Recrystallization and twinning enhancing mechanical property of laser directed energy deposited CoCrFeNi high entropy alloys induced by synchronous ultrasonic impact

Abstract

Synchronous ultrasonic impact treatment is proposed to induce recrystallization and twinning of laser directed energy deposited CoCrFeNi high entropy alloy, which remarkably refine microstructure and enhance the mechanical properties of deposited alloy. The mechanism of ultrasonic impact influence on microstructure has been investigated through multi-scale characterization, and the contribution of multiple mechanisms to the strength has also been discussed in detail. Ultrasonic impact causes intense plastic deformation of the previously deposited layer, recrystallization occurs during the sequent directed energy deposition process, the average grain size reduces by 74 %, meanwhile, annealing twins are generated. Ultimate tensile strength and yield strength of deposited sample increase dramatically while maintaining the plasticity. Dislocation strengthening, grain boundary strengthening and unique annealing twins strengthening contribute to the yield strength enhancement, and the related strengthening contribution are calculated as 49 MPa, 50 MPa and 44 MPa, respectively. Molecular dynamics calculations reveal the presence of annealing twins has a detrimental effect on the growth of deformation twins while it is beneficial to the dislocation multiplication due to more sources of dislocations and additional activation of slip systems during tensile test. The proposed recrystallization and twinning induced by ultrasonic impact offer a novel approach to improving the microstructure and mechanical properties of high entropy alloys by laser directed energy deposition.