Twinning is an important deformation mode in hexagonal close packed (HCP) metals, including magnesium alloys. Precipitates are used to provide strengthening in many of these alloys. The effect of ...precipitates in strengthening against deformation by slip is well understood, but this is not the case for twinning. Recent studies have indicated that precipitates are usually not sheared by twins, but the Orowan law for strengthening against slip by dislocation bowing does not give a good prediction when applied to twinning. It has therefore been proposed that the dominant strengthening effect inhibiting thickening of a twin arises from an additional back-stress that results from embedding a unsheared precipitate in twinned matrix. The present paper uses an Eshelby model to assess the influence of precipitate shape and habit on the internal stresses that arise from embedding a non-shearing precipitate in a {101¯2} twin (the dominant twin type). It is demonstrated that the elastic stresses generated easily exceed the critical resolved shear stress for activation of slip and therefore plastic relaxation is to be expected. In all cases, the predicted plastic zone is confined to a region local to the particle. The implications of these predictions for design of precipitation strengthened HCP alloys are discussed.
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The unique combination of very large strains, high temperatures and high strain rates inherent to friction stir welding (FSW) and friction stir processing (FSP) and their dependency on the processing ...parameters provides an opportunity to tailor the microstructure, and hence the performance of welds and surfaces to an extent not possible with fusion processes. While a great deal of attention has previously been focused on the FSW parameters and their effect on weld quality and joint performance, here the focus is on developing a comprehensive understanding of the fundamentals of the microstructural evolution during FSW/P. Through a consideration of the mechanisms underlying the development of grain structures and textures, phases, phase transformations and precipitation, microstructural control across a very wide range of similar and dissimilar material joints is examined. In particular, when considering the joining of dissimilar metals and alloys, special attention is focused on the control and dispersion of deleterious intermetallic compounds. Similarly, we consider how FSP can be used to locally refine the microstructure as well as provide an opportunity to form metal matrix composites (MMCs) for near surface reinforcement. Finally, the current gaps in our knowledge are considered in the context of the future outlook for FSW/P.
Deformation Enhanced Diffusion in Aluminium Alloys Robson, J. D.
Metallurgical and materials transactions. A, Physical metallurgy and materials science,
10/2020, Letnik:
51, Številka:
10
Journal Article
Recenzirano
Odprti dostop
Deformation introduces defects such as dislocations and excess vacancies that can strongly influence diffusion rates in aluminium alloys. This is of great importance in understanding the effect of ...deformation on processes such as precipitation hardening, which can be accelerated by orders of magnitude by the defects introduced. In this work, a simple and widely used classical model has been employed to explore the effect of process variables on diffusion enhancement due to deformation-induced excess vacancies and dislocations. It is demonstrated that in aluminium alloys, the strain rates and temperatures used in processing and testing encompass a range of regimes. At low strain rate or high temperature, a steady state becomes established in which the deformation enhancement depends on strain rate but not strain. Conversely, at high strain rate or low temperature, deformation enhancement is insensitive to strain rate but increases with strain. For all conditions, the effect of excess vacancies is much stronger than the direct effect of pipe diffusion along dislocations. The predicted deformation-induced vacancy concentration can exceed that expected after rapid quenching by an order of magnitude, but the lifetime of the deformation-induced vacancies is much shorter. Finally, limitations of the classical model and suggestions for improvement are proposed.
Magnesium–zinc–zirconium (ZK) alloys are precipitation strengthened by the addition of zinc and grain refined by zirconium. In ZK alloys these mechanisms interact since some Zn can be precipitated ...into Zn–Zr intermetallics. This reduces the fraction of age hardening precipitates. An experimental study has confirmed the presence of these two particle types in ZK60, a commercial Mg–Zn–Zr alloy. A model has been developed to consider precipitation and strengthening in ZK alloys accounting for this interaction. It has been shown that whilst Zr additions will increase strength by grain refinement this is balanced by a loss of age hardening response. It is predicted that if the same fine grain size could be achieved without Zr then a significant strength increase of around 15% would be expected. Therefore, in wrought applications where the final grain size is determined by recrystallisation, the use of Zr grain refiner in ZK alloys does not lead to optimum strengthening. The model was then applied to explore strategies to increase strengthening in Mg–Zn alloys.
► The development of a fine, stable, particle distribution through suitable heat treatment. ► Particles do not prevent recrystallization during deformation and post-deformation annealing. ► Particles ...can be used to manipulate recrystallization by slowing the kinetics.
Dispersoid particles are widely used in wrought aluminium alloys to control grain structure during thermomechanical processing. The aim of this work was to investigate whether this approach could be utilized in wrought magnesium alloys to obtain better control of recrystallization. A binary magnesium–manganese alloy was heat treated to produce a fine dispersion of manganese precipitates. The effect of this dispersion on dynamic and static recrystallization during channel die deformation (at a slow strain rate), hot rolling, and annealing was studied and compared with that of an alloy free of fine particles. It was found that the presence of particles did not suppress dynamic recrystallization during channel die deformation. Fine particles did lead to a much reduced recrystallized fraction after hot rolling, attributed to a retardation of static recrystallization kinetics. Although the presence of pinning particles greatly slowed recrystallization kinetics on annealing, for no conditions studied was it possible to prevent recrystallization of the as-deformed structure using particles.
The microstructural evolution in AA7050 during post homogenization cooling, and its influence on behaviour during preheat, hot rolling, and solution treatment, has been investigated. It is shown that ...slow cooling, at a rate typical of industrial processing, leads to a complex sequence of large, heterogeneously nucleated precipitates. In particular, S-phase particles grow to over 7
μ
m in diameter. These particles are retained during preheat to rolling, and are above the critical size required to act as particle stimulated nucleation sites for recrystallization. Furthermore, by removing solute they prevent the formation of fine particles during preheat. This leads to a final recrystallized fraction that is over four times larger in homogenized and slow cooled material than in material quenched rapidly from homogenization.
Abstract Background Pheochromocytoma is associated with catecholamine-induced cardiac toxicity, but the extent and nature of cardiac involvement in clinical cohorts is not well-characterized. ...Objectives This study characterized the cardiac phenotype in patients with pheochromocytoma using cardiac magnetic resonance (CMR). Methods A total of 125 subjects were studied, including patients with newly diagnosed pheochromocytoma (n = 29), patients with previously surgically cured pheochromocytoma (n = 31), healthy control subjects (n = 51), and hypertensive control subjects (HTN) (n = 14), using CMR (1.5-T) cine, strain imaging by myocardial tagging, late gadolinium enhancement, and native T1 mapping (Shortened Modified Look-Locker Inversion recovery ShMOLLI). Results Patients who were newly diagnosed with pheochromocytoma, compared with healthy and HTN control subjects, had impaired left ventricular (LV) ejection fraction (<56% in 38% of patients), peak systolic circumferential strain (p < 0.05), and diastolic strain rate (p < 0.05). They had higher myocardial T1 (974 ± 25 ms, as compared with 954 ± 16 ms in healthy and 958 ± 23 ms in HTN subjects; p < 0.05), areas of myocarditis (median 22% LV with T1 >990 ms, as compared with 1% in healthy and 2% in HTN subjects; p < 0.05), and focal fibrosis (59% had nonischemic late gadolinium enhancement, as compared with 14% in HTN subjects). Post-operatively, impaired LV ejection fraction typically normalized, but systolic and diastolic strain impairment persisted. Focal fibrosis (median 5% LV) and T1 abnormalities (median 12% LV) remained, the latter of which may suggest some diffuse fibrosis. Previously cured patients demonstrated abnormal diastolic strain rate (p < 0.001), myocardial T1 (median 12% LV), and small areas of focal fibrosis (median 1% LV). LV mass index was increased in HTN compared with healthy control subjects (p < 0.05), but not in the 2 pheochromocytoma groups. Conclusions This first systematic CMR study characterizing the cardiac phenotype in pheochromocytoma showed that cardiac involvement was frequent and, for some variables, persisted after curative surgery. These effects surpass those of hypertensive heart disease alone, supporting a direct role of catecholamine toxicity that may produce subtle but long-lasting myocardial alterations.
A PANorama of PAX genes in cancer and development Eccles, Michael R; Robson, Ewan J. D; He, Shu-Jie
Nature reviews. Cancer,
200601, 2006-Jan, 2006-1-00, 20060101, Letnik:
6, Številka:
1
Journal Article
Recenzirano
Populations of self-renewing cells that arise during normal embryonic development harbour the potential for rapid proliferation, migration or transdifferentiation and, therefore, tumour generation. ...So, control mechanisms are essential to prevent rapidly expanding populations from malignant growth. Transcription factors have crucial roles in ensuring establishment of such regulation, with the Pax gene family prominent amongst these. This review examines the role of Pax family members during embryogenesis, and their contribution to tumorigenesis when subverted.
The second phase particles (SPPs) that occur in Zircaloys can undergo amorphization and loss of iron in-reactor, which can have a profound effect on corrosion resistance and irradiation growth. In ...this paper, a classical model is presented that predicts the interaction between these processes, based on a previously proposed physical mechanism that considers iron release and transport from the SPP as the controlling factor. It is assumed that iron is released into a mobile state by a homogeneous and continuous amorphization and recrystallization process. As iron is released, it diffuses out of the SPP, causing the relative stability of the amorphous phase to increase, until a persistent amorphous layer forms. The model predicts the growth rate of this layer and the rate at which iron is lost from the SPP. It has been used to explore the effect of flux, temperature, and initial SPP iron concentration on the amorphous layer growth and iron loss kinetics. It is predicted that whilst iron loss is correlated with the thickness of the amorphous layer, the relationship is not a simple scaling, and depends on conditions. The model is also able to explain the observed occurrence of an amorphous layer only in an intermediate temperature range as a natural consequence of the competition between the same fundamental mechanisms.
High power ultrasonic spot welding (USW) is a low heat input solid-state joining process that may offer a solution for welding difficult dissimilar-material couples, like magnesium (Mg) to aluminium ...(Al) for automotive body applications. However, the high strain rate dynamic deformation in USW has been claimed to accelerate inter-diffusion rates in dissimilar joints. The interfacial reaction between Al, AA6111, and Mg AZ31 alloys has been studied as a function of welding energy. For the optimum welding condition of 600J (0.4s) the reaction layer thickness was already ∼5μm thick. Intermetallic reaction centres were found to nucleate within microwelds at the interface at very short welding times and spread and grow rapidly to form a continuous layer, composed of two sub-layers of Al12Mg17 and Al3Mg2. Interface liquation was also found for longer welding times at temperatures below the recognised lowest eutectic reaction temperature in the Al–Mg binary system. Modelling has been used to show that the solid state reaction kinetics were over twice the rate expected from parabolic growth predictions made using rate constants obtained under static test conditions. The reasons for this discrepancy and the depressed melting reaction are discussed.
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