Deformation of metallic materials with real microstructures in small scale

Abstract

It is well-known that precipitated alloys are stronger than their pure metallic counterparts, but their creep behavior under nano-contact loadings is generally unknown. In particular, duralumin 2025, a precipitated aluminum alloy which is significantly harder than pure aluminum, exhibits a prominent indentation creep within submicron indent size at room temperature, with the shear viscosity approaching that of pure aluminum. However, at indentation depths increased to over ~1µm, the shear viscosity of duralumin becomes significantly higher than pure Al. The results suggest that for small indents with submicron depths, indentation creep occurs in a diffusive manner which is insensitive to the precipitated-microstructure. In the micro size regime, all crystalline metals studied to-date exhibit a “smaller-is-stronger” size effect. Here, an unusual weakest-size phenomenon is found in the precipitated duralumin 2025, i.e., below a critical size of ~7µm, the strength increases as the size decreases, while above this size, the strength increases toward the bulk value with increasing size. At the critical size, strain-hardening is also slowest and the room-temperature creep is fastest. The reduction of strength at the weakest size is more significant for the peak-aged state of duralumin than its naturally aged state. Theoretical modeling shows that at the weakest size, both strengthening mechanisms of precipitation hardening and dislocation starvation are ineffective. The present results indicate that the conventional precipitation hardening is not applicable in the micro-regime, and the common “smaller-is-stronger” understanding will break down. An unusual ‘smaller-being-weaker’ phenomenon is observed in compression of eutectic Sn/Pb micropillars, namely, in the specimen-size regime below ~3.5μm, the strength decreases with the size decreasing, while above this size the strength trends to approach the bulk value. The characteristic lamellar microstructure of eutectic alloys in micro-devices may impose an internal length scale that leads to an inverting behavior rather than the conventional ‘smaller-being-stronger’ size effect. Theoretical modeling based on a continuum dislocation model with two-dimensional dislocation dynamics simulations indicate that the phase boundaries of alternate lamellae work inefficiently in blocking dislocations in the small-size regime where ‘smaller-being-weaker’ scenario is at play, but they come into play and produce significant strengthening in micro-pillars larger than ~3.5μm. The present result is an important supplement to conventional knowledge on size effects, and provides important implications on solder-joint design in micro-devices. Strain localization and catastrophic fracture of bulk metallic glass is investigated through tensile tests on Cu/Zr-based glassy micro-wires. The result shows that dense shear bands occur close to the fracture locations, implying that accelerating slip events results in successive slip localization and final fracture. A statistic model is proposed to account for the stochastic deformation from incipient yield to eventual fracture. Molecule dynamics simulation is also used to prove the close spatial and temporal correlation of slip events. The theory indicates that, if the intrinsic emission rate of the slip events grows along the strain path, due to the occurrence of new slip events triggered by prior ones, the successive occurrence of slip events accelerates rapidly to an asymptotic state i.e., the condition of slip localization, which results in catastrophic fracture.