Precipitation behaviors, texture and tensile properties of an extruded Mg-7Y-1Nd-0.5Zr (wt%) alloy bar with large cross-section

Abstract

Precipitation behaviors, texture and tensile properties of an extruded Mg-7Y-1Nd-0.5Zr (wt%) (WE71) alloy bar with large cross-section of 230mm×140mm were investigated by hardness test, tensile test, optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD) macro-texture measurement. The bar was manufactured industrially through a procedure of “multi-direction forging (MDF)+extrusion+on-line quenching+T5 aging”. Totally different age-hardening behaviors are shown during T5 aging at 200 and 235°C. In the first 100h, T5 aging at 235°C brings about 13% increases in hardness, while T5 aging at 200°C results in 47% increase. During T5 aging at 200°C, β′ precipitates homogeneously nucleate within the matrix with high number density; however, during T5 aging at 235°C, β′ precipitates heterogeneously nucleate on discrete and sparse dislocations, resulting in chain-like arrangement of β′ precipitates with broad precipitate free zones in matrix. XRD macro-texture measurement illustrates that basal texture intensity of WE71 bar is much weaker than Mg-8Al-0.5Zn-0.15Mn (wt%) (AZ80) bar; the maximum basal texture intensities in the outer (O) and center (C) of WE71 bar are all about 3, while those of AZ80 bar are 17 and 14, respectively. EBSD micro-texture measurement demonstrates that the maximum texture intensities of C and O are 5.3 and 3.5, respectively. O has higher tensile properties than C because there are more un-dynamic-recrystallization (un-DRX) grains and thus larger average grain size in C. While stretching at room temperature (RT), extrusion direction (ED) in O has the best tensile properties, i.e. ultimate tensile strength (Rm)=368MPa, elongation (A)=5%, and normal direction (ND) in C has the lowest tensile properties, i.e. Rm=255MPa, A=2%. While stretching at 200°C, strength does not degrade much; ED in O still has the best tensile properties, i.e. Rm=326MPa, A=7%, and ND in C still has the lowest tensile properties, i.e. Rm=263MPa, A=4%.