Xinzhong Li

Well-aligned in situ composites in directionally solidified Fe–Ni peritectic system

Well-aligned in situ composites obtained in directionally solidified Fe–Ni peritectic system are formed by nonisothermal cellular coupled growth instead of isothermal coupled growth because of morphological instabilities. Peritectic coupled growth, with a plane interface like eutectic coupled growth, is always unstable due to the influence of peritectic reaction around the liquid/δ∕γ trijunctions. However, cellular nonisothermal peritectic coupled growth, in which one of the two solid phases bulges towards the liquid ahead of the other one, can reach a steady state and produce well-aligned in situ composites under proper growth conditions and sample composition.

Effect of peritectic reaction on dendrite coarsening in directionally solidified Sn–36 at.%Ni alloy

Effect of peritectic reaction on dendrite coarsening was investigated in directionally solidified Sn–36xa0at.%Ni peritectic alloys at different growth rates (2~200xa0μm/s) under constant temperature gradient. A coarsening model was used to characterize the coarsening process in terms of both the secondary dendrite arm spacing (λ2) of the primary Ni3Sn2 phase and the specific surface area (SV) of dendrites. It was shown that peritectic reaction could retard the increase of λ2 and decrease of SV during coarsening, which resulted from decelerating solute transport rate between adjacent dendrite arms caused by the peritectic phase enclosing the primary phase. The kinetics of the peritectic reaction that was found to be crucial to determine the coarsening process was characterized by the reaction constant (f) which not only changed with growth rates but also with solidification time in the real solidification process at a given growth rate.

A study on the microstructure of Alnico8 alloy thermomagnetically treated at various temperatures

Abstract The microstructure evolution of the Alnico8 alloy thermomagnetically treated at the temperatures of 800, 850, 865xa0°C was investigated by transmission electron microscope. The microstructure in both as-homogenized and thermomagnetically treated specimens consists of the rod-like Fe–Co rich phase and Ni–Al rich phase. During thermomagnetical treatment, the Fe–Co rich phase was coarsened to a certain size firstly, then split into smaller ones, which resulted in a more homogenous and finer microstructure. The result also indicates that the temperature of thermomagnetical treatment has great effect on the splitting process of Fe–Co rich phase. Based on the point of view of energy, the splitting process of Fe–Co rich phase was analyzed.

Effect of peritectic reaction on the migration of secondary dendrite arms in the presence of tertiary dendrites: analysis of a directionally solidified Sn–36 at.%Ni peritectic alloy

Directional solidification experiments have been performed on Sn–36xa0at.%Ni peritectic alloy in a constant temperature gradient at different growth velocities. Experimental result shows that a “sawtooth” morphology forms on secondary dendrite arms during the migration of secondary dendrites in the presence of tertiary dendrite arms. A theoretic model is therefore proposed to describe the formation of this “sawtooth” morphology in the peritectic solidification with tertiary dendrite arms taken into consideration. The migration of secondary dendrite arms is caused by remelting/solidification at the hot/cold sides of a liquid pool between secondary dendrite arms, which is a form of temperature gradient zone melting. And, the “sawtooth” morphology is ascribed to the difference in remelting velocity at the hot side of liquid pool during the migration of secondary dendrite arms due to the presence of tertiary dendrite arms. In addition, the proceeding of peritectic reaction can accelerate the formation of “sawtooth” morphology.

Isothermal Peritectic Coupled Growth in Directionally Solidified Cu-20 wt pct Sn Alloy

Isothermal peritectic coupled growth (PCG) has been experimentally observed in directionally solidified Cu-20xa0wtxa0pct Sn alloy at pulling rate of 1xa0μm/s under a temperature gradient up to 45xa0K/mm. The PCG growth interface temperature is determined above the equilibrium peritectic temperature TP. Diffusion coupling at and parallel to the solid/liquid interface necessary for PCG growth above TP is first experimentally proven.