High-temperature deformation behavior of additively manufactured niobium alloys from in-house gas-atomized feedstock

Powder metallurgy (PM) Ti6Al4V alloy was prepared from HDH alloy powder by vacuum pressureless sintering. To study its high-temperature deformation behaviour, isothermal compression testing was carried out with temperature range of 900–1050°C, strain rate range of 0.01–10 s−1, and compression ratio of 60%. Based on the hot deformation data, the corresponding constitutive equation and hot processing map were clarified. The deformation activation energy for PM Ti6Al4V alloy was 606.5 kJ mol−1. Its deformation instability zone was very small, locating in the range of 960∼990°C with strain rate >1 s−1. PM Ti6Al4V alloy had large hot working window and good hot workability. The ideal hot processing domain was at 970–990°C and 0.05–0.01 s−1. Besides, the domain at 1000–1040°C and 1–10 s−1 was also suitable for hot processing. This study proposed an effective guidance for the hot deformation (e.g. forging, extrusion, rolling) of PM Ti6Al4V alloy.

Effect of short-range order on the high-temperature plastic deformation behavior of Ni–Cr–W, Ni–W, and Ni–Re single crystals

The Cu-Ni-Al powders are prepared by atomization method, are hot pressing sintered in the vacuum furnace to prepare the porous Cu-Ni-Al alloy. The microstructure, high-temperature compressive deformation behavior and its effect factors are investigated,respectively. The deformation mechanism of the porous Cu alloy is also analyzed and discussed. The results show that the compression strength, elastic modulus and yield strength decrease with increasing temperature and decreasing strain rate. The compression deformation process is classified in terms of three stages of linear elastic deformation, yield platform area with pore wall buckling, collapse, and plastic deformation with pore densification. In the high temperature compression deformation process, stress concentration and deformation are preferred to occur at the areas with pore concentration and thinner pore walls. The compressive mechanical model of the porous Cu alloy is established using the method of linear regression. It is obvious that the calculation curves is in accordance with the experimental curves in the first and second stages of the deformation process, however, at the third stage,the former is above the experimental curves, which is possibly attributed to the inhomogeneous pore distribution of the porous Cu alloy.

A high Nb containing TiAl alloy was prepared from the pre-alloyed powder of Ti-45Al-8.5Nb-0.2B-0.2W-0.02Y (at%) by spark plasma sintering (SPS). Its high-temperature mechanical properties and compressive deformation behavior were investigated in a temperature range of 700 to 1050°C and a strain rate range of 0.002 to 0.2 s−1. The results show that the high-temperature mechanical properties of the high Nb containing TiAl alloy are sensitive to deformation temperature and strain rate, and the sensitivity to strain rate tends to rise with the deformation temperature increasing. The hot workability of the alloy is good at temperatures higher than 900°C, while fracture occurs at lower temperatures. The flow curves of the samples compressed at or above 900°C exhibit obvious flow softening after the peak stress. Under the deformation condition of 900–1050°C and 0.002–0.2 s−1, the interrelations of peak flow stress, strain rate, and deformation temperature follow the Arrhenius’ equation modified by a hyperbolic sine function with a stress exponent of 5.99 and an apparent activation energy of 441.2 kJ·mol−1.

A comparative study on Arrhenius-type constitutive model and artificial neural network model to predict high-temperature deformation behaviour in Aermet100 steel

Molecular dynamics (MD) simulations were performed to examine the temperature-dependent elastic properties and high-temperature deformation behavior of a Cu64.5Zr35.5 amorphous alloy. From the simulations we find that the elastic constants of the amorphous solid and supercooled liquid exhibit an approximately linear temperature dependence. The predicted temperature dependence of the Young’s modulus for the amorphous solid obtained from the MD simulations is in good agreement with experimental measurements using dynamic mechanical analysis. Furthermore, the high-temperature plastic deformation behavior determined by MD simulations is qualitatively in good agreement with results from plastic deformation experiments performed on 1 mm diameter Cu64.5Zr35.5 metallic glass rods at 698 K. Notably, the MD simulations reveal that the flow softening regime of the stress-strain curve corresponds to an increase in the free volume in the atomic structure. Moreover, the simulations indicate that the atomic mobility significantly increases within the same regime.

High-temperature deformation behaviour and processing map of near eutectic Al–Co–Cr–Fe–Ni alloy

The high-temperature deformation behavior and microstructural changes of harmonic structure composites with WC-Co alloys and high-speed steel (HSS) were investigated in detail. A harmonic structure composite was fabricated by consolidating the mechanically milled powder having WC-Co and HSS powder. The harmonic structure composite demonstrates the microstructure composed of network area (WC-Co) and dispersed area (HSS). The harmonic structure composite shows a sufficient compressive strength in the compression tests at 773 K, but the compression strength decreases at temperatures of above 873 K. The 0.2% proof stress at high temperature almost unchanged even if the network area fraction changed. Furthermore, the network area plays an important role in the high temperature deformation of harmonic structure composites. These results suggest that the formation of voids for WC-Co boundary sliding and poor sintering is an important factor in stress reduction in the high-temperature compression of harmonic structure composites.

It is of great significance to reveal deformation behavior coupling with both the high-temperature effect and microstructure for the in-service welding instability zone. In the present paper, microstructure evolution and high-temperature deformation behavior at the representative temperatures 900 °C and 1100 °C inside the in-service welding instability zone were investigated based on the in-situ high-temperature laser scanning confocal microscope (LSCM). The results indicated that a sufficient condition of carbide precipitation is to hold it at 900 °C for a certain time. However, the phenomenon of carbide precipitation failed to be presented during continuous heating to 1150 °C. Although the slip and twining deformation occurred inside austenite grain, the fracture mechanism gave priority to intergranular cracking, which could be ascribed to the carbides with higher hardness and the grain boundary weakening caused by the temperature effect.

Dynamic mechanical relaxation processes of amorphous alloys are very important to understand plastic deformation, glass transition phenomenon, diffusion behavior and crystallization. How to establish the correlation between mechanical properties and mechanical relaxation modes is one of key issues. In the current research, with the help of dynamic mechanical analysis (DMA), dynamic mechanical behavior of Zr$_{50}$Cu$_{40}$Al$_{10}$ bulk metallic glass from room temperature to supercooled liquid region was probed. In parallel, based on the uniaxial tensile tests, high-temperature flow behavior of Zr$_{50}$Cu$_{40}$Al$_{10}$ metallic glass around glass transition temperature were investigated. Dynamic mechanical behavior and high temperature deformation behavior were discussed in the framework of quasi-point defects theory. The results demonstrated that main $\alpha $ relaxation process of metallic glass can be well described by the quasi-point defects theory. Based on internal friction of Zr$_{50}$Cu$_{40}$Al$_{10}$ metallic glass, activation energy of elementary movement of atoms $U_\beta$ is 0.63 eV. In addition, correlation factor $\chi $ corresponding to concentration of the quasi-point defects in solid glass remains almost constant below the glass transition temperature. When the temperature above the glass transition temperature, the correlation factor $\chi $ increases by increasing the temperature (below the crystallization temperature). Finally, high temperature flow behavior in tensile mode near the glass transition temperature of Zr$_{50}$Cu$_{40}$Al$_{10}$ metallic glass was studied. The normalized viscosity decreases with increasing strain rate at low temperatures or high strain rates, indicating a non-Newtonian flow behavior. Whereas Newtonian flow behavior is observed at higher temperatures and lower strain rates. The apparent viscosity is affected by temperature and strain rate. High-temperature flow behavior of Zr$_{50}$Cu$_{40}$Al$_{10}$ metallic glass was described by stretched exponential function and free volume theory. Specifically, experimental master curve of the high temperature flow behavior of metallic glass is in good agreement with the prediction of the quasi-point defects theory, which provides a new insight on understanding of viscous effects during high temperature deformation of solid glasses.

The effect of the microstructural change on the high-temperature tensile deformation behavior in the supercooled liquid region is investigated in the pre-annealed Zr 65 Al 10 Ni 10 Cu 15 bulk metallic glass. The microstructure before the tensile test on the specimen annealed at 673 K for 1.8ks shows that a small amount of icosahedral phase precipitates and a large amount of amorphous phase still remains. On the contrary, the specimen annealed at 673 K for 2.7 ks shows that a large amount of icosahedral phase precipitates and the presence of amorphous phase is hardly noticeable. From the fact that flow stress of the specimen annealed at 673 K for 2.7 ks increases in comparison with that of asreceived specimen as well as the specimen annealed at 673 K for 1.8 ks in the tensile test, it seems that the deformation behavior in the tension tests is greatly influenced by the difference in the degree of the precipitation of icosahedral phase. Since the m value is about 0.5 and many grain boundaries of icosahedral phase are formed in the specimen annealed at 673 K for 2.7 ks, the interaction between the icosahedral phase particles must be the cause of increase in initial stress. Accordingly, the microstructure of the gage section after tensile test in the specimen annealed at 673 K for 1.8 ks shows that the large amount of icosahedaral phase precipitates in the amorphous matrix. The strain hardening must be caused by the interaction between the icosahedral phase particles whose precipitation and growth are enhanced by deformation.

Role of heat treatment conditions in the high-temperature deformation behavior of laser-powder bed fused Fe–Cr–Ni–Al maraging stainless steel

High-temperature tensile deformation behavior of directionally solidified nickel base superalloy CM 247 DS is studied by conducting tensile tests in temperature range RT-955 °C employing a constant strain rate of 10−3 s−1 and carrying out extensive electron microscopic examinations to understand the concomitant substructural evolution. The alloy exhibits yield strength anomaly (YSA) behavior like many other superalloys, and the yield strength maxima occur at 750 °C. However, unlike in most of the superalloys, ductility continuously increases with temperature. The deformation behavior of the alloy changes significantly with temperature. Transmission electron microscopic examination confirmed that at lower temperature (≤ 750 °C), γ′ shearing is the dominant deformation mechanism; whereas at temperatures above 750 °C, thermally activated dislocation looping around γ′ precipitate is dominant. Substructures evolved during deformation at 750 °C consists mainly of superlattice stacking faults (SSFs) inside γ′ precipitates, whereas at 850 °C uniform dislocation tangles are observed in γ matrix. Superlattice stacking faults result from shearing of γ′ precipitate by a/3〈112〉 dislocations, which arise from the decomposition of a/2〈110〉 matrix dislocations. YSA in this alloy is attributed to dislocation interactions inside γ′; however, the enhanced ductility even at 750 °C is due to formation of SSFs.

The high temperature deformation and microstructural evolution of Ti-6Al-7Nb biomedical alloy under high strain rate loading condition are investigated by means of a split-Hopkinson bar and a clam-shell radiant-heating furnace. Impact tests are performed at strain rate ranging from 1×103 to 3×103 s-1 and temperatures between 25℃ and 700℃. The experimental results indicate that the flow response of Ti-6Al-7Nb alloy is related to temperature and strain rate. At a constant temperature, plastic stress, work hardening rate and strain rate sensitivity all increase with the increasing strain rate, while the thermal activation volume decreases. However, at a constant strain rate, plastic stress, work hardening rate and strain rate sensitivity decrease with increasing temperature, while the thermal activation volume increases. It is found that high temperature and high strain rate deformation behavior of Ti-6Al-7Nb alloy can be adequately described using the Combined Johnson-Cook and Zerilli-Armstrong constitutive equation. The fracture analysis results indicate that the Ti-6Al-7Nb specimens fail predominantly as the result of intensive localized shearing. Furthermore, it is shown that the flow localization effect leads to the formation of adiabatic shear bands. The fracture surfaces of the deformed Ti-6Al-7Nb specimens are characterized by a dimple like structure. The density of the dimples increases with an increasing strain rate or increasing temperature. Transmission electron microscopy observations reveal that the dislocation density increases with an increasing strain rate, but decreases with an increasing temperature. The strengthening effect observed at higher strain rates and lower temperatures is attributed to a greater dislocation density. A linear relationship between the square root of the dislocation density and the true stress is also found.

6061-T6 aluminum alloy has been used extensively as structural materials for internals of experimental nuclear reactors, aircraft fittings, amour systems and high-speed machinery for years due to its superior mechanical properties. In this study, the high temperature deformation and micro-structural evolution of 6061-T6 aluminum alloy under high strain rate loading condition are investigated by means of a split-Hopkinson bar. The specimens with longitudinal direction are heated using a clam-shell radiant-heating furnace. Impact tests are performed at strain rate ranging from 1×103 to 5×103 s-1 and temperatures between 100℃ and 350℃. The experimental results indicate that the flow response of 6061-T6 aluminum alloy is related to temperature and strain rate. At a constant temperature, plastic stress, work hardening rate and strain rate sensitivity all increase with the increasing strain rate, while the thermal activation volume decreases. However, at a constant strain rate, plastic stress, work hardening rate and strain rate sensitivity decrease with increasing temperature, while the thermal activation volume increases. The observed that high temperature and high strain rate deformation behavior of 6061-T6 aluminum alloy can be adequately described using the Zerilli-Armstrong constitutive equation. Transmission electron microscopy observations reveal that the dislocation density increases with an increasing strain rate, but decreases with an increasing temperature. The strengthening effect observed at higher strain rates and lower temperatures is attributed to a greater dislocation density. The stacking fault is also found in high temperature and high strain rate. A linear relationship between the square root of the dislocation density and the true stress is also found.