Experimental Quantification of the Phonon Drag Deformation Mechanism in Metals at Extreme Strain Rates

Abstract

Extreme strain rate deformations, above 10⁶ s⁻¹, are seen across many fields of science and engineering; from meteorite impacts and impact induced crystallographic phase changes to high-speed machining and additive manufacturing. Despite the range of applications, many common high-rate impact experiments are intrinsically limited to strain rates of only 10⁴ s⁻¹ before complicating the material deformation with a superimposing state of shock due to high impact pressures. However, recent advances in optically driven microballistics using laser induced projectile impact tests have provided a new quantitative look into extreme mechanics of materials, at rates above 106 s-1 and well below the onset of shock effects. As deformation strain rates increase, additional strengthening mechanisms in metals become available, leading to a change in the underlying physics of dislocation motion and an increase in strength. This thesis first explores the mechanical properties of pure metals when deformed at extreme strain rates − both in ambient conditions and elevated temperatures. Using an array of complimentary characterization methods, two independent measurements of strength, the dynamic strength and dynamic hardness, are assessed. As the temperature is increased from ambient, the strength and hardness of pure metals both increase an appreciable amount. At these deformation rates, conventional thermal softening effects are now in competition with anti-thermal hardening that arises from ballistic transport of dislocations from phonon interactions in the crystal lattice. These effects are quantified systematically and it is shown that the anomalous thermal strengthening seen is, thermodynamically and kinetically, the expected form of plasticity under these impact conditions. Next, the limits of where this anomalous thermal strengthening occur in metals are investigated. First, solute elements are added to pure Ni to evaluate how additional dislocation pinning mechanisms effect the strength at ambient and elevated temperatures during extreme strain rate deformations. The strengthen increase due to solute pinning of dislocations is additive to the other strengthening mechanisms, yet thermally controlled, which provides a transition from ballistic transport of dislocations to thermally activated strengthening at a critical concentration of solutes. Finally, the upper bound of temperature for dislocation phonon drag strengthening is assessed. While it was shown that pure metals increase strength with increasing temperature, this “hotter-is-stronger” trend breaks down as the temperature approaches the melting point of the metal. Using Sn, due to its low melting temperature, the breakdown from “hotter-is-stronger” to “hotter-is-softer” as the initial substrate temperature approaches the melting temperature is systematically explored.