Understanding microstructural evolution and hardness of nanostructured Fe89.5Ni8Zr2.5 alloy produced by mechanical alloying and pressureless sintering

Fe89.5Ni8Zr2.5 alloy was synthesized by mechanical alloying followed by pressureless sintering at various temperatures up to 900 °C. Microstructural evolution as a function of processing temperature was characterized using focused ion beam microscopy, transmission electron microscopy and X-ray diffraction techniques. The dependence of hardness on the microstructure was utilized to study the mechanical changes. The experimental results showed that microstructural stability can be enhanced by segregation of solutes to grain boundaries at low temperatures and by precipitation of second phases at elevated temperatures. Eventually, at higher processing temperatures the stability was lost due to the coarsening of the precipitated second phases leaving behind ultra-fined grained microstructure. Despite the coarsening of the grain size with increasing processing temperatures, the in-situ formed second phases were found to induce an Orowan strengthening effect leading to approximately 5.5 GPa hardness after 1 h sintering at 900 °C.