Role of Microstructural Characteristics in Combination of Strength and Fracture Toughness of Laser Additively Manufactured Ultrahigh-Strength AerMet100 Steel

A good combination of strength and fracture toughness is required for aerospace applications of laser additively manufactured (LAM) AerMet100 steel. To understand the role of microstructure in the combination of strength and fracture toughness of LAM AerMet100 steel, microstructures, fracture toughness properties and corresponding crack-tip fracture behaviors of the as-deposited specimen and five types of heat-treated specimens were mainly investigated. The results indicate the as-deposited microstructure of LAM AerMet100 steel with a high amount of retained austenite has a high K_IC value of ~ 81.7 MPa m^1/2. After a short process of cryogenic treatment and subsequent tempering treatment, the K_IC value of the steel has the lowest value of ~ 60 MPa m^1/2 due to the apparent decrease of austenite and weak cracking resistance, associating with intergranular cracking along the prior austenite columnar grain boundaries and transgranular cracking of intercellular regions. In contrast, the crack-tip fracture mode of the equiaxed-grain tempered martensite microstructure of LAM AerMet100 steel becomes completely transgranular cracking with the main mechanisms of micro-void nucleation and coalescence. Homogenization treatment can apparently decrease the amount and size of large-size alloy carbides and effectively improve the fracture toughness of the steel. Furthermore, proper microstructure modification (including control of large-size carbide, grain refinement and a decrease of retained austenite) by a longer treatment process can prompt tempered LAM AerMet100 steel to achieve a good combination of yield strength (~ 1827 MPa) and fracture toughness (~ 113 MPa m^1/2) compared with the wrought one.