Figure 8 Cross-sectional TEM results of compressive [110] Mo in (a), (b), (c) and (d).
Discussion
The mechanical responses of Mo-Re pillars not only depend on the orientation of grain, but also on Re content, as displayed in Figure 9. Mo and Mo-5Re show weak orientation-dependent behavior. Yield stresses in [100] Mo and Mo-5Re are near to the corresponding [110] ones. In Mo-14Re and Mo-42Re, however, [110] samples are much stronger than [100] samples. The addition of 5wt.% Re soften Mo and lead lower yield stresses in Mo-5Re than in Mo. While, 14wt.% and 42wt.% Re could effectively enhance deformation resistance. [100] Mo shows similar yield stress with [110] Mo, which is inconsistent with previous researches in Mo. Lots of micro-pillar tests in BCC metals have been in Kim’s researches and been widely cited[14,15,17]. In their results, compressive yield stresses in [100] Mo are much higher than in [110] Mo[15]. The divergence may be due to the size difference that diameters in this work are much larger than other researches.
Figure 9 Summary of yield stresses for [100] and [110] Mo and Mo-Re alloys.
Slip planes in pure Mo that is easily activated at room temperature are {110} [18,19]. For example, Kim et al. examined the slip traces in niobium pillars using SEM and found the planes to generally agree with {110} slip for their (001) oriented pillars in both tension and compression [17]. As illustrated in Figure 6 and figure 7, the normal direction of slip planes in [100] Mo and Mo-14Re are both nearly 45°, meaning activated slip places likely to be {110}. The slip planes in [110] Mo-14Re and Mo-42Re may also be {110}, as shown in Figure 7e and Figure 8. Thus, {110} slip systems dominate plasticity in both [100] and [110] Mo and Mo-Re alloys. One group of parallel slip planes activated in [100] and [110] Mo, which may be attributed to the severe shear instability in [110] Mo. Surface micrographs of pillars after compression in [100] Mo-14Re and Mo-42Re exhibit similar slip features, as shown in Figure 5. Two groups of slip planes activated in [110] Mo-14Re, but do not tangle together, as shown in Figure 7f. Multi-slip bands activate and tangle in deformed [110] Mo-42Re, which effectively improve compressive properties. Therefore, the activation of multi-slip systems becomes easier as Re content rises.
Medvedeva et al. have theoretically justified that Re lead solid solution softening in Mo, because non-planar core of the screw dislocation in Mo tends to a planar core under alloying with Re[20]. In experimental results, however, solid solution effect of 5wt.% Re on Mo depends on processed states. Distinct solid solution softening effect happens in both as-worked and stress-relieved Mo-5Re, but does not happen in fully recrystallized ones[6]. In this research, full recrystallization occurs in Mo-5Re after 1100℃ annealing, as shown in Figure 1. It is noted that no microstructural defects such as grain boundary segregation in micro-pillars. Thus, less defects in Mo-5Re pillars may induce mechanical response near to theoretical simulation. Solid solution effect changes as increased Re [20,21]. As shown in Figure 9, hardening effects in [100] Mo-14Re and Mo-42Re are quite weak, which is consistent with the trend in stress-relieved samples. Thus, one assumption is that [100] micro-pillars have similar compressive properties with bulk ones. Another possibility is that bulk Mo-Re alloy have large amount of [100] textures after thermal process [7]. Yield stresses in [110] Mo-14Re and Mo-42Re are substantially enhanced due to activated multi-slip bands.
Conclusions
The compressive responses of Mo and Mo-Re alloys are investigated via micro-pillar tests. Solid solution effects of Re on Mo are complex. Solid solution softening happens in low Re content alloys (Mo-5Re). While, solid solution hardening happens in Mo-Re alloys with high Re content (Mo-42Re). Solid solution effects not only depend on Re content, but also depend on the state of samples. What is more, grain orientation / textures still have large influence on the solid solution hardening of Re. Weak strengthening effects are found in [100] Mo-14Re and Mo-42Re. However, the activation of multi-slip planes induces high strength in [110] Mo-14Re and Mo-42Re. In summary, [110] orientation Mo-Re alloys could be considered in condition where high strengthening effect is required.