γ/γ′-αMo定向共晶合金激光快凝的微观组织研究

STUDY OF MICROSTRUCTURES INγ/γ'-αMo DIRECTIONAL EUTECTOID ALLOY INDUCED BY LASER RAPID MELTING SOLIDIFICATION

对γ/γ′-αMo定向共晶合金进行激光快凝实验.结果表明,激光快凝后,合金组织细化,偏析减少,硬度显著提高.在激光脉冲平行于定向共晶生长方向时,熔凝区重新生长出更细密的定向共晶组织;激光脉冲垂直于定向共晶生长方向时,熔凝区由更致密的胞状晶和枝晶组成.在熔凝区的枝晶组织中,枝晶臂由γ固溶体及从该固溶体析出的细小γ′相组成;枝晶间由含有γ′的γ固溶体和富Mo亚稳相的共晶组成,亚稳相包括[αMo_((ordered))],Ni_3Mo和[Ni_3Mo_((ordered))].

The principle purpose of surface modification by laser rapid melting solidification is to improve the surface properties of materals through the change of their surface microstructures. The γ/γ′-αMo directional eutectoid alloy consisting of a chemical composition （atomic fraction） of 15.41%Al, 2.56%V, 3.25%Cr, 8.41%Mo and balance Ni was chosen in the present study. The master alloy was melted in a vacuum induction furnace, and then the directional eutectoid specimens were prepared in a vacuum solidified furnace. Two ways, i.e. pulse and scan, were used for the surface laser treatment. A finer directional eutectoid structure grows again in the melting zone when the laser pulse is parallel to the eutectoid growth direction. However, when the laser pulse is perpendicular to the eutectoid growth direction, the microstructures of melting zone are composed of much denser cell-like crystalline and dendrites. The dendritic arm in dendrite structure after laser melting consists of the γ- solid solution and the fine γ-participites, and the microstructures in the interdendrites are composed of the γ-solid solution containing γ and the metastable phase precipitates which are enriched by Mo. These metastable phases are the ordered [αMo（ordered）], Ni3Mo and ordered [Ni3MO（ordered）].