Submicrocrystalline Materials Research Articles

Structure features of bulk submicrocrystalline aluminum alloys at high-rate deformation

The structure formation in commercial Al alloys subjected to severe deformation by dynamic channel-angular pressing has been experimentally investigated. The dependences of the structural characteristics and hardness on the deformation rate and scheme are analyzed. The optimal deformation modes for the formation of submicrocrystalline materials with high hardness and strength are found.

Localization of macrodeformation in submicrocrystalline titanium

A study of the deformation behavior of coarse-grained and submicrocrystalline commercially pure titanium has been carried out. The main specific feature of the behavior of the SMC material is the presence in the loading diagrams of a stage of prefracture, where the deformation occurs virtually without strengthening. At this stage, zones of localized plastic flow with different levels of deformation accumulation are observed. The zone with the maximum amplitude of deformation remains immobile fixed and marks the place of future destruction. The remaining regions of localized deformation move with a velocity that is the greater, the further the zone is located from the zone of fracture. It has been established that in the SMC titanium the local loss and the global loss of stability of plastic flow occur simultaneously. In the coarse-grained material the zone of future fracture is revealed even prior to the beginning of the formation of a macroscopic neck, i.e., the local loss of stability occurs comparatively early, although at the global level the material continues to be deformed quasihomogeneously. The results obtained can be used for correcting regimes of pressure treatment of nanostructured and submicrocrystalline materials.

Cyclic strength of submicrocrystalline nickel obtained by nanopowder sintering

The laws of fatigue hardening, softening, and fracture of coarse-grained, submicrocrystalline (SMC), and nanocrystalline nickel-based metallic materials are briefly reviewed. The cyclic strength of SMC nickel obtained by means of hydrostatic pressing has been studied.

Evaluating the mechanical properties of grain-boundary phases in nano- and submicrocrystalline materials using a model of an elastic multilayer periodic medium

It is shown that determination of the mechanical characteristics of grain-boundary phases in nanoand submicrocrystalline materials using the results of sound velocity measurements interpreted within the framework of the Reuss and Voigt models is incorrect. A more general method is proposed for this purpose, which is based on the model of elastic wave propagation in a multilayer periodic medium.

Study of Grain Growth Kinetics in Submicrocrystalline Armco-Iron

The paper is devoted to the problem of thermal stability of ultra-fine grained (submicrocrystalline) materials prepared by severe plastic deformation. A basis of the paper lies in a fact that there is practically no grain growth in submicrocrystalline materials when annealing temperature is less than 0.35Tm. Reasons of high thermal stability of submicrocrystalline materials at low temperatures are widely discussed in literature. One of them is the affect of triple junction drag on grain boundaries motion. During annealing at a low temperature triple junction drag controls microstructure evolution in submicrocrystalline materials, and this phenomenon can be used to improve their thermal stability at high temperatures. The aim of this paper is to investigate grain growth kinetics in a two-step regime, low temperature and high temperature annealing. The experiments on grain growth were performed in submicrocrystalline Armco-iron fabricated by high pressure torsion. It is established that long-time low temperature pre-annealing reduces the grain growth rate in following high temperature annealing by a factor greater than two.

The theory of anomalous grain growth in submicrocrystalline materials produced by severe plastic deformation

A new approach to describing anomalous grain growth based on the idea of transition of migrating grain boundaries into a highly nonequilibrium state is proposed. The effect of the grain-size scatter and the initial dislocation density on the evolution of this process has been considered. Conditions that are required for the realization of an anomalously rapid grain growth have been analyzed.

Synthesis and microstructure of bulk nanocrystalline copper

The synthesis of bulk nanocrystalline copper (NC-Cu) by powder metallurgy is presented, from compaction of nanocrystalline powders to sintering and differential extrusion. At each step of the process, the microstructure is characterized using X-ray diffraction (XRD) analysis, field-emission gun scanning electron microscopy (FEG-SEM), and transmission electron microscopy (TEM). Particular attention is given to the concept of grain size in nanostructured materials and the comparison of results from the different characterization techniques. The fully dense material has a grain size of 100 nm with a microstructure best described in terms of the distribution of high-angle grain boundaries (GBs), twin boundaries, and low-angle GBs. Dislocations occur in half the grains and at most of the twin boundaries. The GBs are shown to be crystalline, and no evidence is found for amorphous interfacial regions. It is proposed that the grain size be defined only in terms of high-angle GBs, excluding low-angle GBs, for the discussion of mechanical properties. In this respect, the microstructure is compared with the NC-Cu material produced by other synthesis techniques. Powder metallurgy (P/M) processing is revealed as an alternative for the production of large-size submicrocrystalline and NC materials.

Production of Bulk Metallic Nano- and Submicrocrystalline Materials by the Method of Severe Plastic Deformation

It is shown that nano- (Dg < 100 nm) and submicrocrystalline (100 nm < Dg < 1 µm) structures can be obtained in low-carbon and corrosion-resistant austenitic and ferritic steels and aluminum and magnesium alloys by severe plastic torsional deformation under hydrostatic pressure and by equal channel angular pressing.

On physical nature of creep acceleration effect in submicrocrystalline materials

An analysis of special features of manifestation of the creep acceleration effect in polycrystalline metals and alloys in the presence of grain-boundary diffusion fluxes of an impurity from an external medium is made. It is noted that in certain cases where the diffusion properties of sub-boundary regions differ greatly from those within the boundary and in the bulk of the grain (e.g., in materials with submicrocrystalline structure (grain size 0.1–0.5 µm) and/or in the case of diffusion along the grain boundary of a strongly segregating impurity), conventional theoretical methods of calculation of the temperature-temporal range of the occurrence of the above effect have to be corrected. Based on the analysis of the experimental data, feasible solutions to the problem are discussed.

Real structure analysis of nanocrystalline and submicrocrystalline materials using X-ray diffraction

Real structure analysis of nanocrystalline and submicrocrystalline materials using X-ray diffraction

Study of Submicrocrystalline Materials by Conventional Powder Diffraction and Diffuse Scattering in Transmitted Wave

Two methods were used for the analysis of submicro-crystalline copper prepared by high-pressure (6 GPa) severe plastic deformation. Conventional powder XRD study was carried in terms of the modified Williamson-Hall (WH) plots and shown large line-broadening anisotropy. A new high-resolution small-angle transmission diffuse scattering method was applied for determination of crystallite size. Two methods were used for the evaluation of the diffuse scattering - calculation of the autocorrelation function of the crystalline shape by the transformation of the measured signal and by the direct fitting. Bimodal block size distribution has to be assumed for annealed samples. This was also confirmed by classical back-reflection film method as well as by the long tails of conventional diffraction profiles.

Modifications de structure et de propriétés de composés intermétalliques par déformation plastique intense

Abstract The processing of intermetallic compounds by severe plastic deformation (SPD) methods is reviewed. In Ni3Al-based massive compounds or multiphase materials this leads to a disordered nanocrystalline structure. In TiAl, high-temperature SPD by multi-step forging produces sub-microcrystalline materials, but room-temperature SPD by high-pressure torsion results in a partial phase transformation. This is also the case for the σ phase of multiphase FeCrCo alloys.

Superplastic flow in nanocrystalline and sub-microcrystalline yttria-stabilized tetragonal zirconia

Tensile deformation of nanocrystalline ZrO2 + 5 mol% Y2O3 at temperatures in the range of 1283–1403 K is described. It is demonstrated (a) that steady state flow is possible at temperatures of the order of 0.42 Tm, where Tm is the absolute melting point, (b) that 70% engineering strain could be obtained at 1403 K (0.46 Tm), and (c) that significant grain boundary sliding was present during deformation. Static and dynamic grain growth as also a decrease in the relative density of the specimen with deformation could be observed. The present results as well as those of Owen and Chokshi concerning superplastic flow in sub-microcrystalline materials taken from literature could be accounted for quantitatively using the grain boundary sliding controlled flow model of Padmanabhan and Schlipf, originally proposed for microcrystalline superplastic alloys.

Glide-Climb Based GBS Model Applied to Submicrocrystalline Superplasticity

Research on superplasticity in metals have clearly established the importance of microstructural evolution combined with several phenomena: (i) dislocation emission and absorption at and near grain boundaries (ii) concurrent grain growth and (iii) grain boundary sliding (GBS) during deformation [1] . The combined evolutionary aspects of these processes is essential to superplasticity mechanism. With current interest in submicrocrystalline metallic alloys produced by severe deformation processes, and their reported large degree of strain hardening under the condition of low temperature superplasticity, it is necessary to critically examine the understanding and modeling of their response in these materials containing subgrains and ultrafine grains. After examining several steady state and evolutionary hardening-recovery models, it has become clear that an existing superplasticity model involving a mix of two mechanisms: glide-climb based deformation in the grain mantle and general dislocation creep within the grain corc (proposed in 1994) appear to capture all essential features of the experimental behavior. Data for both microcrystalline and submicrocrystalline materials have been reviewed relative to our model and presented here.

Study of The Temperature Coefficient of Thermal Expansion and Microstructure of a 36h Submicrocrystalline Invar Alloy

In the last few years, of great interest to investigators has been the physical materiology of submicrocrystalline (SMC) materials (with average grain size of about 0.1 μm). Their elastic, thermal, electrical, magnetic, and other physical properties significantly differ from those of conventional coarse-grained materials [1–4]. In this connection, of great interest are investigations of invar alloys [5]. These alloys, important from the viewpoint of their practical implementation, have a temperature coefficient of thermal expansion close to zero in the temperature range of their application. In addition, they have anomalous elastic, thermal, magnetic, and other physical properties. In the present work, the characteristic features of thermal expansion of the SMC invar alloy are studied.

An Examination of the Deformation Process in Equal-Channel Angular Pressing

AbstractIn the past decade, there has been an important emphasis in materials science on the production and use of ultrafine-grained materials. These materials offer wide-ranging advantages such as an improvement in strength at low temperatures and enhanced superplastic behavior at high temperatures. New techniques to process these materials have been developed. Good properties have been achieved by powder metallurgy and by techniques of bulk processing. Equal-channel angular (ECA) pressing is a bulkprocessing technique which has led to some remarkable achievements in the production of sub-microcrystalline materials with excellent superplastic properties at low temperatures and high strain rates. When considering the currently available physical and mechanical properties of these materials, it appears that some aspects cannot be explained solely by the very small grain size. In fact, the specific mode of deformation occurring in ECA pressing appears to have an influence on the final properties. This paper reviews the details of this metal working technique and then provides an explanation for the observed microstructures, their thermal stability and their remarkable superplastic properties.

Defect microstructure in titanium nitride submicrocrystals

Transmission electron microscopy has been applied to the defect structure of titanium nitride in the submicrocrystalline (SMC) state produced under conditions of deviation from equilibrium in ion-plasma synthesis. The submicrocrystals contain a new type of defect substructure having a continuum disclination density up to 2.5 rad/µm2. Direct structure methods give evidence for a high density of partial disclinations at the SMC grain boundaries in the nitride phase. A novel method has been used to examine substructures having a high defect density, which has been used to estimate the partial disclination density at the submicrocrystal boundaries. The origin of this highly defective state and the effects of it on the properties of SMC materials is discussed.

Elastic properties and ΔE-effect in nanocrystalline Fe-Cu-Nb-Si-B alloy

The effect of heat treatment in the temperature region from room temperature to 470 °C on the elastic modulus, internal friction and ΔE-effect in amorphous and nanocrystalline Fe-Cu-Nb-Si-B alloy was studied. The irreversible changes of Young's modulus and internal friction were observed for as-cast amorphous and as-quench rapidly nanocrystalized specimens. For both types of specimens these changes had a similar two-stage dependence on annealing temperature. The behaviour of elastic modulus and internal friction at the first stage are very similar to their behaviour in nano- and submicrocrystalline materials exibiting the so-called effect of the anomalous elastic modulus reduction. The phenomena observed are considered to be caused by nonequilibrium interfaces formed at the quenching process. The crystallization process is supposed to be responsible for the second stage.

Creep and diffusion parameters in submicrocrystalline metals

We have studied the creep of nickel and copper in a submicrocrystalline (SMC) state in a vacuum and in the presence of a diffusion contact with an impurity (Cu and Al, respectively). It is shown that a reduction of the resistance in the presence of a diffusion contact with an impurity is observed in the SMC materials in the temperature range 398 to 473 K. This range is 200 to 400 K lower than the corresponding range for coarse-grained material. It is shown that in this temperature interval the coefficients of grain boundary diffusion for copper in SMC nickel are 5 to 6 orders of magnitude larger than in the coarse-grained material. We propose that the reduction in the temperature for the manifestation of a creep activation effect in the presence of a diffusion contact with an impurity in SMC materials is caused by the increase in the diffusion permeability of the submicrocrystalline grain boundaries.