Ti-55511合金显微组织对其高温疲劳行为的影响

Influence of microstructure of Ti-55511 alloy on its fatigue behavior at high temperature

利用扫描电子显微分析(SEM)、聚焦离子束切割技术(FIB)、透射电子显微分析(TEM)等方法对比分析了双态及片层组织对Ti-55511合金的高温疲劳性能及其损伤行为的影响机理。结果表明:在350℃、应力比(R)为0、最大加载应力(σ_(max))为500~600 MPa的疲劳加载条件下,片层组织的疲劳寿命(N_(f))明显高于双态组织。双态组织的疲劳裂纹主要萌生于初生α相(α_(P)),而片层组织的疲劳裂纹萌生于晶界处粗大片层α相(α_(L))与β相的界面。在裂纹扩展阶段,双态组织的裂纹尖端区域出现显著的纳米晶化,导致次生α相(α_(s))粒子完全溶解并转化为β相,明显弱化了合金的抗裂纹扩展能力。由于α_(L)相对晶内变形的显著约束作用,片层组织裂纹尖端区域的纳米晶化并不明显,仍然保留了较高密度的α_(s)相粒子,因而具备较高的抗裂纹扩展能力。

The fatigue properties and damage behavior of Ti-55511 alloy with bimodal and lamellar microstructures at high temperature were compared by scanning electron microscopy(SEM),focused ion beam(FIB)technologies and transmission electron microscopy(TEM).The results show that,fatigue loaded at 350℃,load ratio(R)of 0 and maximum stress(σ_(max))of 500−600 MPa,the fatigue lives(N_(f))of lamellar microstructure are significantly higher than those of bimodal microstructure.The fatigue cracks in bimodal microstructure mainly initiate from the primaryαphase(α_(P)),while those in lamellar microstructure generally nucleate at the interface between the coarse lamellarαphase(αL)andβat the grain boundaries(GBs).During the crack propagation,the significant nanocrystallization takes place at the crack-tip region of bimodal microstructure which leads to the complete dissolution and transformation of secondaryα-phase(α_(s))particles intoβ-phase.This weakens the crack growth resistance of the alloy obviously.Due to the evident fencing effect ofαL on intragranular deformation,the nanocrystallization at the crack-tip region of lamellar microstructure is not obvious,and a relatively high density ofαs phase particles are still retained.Therefore,the lamellar microstructure has a relatively higher resistance to crack growth.