Grain-boundary diffusion of nickel in submicrocrystalline molybdenum processed by severe plastic deformation

The depth-concentration profiles of nickel upon diffusion in submicrocrystalline (SMC) molybdenum processed by severe plastic deformation (SPD) have been studied by Auger electron spectroscopy. The coefficients (D _b) and activation energies of the grain-boundary diffusion of nickel in SMC molybdenum were determined in a 973–1123 K temperature interval. The results indicate that a difference between the D _b values in SMC and coarse-grained molybdenum is related to a nonequilibrium state of grain boundaries in the SPD-processed metal.     

Continuous dynamic recrystallization during severe plastic deformation

Severe plastic deformation (strains > 100%) has been shown to create significant grain refinement in polycrystalline materials, leading to a nanometric equiaxed crystalline structure for such metals as aluminum, copper and nickel alloys. This process, termed continuous dynamic recrystallization, is governed by evolution of the dislocation structure, which creates new grain boundaries from dislocation walls. In the proposed model, plasticity occurs which firstly involves dislocation multiplication, leading to strain hardening limited by dynamic recovery. After a critical dislocation density is reached new grain boundaries are formed by condensation of walls of dislocations, creating a new stable configuration that is favored due to a reduction of the system free energy. This evolution of the microstructure continues to develop, with a consequent progressive decrease in the average grain diameter. The proposed model provides a quantitative prediction of the evolution of the average grain size, as well as the dislocation density, during continued plastic strain. The model can be calibrated by use of results from any experiment that involves large plastic deformation of metals, subject to negligible annealing effects. In this paper, the model has been calibrated, and consequently validated, through experiments on machining of Al 6061-T6.     

Solute diffusion during high-temperature plastic deformation in alloys

Solute volume diffusion during high-temperature plastic deformation in a substitutional solid solution alloy is analyzed theoretically. Both deformation-induced supersaturated vacancy enhanced diffusion effect and dislocation pipe diffusion effect are considered in the model. The model is applied to the prediction of deformation-enhanced phosphorus diffusion in γ-Fe. Deformation-induced supersaturated vacancy enhanced diffusion and pipe diffusion can both enhance the overall phosphorus diffusion coefficient, but the former effect plays a predominant role. At a certain temperature, the deformation-enhanced phosphorus diffusion coefficient is mainly dependent on strain and strain rate, and at each strain rate there is a steady state value for the enhanced diffusion coefficient.     

Nanostructuring of metals by severe plastic deformation for advanced properties

Despite rosy prospects, the use of nanostructured metals and alloys as advanced structural and functional materials has remained controversial until recently. Only in recent years has a breakthrough been outlined in this area, associated both with development of new routes for the fabrication of bulk nanostructured materials and with investigation of the fundamental mechanisms that lead to the new properties of these materials. Although a deep understanding of these mechanisms is still a topic of basic research, pilot commercial products for medicine and microdevices are coming within reach of the market. This progress article discusses new concepts and principles of using severe plastic deformation (SPD) to fabricate bulk nanostructured metals with advanced properties. Special emphasis is laid on the relationship between microstructural features and properties, as well as the first applications of SPD-produced nanomaterials.     

Ultrafine grain formation in face centered cubic metals during severe plastic deformation

The evolution mechanisms of new high-angle boundaries as well as ultrafine grains at large strains were studied by means of multidirectional forging (MDF) of pure copper at low temperature and aluminum alloy at high temperature, where dynamic recovery operates as a main restoration process. The structural changes can be characterized by the evolution of deformation bands such as microshear or kink bands at moderate strains. Multidirectional forging accelerates the evolution of many mutually crossing microshear or kink bands developed in various directions. The misorientations between (sub)grains increased gradually with increasing cumulative strain, finally leading to the development of a new fine-grained structure. The dynamic grain formation can be resulted from in situ or continuous dynamic recrystallization which is discussed in detail.     

Mechanisms of grain-boundary diffusion in two-dimensional metals

The mechanisms of diffusion via grain boundaries in two-dimensional metals have been studied by method of molecular dynamics. It is established that the diffusion can proceed by two mechanisms, which results in deviations from the Arrhenius law. The first mechanism is related to the formation of chains of atomic displacements between two dislocation cores along the boundary (for high-angle boundaries, between the regions of contraction and extension); the second mechanism consists in a cyclic exchange of atomic sites near the same dislocation core (for high-angle boundaries, near the defect region).     

Amorphization and crystallization characteristics of TiNi shape memory alloys by severe plastic deformation

Differential scanning calorimetry (DSC) was used to determine the crystallization fraction and rate in TiNi alloys by severe plastic deformation. Results showed that the reverse martensitic transformation peak was not observed during the first heating at the rate of 40 K/min in the as-rolled samples, but one exothermic peak was observed at 620 K, which was associated with the amorphous crystallization process. During the second heating, reverse martensitic transformation was recovered. The onset crystallization temperature was low in the initial stage of crystallization with lower heating rates, but the crystallization fraction was found to increase with increasing temperature. However, the crystallization fraction was almost constant in the initial stage of crystallization with a relatively high heating rate. In all heating rates, the amorphous crystallization rates almost always reached maximum as the volumetric fraction of amorphous crystallization rose to 50%.     

Fatigue limit and crack growth in ultra-fine grain metals produced by severe plastic deformation

The experimental results on fatigue resistance of ultra-fine grain metals produced by severe plastic deformation (SPD) are reviewed with regard to two major characteristics of cyclic damage initiation and failure—fatigue limit and fatigue crack growth rate. The fatigue limit benefits considerably from grain refinement down to submicrocrystalline scale. Factors affecting the fatigue limit are discussed in the light of SPD-processing and resultant ultra-fine grain structure. Contrasting with the fatigue limit, the fatigue crack growth threshold deteriorates after SPD in comparison to that of ordinary polycrystals. Possible mechanisms of fatigue crack initiation and propagation are discussed and the guidelines for manufacturing are provided towards enhancement and optimization of fatigue performance.     

Effect of dynamic recrystallization on the importance of grain-boundary sliding during creep

During creep of polycrystalline materials at elevated temperatures, a certain amount of the strain is accommodated by grain-boundary sliding (GBS). The relative importance of GBS depends on the stress and grain size and sometimes temperature. During high-strain deformation, dynamic recrystallization often occurs with the resultant grain size only related to the stress. In this situation the importance of GBS is then dependent only upon stress and sometimes temperature. In dynamically recrystallized Magnox Al80 deformed atT>0.8T _m, 16 to 23% of the imposed strain is accommodated by GBS. A comparison has been made between the experimental results and some theoretical models for the importance of GBS during creep, modified to take account of recrystallization. The best fit to the data is obtained with the modified form of Langdons model. Deformation mechanism maps constructed with this model suggest that dynamic recrystallization can cause a switch of mechanism from dislocation creep to dominant GBS at intermediate temperature (T<673 K) and low stress. Deformation mechanism maps have also been constructed for calcite based on the data of Schmidet al. These suggest that GBS is an important mechanism in calcite deformed under geological conditions.     

Structure and properties of amorphous and nanocrystalline NiTi prepared by severe plastic deformation and annealing

The formation of homogeneous nanocrystalline structure by nanocrystallization of amorphous NiTi subjected to high pressure torsion is demonstrated. Structural evolution during annealing was investigated and homogeneous nanocrystalline structures with different grain sizes have been obtained by controlled annealing. Nanocrystallization results in the record value of room temperature strength for this material equal to 2650 MPa with an elongation to failure of about 5%. At elevated temperatures of (0.4…0.5)T_m nanocrystalline nitinol showed a high ultimate strength with sufficient elongation (up to 200%). The observation that the shape and the size of grains after deformation remain close to that of the initial state suggests that in nanocrystalline NiTi such mechanism as grain boundary sliding and grain rotation are active and the generation and motion of dislocations play the role of accommodation of stress concentration.     

Factors affecting dynamic recrystallization of metals and alloys

Abstract

The high-temperature mechanical behaviour of copper, Cu–Al alloys, and nickel has been examined using torsional testing with hollow testpieces in conjunction with microstructural observations on deformed and quenched specimens using both optical and electron microscopy. Dynamic recrystallization occurred in these materials as the restoration process during high-temperature deformation. The factors influencing dynamic recrystallization have been considered, including materials of high stacking fault energy. It was found that the regime of dynamic recrystallization and the transition in flow stress behaviour could be reasonably represented in terms of the Zener–Hollomon parameter. In Cu–Al solid solution alloys, although the addition of the solute aluminium into copper lowered the stacking fault energy, dynamic recrystallization was retarded to higher strains due to the reduced mobility of the grain boundary. By mechanical and microstructural analysis of the behaviour of various single phase metals and alloys during dynamic recrystallization, the factors influencing the behaviour (i.e. stacking fault energy (solute elements), Zener–Hollomon parameter (deformation condition), and strain) can be summarized on a three dimensional schematic.

MST/587     

Modelling twinning evolution during plastic deformation in hexagonal close-packed metals

A new model describing the twin size and volume fraction evolution at the grain level is proposed. An evolution equation for the mean twin length on individual grains is expressed in terms of the local grain structure. This includes characterising the grain size and orientation distributions in the deformed specimen. Additionally, the twin volume fraction is predicted by computing the collective twin volume increments on each grain, if the grain structure is known. A twin nucleation-rate equation is proposed; it depends on dislocation activity, and the local twin and grain orientations. The model is applied to describe twinning behaviour in Be, Hf, Mg, Ti and Zr for various loading and texture conditions, including Y addition effects in MgY alloys. Twinning evolution is compared in Mg holding unimodal and bimodal grain size distributions.     

Characterization of nanostructured metals produced by plastic deformation

Nanostructured metals produced by plastic deformation often exhibit characteristic structural features such as elongated morphology, a bimodal misorientation distribution and the presence of interior dislocations. The characterization of these parameters is demonstrated by results of TEM and EBSD analyses of pure Ni processed by high pressure torsion (HPT) and pure Al processed by accumulative roll bonding (ARB). Care needed in selecting sample plane and characterization technique is discussed.     

Effect of the type of loading on plastic deformation and damage in metals and alloys in a plane stress state

The results of analysis of the data obtained in experimental and analytical investigations were used to evaluate the effect of the type of loading on plastic deformation and, consequently, the damage of metals and alloys under a plane stress state.Translated from Problemy Prochnosti, No. 2, pp. 76–85, February, 1996.     

Coupled simulations of texture evolution during deformation and recrystallization of fcc and bcc metals

Thermo-mechanical processing to produce optimum grain structure and texture is essential for the successful utilization of commercial aluminum alloys and steels as sheet products. Several modeling techniques have been developed in the past with a reasonably good predictive capability for bulk deformation textures. However, prediction of texture evolution during recrystallization remains very challenging because of uncertainties involved in predicting the mechanisms that lead to nuclei formation and crystallographic orientations of the nuclei, and the uncertainties involved in predicting the grain boundary properties that determine the growth kinetics of the nuclei. We present some of our recent work in modeling the recrystallization textures following cold deformation in polycrystalline bcc metals and hot-deformation in fcc metals.     

Simple phenomenological expression for plastic deformation of pure metals and single-phase alloys to high strains

Abstract

Recent studies on the deformation of metals indicate that at relatively high strains the flow stress does not increase with further strain. The mathematical expressions usually used to derive the relationship between stress and strain (generally the Ludwik or Hollomon equations) do not predict such a plateau. A new expression (σ = K₁mⁿ(tanh ε/m)ⁿ is proposed, where K and n have the same values as the original Hollomon equation and m is an additional disposable parameter. The new expression reduces to the Hollomon equation at low strains but, by adjusting the value of m, can be made to correspond to the high-strain behaviour. For copper and silver, using values of n = 0·5 and m = 2, the ratio of the flow stresses at unit strain to the saturation flow stresses corresponds to those observed experimentally.

MST/455     

ETMB model investigation of flow softening during severe plastic deformation

In this study, the flow softening of FCC materials through severe plastic deformation (SPD) is investigated using ETMB (Y. Estrin, L.S. Toth, A. Molinari, Y. Brechet) model. To do so, using the model, the equal channel angular pressing (ECAP) processes of two FCC materials, aluminum and copper, are investigated. The correlation between the recovery parameters of ETMB model (the dynamic recovery exponents and coefficients) and the intrinsic characteristics of the materials (the stacking fault energy (SFE) and melting point) is described and compared with the previous claims.     

On a possible mechanism of instability of plastic deformation of metals and alloys at 4.2 K

The fine structure of spontaneous deformation jumps and jumps initiated by both the shock action on a deformation device and a strong fast-changing magnetic field (up to 2.7 MA/m) has been investigated in metals and alloys at liquid-helium temperatures. Oscillography of the signals of changes in the load on the sample and of acoustic emission has made it possible to reveal the relationship between types of jumps and postulate on the mechanism of their appearance.     

The influence of nonequilibrium vacancies generated by high-power laser pulses on the high-speed plastic deformation of metals and alloys

The effect of nonequilibrium vacancies generated by laser pulses on the motion of dislocations under the conditions of high-speed deformation is studied. An analytical expression is obtained for the contribution of these vacancies to the dynamic yield strength of metals and alloys and it is shown that the dynamic drag of dislocations by nonequilibrium vacancies leads to a significant increase in the yield strength.