Strengthening mechanism of friction stir processed and post heat treated NiAl bronze alloy: Effect of rotation rates

In this work, NiAl bronze (NAB) alloys were subjected to friction stir processing (FSP) at a constant traverse speed of 100 mm/min and rotation rates of 600 rpm, 800 rpm, 1000 rpm and 1200 rpm, respectively. Thereafter, heat treatment was performed at 675 °C for 2 h. The effects of rotation rates on strengthening mechanisms of friction stir processed and post heat treated NAB alloy were studied. The results showed that friction stir processed NAB alloy microhardness was increased as the rotation rate increased. During friction stir processing, martensite nanotwins could be formed due to high strain rates and peculiar martensitic structures of NAB alloys. A rotation rate increase, increased peak temperatures and strain rates in friction stir processed NAB alloys, leading to a significant amount of martensite nanotwins formation. As rotation rates increased from 600 rpm to 1200 rpm, in addition to grain refinement effects, the strengthening mechanism of friction stir processed NAB alloys gradually changed from secondary phase strengthening to solid solution, dislocations and nanotwin strengthening. During post heat treatment, discontinuous static recrystallization occurred and β′ phase decomposed into α and κ phases. Highest microhardness values were obtained at the rotation rate of 800 rpm and the uniformly distributed second phases formed during friction stir processing contributed mainly to higher microhardness.