skip to main content

Title: Recrystallization of bulk nanostructured magnesium alloy AZ31 after severe plastic deformation: an in situ diffraction study

The magnesium alloy AZ31, which has undergone high-pressure torsion processing, was subjected to in situ annealing microbeam synchrotron high-energy X-ray diffraction and compared to the as-received rolled sheet material that was investigated through in situ neutron diffraction. While the latter only exhibits thermal expansion and minor recovery, the nanostructured specimen displays a complex evolution, including recovery, strong recrystallization, phase transformations, and various regimes of grain growth. Nanometer-scale grain sizes, determined using Williamson–Hall analysis, exhibit seamless growth, aligning with the transition to larger grains, as assessed through the occupancy of single-grain reflections on the diffraction rings. The study uncovers strain anomalies resulting from thermal expansion, segregation of Al atoms, and the kinetics of vacancy creation and annihilation. Notably, a substantial number of excess vacancies were generated through high-pressure torsion and maintained for driving the recrystallization and forming highly activated volumes for diffusion and phase precipitation during heating. The unsystematic scatter observed in the Williamson–Hall plot indicates high dislocation densities following severe plastic deformation, which significantly decrease during recrystallization. Subsequently, dislocations reappear during grain growth, likely in response to torque gradients in larger grains. It is worth noting that the characteristics of unsystematic scatter differ for dislocations created at high and low temperatures, underscoring the strong temperature dependence of slip system activation.

Graphical Abstract

more » « less
Author(s) / Creator(s):
; ; ; ; ; ; ; ;
Publisher / Repository:
Springer Science + Business Media
Date Published:
Journal Name:
Journal of Materials Science
Medium: X Size: p. 5831-5853
["p. 5831-5853"]
Sponsoring Org:
National Science Foundation
More Like this
    more » « less
  2. Abstract

    Syntectonic microstructural evolution is a well‐known phenomenon in the mantle and lower crust associated with two main processes: grain size reduction through dynamic recrystallization and development of crystallographic preferred orientation (CPO). However, the effects of annealing via static recrystallization on grain size and CPO have been largely overlooked. We investigated mantle annealing by analyzing a suite of kimberlite‐hosted garnet peridotite xenoliths from the Wyoming Craton. We focus on five xenoliths that show microstructures reflecting different degrees of recrystallization, with annealed grains characterized by distinctive faceted boundaries crosscutting surrounding, nonfaceted matrix grains. These textures are indicative of discontinuous static recrystallization (DiSRX). Electron backscatter diffraction analysis further demonstrates a ∼10°–20° misorientation between DiSRXed grains and the matrix grains, resulting in an overall weaker CPO. These characteristics are remarkably similar to microstructures observed in samples that were annealed after deformation in the laboratory. Measurements of the thermal conditions and water contents associated with the last equilibration of the xenoliths suggests that high homologous temperatures (T/Tm > 0.9) are necessary to induce DiSRX. We postulate that annealing through DiSRX occurs under high temperatures after a short episode of intense deformation (years to hundreds of years) with timescales for annealing estimated as weeks to years, significantly slower than the timescale of hours expected for a kimberlitic magma ascent. We conclude that microstructural transformation due to DiSRX will occur during transient heating events associated with mantle upwelling, plumes, and lithospheric thinning.

    more » « less
  3. null (Ed.)
    Grain growth under shear annealing is crucial for controlling the properties of polycrystalline materials. However, their microscopic kinetics are not well understood because individual atomic trajectories are difficult to track. Here, we study grain growth with single-particle kinetics in colloidal polycrystals using video microscopy. Rich grain-growth phenomena are revealed in three shear regimes, including the normal grain growth (NGG) in weak shear melting–recrystallization process in strong shear. For intermediate shear, early stage NGG is arrested by built-up stress and eventually gives way to dynamic abnormal grain growth (DAGG). We find that DAGG occurs via a melting–recrystallization process, which naturally explains the puzzling stress drop at the onset of DAGG in metals. Moreover, we visualize that grain boundary (GB) migration is coupled with shear via disconnection gliding. The disconnection-gliding dynamics and the collective motions of ambient particles are resolved. We also observed that grain rotation can violate the conventional relation R × θ = c o n s t a n t (R is the grain radius, and θ is the misorientation angle between two grains) by emission and annihilation of dislocations across the grain, resulting in a step-by-step rotation. Besides grain growth, we discover a result in shear-induced melting: The melting volume fraction varies sinusoidally on the angle mismatch between the triangular lattice orientation of the grain and the shear direction. These discoveries hold potential to inform microstructure engineering of polycrystalline materials. 
    more » « less
  4. Abstract

    Single‐crystal architectures in glass, formed by a solid‐solid transformation via laser heating, are novel solids with a rotating lattice. To understand the process of lattice formation that proceeds via crystal growth, we have observed in situ Sb2S3crystal formation under X‐ray irradiation with simultaneous Laue micro X‐ray diffraction (μXRD) pattern collection. By translating the sample with respect to the beam, we form rotating lattice single (RLS) crystal lines with a consistently linear relationship between the rotation angle and distance from nucleation site. The lines begin with a seed crystal, followed by a transition region comprising of sub‐grain or very similarly oriented grains, followed by the presence of a rotating lattice single crystal of unrestricted length. The results demonstrate that the primary cause of lattice rotation within RLS crystals is the densification accompanying the glass → crystal transformation, rather than stresses produced from the difference in thermal expansion coefficient of the two phases or paraelectric → ferroelectric transition during cooling to ambient temperature.

    more » « less
  5. Abstract

    The effect of pressure on grain‐growth kinetics of olivine was investigated up to 10 GPa at 1773 K under relatively water‐poor conditions. The results are interpreted using a relationto obtain the activation volume = 5.0 ± 1.1 cm3/mol forn = 2 or = 5.2 ± 1.1 cm3/mol forn = 3. The small activation volume means that grain‐growth kinetics in pure olivine aggregates is fast even in the dry deep upper mantle, implying that grain‐size is controlled by the pinning by other phases or by dynamic recrystallization except for the early stage after the phase transformation from wadsleyite in upwelling materials. The present results are applied to seismic wave attenuation that is likely controlled by grain‐boundary processes. The inferred peak in attenuation just below the oceanic lithosphere‐asthenosphere boundary from the NoMelt array is difficult to be explained by the pressure effects assuming the absorption band behavior because such a model requires a much larger activation volume than determined in this work and it also fails to explain high attenuation in the deep asthenosphere. This suggests that either melt accumulation or other processes such as elastically accommodated grain‐boundary sliding (EAGBS) is responsible for the peak in attenuation. The present results are also applied to EAGBS. We suggest that the deep upper mantle is likely to be relaxed by EAGBS, which implies that the shear velocity of the deep upper mantle can be several percent smaller than that inferred from single crystal elasticity.

    more » « less