| Rolling under cryogenic environment(liquid-nitrogen temperature,77 K),as a relatively novel process,has good performance in regulating the microstructure and mechanical properties of Mg alloys due to its huge potential in tailoring grain size and improving strength property of metallic materials.However,due to the hexagonal close-packed structure of the conventional magnesium alloy sheet,it is prone to form a strong basal texture during the deformation process,which greatly limits their cryogenic rolling performance,and hinders the research on the microstructure,texture evolution and deformation mechanism of the sheet during cryogenically rolling.In the present study,cryogenic rolling experiments with the thickness reduction per pass of 5% at the rolling speed of 30mm/s,300mm/s,400mm/s and 600mm/s are conducted on the applied AZ31 Mg alloy sheet with bimodal non-basal texture,which is fabricated by the newly developed ECAR-CB-A process,and the strength of the sheet was further regulated by annealing treatment,can significantly improve the plastic deformation capacity of the material.Afterwards,a detailed microstructure characterization on these rolled sheets are performed by using OM,XRD and EBSD measurements.The following conclusions can be obtained.(1)The effect of bimodal non-basal texture on microstructure characteristics,texture evolution and deformation mechanisms of AZ31 magnesium alloy sheet rolled at liquid-nitrogen temperature: No edge cracks are observed until the fourth pass of cryogenic rolling(the accumulated thickness reduction reaches about 18.5%)in the applied the AZ31 Mg alloy sheet with bimodal non-basal texture at 400mm/s.The accumulated thickness reduction before the occurrence of edge cracks in AZ31 Mgalloy sheet with typical basal texture is as low as 8%during cryogenic rolling.This obvious enhancement indicates that the introduced bimodal non-basal texture is in favor for the improvement of cryogenic rolling formability.The selective activation of {10-12}ETs variants depends on the angle between c-axis of grains and ND of the sheet.The as-received sheet possesses a bimodal non-basal texture with the basal poles tilt about ± 40°away from ND to RD,which would result in the activation of at least three variants during cryogenic rolling.This issue further leads to the formation of twin-twin interaction among various {10-12}ETs variants.The twinning behaviors as for {10-12}ETs are responsible for the concentration of c-axis of grains towards ND and the formation of TD-component texture at the beginning of cryogenic rolling.At the later stage of cryogenic rolling,SETs preferentially occur in these PETs with c-axis concentrating around TD,forming these rarely reported{10-12}-{10-12}DTs.Meanwhile,the corresponding volume faction increases quickly with the increasing rolling passes to sustain plastic strain.The twinning behaviors with respect to {10-12}-{10-12} DTs are responsible for the disappearance of TD-component texture.The evolution of involved deformation mechanisms can be summarized as follows:numerous {10-12} ETs + a small amount of dislocation slips → enlarged {10-12} ETs +twin-twin interaction among {10-12} ETs + increasing dislocation slips → enlarged {10-12}ETs + twin-twin interaction among {10-12} ETs + the occurrence of {10-12}-{10-12} DTs+ extensive dislocation slips.These twinning behaviors contribute to the improved rolling formability in AZ31 Mg alloy sheet with bimodal non-basal texture during cryogenic rolling.(2)The cryogenic rollability of AZ31 Mg alloy sheet with bimodal non-basal texture will decrease with the increase of the rolling speed.The three groups of samples showed edge cracks after the fifth,fourth and third passes at the rolling speed of 30 mm/s,300mm/s and 600 mm/s respectively.In addition,although the cumulative reduction of samples rolled at higher speeds is the smaller,the edge cracks are the deeper and wider.When the bimodal non-basal textured AZ31 sheet is cryogenically rolled,the ETs will reach saturation at a low rolling speed,resulting in the first pass rolling samples at 30 and300mm/s have the same twin volume fraction and twin boundary fraction.Upon increased rolling speed to 600mm/s,the saturated twins begin to propagate at this time,leading to a larger volume fraction and a smaller boundary fraction.Moreover,a positive correlation between the number of activated twin variants and the rolling speed was also found.For samples rolled at a speed of 30 mm/s,the evolution of involved deformation mechanisms can be summarized as: saturated {10-12} ETs + a small amount of dislocation slips →enlarged {10-12} ETs + twin-twin interaction among {10-12} ETs + increasing dislocation slips;For samples rolled at 300 mm/s,the involved evolution of deformation mechanism is match for that of the former(sample rolled at 30mm/s),but with the increase of plastic strain,a faster increasing rate in the volume fraction of twin and dislocation density.And for the sample rolled at 600mm/s,the evolution of involved deformation mechanisms changes as: saturated and enlarged {10-12} ETs + increasing dislocation slips → enlarged{10-12} ETs + twin-twin interaction among {10-12} ETs + extensive dislocation slips.With the increase of rolling speed from 30 to 600mm/s,massive dislocation slips are activated to sustain plastic strain.Meanwhile,the growth rate of twins is enhanced,and the combined effect of the two modes makes the double peaks quickly converge to form a basal texture,which leads to a reduction in rollability.In addition,the interaction of dislocation slips and twinning can also cause the texture hardening effect,which further leads to the poor formability of the sheet rolled at high speed. |
