In situ TEM observations and molecular dynamics simulations of deformation defect activities in Mg via nanoindentation

In this work, we performed in situ nanoindentation in TEM to capture the real-time dislocation and twinning activities in pure Mg during loading and unloading. We demonstrated that the screw component of dislocations glides continuously, while the edge components rapidly become sessile during loading. The twin tip propagation is intermittent, whereas the twin boundary migration is more continuous. During unloading, we observed the elastic strain relaxation causes both dislocation retraction and detwinning. Moreover,we note that the plastic zone comprised of dislocations in Mg is well-defined, which contrasts with the diffused plastic zones observed in face-centered cubic metals under the nanoindentation impressions. Additionally, molecular dynamics simulations were performed to study the formation and evolution of deformation-induced crystallographic defects at the early stages of indentation. We observed that,in addition to dislocations, the I1stacking fault bounded with a <1/2c+p> Frank loop can be generated from the plastic zone ahead of the indenter, and potentially serve as a nucleation source for abundant dislocations observed experimentally. These new findings are anticipated to provide new knowledge on the deformation mechanisms of Mg, which are difficult to obtain through conventional ex situ approaches. These observations may serve as a baseline for simulation work that investigate the dynamics of dislocation slip and twinning in Mg and alloys.

Effects of dynamic recrystallization and strain-induced dynamic precipitation on the corrosion behavior of partially recrystallized Mg-9Al-lZn alloys

The corrosion susceptibility of recrystallized and un-recrystallized grains in equal channel angular pressed(ECAPed)Mg-9Al-lZn(AZ91)alloys immersed in chloride containing media was investigated through immersion testing and an electrochemical microcell technique coupledwith high resolution techniques such as scanning Kelvin probe force microscopy(SKPFM),transmission electron microscopy(TEM),andelectron backscatter diffraction(EBSD).During ECAP,dynamic recrystallization(DRX)and strain-induced dynamic precipitation(SIDP)simultaneously occurred,resulting in a bimodal grain structure of original elongated coarse grains and newly formed equiaxed fine grainswith a large volume fraction ofβ-Mg17Al12 precipitates.Corrosion preferentially initiates and propagates in the DRXed grains,owing tothe greater microchemistry difference between theβ-Mg17Al12 precipitates formed at the DRXed grain boundaries and the adjacentα-Mgmatrix,which induces a strong microgalvanic coupling between these phases.Additionally,the weaker basal texture of the DRXed grainsalso makes these grains more susceptible to electrochemical reactions than the highly textured un-DRXed grains.The influence of dynamicrecrystallization and dynamic precipitation was also studied in ECAPed alloys with differenl levels of deformation strain through corrosion andelectrochemical techniques.Increasing the strain level led to a more uniform corrosion with a shallow penetration depth,lower corrosion ratevalues,and higher protective ability of the oxide film.Furthermore,higher levels of strain resulted in greater hardness values of the ECAPedalloys.The superior corrosion resistance and strength of the ECAPed alloys with increasing strain level was attributed to the combination ofsmaller DRXed grain size,higher DRX ratio,and higher volume fraction of uniformly distributed fineβ-Mg17Al12 precipitates.c 2020 Published by Elsevier B.V.on behalf of Chongqing University.

Microstructure and mechanical properties of spark plasma-sintered La_(2)O_(3) dispersion-strengthened W-Ni-Fe alloy

An investigation of lanthanum oxide(La_(2)O_(3))addition to tungsten heavy alloy(WHA)with a ternary composition of W-7 Ni-3 Fe was reported in this study.The mixed powders were sintered using spark plasma sintering(SPS)technique.La_(2)O_(3)was added in increments of 0.25 wt%,0.50 wt%,0.75 wt%and 1.00 wt%to WHA,respectively.The sintered samples were characterized for microstructural evolution and mechanical properties.The influences of La_(2)O_(3)addition on density,grain size,hardness,ultimate tensile strength(UTS)and ductility on W-7 Ni-3 Fe system were discussed in this study.The highest relative sintered density of 87.95%was obtained for 0.25 wt%La_(2)O_(3)addition to W-7 Ni-3 Fe.The lowest grain size of 7.89μm was observed for 1.00 wt%La_(2)O_(3)addition.Similarly,the highest hardness and UTS of HV 533 and1110 MPa,respectively,were also obtained for the same composition.Scanning electron microscopy(SEM)and energy-dispersive spectroscopy(EDS)of the samples revealed homogenous distribution of La_(2)O_(3)in the alloy matrix.Fractography of the sintered alloy samples revealed W-W intergranular fracture.