EBSD Study On Microstructures Of Zr-2.5Nb Alloy After Heat Treatment And Pulsed Laser Surface Modification

In this work,various characterization and analysis techniques such as transmission electron microscope(TEM),X-ray diffraction(XRD),microhardness test,electron channeling contrast(ECC)and especially electron backscatter diffraction(EBSD)technique in a field emission gun scanning electron microscope(FEGSEM)were jointly employed to investigate a Zr-2.5Nb alloy,which was a forged rod.The microstructures of the as-received Zr-2.5Nb alloy were characterized meticulously in composition,morphology and orientation,etc.The effects of different ?-cooling rates(water cooling,air cooling and furnace cooling)on microstructures and mechanical properties(hardness)were studied.Finally,pulsed laser was employed to conduct the surface modification treatment on Zr-2.5Nb alloy,and the evolution of the microstructure was studied with their influence on the hardness was also analyzed.The main conclusions are drawn as follows:(1)The microstructure of the as-received Zr-2.5Nb alloy is composed of bulk ?grains in equiaxed or plate shape(majority)and thin ? films surrounding the bulk grains(minority),with their average grain size and thickness measured to be 1.4 ?m and 72 nm,respectively.Grain boundary analyses reveal a number of low angle boundaries for the ?phase and most of them belong to deformation-induced dislocation boundaries.All Burgers boundaries generated by phase transformation are also separately measured and analyses of relative proportions between them suggest very weak(if any)variant selection ???cooling,which should relate to deformation-induced augment of nucleation rate of ?phases.Compared to previous work in literature,more satisfactory indexing of fine ?phases(down to nanoscale)is attained by the FEGSEM-based EBSD.Examples are presented to clearly reveal well-obeyed Burgers orientation relationships between adjacent? and ? phases.(2)Zr-2.5Nb alloy specimens cooled from ?-phase by water,air and furnace(WC,AC,FC)have complete martensite,basket-weave widmanst?tten,and lenticular structures,respectively.Through the combined use of TEM and SEM,it was determined that there were {10(?)1} twins in WC specimen.The second phase in AC and WC specimen was determined by the EDS technique.The EBSD characterization of the AC specimen was implemented,and all of the 12 variations transformed from the same ? grains according to the Burgers orientation relationship,suggesting very weak(if any)variant selection in AC process.The width of ? plates decreases with the increase of cooling rate,leading to higherhardness.This can be attributed to twins lamellar,fine grain strengthening and solid solution strengthening.(3)After laser surface treatment(LST)at powers of 400 W,100W,50 W,four kinds of modified microstructures were obtained: uniform ?-plate structures(UPS);an unreported bimodal ?-plate structure(BPS),which are featured by coase laths in the core and surrounded thin laths;non-equilibrium hybrid microstructure consisting of both prior ? and? phases and ? plates transformed from new ? phases;and another kind of hybrid microstructure consisting of prior ? phases and new ? plates transformed from new ?phases.In addition,the uniform ?-plate structures(UPS)is also called as "twinned martensite",and both of uniform ?-plate structures(UPS)and bimodal ?-plate structure(BPS)are completely comprised of ? phase.The existence of non-equilibrium hybrid microstructure is convenient to the characterization of the ??? phase transformation.the The origin of the non-equilibrium hybrid microstructure is attributed to the specific thermodynamic conditions induced by the pulsed laser treatment.ECC observation shows that newly formed ? phases during laser heating prefer to nucleate and grow at the expense of edges rather than interiors of prior ? grains.EBSD analyses further reveal that orientations of the new ? phases are not determined by the prior ? grains according to the Burgers relationship but maintain those of the prior ? phases in an eptiaxial growth way.A large number of nano-twins were detected in the UPS zone of the 50 W laser power treatment specimen,which was {10(?)1}?(?)012?compressive twins.