位错对Ti-6Al-4V合金α相形核及微织构形成的影响

INFLUENCES OF DISLOCATIONS ON NUCLEATION AND MICRO-TEXTURE FORMATION OFα PHASE IN Ti-6Al-4V ALLOY

借助Willis-Steeds-Lothe方法计算了单根长直刃型与螺型位错应力场,并采用相场动力学方法模拟了含位错Ti-6Al-4V合金中b→a转变过程,探索长直位错应力场下共格a相的形核及对微织构形成的影响.研究表明,刃型位错应力场与择优a变体间相互作用能中,正应力分量S33起主要作用;螺型位错时切应力分量S23作用最大.刃型位错应力场对a变体选择的作用要强于螺型位错.刃型位错下择优变体以V1与V7为主,螺型位错时以变体V7,V10及V12为主,且V1/V7,V1/V4/V6是刃型位错下主要出现的变体组合类型,而螺型位错时则以V7/V10/V12组合为主.含位错体系的微观组织由位错应力场与a变体之间相互作用能,以及a变体之间弹性相互作用能共同决定.位错周围的应力场可导致界面能较高的界面类型出现.

ABSTRACT Titanium alloys are widely applied in aerospace, chemical and other related industries. The α＋β alloys may obtain various microstructures and mechanical properties simply by varying their thermomechanical processing. Ti-6Al-4V alloy is the most common α＋β titanium alloy. Its strength, ductility, fracture toughness and fatigue properties depend strongly on the microstructure especially texture. The understanding of the formation mechanisms of a micro-texture during processing is necessary for the optimization of the mechanical properties. In this work, the nucleation of a precipitates and micro-texture formation process under the influence of dislocations during the β→α transformation in Ti-6Al-4V alloy was simulated by phase field method. The stress field of an infinite straight dislocation was calculated by Willis-Steeds-Lothe method and used as input of the phase field model. It was shown that the normal stress component S33 plays a dominant role in a variants nucleation in the presence of edge dislocation, while the shear stress component S23 is the most important one for screw dislocation. The effect of edge dislocation on α variant selection is generally stronger than that of screw. V1 and V7 are the main variants selected by the edge dislocation while V7, V10 and V12 dominate around the screw dislocation, with V1/V7, V1/V4/V6 being the main variant cluster types around the edge dislocation, and V7/V10/V12 being the primary one for the screw dislocation. In a system with the presence of dislocations in the parent phase, the precipitate micro- structure is determined by the combined effect of elastic interactions between the dislocation and different variants of a low symmetry precipitate phase, and elastic interactions among different variants. Variants with interfaces of relatively high energy may appear because of variants selection by dislocations.