Improvement of High Temperature Strength of NiAl Strengthened Ferritic Heat-resistant Alloys by Grain Boundary Coverage with χ Phase

The effect of χ phase precipitated at the grain boundary of matrix on the high temperature mechanical properties of ferritic Fe-Cr-Ni-Al-Mo alloys containing nano-sized B2-type NiAl precipitates was investigated. It was found that the grain boundary of Fe-22Cr-6Ni-6Al-3Mo alloy was covered with the χ phase approximately 95% by a heat treatment at 800°C for 50 h. However, the average particle diameter of the NiAl precipitates increases to approximately 300 nm during this heat treatment. We found that the NiAl precipitates can be refined to approximately 27 nm by additional heat treatment above the dissolution temperature of the NiAl precipitates followed by air cooling. The alloy with nano-sized NiAl precipitates and the χ phase exhibits a yield stress of 235 MPa at 700°C. The suppression of intergranular fracture by grain boundary strengthening caused by the χ phase and precipitation strengthening due to the nano-sized NiAl precipitates are responsible for the high strength of the alloy at high temperatures.