D.A. Hughes

Bio: D.A. Hughes is an academic researcher from Sandia National Laboratories. The author has contributed to research in topics: Grain boundary & Recrystallization (metallurgy). The author has an hindex of 4, co-authored 4 publications receiving 2167 citations.

Current issues in recrystallization: a review

Abstract: The current understanding of the fundamentals of recrystallization is summarized. This includes understanding the as-deformed state. Several aspects of recrystallization are described: nucleation and growth, the development of misorientation during deformation, continuous, dynamic, and geometric dynamic recrystallization, particle effects, and texture. This article is authored by the leading experts in these areas. The subjects are discussed individually and recommendations for further study are listed in the final section.

Microstructural evolution in a non-cell forming metal: AlMg

Abstract: Microstructural evolution in Al + 5.5 at.% Mg lightly deformed by rolling was studied using transmission electron microscopy. Observations show that the dislocations are organized into a Taylor lattice containing multiple burgers vectors and having alternating misorientations along {111} slip planes. These observations are in contrast to the usual description of these structures as “random” dislocation tangles. With increasing strain, a grain is subdivided into several domains of differently oriented Taylor lattices. The newly observed boundaries between different domains are long single dislocations walls formed along {111}. As a further evolutionary progression, the single walled domain boundaries develop into microbands. The observed grain subdivision parallels that observed in cell forming metals. Subdivision occurs to accommodate strain using fewer slip systems than required by the Taylor criterion.

Microstructure and local crystallography of cold rolled aluminium

Abstract: High-purity aluminium (99.996%) has been deformed by cold-rolling to intermediate reductions (50 and 60%). The microstructure has been characterized using transmission electron microscopy (TEM) and the local crystallographic orientations have been analysed by convergent beam electron diffraction (CBED) using a semi-automatic technique. A number of dislocation configurations such as microbands (MBs), S-bands and lamellar boundaries (LBs) have been characterized in terms of crystallographic and macroscopic orientations and morphology. For a large number of dislocation boundaries, the angle/axis pairs have been calculated and classified. The microstructural and crystallographic information is combined into orientation images which are related to a framework for the microstructural evolution common to medium and high stacking fault energy f.c.c. polycrystals. This framework consists of a grain subdivision by dislocation boundaries and it is discussed how these boundaries evolve especially how intragranular high angle boundaries can form. The effect of annealing on the deformation microstructure and intragranular nucleation is finally discussed.

Friction Stir Welding and Processing

Abstract: Friction stir welding (FSW) is a relatively new solid-state joining process. This joining technique is energy efficient, environment friendly, and versatile. In particular, it can be used to join high-strength aerospace aluminum alloys and other metallic alloys that are hard to weld by conventional fusion welding. FSW is considered to be the most significant development in metal joining in a decade. Recently, friction stir processing (FSP) was developed for microstructural modification of metallic materials. In this review article, the current state of understanding and development of the FSW and FSP are addressed. Particular emphasis has been given to: (a) mechanisms responsible for the formation of welds and microstructural refinement, and (b) effects of FSW/FSP parameters on resultant microstructure and final mechanical properties. While the bulk of the information is related to aluminum alloys, important results are now available for other metals and alloys. At this stage, the technology diffusion has significantly outpaced the fundamental understanding of microstructural evolution and microstructure–property relationships.

Multicomponent intermetallic nanoparticles and superb mechanical behaviors of complex alloys

Abstract: Alloy design based on single-principal-element systems has approached its limit for performance enhancements. A substantial increase in strength up to gigapascal levels typically causes the premature failure of materials with reduced ductility. Here, we report a strategy to break this trade-off by controllably introducing high-density ductile multicomponent intermetallic nanoparticles (MCINPs) in complex alloy systems. Distinct from the intermetallic-induced embrittlement under conventional wisdom, such MCINP-strengthened alloys exhibit superior strengths of 1.5 gigapascals and ductility as high as 50% in tension at ambient temperature. The plastic instability, a major concern for high-strength materials, can be completely eliminated by generating a distinctive multistage work-hardening behavior, resulting from pronounced dislocation activities and deformation-induced microbands. This MCINP strategy offers a paradigm to develop next-generation materials for structural applications.

Three-dimensional X-ray structural microscopy with submicrometre resolution

Abstract: Advanced materials and processing techniques are based largely on the generation and control of non-homogeneous microstructures, such as precipitates and grain boundaries. X-ray tomography can provide three-dimensional density and chemical distributions of such structures with submicrometre resolution; structural methods exist that give submicrometre resolution in two dimensions; and techniques are available for obtaining grain-centroid positions and grain-average strains in three dimensions. But non-destructive point-to-point three-dimensional structural probes have not hitherto been available for investigations at the critical mesoscopic length scales (tenths to hundreds of micrometres). As a result, investigations of three-dimensional mesoscale phenomena--such as grain growth, deformation, crumpling and strain-gradient effects--rely increasingly on computation and modelling without direct experimental input. Here we describe a three-dimensional X-ray microscopy technique that uses polychromatic synchrotron X-ray microbeams to probe local crystal structure, orientation and strain tensors with submicrometre spatial resolution. We demonstrate the utility of this approach with micrometre-resolution three-dimensional measurements of grain orientations and sizes in polycrystalline aluminium, and with micrometre depth-resolved measurements of elastic strain tensors in cylindrically bent silicon. This technique is applicable to single-crystal, polycrystalline, composite and functionally graded materials.