On the high-pressure phase stability and elastic properties of β-titanium alloys

The Characteristic of {101¯2}<101¯1¯> Twin of Ti-10V-2Fe-3Al under Planar Wave Detonation

The microstructure evolution of the twin of TB6 (Ti-10V-2Fe-3Al) under planar wave detonation was studied. The initial microstructure of the alloy consists of an α and β phase. It is found that twin deformation is operated in only the α phase due to the limited slip system in this phase. α grains are mainly rotated from {101¯0} to {0002} during the deformation due to the {101¯2}<101¯1¯> twin. Twin variant selection is found in this study, and the orientation of all {101¯2} twins is oriented at {0002} in different α grains with different deformation degrees. The twin variant selection is well explained based on the strain relaxation along the loading axis and the Schmid factor for twinning shear.

Computational Study of the Influence of α/β-Phase Ratio and Porosity on the Elastic Modulus of Ti-Based Alloy Foams

This work aims to perform a computational analysis on the influence that microstructure and porosity have on the elastic modulus of Ti-6Al-4V foams used in biomedical applications with different α/β-phase ratios. The work is divided into two analyses, first the influence that the α/β-phase ratio has and second the effects that porosity and α/β-phase ratio have on the elastic modulus. Two microstructures were analyzed: equiaxial α-phase grains + intergranular β-phase (microstructure A) and equiaxial β-phase grains + intergranular α-phase (microstructure B). The α/β-phase ratio was variated from 10 to 90% and the porosity from 29 to 56%. The simulations of the elastic modulus were carried out using finite element analysis (FEA) using ANSYS software v19.3. The results were compared with experimental data reported by our group and those found in the literature. The β-phase amount and porosity have a synergic effect on the elastic modulus, for example, when the foam has a porosity of 29 with 0% β-phase, and it has an elastic modulus of ≈55 GPa, but when the β-phase amount increases to 91%, the elastic modulus decreases as low as 38 GPa. The foams with 54% porosity have values smaller than 30 GPa for all the β-phase amounts.

β Grain Size Inhomogeneity of Large Scale Ti-5Al-5V-5Mo-3Cr Alloy Bulk after Multi-Cycle and Multi-Axial Forging in α + β Field

In order to fabricate homogeneous large-scale Ti-5Al-5V-5Mo-3Cr (Ti-5553) alloy bulk with fine and equiaxial β grain, we performed a series of multi-axial α + β field forging with 62 forging cycles on the large-scale Ti-5553 billet by using 12.5 MN high-speed hydraulic press. The β-annealed microstructure was the starting microstructure of the billet. After the 6th forging cycle, β grain deformed dramatically, and the grain-boundary network developed within the irregular β grain. As the forging cycle increased to 44, the volume fraction of the fine and equiaxial β grain that is less than 20 μm, which is caused by dynamic recrystallization, increased gradually. However, the incomplete dynamic recrystallization region within the original β grain could not be eliminated. As the forging cycle further increased, the volume fraction of the fine and equiaxial β grain did not increase. In contrast, the abnormal grain growth of the β phase occurred during 50th~62nd forging cycle. Here, we attribute the formation of the incomplete dynamic recrystallization region and the abnormal grain growth of the β phase to the high deformation rate of the α + β forging. The refining behavior of β grain and the abnormal coursing β grain, which is found during the multi-cycle multi-axial forging of large-scale Ti-5553 alloy billet, are seldom reported in the isothermal compression of small-scale Ti-5553 alloy specimen. The findings of the paper are instructive for improving the sub-transus forging strategy that is used to fabricate the large-scale homogeneity Ti-5553 alloy billet with fine and equiaxial β grain.

Hot Deformation Behavior, Processing Maps and Microstructural Evolution of the Mg-2.5Nd-0.5Zn-0.5Zr Alloy

Isothermal hot compression experiments were conducted on Mg-2.5Nd-0.5Zn-0.5Zr alloy to investigate hot deformation behavior at the temperature range of 573-773 K and the strain rate range of 0.001 s^-1-10 s^-1 using a Gleeble-3500D thermomechanical simulator. The results showed that the rheological curve showed a typical work hardening stage, and there were three different stages: work hardening, transition and steady state. A strain compensation constitutive model was established to predict the flow stress of the Mg-2.5Nd-0.5Zn-0.5Zr alloy, and the results proved that it had high predictability. The main deformation mechanism of the Mg-2.5Nd-0.5Zn-0.5Zr alloy was dislocation climbing. The processing maps were established to distinguish the unstable region from the working region. The maps showed that the instability generally occurred at high strain rates and low temperatures, and the common forms of instability were cracking and flow localization. The optimum machining range of the alloy was determined to be 592-773 K and 0.001-0.217 s^-1. With the increase in deformation temperature, the grain size of the alloy grew slowly at the 573-673 K temperature range and rapidly at the 673-773 K temperature range.

Evaluation of Surface Integrity in 18CrNiMo7-6 Steel after Multiple Abrasive Waterjet Peening Process

Abrasive waterjet peening (AWJP) as an important surface strengthening method can effectively improve surface properties. In this study, after multiple AWJP, the distribution of compressive residual stress and roughness on the surface of 18CrNiMo7-6 steel has been evaluated by an X-ray diffraction (XRD) method and a 3D surface topography system, respectively. Compared with the single AWJP, multiple AWJP can obviously increase the surface residual stresses (−1024 MPa to −1455 MPa) and the depth of maximum compressive residual stress (100 μm to 120 μm), as well as make the stress distribution more uniform. In terms of the surface roughness, multiple AWJP influences its uniform distribution and reduces the surface roughness (Sa = 0.69 μm), compared with a single AWJP (Sa = 2.96 μm), due to the smaller shot balls and a uniform deformation during multiple AWJP. In addition, we have studied the effects of multiple AWJP on the hardness of the surface layer. The results show that multiple AWJP increases the hardness by up to 15.9%, compared to the single AWJP. These studies provide useful insight into improving the surface properties of 18CrNiMo7-6 steel by multiple AWJP.

Role of Vanadium Addition on Hot Deformation Behavior of Aluminum Alloy 5083

The effect of V addition on the hot deformation behavior of AA5083 was investigated. Single axial compression tests were conducted on the cast and homogenized samples with strain rates ranging from 0.01 to 10 s−1 and deformation temperatures ranging from 300 to 450 °C. The results showed that the contents of V (0–0.10, in wt.%) do not change the grain size of alloy 5083 significantly in the as cast and homogenized conditions, but the formation of fine Al3V particles in the alloy with an addition of 0.05 wt.% V can increase the flow stress, and its activation energy is 10.0% higher than that of V-free alloy 5083. The processing maps show that the appropriate process domain for alloy 5083 with 0.05 wt.% V changes at different true strains. The mechanism for deformation softening is discussed as well.

High-Pressure Induced Phase Transitions in High-Entropy Alloys: A Review

High-entropy alloys (HEAs) as a new class of alloy have been at the cutting edge of advanced metallic materials research in the last decade. With unique chemical and topological structures at the atomic level, HEAs own a combination of extraordinary properties and show potential in widespread applications. However, their phase stability/transition, which is of great scientific and technical importance for materials, has been mainly explored by varying temperature. Recently, pressure as another fundamental and powerful parameter has been introduced to the experimental study of HEAs. Many interesting reversible/irreversible phase transitions that were not expected or otherwise invisible before have been observed by applying high pressure. These recent findings bring new insight into the stability of HEAs, deepens our understanding of HEAs, and open up new avenues towards developing new HEAs. In this paper, we review recent results in various HEAs obtained using in situ static high-pressure synchrotron radiation x-ray techniques and provide some perspectives for future research.

Austenite Decomposition and Precipitation Behavior of Plastically Deformed Low-Si Microalloyed Steel

The aim of the present study is to assess the effects of hot deformation and cooling paths on the phase transformation kinetics in a precipitation-strengthened automotive 0.2C–1.5Mn–0.5Si steel with Nb and Ti microadditions. The analysis of the precipitation processes was performed while taking into account equilibrium calculations and phase transitions resulting from calculated time–temperature–transformation (TTT) and continuous cooling transformation (CCT) diagrams. The austenite decomposition was monitored based on thermodynamic calculations of the volume fraction evolution of individual phases as a function of temperature. The calculations were compared to real CCT and DCCT (deformation continuous cooling transformation) diagrams produced using dilatometric tests. The research included the identification of the microstructure of the nondeformed and thermomechanically processed supercooled austenite products formed at various cooling rates. The complex interactions between the precipitation process, hot deformation, and cooling schedules are linked.

Phase Transformation Behavior of a β-Solidifying γ-TiAl-Based Alloy from Different Phase Regions with Various Cooling Methods

The phase transformation behavior of Ti-42Al-5Mn (at.%) alloy from different phase regions with various cooling rates was investigated based on electron probe micro analyzer-backscattered electrons (EPMA-BSE). It is shown that β→α2′ takes place when this alloy is cooled at a high rate, such as water quenching (WQ), oil cooling (OC), from β single phase. With the decreasing cooling rate to air cooling (AC), β→α2′ is restrained and β→γ is promoted by forming γ platelets. The room-temperature microstructure is βo + α2 when alloy cooled (WQ and OC) from (β + α) dual-phase. However, under AC, β→γ occurs and γ platelets form. It should be noted that α2→γ happens when this alloy cooled from 1180 °C (>Teut) by OC and AC, forming an incomplete lamellae (α2/γ) structure in the α2 phase. However, when the alloy cooled from 1100 °C (<Teut), α2/γ→βo,sec occurs and complete lamellae generates in α2 phase.

Pressure Impact on the Stability and Distortion of the Crystal Structure of CeScO3

The effects of high pressure on the crystal structure of orthorhombic (Pnma) perovskite-type cerium scandate were studied in situ under high pressure by means of synchrotron X-ray powder diffraction, using a diamond-anvil cell. We found that the perovskite-type crystal structure remains stable up to 40 GPa, the highest pressure reached in the experiments. The evolution of unit-cell parameters with pressure indicated an anisotropic compression. The room-temperature pressure-volume equation of state (EOS) obtained from the experiments indicated the EOS parameters V₀ = 262.5(3) ų, B₀ = 165(7) GPa, and B₀' = 6.3(5). From the evolution of microscopic structural parameters like bond distances and coordination polyhedra of cerium and scandium, the macroscopic behavior of CeScO₃ under compression was explained and reasoned for its large pressure stability. The reported results are discussed in comparison with high-pressure results from other perovskites.