Mechanical Properties And Deformation Mechanism Of Directionally Solidified Alloys Of Mg-Zn-Y Alloy At High Temperature

Mg-6.52Zn-0.67Y(wt,%)alloy with columnar crystal structure was prepared by directional solidification technique.Mg-6.52Zn-0.67Y alloy was tested by CMT5105 universal testing machine at 25?,150?,200?,250? and 300?,XRD,SEM and EBSD were used to analyze the microstructure,deformation microstructure and fracture morphology of Mg-6.52Zn-0.67Y alloy during directional solidification,crystal orientation at different tensile temperature.The deformation mechanism and fracture mechanism of Mg-6.52Zn-0.67Y alloy in directional solidification are discussed.The microstructure of Mg-6.52Zn-0.67Y alloy prepared by directional solidification is a parallel growth with one arm,the vertical grain boundaries are straight,and no transverse grain boundaries are produced,and the crystal structure of the<0001>is selected as the columnar grain.The grain boundary phase and transgranular granular phases is I-Mg3Zn6Y quasicrystal phase.?b of Mg-6.52Zn-0.67Y alloy with columnar crystal structure at room temperature is 196MPa,and the ? is 13%,the strength and the plasticity of Mg-6.52Zn-0.67Y alloy are increased by 51%and 760%respectively compared with those by conventional casting.With the composition of the conventional cast alloy,?b is 129MPa,? is 2%.With the increase of the tensile temperature,the strength decreases and the plasticity increases.When the temperature rise from 150? to 200?,the 8 is from 27%to 29%,respectively,?b is from 141MPa to 106MPa,when temperature rise to 300?,the 8 is 37%.Results of EBSD analysis show that,at 150?,the columnar crystals are in dense,roughly parallel arrangement,the average width of {1012} tensile twins is 2?m,at 200?,when stretching at 200? the twin density decreases,the twin size increases(the width is about 17m),and the columnar crystals are segmented into lamellar structures by the tensile twins arranged in parallel with each other.In addition,at 150? and 200?temperature,grain orientation of small angle grain boundary misorientation less than 15°the proportion are 31%and 49%,tension twinning,respectively 23%and 21%,which indicates that the main deformation mechanisms is dislocation and {1012} stretch twins.At 250?,the number of twin obviously reduce,tension twinning in the proportion drop to 8%,both inside the twinning and twin grain boundaries have a small amount of recrystallization grain formation,when the tensile temperature rise to 300?,there is no obvious twin in the deformed microstructure,and there exist a large aggregation of recrystallization grains in both the grain boundary and grain,and the size of the recrystallization grain is 5?9?m.Grain misorientation less than 15° in the direction of small-angle grain boundaries was 26%,the proportion between 20°-40° was 47%,tension twinning was 1%.(0001)on a polar diagram have obvious texture cone,cone slip begins to predominate.SEM tensile fracture analysis shows that the room temperature tensile fracture are cleavage step and clear rivers,its fracture mechanism is cleavage fracture.At 150? and 200?,tensile fracture mainly by cleavage plane,the tear ridge and irregular superficial toughness nest structure and the changes in fracture mechanism form the quasi cleavage fracture mechanism,with the further increase of the tensile temperature,at 250? and 300?,the fracture consist of larger and deeper dimples,the fracture mechanism change from quasi cleavage fracture to ductile fracture mechanism of micro voids.We try to add Zr element to Mg-Zn-Y alloy in directional solidification in order to get better performance.We found that the microstructure of Mg-3.73Zn-0.34Y-0.1Zr alloy and the relatively performance are good after a lot of experiments are done,the cone(1012)of Mg alloy in directional solidification is preferred crystal plane more likely,the room temperature strength is 170MPa,elongation is 20%.