Structural and mechanical behaviour of severe plastically deformed high purity aluminium sheets processed by constrained groove pressing technique

High purity aluminium sheets (∼99.9%) are subjected to intense plastic straining by constrained groove pressing method successfully up to 5 passes thereby imparting an effective plastic strain of 5.8. Transmission electron microscopy studies of constrained groove pressed sheets divulged significant grain refinement and the average grain sizes obtained after five pass is estimated to be ∼0.9 μm. In addition to that, microstructural evolution of constrained groove pressed sheets is characterized by X-ray diffraction peak profile analysis employing Williamson–Hall method and the results obtained fairly concur with electron microscopy findings. The tensile behaviour evolution with increased straining indicates substantial improvement of yield strength by ∼5.3 times from 17 MPa to 90 MPa during first pass corroborated to grain refinement observed. Marginal increase in strengths is noticed during second pass followed by minor drop in strengths attributed to predominance of dislocation recovery is noticed in subsequent passes. Quantitative assessment of degree of deformation homogeneity using microhardness profiles reveal relatively better strain homogeneity at higher number of passes.     

Galvanomagnetic properties of plastically deformed InSb single crystals

The dc Hall effect, dc conductivity and mobility have been studied on deformed and undeformed samples ofn-type InSb from liquid nitrogen temperature to room temperature. These studies have shown that the Hall coefficient values of deformed samples do not differ much from undeformed sample, but a considerable amount of change was observed in mobility, suggesting that equal number of donor and acceptor type dislocations are introduced during the deformation process. In addition the mobility variation of the deformed samples with temperature has shown a peak in 170–300°K range. The dislocation mobility (μ _D) is deduced from the observed mobilities of deformed and undeformed samples. The plotμ _D vs T has two regions, region 1 being independent of temperature and region 2 having a linear increase with temperature. Theβ factor obtained from region 2 is found to be almost equal to the one calculated from Dexter and Seitz model. The dislocation densities at room temperature are also calculated for the deformed samples using the above model.     

Relationship between rupture stress and microstructure parameters of plastically deformed heat-resistant steel

The paper addresses the influence of plastic prestraining in tension and combined tension at 150 and 350°C on the rupture stress in 15Kh2MFA steel upon heat treatment that simulates irradiation embrittlement of materials in a WWER-440 type reactor towards the end of its lifetime. The dependence of rupture stress on the dislocation density in the material upon plastic prestraining is studied. It is found out that as the plastic prestraining in tension and combined tension grows, the dislocation density within small-angle boundaries increases and thus results in a larger rupture stress. An increase in rupture stress in the case of prestraining at 350°C is more intensive than that at 150°C. We analyze the microcrack nucleation and growth micromechanisms in 15Kh2MFA steel during tensile plastic deformation and discuss the effect of the material substructure on the microcrack arrest. __________ Translated from Problemy Prochnosti, No. 4, pp. 19–30, July–August, 2007.     

Transmission electron microscopy characterization of TiN/SiNx multilayered coatings plastically deformed by nanoindentation

Plastic deformation of TiN_5 nm/SiN_0.5 nm multilayers by nanoindentation was investigated by transmission electron microscopy in order to identify deformation mechanisms involved in film failure resulting from severe plastic deformation. The TiN layers exhibited a crystalline fcc structure with a [002] preferential orientation; further crystal growth was interrupted by the amorphous SiN_x layers. After severe plastic deformation collective vertical displacement of slabs of several TiN/SiN_x-bilayers, which resulted from shear sliding at TiN/TiN grain boundaries, was observed. They are, together with horizontal fractures along the SiN_x layers, vertical cracks under the indenter tip following the TiN grain boundaries and delamination from the substrate, the predominant failure mechanisms of these coatings. The deformation behaviour of these films provides an experimental support for the absence of dislocation activity in grains of 5 nm size.     

Instability origin of subgrain formation in plastically deformed materials

The review is focused on two methods of formulation and solution of the subgrain formation problem: an energetic approach and a model of incremental deformations. Both methods are based on a reduced single slip version of crystal plasticity. The mathematical analysis of the energetic approach is done for a single slip model only; in the incremental approach the deformation are assumed small, hence, multi slip can be treated as a sum of single slips. The energetic approach has been employed in analysis of the crystal plasticity model of shear and kink bands. The incremental higher strain gradient model provides an insight into an initial stage of the subgrain formation and the mechanism controlling the subgrain size.     

Fracture initiation at a sphrerical inclusion in a matrix plastically deformed by shear

The critical strain for fracture initiation of a metallic material with a spherical inclusion has been analysed using EaheIby,s inclusion method forthree types of fracture initiation models including the recovery effect by diffusion of atoms. When the elastic constant of inclusion approaches that of the matrix, the critical strain for fracture initiation becomes large in the case of uniform shear deformation of the matrix. It was found that the critical strain becomes large due to the diffusion of atoms, especially for inclusions of small size and a large elastic constant. The model in which the inclusion is cracked by the localized shear deformation can explain the observed inclusion size dependence of the strain for fracture initiation. The inclusion size dependence of the critical strain for fracture initiation by uniform shear deformation of the matrix is different from that by localized shear deformation. Therefore, it is important to know which mechanism governs the fracture.     

Residual stresses in a transversely plastically deformed body containing thin tunnel inclusions

Summary Singular integral equations have been used to solve for the approximate residual-stress concentration in the matrix around thin tunnel inclusions; we consider the cases of one isolated inclusion as well as periodic systems of elliptical ones placed coplanar or in parallel planes. Closed solutions are obtained. Laboratory tension tests have been used in estimating the residual stresses in a steel matrix containing a tunnel copper elliptical inclusion.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 25, No. 2, pp. 19–23, March–April, 1989.     

Influence of dislocation on magnetic domains in plastically deformed 3% Si-Fe

Characteristic layer-like magnetic domains are observed on many large crystals of a polycrystalline bioriented type     

An alternative representation of stress in plastically deformed bodies and its interpretation

The stresses occurring in elastic-plastic problems with spherical or cylindrical symmetry can be represented by uniform expressions containing integrals over the radius with the equivalent plastic strain as a factor of the integrand. These terms, which may be interpreted as compatibilizing stresses, are closely related to the residual stresses.     

Improvement of mechanical properties in severely plastically deformed Ni–Cr alloy

This study was carried out to evaluate the grain refining and mechanical properties in alloys that undergo severe plastic deformation (SPD). Conventional rolling (CR) and cross-roll rolling (CRR) were introduced as methods for SPD, and a Ni–20Cr alloy was selected as the experimental material. The materials were cold rolled to 90% thickness reduction and subsequently annealed at 700 °C for 30 min to obtain the fully recrystallized microstructure. The annealed materials after cold rolling were assessed through electron backscattered diffraction (EBSD) analysis to investigate the grain boundary characteristic distributions (GBCDs). The CRR process was more effective than the CR process in developing grain refinement; the grain size decreased from 70 μm in the initial material to 4.2 μm (CR) and 2.4 μm (CRR), respectively. The grain refinement affected mechanical properties such as microhardness, yield, and tensile strength, which were significantly increased relative to the initial material.     

Workability characteristics and mechanical behavior modeling of severely deformed pure titanium at high temperatures

In the present study, compression tests were performed at temperatures of 600–900 °C and at strain rates of 0.001–0.1 s⁻¹ to study the deformation and workability characteristics of commercially pure titanium after severe plastic deformation (SPD). It was found that the effects of temperature and strain rate are significant in dictating the steady state flow stress levels and the strain values corresponding to peak flow stress. The strain rate sensitivity (m) during hot compression of severely deformed Ti was shown to be strongly temperature dependent, where m increased with the increase in deformation temperature up to 800 °C. High temperature workability was analyzed based on the flow localization parameter (FLP). According to the FLP values, deformation at and below 700 °C is prone to flow localization. The flow behavior was predicted using Arrhenius type and dislocation density based models. The validities of the models were demonstrated with reasonable agreement in comparison to the experimental stress–strain responses.     

Vabrations and permanent displacements of a pin-ended beam deformed plastically by short pulse excitation

The paper discusses and illustrates anomalous computed response of a structure to pulse loading, in which plastic deformations in the first phase of response lead to compressive forces in the subsequent motion. In special circumstances, phenomena related to dynamic buckling arise. These strongly affect the elastic vibrations and permanent displacements. They are implicated in large disagreements observed between solutions by different computer codes for the response of a pin-ended beam to a transverse pressure pulse. This paper treats these questions partly by study of the vibration properties of a simpler (Shanley) model, as affected by dynamic plastic instabilities; and partly by examining the role of finite element modelling in the uniform beam problem.     

Hot deformed anisotropic nanocrystalline NdFeB based magnets prepared from spark plasma sintered melt spun powders

Anisotropic magnets were prepared by spark plasma sintering (SPS) followed by hot deformation (HD) using melt-spun powders as the starting material. Good magnetic properties with the remanence J_r > 1.32 T and maximum of energy product (BH)_max > 303 kJ/m³ have been obtained. The microstructure evolution during HD and its influence on the magnetic properties were investigated. The fine grain zone and coarse grain zone formed in the SPS showed different deformation behaviors. The microstructure also had an important effect on the temperature coefficients of coercivity. A strong domain-wall pinning model was valid to interpret the coercivity mechanism of the HDed magnets. The increase of stray field and weakening of domain-wall pinning effects were the main reasons of the decrease of the coercivity with increasing the compression ratio. The influences of non-uniform plastic deformation on the microstructure and magnetic properties were investigated. The polarization characteristics of HDed magnets were demonstrated. It was found out that the HDed magnets had better corrosion resistance than the counterpart sintered magnet.     

In-plane anisotropic strain of elastically and plastically deformed III-nitrides on lithium gallate

We have investigated both elastically and plastically deformed GaN films on lithium gallate, LiGaO₂, by molecular beam epitaxy. The in-plane lattice parameters were determined from high resolution X-ray diffraction and indicated two different groups of in-plane lattice parameters, influenced by the a- and b-axis of LiGaO₂. The measured in-plane lattice parameters indicate that there exist both compressive and tensile strains of in-plane GaN along the a- and b-axis of LiGaO₂, respectively. This anisotropic strain in GaN films forms a slight distortion of the basal-plane hexagonal structure of GaN films, leading to a different critical thickness of 4.0 ± 0.17 and 7.8 ± 0.7 nm along the a- and b-axis of LiGaO₂, respectively.     

Ion implantation effect on atomic structure of deformed Si, Ir, W, Ni and Cu

It has been revealed, that in Ir subjected to severe plastic deformation, an ultrafine grained structure (UFG) is formed (the grain size of 20-30 nm). Practically no defects have been detected within the grains, while, in the case of Ar⁺ implantation, the subgrain structure with characteristic sizes of about 3-5 nm is formed; defects have been detected within subgrains.The subgrain structure was also revealed in UFG Ni and Cu after severe plastic deformation (SPD) (subgrain size of 3-15 nm), but in the latter case the observed boundary region is broader and subgrain is highly disoriented.     

Decomposition of deformed α′(α″) martensitic phase in Ti–6Al–4V alloy

ABSTRACT

The process of martensitic α′(α″) phase decomposition in titanium alloys has not been sufficiently characterised in the literature – especially in terms of plastically deformed martensite. The research results of water-quenched Ti–6Al–4V alloy, subsequently cold deformed in compression test and tempered at the temperature range of 600–900°C for 1 and 2?h were presented in the paper. Light and scanning electron microscopy observations revealed the influence of plastic deformation on tempered martensite laths morphology – particularly at the temperature of 900°C – it favoured their fragmentation and spheroidisation. The effect of plastic deformation on characteristic temperatures of α′(α″)→α?+?β phase transformation, phase composition and alloying elements distribution in phase constituents of Ti–6Al–4V alloy was identified and evaluated too.

This paper is part of a thematic issue on Titanium.     

Enhanced grain refinement and mechanical properties in a severely plastic deformed Ni-30Cr alloy

The present study was carried out to evaluate the microstructures and mechanical properties of severely deformed Ni-30Cr alloys. Cross-roll rolling (CRR) was introduced as a severe plastic deformation (SPD) process. Ni-30Cr alloy sheets were cold rolled to 90% thickness reduction and subsequently annealed at 700 °C for 30 min to obtain the recrystallized microstructure. Electron back-scattered diffraction (EBSD) was introduced to analyze grain boundary character distributions (GBCDs). The application of CRR to the Ni-30Cr alloy effectively enhanced grain refinement through heat treatment; consequently, the average grain size was significantly refined from 33 μm in the initial material to 0.6 μm. This grain refinement directly improved the mechanical properties, in which yield and tensile strengths significantly increased relative to those of the initial material. We systematically discuss the grain refinement and accompanying improvement in mechanical properties in terms of the effective strain imposed by CRR relative to conventional rolling (CR).     

New aspects of the β–α polymorphic transition in plastically deformed isotactic polypropylene studied by microindentation hardness

The β–α polymorphic transition in plastically deformed isotactic polypropylene (iPP) was characterized by means of microindentation hardness. For this purpose microindentations were mapped onto the surface of the necking zone of a tensile loaded injection molded β-iPP “dumb-bell” specimen. Results evidence a sharp decrease of the H-values instead of the expected H-increase due to the β–α polymorphic transition. Far away from the necking zone an H-increase is detected. It is shown that the destruction of the starting isotropic spherulitic structure and the decrease of crystallinity in the necking zone gives rise to lower H-values. However, at larger distances from the neck, the emerging fibre structure induces a better chain orientation that results in a slight H-increase. Analysis of the isotropic and necked samples before and after their annealing using DSC and WAXS supports the assumption regarding the role of the microvoids in decreasing the hardness value.