A NiAl-Cr(Mo) composite containing Hf was directionally solidified under Ar
atmosphere in Al2O3-SiO2 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 Ni2AlHf. 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 so
lidification. The G-phase precipitates were cuboidal in shape and coherent
with NiAl matrix. After the HIPed and aged treatment, the G-phase precipita
tes completely disappeared and the amount of intercellular Ni2AlHf phase wa
s partially reduced. The mechanical properties were measured by tensile and
compressive tests within the temperature range of 25 similar to 1100 degre
esC. 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 t
hat of many NiAl-based alloys and possible strengthening mechanism were dis
cussed. (C) 2001 Elsevier Science Ltd. All rights reserved.