Microstructure Evolution and Recrystallized Behavior of Friction Stir Welding Twin-Induced Plasticity Steel

The restoration mechanism of twin-induced plasticity(TWIP)steel during friction stir welding(FSW)changed with the degree of the deformation,and the microstructure evolution and dynamic recrystallization are complex and unclear.In this paper,the electron backscattered diffraction and transmission electron microscopy techniques were used to evaluate the dynamic grain structure of FSW joint of TWIP steel.The results showed that the dynamic recrystallization mechanisms in TWIP steel during FSW contained discontinuous dynamic recrystallization(DDRX)and continuous dynamic recrystallization(CDRX).The recrystallization mechanism transitioned from DDRX at the initial deformation stage to DDRX and CDRX at the middle deformation stage,eventually becoming primarily CDRX at the end deformation stage.Numerous annealing twin boundaries(ATBs)were formed within the joint,and the straight ATBs primarily resulted from grain growth accidents,while cluster-shaped ATBs were formed through re-excitations and decomposition of specific grain boundaries.

Quasi-in-situ investigation on complete lamellar fragmentation of β-solidified TiAl alloy during uniaxial isothermal compression

The coarse as-cast lamellar microstructure in TiAl alloys is difficult to be broken completely by thermomechanical processing. Some remnant lamellar colonies in the deformed microstructure seriously affect the microstructural homogeneity and deteriorate the properties. In this study, it is found that by isothermal compression at 1230 °C and 1250 °C, the lamellar colonies of Ti-43.5Al-4Nb-1Mo-0.1B (TNM) alloys can be completely broken. This is attributed to the weakened anisotropic deformation behavior of the lamellar colonies due to the isothermal holding treatment before deformation. The deformation behavior at 1230 °C was investigated by quasi-in-situ experiments. It is observed that the regions near lamellar colony boundaries first undergo dynamic recrystallization at small strain, while the lamellar colonies gradually break down with increasing strain. The adequate fragmentation of lamellar colonies mainly depends on the recrystallization of α lamellae (αL). The isothermal holding at 1230 °C leads to an increase in the content and thickness of αL, which allows it to assume more deformation and promotes its recrystallization by reaching critical strain. The interrupted γ lamellae (γL) formed by decomposition during isothermal holding facilitates the occurrence of α recrystallization within the lamellar colonies by hindering dislocation movement. In addition, recrystallized γ grains (γR) are gradually dissolved by the formation of α precipitates inside them through the γ → α phase transformation and the subsequent consumption of α precipitates by the recrystallized α grains.

Microstructural influences on the high cycle fatigue life dispersion and damage mechanism in a metastable β titanium alloy

In this work,the effect of microstructure features on the high-cycle fatigue behavior of Ti-7Mo-3Nb-3Cr-3Al(Ti-7333)alloy is investigated.Fatigue tests were carried out at room temperature in lab air atmosphere using a sinusoidal wave at a frequency of 120 Hz and a stress ratio of 0.1.Results show that the fatigue strength is closely related to the microstructure features,especially theα_(p) percentage.The Ti-7333 alloy with a lowerα_(p) percentage exhibits a higher scatter in fatigue data.The bimodal fatigue behavior and the duality of the S-N curve are reported in the Ti-7333 alloy with relatively lowerα_(p) percentage.Crack initiation region shows the compoundα_(p)/βfacets.Facetedα_(p) particles show crystallographic orientation and morphology dependence characteristics.Crack-initiation was accompanied by faceting process across elongatedα_(p) particles or multiple adjacentα_(p) particles.These particles generally oriented for basal slip result in near basal facets.Fatigue crack can also initiate at elongatedα_(p) particle well oriented for prismatic slip.Theβfacet is in close correspondence to{110}or{112}plane with high Schmid factor.Based on the fracture observation and FIB-CS analysis,three classes of fatigue-critical microstructural configurations are deduced.A phenomenological model for the formation ofα_(p) facet in the bimodal microstructure is proposed.This work provides an insight into the fatigue damage process of theβprecipitate strengthened metastableβtitanium alloys.