Uncovering the hierarchical clusters in the heat-affected zone of an electron beam weldedα/βtitanium alloy joint

Using high-resolution transmission Kikuchi diffraction(TKD)and transmission electron microscopy(TEM),we examined the hierarchical clusters that form in situ in the heat-affected zone(HAZ),which are com-monly referred to as“ghost”structures,of bimodal titanium alloy Ti-5Al-2Sn-2Zr-4Mo-4Cr(wt%,TC17).The ghost structures are enriched with Al elements but poor in Mo and Cr compared to the surroundingβmatrix.TKD results show that the ghost structure in middle-HAZ mainly consists ofα_(L)laths with a high-angle grain boundary,which exhibits the classic Burgers orientation relationship(BOR)with the host matrix,while it encircles theα_(P)grains in far-HAZ.And the ghost structure is evidenced to form via in-complete martensitic transformation.TEM results further confirm that the ghost structure is composed ofαL and tinyβ_(L)laths with BOR,with the former being enriched with Al and poor with Cr and Mo,while the latter is the opposite.Interestingly,twoα_(L)variant clusters with a check-mark morphology are fre-quently observed viewed along[0001]_(αL)//[110]_(βL)directions,which are dominated by the crystallographic and geometrical relationships betweenαandβphases.Based on the microstructural characterization,it is hypothesized that the ghost structure is transformed from the initialα_(P)phase,due to the coupling ef-fect of high thermal stress(which induces the formation of a large number of dislocations)and element diffusion caused by sudden temperature increase and plunge cooling in the HAZ during the welding pro-cess.

High-resolution characterization of the fretting corrosion of Alloy 690 in the simulated secondary water of pressurized water reactor

The worn scars on Alloy 690 after the fretting corrosion testing in simulated pressurized water reactor(PWR)secondary water have been comprehensively analyzed by scanning transmission electron microscopy(STEM)and transmission Kikuchi diffraction(TKD).The high-quality characterization results experimentally show that the fretting wear accelerates the corrosion of Alloy 690 in two approaches.The first one is to break the integrity of the oxide scale by introducing cavities at the oxide grain boundaries.The second one is to alter the microstructure of the underneath matrix,forming a nano-grained matrix layer.The increased grain boundary density in this layer can accelerate the consumption of Cr in the near-surface matrix.The loss of oxide scale integrity and the accelerated Cr consumption are believed to contribute to the deteriorated corrosion resistance of Alloy 690 during the fretting corrosion process.