预压缩镍基单晶合金拉伸蠕变期间的组织演化

MICROSTRUCTURE EVOLUTION OF PRE-COMPRESSION NICKEL-BASED SINGLE CRYSTAL SUPERALLOY DURING TENSILE CREEP

采用预压应力处理使镍基单晶合金中的γ′相转变成P-型筏状结构,通过拉伸蠕变曲线测定和组织形貌观察,研究了该合金拉伸蠕变中的组织演化.结果表明:在拉伸蠕变初期,合金中的P-型筏状γ′相转变为N-型筏状结构.由于高温拉应力导致γ′/γ两相中元素平衡浓度发生变化及P-型筏状γ′相的不均匀粗化,促使P-型筏状γ′相发生分解出现沟槽;沟槽区域溶质元素化学位的提高引起的元素定向扩散是γ′相逐渐溶断成类立方体结构的主要原因.切应力分量使立方γ′相与应力轴垂直界面的晶格收缩可排斥较大半径的Al和Ta原子,拉伸张应力使平行于应力轴界面的晶格扩张可诱捕较大半径的Al和Ta原子,是促使γ′相定向生长成为N-型筏状的主要原因.其中,在拉应力作用下类立方γ′相不同界面的应变能密度变化是元素扩散及γ′相定向粗化的驱动力。

By means of pre compression treatment, the cubic γ′ phase in a single crystal nickelbased superalloy transformed into the P type rafted structure and its evolution during tensile creep was investigated by measuring creep curve and microstructure observation. Results show that the P type structure is transformed into the N type structure in the initial stage of the tensile creep. Under the effect of the tensile stress at high temperature, the changes of the equilibrium concentration of elements in the γ′/γ phases promoted the unhomogeneous coarsening of the γ′ phase, and the further decomposition of the P-type raft induced the groove structure, in which the chemical potential of solute element enhanced, resulting in the directional diffusion of the elements and dissolution of the P type rafted γ′ phase, so the cubical like γ′ phase formed. The shear stress induced the lattice constriction in the horizontal interfaces of the cubical-like γ′ phase, and the Al, Ta atoms with bigger radius are repelled out, while the tension stress induced the lattice expansion in the side interfaces of the cubical-like γ′ phase and the Al, Ta atoms with the bigger radius are trapped, which promotes the directional growing of γ′ phase into the N type rafted structure. Thereinto, the change of the strain energy density in different interfaces of the cubical like γ′ phase is also the driving force of the elements diffusion and the γ′ phase directional growth.