Abstract
The effects of Ti addition and heat treatment on the microstructure evolution and mechanical properties of NiAl-Cr(Mo) hypereutectic alloy were studied. The microstructure of NiAl-32Cr-6Mo alloy consists of Cr(Mo) dendrites and NiAl + Cr(Mo) eutectic. Ti addition would lead to the formation of Ni2AlTi precipitated phase. With the increasing Ti content, the number of Ni2AlTi phase increases. After heat treatment, the phase constituent of the alloy does not change. The hardness, room-temperature and high-temperature yield strength of the as-cast NiAl-32Cr-6Mo-1Ti alloy obviously increase due to the precipitation strengthening and solid solution strengthening, but the increasing primary phase and irregular microstructure result in the decreasing strength of NiAl-32Cr-6Mo-3Ti alloy. The yield strength and hardness of the alloy decline obviously after heat treatment while the ductility increases due to the uniform microstructure and increasing number of primary phase. All alloys show typical brittle quasi-cleavage fracture at room temperature. The number of cleavage steps and tearing ridges in the Ti-doped alloys increases significantly, and the fracture surface becomes more rugged. In addition, multiple cleavages are observed in the NiAl phase, which shows that the NiAl phase also plays a role in strengthening the alloy.
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