Study On Three-dimensional Creep Anisotropy And Enhancement Methods Of Creep Resistance Of Mg-Y Alloys

Due to the excellent room-temperature mechanical properties and high-temperature creep resistance,magnesium alloys containing rare earth elements(Mg-RE alloys)have broad application prospects in transportation and aerospace industries.As one of the most representative RE elements,yttrium(Y)shows excellent solution strengthening and precipitation strengthening effects,which can significantly highlight the high-temperature properties of Mg alloys.In the present study,compressive creep tests were conducted on hot-rolled Mg-x Y(x=0,0.15,1.5,4.5,7.5 and 10.5wt%)alloy sheets along the rolling direction(RD),transverse direction(TD)and normal direction(ND),respectively.Microstructure evolutions were analyzed by using optical microscopy(OM),electron backscattering diffraction(EBSD),scanning electron microscopy(SEM),transsmissin electron microscopy(TEM)and X-ray diffraction(XRD)apparatus in detail.The effects of Y content on the compressive creep behaviors and three-dimensional creep anisotropy were carefully investigated.According to the microstructure characteristics of Mg-Y alloys with low,medium and high Y contents,the corresponding enhancement methods of creep resistance were proposed.1.Loading direction and Y content both exhibited great impacts on the creep hehaviors of Mg-Y alloys at 523 K.Strong creep anisotropy was observed in the 0Y,0.15 Y,1.5Y,and 4.5Y alloys,and the sequence of creep resistance was ND > TD ≥ RD.Heavy cross-slip and {10-12} twinning deteriorated the creep resistance along the RD and TD.Dislocation climb and pyramidal <c+a> slip dominated the creep behavior along the ND and produced higher creep resistance.Conversely,the creep mechanisms,namely,cross-slip and pyramidal <c+a> slip,showed no disparity among the three directions in the 7.5Y and 10.5Y alloys,and the indiscriminate creep mechanisms resulted in creep isotropy.As the Y content increased,the creep resistance was increased regardless of the loading direction.Solution strengthening became obvious above 0.15%Y,and more active pyramidal <c+a> dislocations began to improve the creep resistance from1.5%Y.The improved hindering effect on twinning was also helpful to increase the creep resistance along the RD and TD.When the Y content exceeded 7.5%,dynamic precipitation strengthening further enhanced the creep resistance.2.Addition of dilute Mn into 1.5Y alloy barely changed the creep mechanisms along the RD and TD,including cross-slip and {10-12}twinning.While the creep mechanisms changed from dislocation climb and pyramidal < c + a > slip to cross-slip and pyramidal < c + a > slip.Strong creep anisotropy was still existed and the sequence of creep resistance was ND > RD ≥ TD.The enhancement of creep resistance at 523 K by dilute Mn addition was closely related to that the dynamic precipitation of α-Mn particles could strongly impede the mobilities of dislocations and grain boundaries.Moreover,the solute segregation of Mn along the dislocation lines also delayed the continuous movement of dislocations and contributed to the increased creep resistance.With applied stress increasing,the increased twin amounts along the RD and TD were responsible for the strong creep anisotropy.Nevertheless,the dynamic precipitation of α-Mn particles obviously inhibited the thickening of twins,which was beneficial for the enhanced creep resistance and decreased creep anisotropy.3.After pre-compression along the TD,the creep resistance of 4.5Y alloy along the different directions at 523 K obviously changed due to the significantly changed creep mechanisms.Specifically,the frequent twin intersections between newly formed twins and preset twins resulted in the stupendous improvement of creep resistance along the RD,and the detwinning instead of further twinning led to the slightly enhanced creep resistance along the ND.Conversely,the adjacent twins merged into thick ones and heavy cross-slip along with stepped pyramidal dislocations accelerated the creep strain and deteriorated the creep resistance along the TD.As a result,the sequence of creep resistance tended to be RD > ND >TD after pre-compression.Although the creep resistance along the TD was reduced after pre-compression,the enhancement of creep resistance along the RD was more obvious.Thus,the pre-compression was manifested to be a successful method to improve the creep properties along the worst direction(RD)of 4.5Y alloy.4.The effects of grain size on the creep behaviors of 4.5Y alloy was closely related to the creep temperature.When crept at 493 K,grain boundary sliding(GBS)was inhibited.Grain boundary strengthening in fine-grained samples and {10-12} twinning in coarse-grained samples led to that the creep resistance along the RD was decreased with grain size increasing,while the creep resistance along the ND barely exhibited dependence on grain size due to twinning was inhibited by the improper grain orientation.Consequently,the creep anisotropy was gradually increased with grain size increasing.When crept at 523 K,the activation of GBS was promoted and the contribution of GBS to creep strain was gradually decreased,which resulted in that the creep resistance was increased with grain size increasing regardless of loading direction.Meanwhile,the deterioration effects of {10-12} twinning along the RD was gradually increased,leading to the increased creep anisotropy.5.The static precipitates induced by aging treatment strongly retarded the cross-slip and interacted with pyramidal dislocations,which had strong strengthening effect on the creep resistance of 10.5Y alloy.Besides,no precipitate-free zone(PFZ)was formed at 493 K,but PFZs were formed roughly parallel to the compressive direction in both AS and AA samples at 523 K.The wider PFZs in AS sample deteriorated its creep resistance.At 553 K,PFZs were uniformly distributed along the grain boundaries in both AS and AA samples,which reduced the gap of creep resistance between AS and AA samples.