片层组织钛合金中相界面诱导保载疲劳裂纹形成

钛合金具有比强度高、抗疲劳性能好和耐腐蚀性优异等特点,已广泛应用于航空、能源、化工和医药等领域,特别适合于制造航空发动机的压气机盘、风扇盘和叶片等部件.然而,在飞机运行过程中航空发动机部件通常受保载疲劳的影响,这会显著降低部件的使用寿命并威胁到航空安全,因此研究钛合金的保载疲劳行为至关重要.本文研究了片层组织钛合金(α板条和β板条交替排列)在室温保载疲劳过程中的裂纹形核过程.集束中的α板条和β板条变形不匹配,α板条中的位错滑移容易造成在片层界面处产生大量残余位错.界面残余位错引起的局部拉应力促进集束中微孔的形成和连接,从而产生保载疲劳裂纹.这项工作提供了片层组织钛合金在保载疲劳过程中的裂纹形核机制,同时也有助于认识片层材料的开裂行为,对材料的组织发展有指导意义.

Temperature-gradient induced microstructure evolution in heat-affected zone of electron beam welded Ti-6Al-4V titanium alloy

The heat-affected zone(HAZ) of electron beam welded(EBW) joint normally undergoes a unique heat-treating process consisting of rapid temperature rising and dropping stages, resulting in temperature-gradient in HAZ as a function of the distance to fusion zone(FZ). In the current work,microstructure, elements distribution and crystallographic orientation of three parts(near base material(BM) zone, mid-HAZ and near-FZ) in the HAZ of Ti-6Al-4V alloy were systematically investigated. The microstructure observation revealed that the microstructural variation from near-BM to near-FZ included the reduction of primary α(αp) grains, the increase of transformed β structure(βt) and the formation of various α structures. The rim-α, dendritic α and abnormal secondary α(αs) colonies formed in the mid-HAZ, while the "ghost" structures grew in the near-FZ respectively. The electron probe microanalyzer(EPMA) and electron back-scattered diffraction(EBSD) technologies were employed to evaluate the elements diffusion and texture evolution during the unique thermal process of welding. The formation of the various α structures in the HAZ were discussed based on the EPMA and EBSD results. Finally, the nanoindentation hardness of "ghost" structures was presented and compared with nearby βt regions.

Fatigue crack tip plastic zone ofα+βtitanium alloy with Widmanstatten microstructure

The recent studies had focused on the fatigue crack propagation behaviors of α＋β titanium alloys with Widmanstatten microstructure. The fascinated interest of this type of microstructure is due to the superior fatigue crack propagation resistance and fracture toughness as compared to other microstructures,which was believed to be related to the fatigue crack tip plastic zone（CTPZ）. In this study, the plastic deformation in fatigue CTPZ of Ti-6 A1-4 V titanium alloy with Widmanstatten microstructure was characterized by scanning electron microscope（SEM） and electron backscatter diffraction（EBSD）. The results showed that large-scale slipping and deformation twinning were generated in fatigue CTPZ due to the crystallographic feature of the Widmanstatten microstructure. The activation of twinning was related to the rank of Schmid factor（SF） and the diversity of twin variants developing behaviors reflected the influence of SF rank. The sizes of CTPZ under different stress intensity factors（K） were examined by the white-light coherence method, and the results revealed that the range of the plastic zone is enlarged with the increasing K（or crack length）, while the plastic strain decreased rapidly with the increasing distance from the crack surface. The large-scale slipping and deformation twinning in Widmannstatten microstructure remarkably expanded the range of fatigue CTPZ, which would lead to the obvious larger size of the observed CTPZ than that of the theoretically calculated size.

Effects of heating rate on the alloy element partitioning and mechanical properties in equiaxedα+βTi-6Al-4V alloy

The effects of heating rate on the alloy element partitioning and mechanical properties during the phase transformation ofα→βin Ti-6Al-4 V alloy under solution treatment have been investigated by the experiments and phase field simulations,which reveal the evolutions of microstructure and compositions at the non-equilibrium state and well verify the experimental results.The specific results indicate that the compositions measured through electron probe micro-analysis(EPMA)under a lower heating rate are close to the equilibrium ones corresponding to the solution temperature.Heating up to the target solution temperature,as the heating rate increases,the Al content decreases and V increases in the primaryα(α_(p))grain with a larger size,the volume fraction ofα_(p)increases and the composition gradient betweenα_(p)andβphases gets steeper.The interrelated relationship among the diffusion,compositions,solution temperature and free energy of the system has been discussed in detail.Moreover,increasing the heating rate(~20.0 K/min)may help to improve the mechanical properties of the alloy by mainly adjusting theα_(p)/β;volume fractions,α_(p)particle size and secondaryα(α_(p))size during the process of heating up to the solution temperature.These results may shed some light on the optimization of the knowledge-based heat treatment route.

Influence of Processing Conditions on Microstructure and Mechanical Properties of Large Thin-Wall Centrifugal Ti–6Al–4V Casting

In this work, the effects of mould pre-heating temperatures and hot isostatic pressing （HIPping） process on the microstructural characteristics and mechanical properties, including static tensile and damage- tolerance properties of large thin-wall cylindrical Ti-6Al-4V casting, have been studied. The experimental results show that with the increasing mould pre-heating temperature from 673 to 873 K, the casting mi- crostructures change from a mixture of Widmanstatten and colony microstructure to a primary colony. The centre of the thick wall section has relatively coarse microstructure than the edge and thin section. Lower mould pre-heating temperature brings about more porosities. HIPping process, which not only reduces the casting pores effectively but also increases the prior β grain boundary cohesion and coars- ens the microstructure, is essential to improving the ductility of the casting. Due to the oxygen contamination and finer microstructure on the surface, micro-hardness profiles on the cross section present a decreas- ing tendency from the surface to inner. The thickness of the reaction layers for the different mould pre-heating temperatures is nearly the same （-450 μm）. On the whole, the tensile strength and micro- hardness decrease with increasing mould pre-heating temperature from 673 to 873 K. However, the fracture toughness and fatigue crack growth resistance of the castings increase with increasing mould pre- heating temperature.

Characterization ofα_(2)Precipitates in Ti–6Al and Ti–8Al Binary Alloys:A Comparative Investigation

Transmission electron microscopy(TEM)and atom probe tomography(APT)techniques were used to investigate the nanoscale orderedα_(2)(Ti_(3)Al)precipitates in Ti–Al binary alloys.Ti–6Al and Ti–8Al binary alloys were solution treated and aged to obtain Widmanstatten microstructure and promoteα_(2)precipitates.The TEM results displayed strong short-range ordering ofα_(2)precipitates in Ti–8Al alloy,while no evidence of the superlattice reflections ofα_(2)in Ti–6Al alloy.The results acquired from APT showed theα_(2)clusters and atoms distribution at the interface between the matrix andα_(2)precipitates.The size and morphology ofα_(2)particles in Ti–8Al alloy,respectively,obtained by TEM and APT are closely consistent.Meanwhile,the APT results displayed tiny size clusters in Ti–6Al alloy,which supposed to give evidence of the initial ordering process ofα_(2)precipitates in the absence of correlative results from TEM.