Ma Chaoli

Phase Equilibria in Nb-Si-Mo Ternary Alloys at 1 273 K and 2 073 K

Phase equilibrium in Nb-Si-Mo ternary alloys (<37.5 at.% Si) at 1 273 K and 2 073 K is investigated by using X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The partial isothermal section at 1 273 K, which contains four single-phase regions, five two-phase regions and two three-phase regions, is basically the same as that at 1 973 K. However, when the temperature increases to 2 073 K, the three-phase region of Nbss+α-(Nb(Mo))5Si3+β-(Nb, Mo)5Si3 obviously moves towards the Nb-rich corner. This suggests that Nb-Si-Mo ternary alloys remain stable at least up to 1 973 K.

Liquid-Solid Phase Equilibria of Nb-Si-Ti Ternary Alloys

Abstract To determine the liquid–solid phase equilibria of the Nb-Si-Ti ternary system, Nb-Si-Ti alloys of different compositions are prepared. By means of scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron probe microanalysis (EPMA), the phases in the alloys, such as Si-based solutions, Ti(Nb)Si, Ti(Nb)Si2, Nb(Ti)Si2, Ti(Nb)5Si4, Nb(Ti)5Si3, Ti(Nb)5Si3, Nb(Ti)3Si and Nb-based solutions are identified, and the phase evolution is analyzed. As a result, the microstructural and microchemical evidence provides a clear definition of the Nb-Si-Ti liquidus surface projection and indicates that the ternary phase diagram has seven transition reactions.

Effect of Cold Expansion on High Cycle Fatigue of 7A85 Aluminum Alloy Straight Lugs

Abstract The cold expansion method was proposed to improve the high cycle fatigue (HCF) property of 7A85 aluminum alloy straight lugs. The microstructure, the microhardness and the residual stress near the hole wall of cold expanded lugs were investigated. The results indicate that a plastic deformation layer with about 150 μm in thickness is formed on the surface of the hole wall. Within the plastic deformation layer, the microhardness and the compressive residual stress are high, and the grains are stretched along the moving direction of the mandrel and compressed perpendicular to it. The fatigue life and fatigue strength were found to be improved significantly by cold expansion (CE). The fatigue crack initiation and propagation of cold expansion specimens were analyzed compared with those of the specimens without cold expansion (WCE). The mechanism of the improved fatigue property for cold expanded 7A85 aluminum alloy straight lugs was discussed.

Effect of cooling rate on morphology of primary particles in Al-Sc-Zr master alloy

Al-1.0%Sc-1.0%Zr (mass fraction) master alloy was prepared at different cooling rates. The morphology and thermodynamics data of the primary particles of the master alloy were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). It shows that the primary particles are dendrite-shaped particles comprised of several attached small cubic, cusped-cubic or crucifer shape particles at slow cooling rate. However, the primary particles are separated with crucifer shape at intermediate cooling rate, and they are cubic with cusped-cubic shape at high cooling rate. Meanwhile, the separated and attached particles present Al3Sc/Al3Zr1-xScx core-shell structure. The formation mechanism of the structure was systematically investigated by a mathematical model.

Phase Transformation in a β-Ti Alloy with Good Balance Between High Strength and High Fracture Toughness

Abstract This article studies the phase transformation of the metastable β-Ti-Al-Mo-V-Cr-Zr alloy (Ti-1300) to disclose the morphological reason for its high strength and high fracture toughness. It has been found that its ultrahigh strength (ultimate tensile strength exceeds 1 400 MPa) owes mainly to the spheroidization of the α-phase, while the high fracture toughness (exceeds 81 MPa·m½) to the special lath-shaped α-particles. Compared to the needle-shaped second α-articles, the coarser lath-shaped ones remove the stress concentration at the lath tips and consequently benefit improvement of fracture toughness. The article also describes shape evolution of the α-particles during aging thermodynamically and kinetically, and suggests an optimized aging processing to achieve an ideal balance between high strength and high toughness for this alloy.

Effect of Aging Time at High Temperature on Microstructural Evolution Behavior of a Nickel-Based Single Crystal Superalloy

Abstract The microstructural evolution behavior of one kind of nickel-based single crystal superalloy aged at elevated temperature was investigated. The short-term exposures were performed at 980, 1050 and 1100 °C for 20, 50 and 100 h, and long time agings were carried on from 200 to 800 h at 1100 °C. Microstructure observation was carried out by field emission scanning electron microscope (FE-SEM). Electron Probe Micro Analyzer (EPMA) was employed to determine the composition of TCP phase. The microhardness was tested to all the aged specimens. The results show that in short-term aging, the coarsening of γ ′ particles in this superalloy follows the standard r 3 -t kinetics of diffusion-controlled particle growth and coarsening rate constants were calculated. During long-term aging, for samples exposed for 400 h at 1100 °C, the rafted structure has developed completely and TCP precipitates has formed. The composition of TCP phase is mainly composed of Re and W. In addition, under some conditions of short time aging, TCP phases are found to precipitate from the alloy. The microhardnesses of all the aged specimens exhibit no obvious changes.