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

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 beta grain, alpha colony, and alpha plate. The multi-level lamellar microstructure of TC21 alloy, which was prepared by solution treatment at 980 degrees C and annealed temperature ranging from 720 degrees C to 820 degrees 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 a colony (d(c)) and a plate (d(alpha)) increase gradually with the annealing temperature. Although the sizes of alpha colonies and alpha plates obey the Hall-Petch relationship with respect to strength, the distribution of points between d(alpha)(-1/2) and strength is rather dispersed than d(c)(-1/2). Thus, alpha colony is the effective controlling unit of strength of the alloy. Meanwhile, alpha 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 alpha plate during the crack propagation. Moreover, an intensity relationship exists for the crack growth resistance with the increasing size of alpha colony, which cause rough propagation path by shearing amount of a plates and result in high toughness with increasing annealing temperature, thereby providing the reasonable evidence that alpha plates is the effective control unit of fracture toughness.