Effect of Gradient Heat Treatments on Microstructure and Microhardness of Nb-Si Based Ultrahigh Temperature Alloys
Xiping Guo*, Xiangcheng Kong, Yanqiang Qiao
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China

EXTENDED ABSTRACT: Nb-Si based ultrahigh temperature alloys are expected to be used as high temperature structural materials in the temperature range of 1200~1400℃ because of their high melting points, low densities and good high temperature strength. Appropriate heat treatment process can homogenize the alloy composition, accommodate the microstructure and obtain stable phases, and then improve its mechanical properties. At present, the research methods of material have been changed from traditional “trial-and-error” to “material genetic engineering”. One of the remarkable characteristics of “material genetic engineering” method is the high throughput of preparation and analysis processes. As a high throughput research method, gradient heat treatment can quickly optimize the heat treatment process of alloys. In this report, the master alloy ingot of Nb-Si based ultrahigh temperature alloy was prepared by vacuum nonconsumable arc melting and then high frequency induction melting. The Nb-Si based ultrahigh temperature alloy was integrally directionally solidified at 2050℃. The axial temperature gradient of vertical samples in the directional solidification furnace was measured by “pulling method”, and then the induction-melted and directionally solidified samples were heat treated at a temperature gradient for 10 and 50 hours respectively. The effects of heat treatment temperature and time on the constituent phases, microstructure, composition and microhardness of the alloy were studied, and the heat treatment process parameters of the alloy was optimized rapidly.