D.Q. Yi

Twinning and orientation relationships of T-phase precipitates in an Al matrix

The recent increasing interest of T-phase in Al alloy has been switched to its twins. In this study, we employed high resolution transmission electron microscopy to study and compare the morphology and orientation relationships (OR) of T-phase and its twins in an Al–Cu–Mg–Mn alloy. It is found that T-phase tends to form on the {403}Al habit planes and exhibit a rod-like shape, with it longitudinal axis, [010]T, being parallel to the matrix [010]Al direction. Three different OR types are determined between T-phase and Al matrix, namely, {200}T〈010〉T//{200}Al〈010〉Al (OR-I), {200}T〈010〉T//

Concurrent formation of two different type precipitation-free zones during the initial stage of homogenization

\{ 40\bar{3}\}_{\text{Al}}

The effect of point defect on mechanical properties of MoSi2

〈010〉Al (OR-II), and {200}T〈010〉T//{301}Al〈010〉Al (OR-III). OR-II is the most widely observed OR, while OR-I and III can form from the OR-II by twinning. During the twinning, the cross-section of T-phase transforms from a parallelogram-like shape into a shell-like shape. Further analyses on the shell-like T-twins strongly suggest that tenfold twins could form directly from the successive twinning of an individual T crystal.

Effect of α phase on fatigue crack growth of Ti-6242 alloy

A biomedical β titanium alloy (Ti-7Nb-10Mo) was designed and prepared by vacuum arc self-consumable melting. The ingot was forged and rolled to plates, followed by quenching and aging. Age-hardening behavior, microstructure evolution and its influence on mechanical properties of the alloy during aging were investigated, using X-ray diffraction, transmission electron microscopy, tensile and hardness measurements. The electrochemical behavior of the alloy was investigated in Ringers solution. The microstructure of solution-treated (ST) alloy consists of the supersaturated solid solution β phase and the ωath formed during athermal process. The ST alloy exhibits Youngs modulus of 80 GPa, tensile strength of 774 MPa and elongation of 20%. The precipitation sequences during isothermal aging at different temperatures were determined as β+ωath→β+ωiso (144 h) at Taging=350-400 °C, β+ωath→β+ωiso+α→β+α at Taging=500°C, and β+ωath→β+α at Taging=600-650 °C, where ωiso forms during isothermal process. The mechanical properties of the alloy can be tailored easily through controlling the phase transition during aging. Comparing with the conventional Ti-6Al-4V alloy, the Ti-7Nb-10Mo alloy is more resistant to corrosion in Ringers solution. Results show that the Ti-7Nb-10Mo alloy is promising for biomedical applications.

A Scandium Modified Al-0.7 Fe Binary Alloy: Improvement of Creep-Resistance Property

Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy were used to study the microstructure evolution of Al–Cu–Mg alloy during the initial stage of homogenization. It was found that two types of precipitation-free zones (PFZs) can form concurrently: one near grain boundaries and the other at the grain centres. Depth profile analyses of solute concentrations and dislocation-loop distributions strongly suggested that the formations of the two type of PFZs are different, due solely and exclusively to solute and vacancy depletion, respectively. A mechanism model was proposed to explain the concurrent formation of the two different type of PFZs during the initial stage of homogenization.

A Pseudo-Binary Diagram of the (Bi,Pb)-Sr-Ca-Cu-O System

Abstract Orthorhombic α″ phase often forms in Ti alloys through martensite transformation during quenching. Here, we report the formation of orthorhombic α″ phase in a low modulus β-Ti alloy during isothermal ageing. The coexists with α and ω phases, and remains stable for 120 h during ageing at 673 K. The phase is highly coherent with the β matrix, sharing the same basic orientation relationship as phase: //[0 0 2]α″, [1 0 0]β//[1 0 0]α″, [0 1 1]β//[0 1 0]α″. But differing from orthorhombic phase, the lattice constants of phase are determined as a = 0.352 nm, b = 0.494 nm, and c = 0.464 nm.

Effects of inclusions, grain boundaries and grain orientations on the fatigue crack initiation and propagation behavior of 2524-T3 Al alloy

The effect of point defect on mechanical properties of MoSi2 has been investigated by the first-principles. The elastic constants, mechanical modulus, hardness and thermodynamic properties of MoSi2 with four different single point defects have been calculated. By comparing with the defect-free MoSi2, it is found that the bulk modulus/shear modulus ratio (B/G) of MoSi2 with a single point defect increases slightly while the Debye temperature decreases drastically, which indicates that MoSi2 with some point defects have relatively good ductility. The calculated three-dimensional (3D) contours of elastic modulus and these projections on the (001) and (010) planes show that the directionality of Young’s modulus and shear modulus is unapparent for MoSi2 with a point defect (VMo and MoSi) but it is relatively obvious for MoSi2 with VSi and SiMo. It suggests that VSi and SiMo can strengthen the anisotropy in elasticity. The electronic properties of C11b MoSi2 with different single point defects have been studied to reveal further the influencing mechanism of point defect on mechanical properties. This work should help reveal the interrelation between intrinsic defects and service performance of MoSi2.

Effects of cold deformation on microstructure, texture evolution and mechanical properties of Ti–Nb–Ta–Zr–Fe alloy for biomedical applications

Fatigue crack growth as a function of a phase volume fraction in Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) alloy was investigated using fatigue testing, optical microscopy, scanning electron microscopy, and transmission electron microscopy. The α + β annealing treatments with different solid solution temperatures and cooling rates were conducted in order to tailor microstructure with different a phase features in the Ti-6242 alloy, and fatigue crack growth mechanism was discussed after detailed microstructure characterization. The results showed that fatigue crack growth rate of Ti-6242 alloy decreased with the decrease in volume fraction of the primary a phase (αp). Samples with a large-sized α grain micro-structure treated at high solid solution temperature and slow cooling rate have lower fatigue crack growth rate. The appearance of secondary a phase (αs) with the increase of solid solution temperature led to crack deflection. Moreover, a fatigue crack growth transition phenomenon was observed in the Paris regime of Ti-6242 alloy with 29.8% αp (typical bi-modal microstructure) and large-sized a grain microstructure, owing to the change of fatigue crack growth mechanism.