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

Precipitation behaviors, texture and tensile properties of an extruded Mg-7Y-1Nd-0.5Zr (wt%) (WE71) alloy bar with large cross-section of 230 mmx140 mm 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 degrees C. In the first 100 h, T5 aging at 235 degrees C brings about 13% increases in hardness, while T5 aging at 200 degrees C results in 47% increase. During T5 aging at 200 degrees C, beta' precipitates homogeneously nucleate within the matrix with high number density; however, during T5 aging at 235 degrees C, beta' precipitates heterogeneously nucleate on discrete and sparse dislocations, resulting in chain-like arrangement of beta' 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-0.5Zn0.15Mn (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 (R-m)=368 MPa, elongation (A)=5%, and normal direction (ND) in C has the lowest tensile properties, i.e. R-m=255 MPa, A=2%. While stretching at 200 degrees C, strength does not degrade much; ED in O still has the best tensile properties, i.e. R-m=326 MPa, A=7%, and ND in C still has the lowest tensile properties, i.e. R-m=263 MPa, A=4%.