The Natural Aging Effect on Hardenability in Al-Mg-Si: A Complex Interaction between Composition and Heat Treatment Parameters

The technological relevance of Al-Mg-Si alloys has been rapidly growing over the last decade. Of particular interest to current and future applications is the problematic negative effect of prior natural aging on subsequent artificial age hardening. The influence of natural aging is dependent on both processing and compositional variables and has origins that are far from well-understood. This work examines the hardenability of 6000 series alloys under a wide range of conditions, paying particular attention to the natural aging effect. Experimental variables include alloy composition (Mg + Si, Mg/Si), cooling rate after solutionization, and duration of prior natural aging. Hardenability was evaluated with full hardness and conductivity aging curves for each condition, as well as select Transmission Electron Microscopy (TEM). Results are discussed based on the actions of naturally aged solute clusters during artificial aging. In particular, a complex interaction between vacancy concentration, cluster stability, and precipitation driving force is suggested

On the mechanisms of stress relaxation and intensification at the lithium/solid-state electrolyte interface

Under electrochemical cycling, stress intensification and relaxation within small volumes at the lithium/solid-state electrolyte (SSE) interface are thought to be critical factors contributing to mechanical failure of the SSE and subsequent short-circuiting of the device. Nanoindentation has been used to examine the diffusion-limited pressure lithium can support in the absence of active dislocation sources at high homologous temperatures. Based on the underlying physics of this deformation mechanism, a simple perturbation model coupling local current density, elastic stress, and diffusional creep relaxation is introduced. Combining this analysis with the indentation results, it is possible to describe a defect length scale which is too large for effective diffusional creep relaxation, but too small for efficient dislocation multiplication. In this instance, the properties of the SSE may become critical, and relevant indentation results of the SSE are described. The final outcome of the proposed analysis is a newly developed deformation mechanism map

Investigation of Al-Zn-Zr and Al-Zn-Ni alloys for high electrical conductivity and strength application

Al-Zn-TM (TM=Transition metals) alloys are developed with an integrated computational material engineering (ICME) strategy. Al-Zn-Ni and Al-Zn-Zr are determined to have promising electrical conductivities via a series of ab initio density functional theory (DFT) simulations assessing combinations of Al-TM and Al-Zn-TM. The computed enthalpies of formation are used to identify the zero-temperature equilibrium precipitate phase in both alloys with increasing levels of Zn content, with a particular focus of finding Zn content levels that result in a precipitate L12 structure. The corresponding microhardness and electrical conductivity measurements of both alloys are evaluated. Transmission Electron Microscopy (TEM) is used to examine the morphology of the Al3-xZnxNi and Al3-xZnxZr precipitates formed in the respective alloys and their structures were confirmed as L12 by selected area electron diffraction (SAED). Through qualitative chemical analysis, it is demonstrated that Ni and Zr are not present in the matrix but are completely used up in forming the respective precipitate phases in both alloys

Nanoindentation of high-purity vapor deposited lithium films: A mechanistic rationalization of the transition from diffusion to dislocation-mediated flow

© Materials Research Society 2018Â. Nanoindentation experiments performed in high-purity vapor deposited lithium films at 31 °C reveal a strain rate and length scale dependence in the stress at which pop-in type events signal an abrupt transition from diffusion to dislocation-mediated flow. The stress level at which the transition to dislocation-mediated flow occurs varies with the strain rate and ranges from 88 to 208 times larger than the nominal yield strength of bulk, polycrystalline lithium. Variation in the indentation strain rate reveals the relationship between the stress required to initiate the transition and the length scale at which the transition occurs follows the power-law relation, hardness × depth1.17 = 1.545 N/m0.83, where the magnitude of the exponent and constant reflect the defect structure of the film. A rationalization of the transition is provided through direct comparisons between the measured cumulative distribution function (CDF) and the CDF hypothesized for the activation of a Frank-Read source

Nanoindentation of high-purity vapor deposited lithium films: A mechanistic rationalization of diffusion-mediated flow

© 2018 Materials Research Society. Nanoindentation experiments performed in 5 and 18 μm thick vapor deposited polycrystalline lithium films at 31 °C reveal the mean pressure lithium can support is strongly dependent on length scale and strain rate. At the smallest length scales (indentation depths of 40 nm), the mean pressure lithium can support increases from ∼23 to 175 MPa as the indentation strain rate increases from 0.195 to 1.364 s-1. Furthermore, these pressures are ∼46-350 times higher than the nominal yield strength of bulk polycrystalline lithium. The length scale and strain rate dependent hardness is rationalized using slightly modified forms of the Nabarro-Herring and Harper-Dorn creep mechanisms. Load-displacement curves suggest a stress and length-scale dependent transition from diffusion to dislocation-mediated flow. Collectively, these experimental observations shed significant new light on the mechanical behavior of lithium at the length scale of defects existing at the lithium/solid electrolyte interface