Z. Chen

In situ analysis of the tensile deformation mechanisms in extruded Mg-1Mn-1Nd (wt%)

An extruded Mg–1Mn–1Nd (wt%) (MN11) alloy was tested in tension in an SEM at temperatures of 323 K (50°C), 423 K (150°C), and 523 K (250°C) to analyse the local deformation mechanisms through in situ observations. Electron backscatter diffraction was performed before and after the deformation. It was found that the tensile strength decreased with increasing temperature, and the relative activity of different twinning and slip systems was quantified. At 323 K (50°C), extension twinning, basal, prismatic ⟨a⟩, and pyramidal ⟨c + a⟩ slip were active. Much less extension twinning was observed at 423 K (150°C), while basal slip and prismatic ⟨a⟩ slip were dominant and presented similar activities. At 523 K (250°C), twinning was not observed, and basal slip controlled the deformation.

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.

On the Controversy About the Presence of Grain Boundary Sliding in Mg AZ31

Highly-textured, rolled AZ31 sheet material shows a significant drop in the plastic anisotropy (r-value; r=ew/et) in tension between 25°C and 200°C. This behavior was initially explained as a result of the increased activity of non-basal slip with increased temperature. Other authors suggested, however, that the mechanism responsible for this phenomenon was the activation of grain boundary sliding (GBS). Here, in-situ tensile tests have been carried out in an SEM at various temperatures in order to obtain further evidence of the role of GBS during moderate to high temperature deformation of Mg alloys, which remains highly controversial.

Tensile and Creep Deformation Mechanisms in Rolled AZ31

Tensile experiments were performed on a rolled AZ31 alloy in an SEM at 323K (50°C), 423K (150°C), and 523K (250°C) in order to analyze the deformation mechanisms in-situ. Electron backscatter diffraction (EBSD) was performed both before and after deformation. The mechanical anisotropy was considerably reduced with temperature. Extension twinning was observed at 323K (50°C), but disappeared at 423K (150°C), indicating that the CRSS of non-basal systems becomes smaller than that of twinning at T to mainly prismatic slip. This is consistent with a decrease of the CRSS of non-basal slip systems with increasing temperature. In-situ tensile-creep experiments, performed at approximately the yield stress at 423K (150°C), indicated less slip and more grain boundary cracking occurs under creep deformation as compared to the higher-stress tensile experiments.

The Effect of Thermomechanical Processing on the Creep Behavior and Fracture Toughness of Thixomolded® AM60 Alloy

Creep and fracture toughness experiments were performed on a commercially available magnesium-aluminum alloy (AM60) after three processing treatments: (1) As-Thixomolded® (as-molded), (2) Thixomolded® then thermomechanically processed (TTMP), and (3) Thixomolded® then TTMP then annealed (annealed). The conventional tensile-creep experiments were performed at applied stresses ranging between 20-75MPa and temperatures between 373-473K (100-200°C). In-situ tensile-creep tests were performed on selected samples. The as-molded material exhibited creep resistance superior to the thermomechanically processed materials. Creep experiments indicated grain boundary cracking, and grain size was expected to be an important microstructural parameter that affected the creep behavior. Fracture toughness experiments were performed at room temperature (RT) on single edge notched tension (SENT) samples. The TTMP and annealed materials exhibited fracture toughness values almost twice that of the as-molded material.