Effect of V Addition on Microstructure and Mechanical Properties in C–Mn–Si Steels after Quenching and Partitioning Processes

Three C-Si-Mn Q&P steels with different V addition after one-step and two-step quenching and partitioning (Q&P) processes were investigated by means of optical microstructure observation, X-ray diffraction (XRD) measurement, transmission electron microscopy (TEM) characterization and particle size distribution (PSD) analysis. The effect of V addition on strength and ductility of the steels was elucidated by comparative analysis on the microstructure and mechanical properties as functions of partitioning time and temperature. For one-step Q&P treatment, the mechanical properties were mainly controlled by the tempering behavior of martensite during partitioning. V addition was helpful to mitigate the deterioration of mechanical properties by precipitation strengthening and grain refinement strengthening. For two-step Q&P treatment, the satisfying plasticity was attributed to the transformation-induced plasticity (TRIP) effect of retained austenite maintaining the high work hardening rate at high strain regime. The higher volume fraction of retained austenite with high stability resulted from the refined microstructure and the promoted carbon partitioning for the steel with 0.16 wt% V addition. However, the carbon consumption due to the formation of VC carbides led to the strength reduction of tempered martensite.

A Strategy for Dimensionality Reduction and Data Analysis Applied to Microstructure–Property Relationships of Nanoporous Metals

Nanoporous metals, with their complex microstructure, represent an ideal candidate for the development of methods that combine physics, data, and machine learning. The preparation of nanporous metals via dealloying allows for tuning of the microstructure and macroscopic mechanical properties within a large design space, dependent on the chosen dealloying conditions. Specifically, it is possible to define the solid fraction, ligament size, and connectivity density within a large range. These microstructural parameters have a large impact on the macroscopic mechanical behavior. This makes this class of materials an ideal science case for the development of strategies for dimensionality reduction, supporting the analysis and visualization of the underlying structure–property relationships. Efficient finite element beam modeling techniques were used to generate ~200 data sets for macroscopic compression and nanoindentation of open pore nanofoams. A strategy consisting of dimensional analysis, principal component analysis, and machine learning allowed for data mining of the microstructure–property relationships. It turned out that the scaling law of the work hardening rate has the same exponent as the Young’s modulus. Simple linear relationships are derived for the normalized work hardening rate and hardness. The hardness to yield stress ratio is not limited to 1, as commonly assumed for foams, but spreads over a large range of values from 0.5 to 3.

Novel Candidates for Palladium Alloys – Enhanced Strength and Ductility - Generalized Stacking Fault Energy Calculations

Generalized stacking fault energies of palladium alloys were calculated using the density functional theory. The stacking fault energy of palladium alloys is correlated with the valence electron of the transition metal element. The twinning tendency is also modified by the presence of an alloying element in the plane of deformation. The obtained results suggest that Pd –transition metal alloys with elements such as Cr, Mo, W, Mn, Re are expected to exhibit high work hardening rate due to the tendency to emit of the partial dislocations and mechanical twins, which results in increased strength and ductility.

Overcoming Strength-Ductility Trade-Off at Cryogenic Temperature of Low Carbon Low Alloy Steel via Controlling Retained Austenite Stability

Stress–strain behavior of a low carbon low alloy multiphase steel with ferrite, tempered bainite, and retained austenite was studied at different cryogenic temperatures. Results indicated that both strength and ductility were enhanced with decreasing tensile testing temperature. The enhancement of both strength and ductility was attributed to the decreased mechanical stability of retained austenite with decreasing temperature, resulting in sufficient transformation induced plasticity (TRIP) effect for increasing work hardening rate.

Influence of Al Additions on the Microstructure and Mechanical Properties of a C and Si-Free High-Mn Steel

The lightweight Fe–Mn–Al–C steels have drawn considerable attention from the literature due to their outstanding combination of high ductility and specific strength. Although the mechanical behavior of such steels has been extensively studied, the effect of Al when no C and Si are added has not been investigated in detail. For this reason, the main objective of this work was to study the microstructural evolution and mechanical behavior of carbon and silicon-free high-Mn steels with different aluminum contents. Alloys with 0, 2.5, and 5 wt. % Al were processed by spray forming to ensure high homogeneity and a fully austenitic microstructure. Cold rolling and annealing were performed to obtain a fine grain-sized material. The mechanical properties were similar regardless of the Al content, especially the work hardening rate. Deformation twinning and strain-induced phase transformation were not observed for any of the compositions. Additionally, a dislocation cell-like structure was observed for all of the alloys indicating that the Al additions did not change considerably the dislocation behavior, even though it considerably changed the estimated Stacking Fault Energy (SFE) value for all the alloys studied in this work.

Strain-hardenability of new strengthened TRIP/TWIP titanium alloys

A new Ti-Cr based alloy has been developed to reach a TWIP (TWinning Induced Plasticity) effect as the main deformation mechanism. This new composition, involving Fe addition, was derived from a classical TRIP/TWIP alloy Ti-8.5Cr-1.5Al (wt%) (TCA). The main objective is to achieve an optimized strength/hardenability combination by limiting the TRIP (TRansformation Induced Plasticity) effect whose critical resolved shear stress lowers the plasticity threshold. This new alloy Ti-7Cr-1Al-xFe (wt%) (TCAF) displays excellent mechanical properties, with an increased yield strength (with respect to TCA alloy), a very high work-hardening rate and an extremely high fracture strength (UTS=1415MPa), while maintaining an excellent ductility (ε=0.38 at fracture). Both mechanical (tensile tests) and microstructural characterization at different scales (EBSD, XRD) have been performed, evidencing a dense network of fine {332}<113> mechanical twins as well as the presence of stress-induced martensite plates at twins intersections, as a secondary mechanism.

The Deformation Behavior of Oxygen-Modified Ti-29Nb-13Ta-4.6Zr(wt.%)

A series of Ti-29Nb-13Ta-4.6Zr(wt.%) {TNTZ} alloys containing either 0.1, 0.3 or 0.7(wt.%) oxygen (O) were room-temperature tensile tested inside a scanning electron microscope to evaluate the effect of O on the deformation evolution. The deformation modes observed for TNTZ-0.1O, which exhibited the largest elongation-to-failure and lowest strength of all the alloys, were deformation-induced α”-martensitic transformation, {332}<113> twinning, and <111> slip. For the other two alloys, <111> slip was the dominant deformation mode, where TNTZ-0.7O exhibited more homogeneous and extensive slip, a higher frequency of cross slip, and a higher work-hardening rate, all of which contributed to both its strength and elongation-to-failure being greater than that for TNTZ-0.3O. TNTZ-0.3O exhibited the greatest tendency for cracking, which generally occurred on grain boundaries perpendicular to the tensile axis, leading to the lowest elongation-to-failure of all the alloys.