显微组织对近α钛合金BT-20疲劳裂纹扩展的影响

Influence of Microstructure on the Fatigue Crack Growth in Near-α Ti Alloy BT-20

通过显微镜及电子背散射衍射试验对近α钛合金BT-20的篮网、双态和魏氏组织微观结构及晶粒尺寸进行测定,并采用拉伸与疲劳裂纹扩展试验进行力学性能以及裂纹扩展研究,讨论微观组织对裂纹扩展速率、路径和断口形貌的影响.结果表明,双态组织的强度和延伸率最好,魏氏组织最差;双态组织的断口为解理断裂,裂纹碰到大尺寸初生α晶粒时发生穿晶失效,形态呈直线状,从而具有较高的扩展速率;篮网和魏氏组织的断口形式为沿相界断裂,裂纹扩展路径在通过α晶粒集束边界时改变方向,路径呈曲线状,扩展速率相对较低,从而具有较好的耐裂纹性能.

The lamellar,bimodal,acicular microstructures and grain morphology of BT-20 near-αTi alloy were observed by optical microscopy and electron backscatter diffraction. Tensile and fatigue crack growth tests were carried out to analyze the mechanical property and the influence of microstructures on fatigue crack growth rate,path and fractography. The results showed that the bimodal microstructure has the highest strength and best ductility,and acicular microstructure exhibits the lowest strength and poor ductility. The fractography of bimodal microstructure is cleavage fracture. The transgranular fracture happens when crack encounters the large primary αgrain,which results in a relatively straight profile of the crack and explains the higher crack growth rate. The fractography of the lamellar and acicular microstructures is intergranular fracture.The crack path changes direction when encounters boundaries of two colonies of α grains,which results in tortuosity of the crack,lower crack growth rate and a better crack growth resistance.