Xiaohua Min

Enhanced uniform elongation by pre-straining with deformation twinning in high-strength β-titanium alloys with an isothermal ω-phase

It is shown that pre-straining with deformation twinning is a novel approach to enhancing the uniform elongation of a high-strength β-type Ti–15Mo alloy (mass%) with isothermal ω-phase precipitation. Pre-existent mechanical {3 3 2}⟨1 1 3⟩ twins hinder the early onset of plastic instability (necking) after yielding, which is often caused by the presence of the isothermal ω-phase, and enhance the uniform elongation markedly from 0% to 13% at a yield strength level of 900 MPa.

Study of {332} twinning in a multilayered Ti-10Mo-xFe (x = 1–3) alloy by ECCI and EBSD

Abstract We have investigated the propagation of {332}<113> twins in a multilayered Ti-10Mo-xFe (x = 1–3) alloy fabricated by multi-pass hot rolling. The material contains a macroscopic Fe-graded structure (about 130 μm width) between 1 and 3 wt% Fe in the direction perpendicular to rolling. We observe strong influence of the Fe-graded structure in the twin propagation behavior. The propagation of {332}<113> twins that are nucleated in Fe-lean regions (~1 wt% Fe) is interrupted in the grain interiors at a specific Fe content, namely, about 2 wt% Fe. We ascribe this effect to the role of Fe content in solid solution on the stress for twin propagation. The interruption of twins in the grain interiors results in the development of characteristic dislocation configurations such as highly dense dislocation walls (HDDWs) associated to strain localization phenomena. The nucleation and propagation of these dislocation configurations is ascribed to the underlying plastic accommodation mechanisms of the stress field at the twin tips. We find that the crystallographic alignment of HDDWs is determined by the stress field at the twin tips and the deformation texture. The excellent plastic accommodation at the interrupted twin tips allows attaining the good ductility of the present material (total elongation of 28%).

Strength evaluation of α and β phases by nanoindentation in Ti–15Mo alloys with Fe and Al addition

Abstract The strengths of the α precipitate and the β matrix were evaluated by nanohardness in the Ti−15Mo−1Fe and Ti−15Mo−3Al alloys and compared to those of the Ti−15Mo alloy. The α phases with similar size (a long axis of a few micrometres and a short axis of a few hundred nanometres), distribution and volume fraction were obtained in three alloys by adjusting the aging temperature. Analyses by SEM-EDS confirmed that Fe and Mo were enriched in the β phase and depleted in the α phase, while Al was enriched in the α phase and depleted in the β phase. Tensile tests were carried out, and the tensile strength was shown to be higher in the Ti−15Mo−1Fe and Ti−15Mo−3Al alloys than in the Ti−15Mo alloy. The nanohardness measurements indicated that the α phase was softer than the β phase in both Ti−15Mo−1Fe and Ti−15Mo alloys, while it was harder in the Ti−15Mo−3Al alloy. The increased tensile strength was mainly caused by the strength of the Fe enriched β phase in the Ti−15Mo−1Fe alloy and by the strength of the Al enriched α phase in the Ti−15Mo−3Al alloy.

Microstructure, mechanical properties and springback behaviour of Ti‑6Al‑4V alloy connection rod for spinal fixation device

The effect of annealing condition on microstructure, mechanical properties and springback behaviour was examined in the connection rod of Ti-6Al-4V alloy for spinal fixation devices. Compared with the deformed microstructure in the sample before annealing, relatively few equiaxed grains were present after annealing at 1003 K after 1.8 ks, and a considerable amount appeared at 7.2 ks. When annealing time was extended to 36 ks, the recrystallised grains further grew. Vickers hardness, tensile strength and bending strength decreased with increasing annealing time, whereas the elastic and bending moduli showed no significant change with annealing time of up to 7.2 ks and then slightly decreased at 36 ks. The springback ratio was closely associated with strength and modulus and applied bending deflection. The springback ratio reached the highest and lowest values in the sample before and after annealing for 7.2 ks, respectively. A good combination of strength, modulus and springback ratio was obtained in the sample after annealing for 7.2 ks.

First-principles study of phase stability and elastic properties in metastable Ti-Mo alloys with cluster structure

ABSTRACT This paper provided a novel approach for evaluating phase stability and elastic properties in metastable Ti–Mo alloys with low Mo content by first-principles combined with cluster structure. In 54-atom body-centered-cubic supercell by substituting Ti atoms with 2–7 Mo atoms (7.1–23.0 wt% Mo), individual cluster structure of β-phase was constructed by ‘-Mo-Ti-Mo-’ cluster unit having the lowest cohesive energy. The distorted supercell was more stable than undistorted one at a low Mo content. With increasing Mo content, the density of state at Fermi level decreased, and bonding electron number increased, indicating β-phase stability was gradually promoted. Tetragonal shear elastic constant (C′ = (C11 – C12)/2), shear modulus (G111) and anisotropy factor (A = C44/C′) exhibited a fluctuation with Mo addition, while the change trend of A was opposite to C′ and G111. Calculated Young’s modulus exhibited similar changing trend to the C′, implying that the softening of C′ resulted in low Young’s modulus of β-phase. Measured Young’s modulus exhibited significant difference from calculated one, which was mainly caused by formation of α″-martensite and ω-phase. The values of C′, G111 and A were considered to associate with not only elastic properties of β-phase itself but also transition from β-phase to α″-martensite and/or ω-phase.

Quantitative Analysis of Twinning-Induced Plasticity (TWIP) in Beta-Titanium Alloy

Work hardening behavior of Ti-15Mo alloy (mass %) was examined through quantitative evaluation of the {332} twinning structural evolution at various plastic strain levels by optical microscope (OM) observations combined with electron backscatter diffraction analyses (EBSD). Based on analyzed images from the optical micrographs, area fraction of twins rapidly increases to 40% at strain of 0.05, and gradually reaches to 52% at strain of 0.12. With further deformation to a strain of 0.17, it has no significant change. The EBSD inverse pole figure maps exhibit that the twins become denser and thicker with increasing strain. A linear relationship is found between true stress and inverse square root of mean free path of dislocations at strain up to 0.12, indicating that the work hardening is mainly dominated by dynamic microstructure refinement induced by the twinning.