循环热处理对激光定向能量沉积Ti-6Al-4V-1Mo合金疲劳裂纹扩展速率的影响

Effect of Cyclic Heat Treatment on Fatigue Crack Growth Rate of Ti-6Al-4V-1Mo Alloy Prepared by Laser Directed Energy Deposition

摘要:

对一种面向激光定向能量沉积的新型Ti-6Al-4V-1Mo钛合金循环热处理前后的疲劳裂纹扩展速率进行了研究，分析了循环热处理前后显微组织的变化及疲劳裂纹扩展试样的断口和扩展路径。结果显示：沉积态与循环热处理试样裂纹扩展速率在稳定扩展区的表达式分别为da/dN=1.8651×10^-8(ΔK)^3.2271和da/dN=1.4112×10^-8(ΔK)^3.1125。与沉积态相比，循环热处理后合金微观组织从均匀编织的网篮组织转变为由近球状α与β转化基体组成的双态组织，初生β晶界处的晶界α相连续性也在循环过程中被打断。循环热处理后初生α的粗化及非连续晶界α相是缓解应力集中、偏转疲劳裂纹的主要原因，由此导致疲劳裂纹扩展。此外，循环热处理试样中更易出现的裂纹分支、二次裂纹及裂纹桥接也是降低疲劳裂纹扩展的驱动力，是减缓裂纹扩展速率的重要原因。

Abstract:

The fatigue crack growth rate of a novel Ti-6Al-4V-1Mo titanium alloy, which is developed for laser directed energy deposition technique, was investigated before and after cyclic heat treatment (CHT). Changes in microstructure, fracture surfaces, and crack growth paths were analyzed before and after CHT. Results indicate that in the stable crack growth region, the growth rates for the as-deposited and cyclic heat-treated specimens follow the relationships da/dN=1.8651×10^-8(ΔK)^3.2271 and da/dN=1.4112×10^-8(ΔK)^3.1125, respectively. Compared with that at the as-deposited state, the microstructure after CHT is transformed from a uniform basket-weave microstructure to a dual-phase microstructure consisting of near-spherical α and β-transformed matrix phases. The cyclic process also disrupts the continuity of the grain boundary α (α_GB) at the primary β-phase grain boundary. The coarsening of primary α and the disruption of α_GB continuity are the primary factors to release stress concentration and promote crack deflection, thereby decreasing the fatigue crack growth rate. Additionally, the increased occurrence of crack branching, secondary cracking, and crack bridging in cyclic heat-treated specimens further reduces the crack driving force and slows the fatigue crack growth rate.