Lamellar α fencing effect for improving stress relaxation resistance in Ti-55511 alloy

Abstract

Abstract The stress relaxation (SRT) behaviour and α phase evolution of Ti-55511 alloy with bimodal and lamellar microstructures were investigated at 350, 400, 450 and 500 °C. The results indicated that, the stress decreased with SRT time and the decreasing rate increased with temperature at the initial stage for both bimodal and lamellar samples. While, the stress relaxation limit (σ∞) of lamellar sample was obviously larger than that of bimodal sample at the same temperature. Dislocation annihilation and coarsening of secondary α (αS) phase always took place during SRT especially at higher temperature, and they were less significant for lamellar sample due to the fencing effect of long lamellar α (αL) on dislocations. Besides, the fragment of globular primary α phase (αp) occurred along subgrain boundaries during SRT at 500 °C which led to obvious refinement of αp. The αL in lamellar structure fractured into many pieces along local shear bands during SRT at 350 °C, and some of them coarsened and spheroidized apparently during SRT at 500 °C.