The Investigation Of Solid-state Phase Transformation Behavior In A β-ZrTiAlV Alloy

Zirconium metal has excellent resistance to radiation damage and atomic oxygen corrosion.In addition,zirconium has a low expansion coefficient,so it has the potential to serve in an alternate temperature environment.Compared with alloy steels and titanium alloys which are widely used in active components of space vehicles,zirconium has more potential applications in aerospace industry.Because of the infinite mutual solubility of zirconium and titanium,titanium element can be added to the zirconium matrix to meet the performance requirement of the structural material.Therefore,Zr-Ti-based alloys have a very broad application prospect in aerospace.In this paper,by means of electron baskscatter diffraction（EBSD）,electron channeling contrast（ECC）,X-ray diffraction（XRD）,optical microscope（OM）and microhardness technologies,we focus on noe novelβ-51.1Zr40.2Ti4.5Al4.2V alloy and systematically investigate the microstructure evolution and the effect of priorβgrain sizes on the variant selection ofαphase during the furnace cooling period as well as the microstructure evolution during room temperature compression progress.The main conclusions are summarized below:β→αtransformation will occur during the furnace cooling progress inβ-ZrTiAlV alloy.During this period,αphase firstly nucleates at the grain boundaries,and the orientation ofα_GB is effected by the adjacent priorβgrains,so theα_GB with{0001}_αpole parallel to the common{110}_βpole of adjacentβgrains will present preferred nucleation atβgrain boundaries.The major nucleation mechanism ofα_WGB is interface instability nucleation mechanism,and sympathetic nucleation mechanism can be rarely observed.Therefore,α_WGB grains nucleate and grow into the interior of priorβgrains by inheriting the orientation ofα_GB.In order to coordinate the strain energy formed during theβ→αtransformation,α_WI developes three-variant cluster by so-called self-accommodation mechanism,the three variants are related to each other by an angle/axis pair 60°/<112?0>.With the decrease of furnace-cooling temperature,the relative volume fraction ofαphase increases,resulting in the increasement of hardness.There is a linear relationship between hardness and the relative volume fraction ofαphase.Theoretical 12αvariants can be obtained during the furnace cooling from 750℃to550℃inβ-ZrTiAlV alloy.Allαvariants formed within one parentβgrain meet the axle/angle relationships predicted by Burgers theory.For smaller priorβgrains,onlyα_GB grain with{0001}_αparalleling to the common{110}_βpole of adjacentβgrains nucleates at the grain boundary,according to the interface instability nucleation mechanism ofα_WGB,the orientation ofα_GB can propogate to the inside of the parentβgrain.As a result of this,αvariant holding the same orientation withα_GB the will widely distribute within the parentβgrain,resulting in stronger variant selection in smaller priorβgrains.For bigger priorβgrains,besides the formation of aforementionedα_GB,α_GB grains with other orientations also nucleate at the grain boundary.Although the former preferentially nucleate,αvariants with different orientations obstruct each other during the next period,soαvariants whose orientations are parallel to the common{110}_βpoles just gather near the grain boundaries,and these variants uniformly distribute inside the parentβgrain,which cause weaker variant selection in bigger priorβgrains.Stress induced martensitic transformation（β→α′）will occur during the room temperature compression progress ofβ-ZrTiAlV alloy.As the compression strain increases,several priorβgrains totally transform toα′phase.{101?2}<101?1>twin is also observed inα′phase and the number of twin increases with the continuous compression.The misorientation distribution near the grain boundaries inα′phase shows that grain boundaries will hinder the growth ofα′phase during compression,so the area near the boundaries will accumulate a lot of strain,which will promote stress induced martensitic transformation in adjacent priorβgrains and form newα′phase.