al-9.0%Si-4.0%Cu-0.4%Mg alloy exhibits the cyclic strain hardening during whole fatigue deformation, while the Al-9.0%Si-4.0%Cu-0.4%Mg-0.3%Sc alloys exhibit the cyclic strain hardening in the initial stage of fatigue deformation and then the stable cyclic stress response in the later stage of fatigue deformation. At the higher total strain amplitudes, the Al-9.0%Si-4.0%Cu-0.4%Mg(-0.3%Sc) alloys exhibit the cyclic strain hardening. The addition of Sc can effectively enhance the cyclic deformation resistance and prolong the fatigue lives of the Al-9.0%Si-4.0%Cu-0.4%Mg alloy with T6 treated state. At the lower total strain amplitudes, the cyclic deformation mechanism of the Al-9.0%Si-4.0%Cu-0.4%Mg(-0.3%Sc) alloys with T6 treated state is the plane slip, while at the higher total strain amplitudes, the cyclic deformation mechanism becomes the wavy slip.
10Cr12Ni3Mo2VN steel is mainly made by forging and usually used to make last stage blades of ultra-supercritical unit, demanding strict standards of microstructure property because of its hard service environment, so it is necessary to conduct deep research on its hot deformation behavior. The hot deformation behavior of 10Cr12Ni3Mo2VN steel was investigated through high temperature compression tests on the Gleeble-1500 thermal-mechanical simulator at 850~1200 ℃ and strain rate range of 0.01~10 s-1. The results show that dynamic recrystallization becomes more prone to happen and recrystallized grain size increases with increasing temperature and decreasing strain rate. Isometric crystal and mixed structure appear after compressed 60% at 1200 ℃ with high and low strain rate respectively. A new method of establishing the hot deformation hyperbolic sine constitutive equation by Levenberg-Marquardt algorithm is proposed. Parameters of the constitutive equations established by traditional linear fitting and Levenberg-Marquardt algorithm have a similar value, and both of the constitutive equations have a high prediction precision, so the method of establishing constitutive equation by Levenberg-Marquardt algorithm is credible. However, Levenberg-Marquardt algorithm can get all parameters at the same time with fewer and simpler steps compared to traditional linear fitting. In addition, the values of critical strain for dynamic recrystallization initiation are determined from the work hardening rate-strain curves and a model related to Zener-Hollomon parameter for predicting critical and peak strain under different deformation paraments is established.
Surface nanocrystallization (SNC) can effectively enhance the surface and global properties of the metallic materials, such as microhardness, intensity, fatigue, wear and corrosion resistances, therefore provides more promising practical industrial applicability. Up to now, several SNC treatment methods were developed based either on the principles of ball impactions or friction sliding, however, difficulty still exists for the surface treatment of large-dimensional samples with high efficiency. Recently, more attentions were focused on the asymmetric rolling, of which upper and lower rolls rotate with different circumferential speeds, and then an extra shear strain was applied to metal sheet in addition to compression strain. The shear strain could refine the grains into micro- or submicro-scales. In order to investigate the possibility to realize the SNC for metal sheet in the rolling process and examine the effects of rolling parameters, silicon steel sheet was rolled by means of asymmetric rolling and conventional rolling respectively, the microstructural evolution in the top surface layer was observed for the samples rolled for different parameters including mismatch speed ratio, rolling reduction and rolling pass. Experimental results show that after the asymmetric rolling, nanocrystallines about 10~50 nm in size with nearly random orientations form in the top-surface layer of sheet. Meanwhile, dislocation cells can be observed after conventional rolling, which indicates that the asymmetric rolling can be utilized for the surface nanocrystallization of the cubic metal sheets. The surface nanocrystallization mechanism induced by asymmetric rolling was summarized as follows: (1) upon the application of repeated shear force, submicro-grains/dislocation cells form through formations, slips, annihilations and recombinations of high density of dislocations; (2) with a further increment of rolling reduction and rolling pass, high density of dislocations in the refined cells/grains developing in above route lead to reduction of grain size and increment of misorientations between the refined grains; (3) nanocrystallines with nearly random orientations form. Larger reduction and multi-passes are necessary for the surface nanocrystallization induced by asymmetric rolling, and the increment of mismatch speed ratio can accelerate the grain refinement process.
Deformation and fracture behaviors as well as their mechanisms of polycrystalline beryllium at room temperature were systematically studied by in situ tensile test in SEM, characterizing fracture cleavage planes by electron backscattered diffraction (EBSD) technique, and twinning deformation analyzing by OM. The results show that slip and twinning deformation of polycrystalline beryllium are difficult to occur under tensile stress at room temperature. Slip bands happen only in some grains with a favorable orientation, and finally twinning deformation grain number accounts for only about 5% of the total grains. There exists the cross slip between (0001) basal plane and {1010} prismatic plane in the deformation process. Microcracks usually initiate at one grain boundary, then propagate by a transgranular way and terminate at the other side of the grain boundary in polycrystalline beryllium. Crack initiation of polycrystalline beryllium is in accordance with Stroh dislocation pile-up crack theory. The growth of microcracks have to depend on different microcracks merging by cleavage steps or tearing way due to a strong blocking effect of grain boundaries on the microcracks propagation. Basal cleavage planes of polycrystalline beryllium are determined to be (0001) and {1010} planes. Both of them are the main paths of cleavage crack initiation and propagation of polycrystalline beryllium. It is not observed that twinning deformation induces nucleation of microcracks.
Different volume fractions of 304 stainless steel capillary tubes/Zr53.5Cu26.5Ni5Al12Ag3 metallic glass composites were prepared using infiltration method. Their properties and deformation behaviors were investigated systematically. The mechanical properties were performed on materials test machine. Surfaces and fracture morphologies were examined using white light interferometer, X-ray 3D imaging and SEM techniques. The results show that the ductility of composites was improved. The compressive strain of composite reaches 20% when the volume fraction is 34%. The deformation involves obvious work hardening. The amount of work hardening depends on the content of tubes. The composite fails in the shear mode along 45°. The split and debonding of tubes and interfaces act as the propagation way of crack. The amount of shear bands increase as the volume fraction increases. The shear deformation of amorphous in tubes falls behind that out tubes.
Nowadays, more and more researchers pay attention to the microwave absorbing materials with the increase of electromagnetic pollution and the development of stealth technology for military platforms. Traditional microwave absorbers, such as ferrite, metal powder, conductive magnetic fibers, magnetic flake powder, and nanoparticles and so on, are facing some common problems such as large specific gravity, narrow absorption bandwidth, large match thickness and poor absorption performance. Composites with core-shell structures are becoming promising microwave absorbing material because such structure can exhibit magnetic and dielectric characteristics through the proper selection of core and shell materials. Thus, Fe(CO)5 deposition on the SrFe12O19 surface has been considered for the fabrication of a new composite that possesses the advantages of these two materials. This new composite might also obtain remarkable microwave absorption through thin layer absorbers in the entire 2~18 GHz frequency range. The Fe-SrFe12O19 composites with core-shell structures were prepared by metal organic chemical vapor deposition (MOCVD) using the SrFe12O19 and iron pentacarbonyl [Fe(CO)5] as the precursors. XRD, SEM, EDS and a vector network analyzer were used to characterize the structure and electromagnetic properties of the samples. The structure and morphology analyses show that the composites have complete core-shell structures with SrFe12O19 as core and Fe layers as shell. Fe nanoparticles were uniformly deposited on the surface of SrFe12O19 with thickness of about 0.5 mm at the reaction temperature of 180 ℃ with N2 flow rate of 30 mL/min for 30 min. Simulation studies show that SrFe12O19 electromagnetic properties changed significantly and the absorbing properties got evidently improvement after Fe deposited on its surface. The samples prepared with deposition time of 30 min have the best absorbing properties. A reflection loss (RL) value exceeding -10 dB in the range of 6.8~18.0 GHz frequency was obtained by selecting an appropriate thickness of the absorber layer from 1.5 to 3.0 mm. Moreover, a minimum RL of -21.2 dB at was obtained for a 2.0 mm thick layer. Fe-deposited SrFe12O19 by MOCVD can significantly improve the electromagnetic properties of SrFe12O19 and Fe-SrFe12O19 composites could be used as an effective microwave absorption material.
Pt-modified aluminide coating has attracted great attention due to its advantage of the integrated property in resisting both high temperature oxidation and hot corrosion. By the presence of Pt, the spallation trend of the grown oxide scale and the detrimental effect of S can be restrained at a very low level. Besides, Pt could promote α-Al2O3 formation and stabilize β-NiAl phase. Thus Pt-modified aluminide (Pt-Al) coating has been widely used in some crucial applications requiring reliability and extended service life. There are mainly PtAl2, β-(Ni, Pt)Al and γ/γ ′-NiPtAl phases existing inside Pt-Al coating. In this work, a single phase PtAl2 coating was prepared on a Ni-based K38G superalloy through pulse-electroplating of Pt and pack aluminization under stepped heating mode. At 1100 ℃ , the isothermal oxidation behavior of the single phase PtAl2 coating was evaluated by thermogravimetric analysis (TGA). Cyclic oxidation test of the PtAl2 coating was performed within a vertical muffle furnace at the same temperature. The results indicate that the singular PtAl2 coating possesses quite good isothermal oxidation resistance. However, its resistance against cyclic oxidation is very poor. The cyclic stress induced by repeated heating and cooling has caused visible detachment of PtAl2 coating layer, and the spallation of PtAl2 in further would lead to a premature failure of the whole coating system. Partial spallation of PtAl2 layer, including undesirable consumption of Al inside β-NiAl nearby the spallation acts the main reason responsible for the final failure. Accordingly, it is not appropriate to apply single phase PtAl2 coating in the high temperature services involving stress and load. The degradation mechanism of the singular PtAl2 coating is investigated by discussing the stress generated from cyclic heating and cooling.
Magnetic refrigeration based on the magnetocaloric effect offers a potential energy saving, atmosphere friendly way to replace vapor-compression refrigeration. In this work, Mn1.2Fe0.8P0.74Ge0.26-xSex (x=0, 0.005, 0.01, 0.015, 0.02, 0.03) compounds were prepared by mechanical milling and subsequent spark plasma sintering (SPS) technique, their crystal structures, phase transition process and magnetocaloric properties were investigated by XRD, DSC, VSM and direct measurement equipment of magnetocaloric effect. The results show that the Mn1.2Fe0.8P0.74Ge0.26-xSex compounds possess a hexagonal Fe2P-type crystal structure. With increasing Se concentration, the lattice parameters a and c change significantly. It causes c/a ratio decreases firstly, keeps unchanging and followed by increasing again. And the Curie temperature (Tc) of the compounds is increased with the decrease of the c/a ratio. Either applied magnetic field or temperature change can induce the magnetic transformation between the paramagnetic phase and ferromagnetic phase. Low substitution of Se for Ge (x≤0.015) in the Mn1.2Fe0.8P0.74Ge0.26-xSex compounds leads to higher Tc, narrower temperature range of the two-phase coexistence (?Tcoex) and larger adiabatic temperature change (?Tad), in addition the thermal hysteresis (?Thys) and entropy change (?SDSC) remain almost unaffected. 0.01 Se substitution leads to 5.6 K increase in Tc, 10.6% decrease in ?Tcoex, and 10% increase in ?Tad. With a further increase in Se content, magnetocaloric properties of compound decrease.
Based on the evolution of dislocation density and volume fraction of twins, a physically based constitutive model of Fe-22Mn-0.6C twinning induced plasticity (TWIP) steel has been developed. By taking the influence of slip inside twins on the plastic deformation and the difference of the average Taylor factors between the twinned regions and matrix regions into account, the plastic strain at the representative element was presented as the weighted sum of matrix slip, twinning and slip in twinned regions in this model. A linear function between yield stress and strain rate with natural logarithm was established by considering the effect of strain rate on thermally activated stress. And then, The Euler method was adopted and the parameters of this model were obtained in order to describe as accurately as the experimental results. The results from the model are in good agreement with the experimental results and the average relative error is only 0.84%. Compared with the model free of slip and the model free of the difference of Taylor factor at twinned regions, the average relative error is reduced 1.1% and 2.9%, respectively. The interaction between two twins and the sliding mechanism and its impact on the macro-deformation were investigated. The results show that there is a negative correlation between gliding rate and twinning rate and slip rate decreases with the increase of twinning rate. When the twins become saturated, the twin rate decreases rapidly, being opposite to the slip rate. The yield stress increases and the rate of strain hardening remains approximately unchanged with the increase of strain rate.
Assorted morphologies of precipitates in fcc/bcc transformation systems have been reported, which often exhibit irrational characteristic. Various models have been developed to explain the habit plane (or the broad facet), but the overall morphologies of the precipitates are seldom explained quantitatively. In this work, the equilibrium cross-section of rod-shape austenite precipitates in duplex stainless steel is determined by atomistic simulation. The input orientation relationship (OR) between fcc and bcc phase is determined according to the O-line criterion (one of the interfaces contains a single set of dislocations), with the condition that the invariant line lies in close-packed plane pair of and the Burgers vector of O-lines is . The obtained OR is close to the K-S OR , , with the deviation of 1.27° from the parallelism in both planes and directions. Interfacial energy of interfaces in various orientations in the zone axis of the invariant line has been calculated. To ensure the reliability of the calculated values, an initial atomic configuration free of interstitial and vacancy is constructed for each interface. The energies of the O-line interface, the interface normal to either or Δg(020) are found to have similar values, each at a local minimum. Based on the calculated interfacial energies, the equilibrium cross-section morphology is determined by the Wulff construction. The result shows that the morphology exhibits a near rhombus cross-section, which agrees consistently with experiments. One of the major facets is the O-line interface, normal to , in agreement with the observation. The other facet is normal to Δg(020) in the calculated result, while it is normal to in experimental results, with about 10o difference between them. The discrepancy between calculated and observed results is probably because the experiments have not reached the equilibrium state. The dislocation structures in these three interfaces are identified from the atomic simulation results by a newly developed method based on the singular value decomposition of the Nye tensor. It confirms that the O-line interface contains a single set of [011]f/2 dislocations with spacing of 1.5 nm. The interface normal to either or Δg(020) contains two sets of dislocations. The dislocation structure in the facet normal to is in good agreement with experimental observation of the non-O-line facet, including the local decomposition of dislocation core to stacking faults.
The window of thermomechanical parameters where dynamic recrystallization occurs can be predicted according to the power dissipation map based on dynamic materials model, and the distribution of thermomechanical parameters in metal forging can be calculated by using finite element (FE) simulation. Thus, the zone of dynamic recrystallization and its evolution in metal forging not only can be simulated and predicted, but also the forging process parameters where dynamic recrystallization occurs can be optimized by the combination of the window of thermomechanical parameters where dynamic recrystallization occurs and finite element simulaton, which provides a new way for realizing the control of microstructure and property of forging. A method for simulating and predicting dynamic recrystallization in metal forging is proposed based on the combination of the thermomechanical parameter window of dynamic recrystallization predicted by power dissipation map and finite element simulation, and the method is already integrated into the commercial FE software Deform 3D. The zone of dynamic recrystallization and its evolution in compression of titanium alloy TC11 at process parameters (1020 ℃, 0.1 s-1), (1050 ℃, 0.1 s-1), (1050 ℃, 10 s-1) and constant strain rate are successfully simulated and predicted by using the modified FE software Deform 3D. The simulated and predicted result is in good agreement with experiment.
External corrosion and stress corrosion cracking (SCC) have been observed in a thin layer electrolyte under disbonded coating shielding cathodic protection (CP). In this work, characteristic and evolution of the thin layer electrolyte environment (potential, pH, etc.) on pipeline steel under disbonded coatings were investigated by microelectrode technology in a simulating crevice cell at various CP levels and different atmosphere conditions (CO2 and O2). The results show that pH of the thin layer electrolyte under the disbondment varies from near neutral to the value as high as 12, depending on the amount of CP current and CO2. CP current on pipeline surface initiates a high pH environment on the steel surface. Due to the CP shielding effect and a high CO2 content, a near-neutral pH environment may exist under disbonded coating. The CP effectiveness and the shielding effect of the disbondment can be estimated by measuring pH of the thin layer electrolyte under disbonded coatings.
Laser etching technique was used to build three microstructures of grid, line and dot on Ti6Al4V alloy surface, and sol-gel method was used to coat SiO2 nanoparticles on the microstructure to build the hydrophobic/superhydrophobic surface with the micro/micro-nano structure. Adhesion area of the chlorella on the surfaces was used to evaluate the antifouling performance of the halobios, and dynamic wash test was used to evaluate the adhesion strength of the chlorella. It is shown that the hydrophobic/superhydrophobic surfaces with the microstructure are in accordance with Wenzel model, the superhydrophobic surfaces with the micro-nano structure are in accordance with Cassie model, and have stronger antifouling and less adhesion strength. The grid surfaces have the strongest superhydrophobic and self-cleaning performance, flowed by the line surfaces and then the dot surfaces. The contact angles decrease, the roll angles increase, the antifouling performance gets small and the adhesion strength increase with the increment of space of the microstructure.
