Q.D. Hu

Quantitatively Analyzing Strength Contribution vs Grain Boundary Scale Relation in Pure Titanium Subjected to Severe Plastic Deformation

Electron backscatter diffraction was used to reveal high- and low-angle grain boundaries (HAGBs, with misorientation ≥15xa0deg, and LAGBs, <15xa0deg) in pure titanium (ASTM grade 2) subjected to equal channel angular pressing. Comprehensive paradigms were developed to present relations of yield strength vs HAGB grain diameter, and LAGB contribution vs LAGB linear intercept. Incorporating grain orientations (against loading axis) into the Hall–Petch relation, we quantitatively investigated the strength contributions by HAGBs and LAGBs, respectively.

Hot deformation characteristics of as-cast high-Cr ultra-super-critical rotor steel with columnar grains

Isothermal hot compression tests of as-cast high-Cr ultra-super-critical (USC) rotor steel with columnar grains perpendicular to the compression direction were carried out in the temperature range from 950 to 1250°C at strain rates ranging from 0.001 to 1 s−1. The softening mechanism was dynamic recovery (DRV) at 950°C and the strain rate of 1 s−1, whereas it was dynamic recrystallization (DRX) under the other conditions. A modified constitutive equation based on the Arrhenius model with strain compensation reasonably predicted the flow stress under various deformation conditions, and the activation energy was calculated to be 643.92 kJ∙mol−1. The critical stresses of dynamic recrystallization under different conditions were determined from the work-hardening rate (θ)–flow stress (σ) and −∂θ/∂σ–σ curves. The optimum processing parameters via analysis of the processing map and the softening mechanism were determined to be a deformation temperature range from 1100 to 1200°C and a strain-rate range from 0.001 to 0.08 s−1, with a power dissipation efficiency η greater than 31%.

Atomic Distance Tuning Effect for Nucleation in Liquid Iron

Liquid structural evolution of iron with various oxides was tracked from above liquidus to undercooling temperatures using an in situ high-energy X-ray diffraction method. The icosahedral-like orders and its enhancement with the decreasing temperature in all the liquids investigated suggest that icosahedral-like orders are not the sole reasons responsible for the variation of undercooling. The reduction of nearest-neighbor distance (r1) tuned by catalyzers contributes to the enhanced nucleation behavior of liquid iron.

Peritectic Solidification Path of the La(Fe,Si)13 Phase in Dual-Phase Directionally Solidified La-Fe-Si Magnetocaloric Alloys

AbstractNaZn13-type La(Fe,Si)13 alloy is a promising magnetocaloric material for solid refrigeration. Currently, several days of high-temperature annealing are typically required to transform both the α-Fe phase and LaFeSi phase to a La(Fe,Si)13 phase by diffusion. However, no bulk casting has yet been reported. In this study, directional solidification is used to prepare LaFe11.6Si1.4 alloys with a dual-phase microstructure consisting of α-Fe and LaFeSi phases. It was found that the volume fraction of La(Fe,Si)13 phase in directionally solidified LaFe11.6Si1.4 alloys can be increased to ~62 pct. It was also found that the volume fraction of the La(Fe,Si)13 phase is dependent primarily on the temperature gradient at different growth rates, which can be the result of their wide temperature range between solidus and liquidus. A significant Si segregation was observed in directionally solidified LaFe11.6Si1.4 alloys. Furthermore, the solidification path was discussed, focusing on the volume fraction increase of the La(Fe,Si)13 phase by directional solidification.n

Applications of the directional solidification in magnetic shape memory alloys

A zone melting liquid metal cooling (ZMLMC) method of directional solidification was applied to prepare highly-oriented Ni52Fe17Ga27Co4 magnetic shape memory alloys. At high temperature gradient and low growth velocity, the well-developed preferred orientation for coarse columnar crystals was obtained. Such a structure leads to a large complete pseudoelastic recovery of 5% at 348 K. Moreover, the pseudoelastic behaviours and the kinetics of the martensitic transformation (MT) are significantly affected by the intersection angle between the loading direction and the grain boundaries.