Microstructure, interface and mechanical property of the DS NiAl/Cr(Mo,Hf) composite

Abstract

A NiAl–Cr(Mo) composite containing Hf was directionally solidified under Ar atmosphere in Al 2O 3–SiO 2 ceramic mold by the standard Bridgman method. The microstructure in the as-grown and hot isostatic pressing (HIP) conditions was studied using SEM, HREM and TEM equipped with EDXS. The composite was mainly composed of three phases, i.e. lamellar Cr(Mo), NiAl matrix and semi-continuously distributed Ni 2AlHf. The common growth direction of NiAl and Cr(Mo) is <111>, and the interface between them was atomically flat with no transition layer. Fine G-phase precipitates were formed in the presence of Si which was from the ceramic shell molds used during the directional solidification. The G-phase precipitates were cuboidal in shape and coherent with NiAl matrix. After the HIPed and aged treatment, the G-phase precipitates completely disappeared and the amount of intercellular Ni 2AlHf phase was partially reduced. The mechanical properties were measured by tensile and compressive tests within the temperature range of 25∼1100°C. The BDTT of the HIPed and aged materials is significantly higher than the as-grown composite. The yield stress of the composite was higher than that of many NiAl-based alloys and possible strengthening mechanism were discussed.