Research On Grain Growth And Mechanical Training Process In Polycrystalline Ni-Mn-Ga Alloys

The rule of grain growth in polycrystalline Ni-Mn-Ga alloys heat treated withdifferent temperatures and time were studied in this paper. The microstructure,composition and phase of the polycrystalline alloys were analyzed by opticalmicroscope （OM）, energy dispersive spectrum （EDS） and X-ray diffraction （XRD）. Therule was finally attained. It shows that the grains grow much easier at the temperature of1100℃than any other applied temperature. But if the Ni atom percent exceeded50at.%in polycrystalline Ni-Mn-Ga alloys, the rule failed. According to the rule, different sizesof grains were obtained through different heat treatments. The microstructure,composition, phase, transformation temperature, grain orientation and twin relationshipwere analyzed by OM, EDS, XRD, differential scanning calorimetry （DSC） andelectron backscatter diffraction（EBSD）. In the view point of inverse pole figure（IPF）, amutil-stage twin structure, which means smaller twin variants stay within larger ones,was observed.In attaintion, variants in polystalline alloys with different orientation wereobviously observed, one-way and two-way alternative compression were applied inalloys with different grain sizes. During compression, the microstructure was observedby OM. The one way training with step increased stresses was applied in alloys withfine grains, which shows that the mobility of twin boundary incresses with increasinggrain size. The two way alternative training was applied in alloys with coarse grains,which reveals that the mobility of twin boundary was different in different directions,0.4%and0.25%stress induced strain were obtained along Y and Z directionsrespectivelly. From the OM and EBSD figures, variants increased with increasingtraining cycle numbers, meanwhile, the proportion of favoriants increased at theexpense of other small variants.Finally, the twin orientation relationship was analyzed by EBSD. And it was alsoobserved that martensite twin variant had alternative distribution.The two twin variantshave a compound twinning relationship with the twinning elements Κ₁=（112）,η₁=[11-1], Κ₂=（11-2）, η₂=[111], P=（1-10）, and s=0.121. The misorientation betweenthem is86.4°, around <110>axis.