Haichang Jiang

Effect of precipitation on internal friction of AZ91 magnesium alloy

The effect of precipitation on the internal friction (IF) of AZ91 magnesium alloy was investigated by using X-ray diffraction (XRD) analysis, scanning electron microscope (SEM) observation, and dynamic mechanical analysis (DMA). Six different states of alloy were prepared by applying different heat treatment processes: as-cast, in-complete solid solution, complete solid solution, micro-precipitation, continuous precipitation and continuous-discontinuous precipitation. It was found that the internal friction of in-completely solid-solutionized, completely solid-solutionized and micro-precipitated specimens showed a similar characteristic, and the grain boundary relaxation is completed depressed due to the Al atoms supersaturated in the alpha-Mg solution. However, a thermal relaxation internal friction peak was observed for continuously precipitated and continuously-discontinuously precipitated specimens at around 438 K and frequency of about 1 Hz, which was attributed to the grain boundaries relaxation. Furthermore, it was found that the relaxation of the beta-Mg(17)Al(12)/alpha-Mg phase interfaces should give its contribution to the background internal friction in the as-cast, continuously precipitated and continuously-discontinuously precipitated specimens.

Effect of post-weld annealing on microstructure and properties of NiTi welding joints

Abstract The effect of post-weld annealing on the microstructure and properties of a Ni50·8Ti49·2 weld seam was systematically investigated. The experimental results indicated that the grain in the welding joint was refined and more and more Ni-rich particles were precipitated with the increasing annealing temperature. The differential scanning calorimeter results showed that the martensitic transformation behaviour changed from A→R→M/M→R→A (A: austenite, R: R phase and M: martensite) to A→M/M→A and A→R→M/M→A with the increasing annealing temperatures. When annealed at 1073 K for 3600 s, the phase transformation temperature of the joint is close to that of the base metal. However, the ultimate tensile strength of the weld seam was not improved by the annealing.

Microstructures and damping capacity of TiNiNbMo quaternary alloys with different Ti/Ni ratios

Abstract The effect of Ti/Ni ratio on the microstructure, phase transformation characteristics and damping capacity of TiNiNbMo quaternary alloys was investigated by scanning electron microscope, differential scanning calorimetry (DSC) and dynamic mechanical analyser. The microstructure observation showed that eutectic is weakened by the increase in the Ti/Ni ratio. The DSC test indicated that TiNiNbMo alloys with different Ti/Ni ratios exhibit multistage transformations including R transformation and B19′ martensite transformation, and the phase transformation temperature range was increased during cooling or heating. Meanwhile, when the Ti/Ni ratio is equal to 1·08, the alloy in martensite phase exhibited much higher damping than that of Ti50·1Ni49·9 binary alloy, which is mainly attributed to the precipitation of fine and dispersed β-Nb particles in the prime NiTi phases.