Strength and fracture toughness of TC21 alloy with multi-level lamellar microstructure

Abstract This study aims to determine an effective microstructure unit that controls the strength and fracture toughness of TC21 alloy with multi-level lamellar microstructure, including original β grain, α colony, and α plate. The multi-level lamellar microstructure of TC21 alloy, which was prepared by solution treatment at 980 °C and annealed temperature ranging from 720 °C to 820 °C, was characterized using scanning electron microscopy, electron backscatter diffraction, optical microscopy, and image analysis software. The relationship between multi-level microstructure and strength and fracture toughness was discussed using the Hall–Petch formula. Results show that the sizes of α colony ( d c ) and α plate ( d α ) increase gradually with the annealing temperature. Although the sizes of α colonies and α plates obey the Hall–Petch relationship with respect to strength, the distribution of points between d α − 1 / 2 and strength is rather dispersed than d c − 1 / 2 . Thus, α colony is the effective controlling unit of strength of the alloy. Meanwhile, α plate is the effective microstructure unit for controlling fracture toughness. It is confirmed by EBSD that the fracture toughness depends on the bending, rotation and shearing of α plate during the crack propagation. Moreover, an intensity relationship exists for the crack growth resistance with the increasing size of α colony, which cause rough propagation path by shearing amount of α plates and result in high toughness with increasing annealing temperature, thereby providing the reasonable evidence that α plates is the effective control unit of fracture toughness.