Dongdi Yin

Characterization of phases in Mg-10Y-5Gd-2Zn-0.5Zr alloy processed by heat treatment

Abstract Characterizations of phases in Mg-10%Y-5%Gd-2%Zn-0.5%Zr (WGZ1052) alloy during heat treatments were investigated by OM, XRD, SEM and EDS. The mechanisms of microstructure evolution were discussed. The results show that, after high temperature heat treatments, the Mg 12 ZnY phases still exist. During solution-treatment at 535 °C, the amount of the long-period stacking order structures decreases. At 545 °C for 20 h and 24 h, there are still remnant Mg 12 ZnY compounds in the Mg matrix, the shape of which does not change and the amount does not decrease obviously.

Influence of Thermal and Thermo-Mechanical Processing on the Creep Resistance of Mg-10Gd-3Y-0,4Zr Alloy

Alloy Mg-10Gd-3Y-0,4Zr in as-cast, as-extruded, cast-T6 (peak aged) and extruded-T5 (peak aged) state was tensile creep tested at 200, 250 and 300 °C and stress 50, 80 and 120 MPa. Comparison of minimal creep rate shows that alloy Mg-10Gd-3Y-0,4Zr in cast-T6 conditions is characterized by an excellent creep resistance, which is higher than that of commercially available Mg-alloys. Creep resistance of as-cast, as-extruded and extruded-T5 alloy Mg-10Gd-3Y-0,4Zr is lower. Cavity nucleation is heavily affected by the amount of secondary phases on the grain boundaries and also by the initial grain size of the microstructure. After extrusion and in the extruded-T5 conditions creep cavitation was not observed, whereas in the as-cast and cast-T6 conditions creep cavitation occurred on the high fraction of grain boundaries.

In-Situ Study of the Tensile Deformation and Fracture Modes in Peak-Aged Cast Mg-11Y-5Gd-2Zn-0.5Zr (Weight Percent)

Tensile deformation and fracture modes in peak-aged cast Mg-11Y-5Gd-2Zn-0.5Zr (wt pct) (WGZ1152) samples at temperatures between 298 K [25 °C, room temperature (RT)] and 623 K (350 °C) (0.33 to 0.69Tm) were studied in situ inside a scanning electron microscope (SEM) using electron backscatter diffraction (EBSD) and slip trace analysis. The ultimate tensile strength (UTS) (265 MPa) and yield strength (YS) (193 MPa) at 523 K (250 °C) were 91 and 80 pct of those at RT, respectively. The observed dominant slip mode transitioned from basal 〈a〉 slip (100 pct) to basal 〈a〉 slip (81 pct) combined with prismatic 〈a〉 slip (12 pct) from RT to 473 K (200 °C). As the temperature increased to 623 K (350 °C), basal 〈a〉 slip (67 pct) and pyramidal 〈c+a〉 slip (25 pct) became the dominant slip modes. The estimated critical resolved shear stress (CRSS) ratio of pyramidal 〈c+a〉 slip/basal 〈a〉 slip (7.3) was lower than that of prismatic 〈a〉 slip/basal 〈a〉 slip (12.7) at temperatures above 573 K (300 °C). Prismatic 〈a〉 slip and pyramidal 〈c+a〉 slip were more active at higher strains for moderate temperatures [473 K to 523 K (200 °C to 250 °C)] and at high temperatures [573 K to 623 K (300 °C to 350 °C)], respectively. A transition in the dominant fracture mode occurred from transgranular cracking (40 pct) combined with intergranular cracking (60 pct) to intergranular cracking as temperatures increased from RT to 623 K (350 °C). The intergranular crack nucleation sites tended to be located at grain boundaries and the interface between the Mg matrix and the large intermetallic grain boundary X phase. Slip bands were associated with transgranular crack nucleation.

Compressive Creep Behaviour of Extruded Mg-10Gd-3Y-0.5Zr (wt.%) Alloy

The compressive creep behaviour of extruded Mg-10Gd-3Y-0.5Zr (wt.%, GW103) alloy was investigated at temperatures from 200 °C to 300 °C and under stresses from 50 MPa to 120 MPa. The peak-aged alloy exhibited minimum creep rates ranging from 1.90×10-9 s-1 to 6.14×10-6 s-1 and the aging treatment exerted a positive effect on its creep performance. The measured stress exponent of the peak-aged extruded alloy varied from 2.0 to 3.4, while the activation energy was 83.4 kJ/mol and 184.3 kJ/mol at low temperature and high temperature regime, respectively. This suggested grain boundary sliding was the primary creep mechanism at low temperature but dislocation-controlled creep dominated at high temperature. XRD patterns and SEM micrographs indicated precipitates increased with creep time, and further dynamic recrystallization occurred. Precipitate free zones (PFZs) were clearly observed near the grain boundaries parallel to the loading direction.

Modification of Eutectic Si in Al-Si-(Ba) Alloy by Inducing a Novel 9R Structure in Twins

The change of twinning morphology plays an important role in the modification of Al-Si alloys, which are widely used in industrial applications. However, the interpretation of this change is still insufficient. In this work, the microstructure of twins was investigated in two kinds of Al-Si alloys with different additions of Ba using high-resolution transmission electron microscopy (HRTEM). Unlike the normal {111} twin that exists in Ba-free alloy, discontinuous twins and multiple twins were observed in the Ba-containing alloy. In addition, the 9R structure formed by the dissociation of twins was firstly observed at the turning of discontinuous twins and the intersection of multiple twins in Al-Si alloys.

Microstructure and Mechanical Properties of Mg-3Y Binary Alloy Processed by Cyclic Extrusion and Compression

Mg-3wt.%Y alloys were processed by cyclic extrusion and compression (CEC) up to 7 passes at different temperatures from 375 to 450 °C, respectively. The microstructure was effectively refined and the mean grain size was decreased from 800 μm to 3–15 μm. Tensile and compressive tests were performed at room temperature at an initial strain rate of 1.11 × 10-3 s-1. The experimental results show that after 7-pass CEC processing the tensile yield strength and elongation-to-failure of Mg-3Y alloy increased simultaneously. Furthermore, the strength differential effect (SDE) of tension-compression of the alloy was weakened, especially the SDE value was only 3.3% when processed at 400 °C.