Deformation Of Metallic Alloys Research Articles

A comprehensive review on the deformation behavior of refractory high entropy alloys at elevated temperatures

Thermo-mechanical processing of refractory high entropy alloys (RHEAs) at high temperatures is very important. It is an effective method of modifying the microstructure, properties, and shaping into final components after casting. Using the Scopus database, 57 articles relating to the hot deformation of refractory high entropy alloys were extracted from 2011 to 2022. Despite the limited number of articles on hot deformation of RHEAs, it is important to find out if the dominant softening mechanisms reported in other metallic alloys are evident. This is the main impetus for this study since the hot deformation behavior has not been comprehensively studied. All the probable mechanisms influencing deformation in metallic alloys, such as work hardening, dynamic recrystallization, and dynamic recovery, have also been observed in RHEAs. The bulging phenomenon, serrated grain boundaries, and necklace-like structures reported in metallic alloys have also been detected in hot deformed RHEAs. Unsafe deformation behavior such as cracks that have been reported in metallic alloys, have also been observed in RHEAs. This review has provided a comprehensive study on the hot working processes of RHEAs and highlighted critical gaps for future research direction with some suggested limitations.

Superplasticity of Metallic Alloys: Some Current Findings and Open Questions

Superplasticity of metallic alloys has been widely investigated for many years but despite numerous studies, some important questions regarding superplastic behavior of metals can be considered as still open. Among others, one can mention the role of grain boundary sliding as contributing mechanism of deformation, the microstructural requirements for getting superplasticity in metallic alloys or the link between mechanisms of deformation and mechanisms of damage. Interesting information were recently obtained by the use of new experimental techniques. One can mention for instance the development of fine grids allowing more detailed studies of the respective contributions of intra-and inter-granular strains in superplastic alloys or fast X-ray micro tomography allowing in situ 3D damage studies. The aim of this paper is to give information about current findings related to superplastic deformation of metallic alloys and to suggest some perspectives for future.

Some guidelines for promoting high temperature deformation of metallic alloys

Capacity to reach large deformation at high temperature is an important issue in many forming processes of metallic alloys. It is well known that a low value of the stress exponent (or a concomitant high value of the strain rate sensitivity parameter) is a key point for controlling resistance to necking. A first way for decreasing the stress exponent is to get superplastic properties but it frequently requires dealing with fine microstructures which can be difficult to produce and to preserve. Moreover, in the case of single phase alloys, like aluminum or magnesium alloys, superplastic deformation generally induces damage which can result in premature fracture or damaged components after forming. The aim of this paper is to give some guidelines for promoting high temperature deformation of metallic alloys, with a particular attention given to superplastic forming. The possibility to reduce the temperature of superplastic forming (SPF) for titanium alloys, the capacity to get a better understanding of the specificities of damage process in the case of superplastic deformation and the ability to get large strains to fracture avoiding the production of fine grains before strain are more specifically discussed.

Recent Experimental and Numerical Developments in Semisolid Deformation

Semisolid deformation in metallic alloys has been studied for many years, both experimentally and through numerical simulations, because mushy zone mechanics plays a key role in the quality of metalcasting processes. For researchers, the topic of semisolid deformation is particularly interesting due to the concomitant phenomena of liquid metal flow, the evolution in fraction solid, and grain coalescence. At the same time, the high temperatures, opacity of the melt, and the strong sensitivity to alloy composition and cooling rates pose strong challenges requiring sustained research effort. This section of JOM explores recent experimental and numerical developments in semisolid deformation, along with their applications for improving industrial processing. Five diverse articles are presented to highlight current cutting-edge research, from experiments that can directly observe grain rearrangement under various loading modes to models that simulate dendrite deformation. The first two articles showcase exciting developments in the use of x-ray radiography to directly observe semisolid micromechanics in thin-sample Al alloy and steel specimens. The article ‘‘In situ Investigation of Dendrite Deformation during Upward Solidification of Al-7wt.%Si’’ by Reinhart et al. examines the phenomena of dendrites bending due to gravity, which can precede fragmentation in the case of well-developed secondary arms. The article ‘‘Synchrotron Radiography Studies of ShearInduced Dilation in Semisolid Al Alloys and Steels,’’ by Gourlay et al. focuses on the phenomena of Reynolds dilatancy during shear deformation in alloys with morphologies ranging from equiaxeddendritic to globular. While x-rays can be used to observe local semisolid deformation, the technique known as neutron diffraction is also being used to provide exciting new experimental results to examine structural transitions within the semisolid region. In the article ‘‘Deformation of Coherency and Rigidity Temperatures in Al-Cu Alloys using In-Situ Neutron Diffraction during Casting,’’ by Drezet et al., measurements are reported that quantify the point at which thermally induced deformations start to generate internal stresses in a casting as a function of cooling rates. This knowledge is required for macro-scale casting process simulations. The final two articles usecomputational modeling to advance our fundamental knowledge of deformation and defects. The article by Yamaguchi and Beckermann, ‘‘Direct Numerical Simulation of Solid Deformation during Dendritic Solidification,’’ combines a polycrystalline phase-field model with a material point stress analysis to numerically simulate the fully coupled dendritic solidification and elasto-viscoplastic deformation behavior of a pure substance. The results indicate that deformation modifies the crystallographic orientation of a dendrite and the subsequent mushy zone solidification behavior. In ‘‘Prediction of Hot Tearing using a Dimensionless Niyama Criterion,’’ by Monroe and Beckermann, the well-known Niyama criterion is extended to include the effect of applied strain. This new criterion can be easily implemented in industrial casting simulation software to predict both shrinkage porosity and hot tearing susceptibility in a casting process. Collectively, this series of articles provides new insight into the effects of phenomena at various length-scales on mechanical deformation and failure during solidification. New experimental techniques and new modeling methods are providing exciting results that are key for improving both fundamental understanding of semisolid deformation and industrial casting processes. The reader is also referred to prior JOM issues for other articles on similar themes. Andre B. Phillion is the guest editor for the Solidification Committee of the TMS Materials Processing & Manufacturing Division, and coordinator of the topic Semisolid Deformation: Experimental and Numerical Developments in this issue. JOM, Vol. 66, No. 8, 2014

Non-localized deformation in metallic alloys with amorphous structure

In bulk or ribbon metallic alloys with monolithic amorphous structure, plastic deformation at room temperature is dominated by highly localized shear banding. Here we report suppressed shear localization under tension in a metallic glass (MG) embedded with pre-existing amorphous shear bands using large-scale atomistic simulations. It is demonstrated that pre-deformation lowers material strength and triggers enhanced strain fluctuation before sample yielding, leading to highly dispersed plastic shearing in the entire sample. Furthermore, the transition in deformation mode from highly localized shear banding to non-localized plastic deformation is associated with the competition between the yield strength of the material and the critical stress required for the formation of mature shear bands in the load-bearing materials. Given that a sufficient amount of glassy heterogeneities can be introduced into MGs under experimental conditions, the obtained results could provide a promising strategy for designing tensile ductile MGs with pure amorphous structure at room temperature.

John Wesley Mitchell. 3 December 1913 — 12 July 2007

John (‘Jack’) Mitchell was a New Zealander who came to Oxford University on a scholarship of the Commission of the 1851 Exhibition. Mitchell’s association with Nevill (later Sir Nevill) Mott FRS during World War II, when they both worked at the Armament Research Department, afterwards led Mitchell to join Mott at Bristol University, where he began the research into the photographic process for which he is best known. His pursuit of an understanding of the mechanism through which a latent image forms led to the important discovery of the decoration by silver particles of individual dislocations in silver halide crystals and the mosaic microstructure of them. In turn the decoration technique provided the first clear experimental evidence of the link between plastic deformation and the creation and movement of dislocations. In 1960 Mitchell was appointed as professor in the Physics Department at the University of Virginia, where, apart from a brief spell in England as the Director of the National Chemical Laboratory, he worked happily for some 40 years. During this time his research group published many papers describing and explaining the mechanisms of plastic deformation in metallic alloys, devising and using state-of-the-art methods, thereby adding much to a wider understanding of mechanical properties and strength. All the time, working alone, Mitchell further developed and refined his photoaggregation theory of the photographic process, gaining worldwide recognition and honours for his effort.

Shear on the Flow Surface of Metallic Crystals

The present article addresses the following question: How is it that shears are so common in the plastic deformation of metallic alloys? An answer is sought in a geometric description of the shear flow when the deformation is produced by slip systems gliding according to the Schmid law. Such flows are represented schematically by what is called “simple shear” and a kinematic study is done of the way these shears can be produced by the joint activity of various slip systems. This implies specific conditions on the glide rates, which can be known analytically thanks to adequate parametrizations. All the possible shears have been calculated in the case of cubic metals deforming with identical critical resolved shear stresses (Bishop and Hill polyhedron). Three dimensional representations are given in the space of the Bunge angles associated with the principal directions of the shears. A special attention has been given to the number of slip systems involved. Most of the shears are not far from some combination of two or three systems. This is quantified by defining the misorientation ω between a shear taken at random and the set of shears produced by the glide on two or three octahedral slip systems. It is found that in most cases, ω<15 deg. The maximum value of ω (30.5 deg) is found for the orientations called Cube and U in rolled metals.

The measurement of prior plastic deformation in metallic alloys using the neutron diffraction technique

The neutron diffraction technique has been used to determine the extent of prior plastic deformation in an aluminium alloy weld and preloaded cracked steel beams. The change in peak width has been shown to be a function of the prior plastic deformation, and is found to be insensitive to the direction of prior straining, material and the orientation of the diffraction vector. Prior plastic deformation has also been shown to influence the reference peak angle. The distributions of prior plastic strain in the steel cracked beam subjected to an overload were found to be in good agreement with results from finite element analyses.

Strain gradients and size effects in nonhomogeneous plastic deformation

Phenomenological constitutive equation for plastic deformation in metallic alloys are usually developed with the basic assumption of uniformity of the deformation field, or by the homogenization of the field over a macroscopic representative element. This, in turn, coupled with simple nondimensional analysis within the classical continuum theory of plasticity, leads to the development of homogeneous constitutive equations that relate the flow stress to some internal variables, such as strain hardening, strain rate, and volume fraction of second phase particles in metal matrix composites. However, there are circumstances where the size of the microstructure significantly influences the overall mechanical properties of the material. This becomes even more crucial when the size of the plastic deformation zone and the magnitude of the strain gradient within it become comparable to the size of the underlying dislocation structure. For example, predicting the width of the shear band in metallic alloys and its scaling with the size of the microstructure is an important issue from a fundamental point of view. A constitutive model that can capture this phenomenon can also provide an explanation of other observed phenomena such as the size of the plastic zone near a crack tip, the dependence of flow stress onmore » particle size in metal matrix composite, the effect of specimen size on the stress-strain curve in pure copper, and the significant increase in flow stress observed in indentation tests when the size of the indenter is in the nanometer range. The objective of this paper is to provide a possible explanation of how a local microscopic strain inhomogeneity resulting from a heterogeneous microstructure can generate a strain gradient effect when the deformation field is averaged over a macroscopic representative element, leading to a specific predictive relation for the strain gradient coefficient c in terms of the materials elastic-plastic moduli and grain size.« less

The physics of superplastic deformation

Abstract Superplasticity is an important mode of deformation in metallic alloys with very small grain sizes (usually less than 10 μm). In general, high elongations are observed over a rather limited range of intermediate strain rates and there is a decrease in the superplastic effect at both high and low strain rates. The major experimental observations in superplastic metals are summarized and the physical mechanisms of flow are discussed with reference to the behavior at high, intermediate and low strain rates, respectively. Superplastic-like behavior has been reported recently in some ceramics but the experimental evidence suggests that the mechanism of flow in these materials in not the as in metals.