Strengthening behavior due to cyclic elastic loading in Pd-based metallic glass

Nanoindentation technique was utilized to unravel the strengthening behavior of Pd-based metallic glass under elastic cyclic loading. This was conducted to ultimately evaluate fatigue behavior which is thought to be a major concern for the structural performance of bulk metallic glasses. The event of shear band formation is considered after subjecting the alloy to loading cycles in the nominal elastic regime, followed by a monotonic deep indent. In all experiments, substantial shift in the load needed to initiate a shear band (indicative of plasticity) was noted. Following prolonged cycling, a fatigue limit was observed evident by the saturation in hardening effect. By varying the amplitude of elastic loading, it appears that a critical loading is needed to initiate strengthening in the bulk metallic glass (BMG); i.e. a specific threshold need to be exceeded for the hardening effect to trigger. This was concluded following successive experiments with increasing number of cycles which proves the effect over multiple cycling. Moreover, the effect of higher loading rates becomes more significant as the successive number of cycles increases. For further assessment, comparative experiments were carried out between holding and cycling in the elastic region. These experiments suggested the necessity of cycling to achieve hardening in the alloy, as no hardening effect was observed following holding experiments. In general, strengthening effect was attributed to the possible development of regions of microplasticity due to the actuating force, without being able to detect these regions in the global load-displacement response. Additional tests were conducted to unravel the irreversibility of these cycling/hardening effects. It was found that slight recovery is possible when no resting time was introduced before the deep indent. This suggests that part of the cycling effects can be reversed which can be linked to the instability of the shear transformation zones (STZs).