Effective Design of Cr-Co-Ni-Ta Eutectic Medium Entropy Alloys with High Compressive Properties Using Combined CALPHAD and Experimental Approaches

Alloy design of Cr-Co-Ni-Ta eutectic medium entropy alloys (EMEAs) was performed through a CALPHAD method coupled with experimental study, with the aim to attain high phase stability as well as excellent mechanical properties. Based on calculated pseudo-binary diagram, CrCoNiTax (x = 0.1, 0.3, 0.4, 0.5, 0.7) medium entropy alloys were investigated. Two phases, FCC solid solution and Laves phase, were identified in the alloys. With increasing Ta content, the volume fraction of hard and brittle Laves phase increased, microstructure changed from hypoeutectic (Ta0.1, Ta0.3) to eutectic (Ta0.4) and then to hypereutectic (Ta0.5, Ta0.7). The stability of phases was assessed by considering the thermodynamic parameter Ω and valence electron concentration (VEC). The eutectic phases become stable when 1.42 < Ω < 0.74 and 7.5 < VEC < 8.25. In addition, based on nanoindentation, the results indicated that solid solution strengthening in γ phase was significantly enhanced, eutectic phase in CrCoNiTa0.4 EMEA was found to process the highest microhardness and elastic modulus. Finally, the hardness of alloys was positively correlated with the content of Ta and the plastic strain of alloys obviously decreased, while the compression strength firstly increased and then decreased. CrCoNiTa0.4 was the most promising alloy with the highest compression strength (2502 MPa) and high plastic strain (20.6%).

Local Mesh Refinement for Correlation of FEA Estimated Plastic Strain to Tests in Areas of High Plastic Strain

The total strain, elastic plus plastic, was measured with strain gages on valve bodies with internal pressure that caused surface yielding. The correlation of the simulated maximum principal strain was compared to strain gage data. A mesh sensitivity study shows that in regions of large plastic strain, mesh elements are required that are an order of magnitude smaller than what is used for linear elastic stress analysis for the same structure. A local mesh refinement was adequate to resolve the local high strain values. Both the location and magnitude of the maximum strain changed with a local mesh refinement. The local mesh refinement requirement was consistent over several structures that were tested. The test and simulation work will be presented along with the mesh sensitivity study. Some results on using an energy stabilization technique to aid convergence will be presented in terms of the impact on the predicted plastic strain.

Arbitrary Lagrangian Eulerian FEA Method to Predict Wavy Pattern and Weldability Window During Magnetic Pulsed Welding

Finite element simulations of high strain rate forming processes have received significant attention over the last decade. For instance, in Magnetic Pulsed Welding (MPW), extremely high plastic strain regions develop. Thus, a traditional pure Lagrangian analysis is not able to accurately model the process due to excessive element distortion near the contact zone. In this study, the Arbitrary Lagrangian Eulerian (ALE) method is used to simulate a MPW process while retaining a high-quality mesh. Also the ALE method was able to numerically predict the necessary process parameters to achieve a wavy pattern region for two Al6061-T6 plates impacted during the MPW process. The captured wavy pattern region in this study can be used as a first estimation of parameters necessary to achieve a successful MPW component and thus reduce trial and error experimental investigations.

Very High Plastic Strain Zones in 304 Stainless Steel Small Punch Specimen Loaded at RT by Martensite Formation and Recrystallization Technique

Very high plastic strain zones with equivalent plastic strain above 0.2, PZ0.2 and above 0.5, PZ0.5 in 304 stainless steel small punch specimens loaded at RT to various level were observed and measured by martensite formation and recrystallization technique, respectively. It is found that both the very high plastic zones are formed ,at middle stage of the small punch test, at first near the outer surface region of the specimen where the loading ball is contacted to the specimen. The zones extend with increasing load toward the inner surface. Thus the contact area part of the specimen with the ball causes a significant strain gradient through thickness. This will be due to the constraint of the plastic deformation near the contact region by the friction force.

The Influence of High Plastic Strain on Precipitation from Solid Solution in Third Generation Al-Li Alloys

Different levels of compression plane strain were applied to third generation aluminum alloys to simulate rolling-type deformation following solution heat treatment. The aluminum alloys utilized were extruded production samples provided by Alcoa in a solution heat treated condition (T4). The alloys included in this study are AA2099-T4 containing 1.78 wt% lithium, and AA2055-T4 containing 1.13 wt% lithium and an additional component of 0.45 wt% silver. Following plane strain compression, the samples were isothermally heat treated at 155 °C for times up to about 7 days. Data presented include hardening behavior values and various electron microscopy techniques using conventional TEM to document subsequent precipitate sequence distributions and general kinetics.

Principle of the Continuous Variable Cross-Section Recycled Extrusion (CVCE) Process

A kind of severe deformation method for refined grain materials named continuous variable cross-section recycled extrusion (CVCE) was proposed. The composition structure and working principle of processing were introduced. First, the cylinder specimen was extruded to circular cone and then the circular cone specimen was compressed into columned one. After that, the specimen was rotated by 180° and repeated the forward procedure. All of procedures mentioned above constitute a cycle. The whole process is repeated again and again. The process can introduce ultra-high plastic strain without any geometrical change. The process has been applied to commercial aluminum (1100), Mg alloy. After several cycles of CVCE, bulk materials with sub-micron grain structure were successfully obtained. The average grain size of AZ31 magnesium alloy is refined from 25μm to 3μm after six cycles and the tensile elongation were improved dramatically.

The Microstructure and Properties of LY12 Aluminum Alloy by Ultra-High Plastic Strain

The ultra-high plastic deformation behavior by laser shock processing on the LY12 aluminum alloy had been investigated. The morphology of the materials had been analyzed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). It was found that the grain refinement layer was formed in the thickness of about 100μm .The dislocation density of LY12 aluminum alloy should be large increased after laser shocking because the accumulation of dislocation was appeared on the grain boundary. With the laser energy density increased there formed subgrain structure and eventually generate ultra-fine grain. Hardness test results also show that the surface hardness obtains a big growth about 60% after laser shock processing. The results showed that the formation of ultra-high plastic strain can improve the surface hardness of LY12 aluminum alloy, and thus effectively improve the comprehensive mechanical properties.

High Temperature Low-Cycle Fatigue, Deformation Microstructure and Final Fracture Behavior of a Nickel-Base Superalloy

The low-cycle fatigue (LCF) properties of a nickel-base precipitation-strengthened superalloy (GH4145/SQ), obtained at a temperature of 538 o C, were reported and discussed in this paper. The properties investigated include cyclic stress response, fatigue life, deformation microstructure and final fracture features as a function of applied strain amplitude. It was shown that the alloy exhibited a pronounced initial hardening followed by continuous softening to failure at high plastic strain amplitudes ( > 0.2% ap ε ), while at low plastic strain amplitudes ( < 0.2% ap ε ) the initial hardening was followed by a well-defined saturation stage. Bilinear behavior with a change of slope at a plastic strain amplitude of about 0.2% was observed in the cyclic stress-strain (CSS) and Coffin-Manson (C-M) plots. TEM observations revealed that slip band density increased with increasing total strain amplitude and precipitate degradation resulting from dislocation-precipitate interactions took place with continuous cyclic straining. The change in the microstructure during cycling is thus responsible for the fatigue hardening / softening behavior of the alloy. SEM examinations indicated that at low plastic strain amplitudes ( < 0.2% ap ε ) crack propagation was basically transgranular, while at high plastic strain amplitudes ( > 0.2% ap ε ) crack propagation exhibited intergranular features, as a whole. The variation in both the number of operating slip systems and the fracture modes with the strain amplitude employed was used to explain the observed two-stage LCF behavior of the present investigated superalloy.