Precipitation and Hardening in Magnesium Alloys Nie, Jian-Feng
Metallurgical and materials transactions. A, Physical metallurgy and materials science,
11/2012, Volume:
43, Issue:
11
Journal Article
Peer reviewed
Open access
Magnesium alloys have received an increasing interest in the past 12 years for potential applications in the automotive, aircraft, aerospace, and electronic industries. Many of these alloys are ...strong because of solid-state precipitates that are produced by an age-hardening process. Although some strength improvements of existing magnesium alloys have been made and some novel alloys with improved strength have been developed, the strength level that has been achieved so far is still substantially lower than that obtained in counterpart aluminum alloys. Further improvements in the alloy strength require a better understanding of the structure, morphology, orientation of precipitates, effects of precipitate morphology, and orientation on the strengthening and microstructural factors that are important in controlling the nucleation and growth of these precipitates. In this review, precipitation in most precipitation-hardenable magnesium alloys is reviewed, and its relationship with strengthening is examined. It is demonstrated that the precipitation phenomena in these alloys, especially in the very early stage of the precipitation process, are still far from being well understood, and many fundamental issues remain unsolved even after some extensive and concerted efforts made in the past 12 years. The challenges associated with precipitation hardening and age hardening are identified and discussed, and guidelines are outlined for the rational design and development of higher strength, and ultimately ultrahigh strength, magnesium alloys
via
precipitation hardening.
Long nanopore reads are advantageous in de novo genome assembly. However, nanopore reads usually have broad error distribution and high-error-rate subsequences. Existing error correction tools cannot ...correct nanopore reads efficiently and effectively. Most methods trim high-error-rate subsequences during error correction, which reduces both the length of the reads and contiguity of the final assembly. Here, we develop an error correction, and de novo assembly tool designed to overcome complex errors in nanopore reads. We propose an adaptive read selection and two-step progressive method to quickly correct nanopore reads to high accuracy. We introduce a two-stage assembler to utilize the full length of nanopore reads. Our tool achieves superior performance in both error correction and de novo assembling nanopore reads. It requires only 8122 hours to assemble a 35X coverage human genome and achieves a 2.47-fold improvement in NG50. Furthermore, our assembly of the human WERI cell line shows an NG50 of 22 Mbp. The high-quality assembly of nanopore reads can significantly reduce false positives in structure variation detection.
Lightweight magnesium alloys are attractive as structural materials for improving energy efficiency in applications such as weight reduction of transportation vehicles. One major obstacle for ...widespread applications is the limited ductility of magnesium, which has been attributed to Formula: see text dislocations failing to accommodate plastic strain. We demonstrate, using in situ transmission electron microscope mechanical testing, that Formula: see text dislocations of various characters can accommodate considerable plasticity through gliding on pyramidal planes. We found that submicrometer-size magnesium samples exhibit high plasticity that is far greater than for their bulk counterparts. Small crystal size usually brings high stress, which in turn activates more Formula: see text dislocations in magnesium to accommodate plasticity, leading to both high strength and good plasticity.
Interface segregation of solute atoms has a profound effect on properties of engineering alloys. The occurrence of solute segregation in coherent twin boundaries (CTBs) in Mg alloys is commonly ...considered to be induced by atomic size effect where solute atoms larger than Mg take extension sites and those smaller ones take compression sites in CTBs. Here we report an unusual solute segregation phenomenon in a group of Mg alloys-solute atoms larger than Mg unexpectedly segregate to compression sites of {10Formula: see text1} fully coherent twin boundary and do not segregate to the extension or compression site of {10Formula: see text2} fully coherent twin boundary. We propose that such segregation is dominated by chemical bonding (coordination and solute electronic configuration) rather than elastic strain minimization. We further demonstrate that the chemical bonding factor can also predict the solute segregation phenomena reported previously. Our findings advance the atomic-level understanding of the role of electronic structure in solute segregation in fully coherent twin boundaries, and more broadly grain boundaries, in Mg alloys. They are likely to provide insights into interface boundaries in other metals and alloys of different structures.
Modern engineering alloys contain multiple alloying elements, but their direct observation when segregated at the atomic scale is challenging because segregation is susceptible to electron beam ...damage. This is very severe for magnesium alloys, especially when solute atoms segregate to form single atomic columns. Here we show that we can image segregation in magnesium alloys with atomic-resolution X-ray dispersive spectroscopy at a much lower electron voltage. We report a co-segregation pattern at twin boundaries in a magnesium alloy with both larger and smaller solutes forming alternating columns that fully occupy the twin boundary, in contrast to previous observations of half occupancy where mixed-solute columns alternate with magnesium. We further show that the solute co-segregation affects the twin migration mechanism and increases the twin boundary pinning. Our work demonstrates that the atomic-scale analysis of the structure and chemistry of solute segregation in metallic alloys with complex compositions is now possible.
The structural and compositional evolution of precipitates in Al-Mg-Si-Cu alloys were systematically investigated by atomic resolution high-angle annular dark-field scanning transmission electron ...microscopy and atom probe tomography. In under-aged alloys, most of precipitates have a disordered structure, with a substructure of β″ (LDC) and Cu sub-unit cluster or C unit cell. After aging to peak strength, disordered precipitates including β″, QP1 and QP2 phases are formed. The disordered QP1 and QP2 phases, which contain the unit cells of Q′ and C phases, respectively, are the precursor phases of Q′ phase in these alloys. The β″ phase can transform into the disordered QP1 phase by incorporating Cu atoms, forming Cu sub-unit clusters and QP lattice. When the alloy is over-aged, the ordering and transformation of QP1 to Q′ occurs by the formation of Cu sub-unit clusters, the ordering of QP lattice, and the ordering of QC lattice. In contrast, the transformation of the disordered QP2 phase are rather sluggish. After sufficient aging, Q′, C and disordered QP2 transform into the Q phase. During the evolution of the precipitates in these alloys, a continuous incorporation of Mg, Si and Cu atoms and release of Al atoms occur. These findings provide new insights in understanding precipitation in Al-Mg-Si-Cu alloys.
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Developing ultra-high strength in rare-earth-free Mg alloys using conventional extrusion process is a great challenge. What is even more difficult is to achieve such a goal at a lower processing ...cost. In this work, we report a novel low-alloyed Mg-2Sn-2Ca alloy (in wt. %) that exhibits tunable ultra-high tensile yield strength (360–440 MPa) depending on extrusion parameters. More importantly, there is little drop in mechanical properties of this alloy even when it is extruded at a speed several times higher than those used in the reported high strength Mg alloys. Examination of as-extruded microstructures of this Ca-containing Mg alloy reveals that the ultra-high strength is mainly associated with the presence of surprisingly submicron matrix grains (down to ∼0.32 μm). The results suggest that the Ca addition promotes accumulations of the pyramidal dislocations, which eventually transform into the low angular grain boundaries (LAGBs). The high number density of LAGBs separate the α-Mg matrix via either discontinuous dynamic recrystallization (DDRX) mechanism in the early stage or the continuous dynamic recrystallization (CDRX) mechanism in the later stage of extrusion, which effectively enhances the nucleation rates of the DRXed grains. More importantly, large amount of Ca segregation along LAGBs, accompanied with dynamically precipitated Mg2Ca nano-phases, are detected in the present non-severely deformed samples. It is the combination of solute segregations and numerous Mg2Ca nano-precipitates that contributes to the formation of the ultra-fine grains via pinning mechanism. The ultrafine grains size, Ca enrichment in most LAGBs, and residual Mg2Ca nano-precipitates would in turn contribute significantly to the enhancement of the yield strength of the as-extruded Mg-2Sn-2Ca (wt.%) alloy. The low content of alloying elements and the fast one-step extrusion process render the present alloys low-cost and thus have great potential in large-scale industry applications.
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The difference in the mechanical behaviors of dilute solid solutions, complex solid solutions and their corresponding strengthening mechanisms, is an evolving field of study. An understanding of the ...mechanisms and formulation of theories of strengthening in the complex atomic energy landscapes could eventually lead to a better understanding of the fundamental behavior of condensed matter itself. In this work we attempt to extract the effect of thermo–mechanical processing on the microstructure–mechanical property linkages of complex concentrated alloys (CCAs) by training machine learning (ML) models using processing information / parameters as features. The effect of processing on the phase morphology and the mechanical properties is studied. The stacking fault energy (SFE) predicted based on CCA composition is used as a benchmark to identify deformation mechanisms that are activated based on the arrangement of the component elements within the distorted CCA lattice. This work presents a novel method that attempts to establish ML based process–structure–property (PSP) linkages that could help capture higher order dependencies that may not be adequately captured by existing relations between mechanical properties, phase evolution, composition and processing information. An assortment of Bayesian–learning models are used to create a framework that captures the evolution of phases, their volume fractions, grain sizes and the corresponding change in mechanical properties of a diverse set of CCA compositions as they encounter various processing conditions. The evolution of the mechanical property with grain size is captured as Hall–Petch relations as an example of possible PSP linkage representations.
•Effect of thermo-mechanical processing on the microstructure-mechanical properties.•A hierarchical model for determining properties over multiple length scale.•Bayesian learning models for predicting phases and their volume fractions.•Hall-Petch relation is verified as an example of PSP linkage representations.
Twin boundaries play an important role in shaping the mechanical properties of Mg and Mg alloys. In this study, we simulate the segregation energies (SEs) of 23 solutes in the profuse {101¯2} and ...{101¯1} twin boundaries in Mg. Almost all solutes have both negative and positive SEs (except Tl), which indicate these solutes prefer to segregate at half of the sites in the twin boundaries. The correlation between the signs of SEs and the solute sites is a strong indication for the importance of the volumetric effects, which is confirmed by our modeling based on Voronoi volume calculations. In order to quantitatively describe the solute segregation, solute coverages of the two twin boundaries are calculated using the Langmuir-McLean model. Convergence tests are performed in order to obtain accurate solute coverages. The converged solute coverages are employed as descriptors for the solute strengthening through twin boundaries. Finally, we show two “treasure maps” for the materials design based on descriptors originated from solute solution effects at twin boundaries and stacking faults.
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•Possible descriptors for mechanical properties of Mg are identified.•The segregation energies of 23 solutes in both the compression and tensile twin boundaries are calculated and modeled.•The mechanical contributions to segregation energies determine the mathematic signs of the latter.
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