Stored energy of a severely deformed interstitial free steel

A Ti-stabilised IF steel subjected to room temperature equal channel angular pressing (ECAP) for 8 passes, route B_C was further cold rolled to 25, 50 and 95% thickness reductions. The evolution of bulk stored energy (350–600 J mol⁻¹) and the associated thermal behaviour was investigated by differential scanning calorimetry (DSC). Local stored energy (5–140 J mol⁻¹) was measured using microhardness, electron back-scattering diffraction (EBSD) and X-ray line profile analysis. The higher stored energy values via calorimetry correspond to energy release from all sources of strain in the material volume as well as Ti precipitation during annealing. An apparent activation energy of 500–550 J mol⁻¹ suggests sluggish recrystallisation due to excess Ti in solid solution.     

Grain refinement in low SFE and particle-containing nickel aluminium bronze during severe plastic deformation at elevated temperatures

The influence of particle size and morphology on grain refinement in low stacking fault energy(SFE)alloys was studied by comparing the grain structures in single-and multi-phase Al-bronze(AB)alloys following equal channel angular pressing(ECAP)between 350 and 500℃.In particular,nickel aluminium bronze(NAB)was chosen as it contained both coarse and fine rounded particles,as well as a lamellar phase which evolved during ECAP.Grain refinement in the single-phase alloy was achieved through dynamic recrystallisation initiated at deformed twin boundaries.By contrast,different mechanisms were observed in the particle-containing NAB.Recrystallisation around the coarse κⅡ particles(～5 μm)was promoted through particle stimulated nucleation(PSN),whereas recrystallisation in the region of the fine κⅣ(～0.4μm)was delayed due to the activation of secondary slip.Grain refinement in areas of the lamellar κⅢ showed significant variation,depending on the lamellar orientation relative to the shear plane of ECAP.As the lamellae deformed,numerous high angle grain boundaries were generated between fragments and served as nucleation sites for recrystallisation,while PSN occurred around spheroidised lamellae.The spreading of the κⅢ particles by ECAP then enhanced the total area of recrystallised grains.     

An experimental verification of the finite element modelling of equal channel angular pressing

Strain hardening of pure copper and friction between the sample and die channels is considered for finite element modelling. To validate the FEM results, the FEM calculated effective strain variations were compared with the hardness measurements. Simulated load–stroke curve and peak load calculations were also compared with the experimentally recorded load–stroke curve and peak load. Different stages of the load–stroke curve of the ECAP process was explained in detail. In over all, good conformity is observed between the FEM calculations and experimental results.     

Microstructure control during severe plastic deformation of Al-Cu-Li and the influence on strength and ductility

The microstructure and mechanical behaviour of an Al-Cu-Li alloy has been examined after processing over a wide range of conditions involving severe deformation at elevated temperatures with prior or subsequent heat treatments. Dislocation cellular or subgrain structures are obtained, with varying degrees of precipitation. High strength, as well as poor ductility, can be correlated with the presence of a high density of fine T₁ phase precipitates, with the dislocation substructure playing a smaller role. Careful control of processing conditions allows a suitable combination of good strength with ductility to be obtained.     

Microstructures and mechanical deformation behaviors of ultrafine-grained commercial pure (grade 3) Ti processed by two-step severe plastic deformation

Ultrafine-grained (UFG) commercial pure (CP, grade 3) Ti was produced using two-step severe plastic deformation, eight passes equal channel angular extrusion (ECAE) and cold rolling (CR) at liquid nitrogen temperature (LNT). Microstructural evolution and mechanical behaviors of UFG CP-Ti have been systematically investigated. After eight passes ECAE, the grain size was refined to sub-micron scale, smaller than 0.5 μm. Subsequent CR at LNT or RT for both UFG and coarse-grained (CG) specimens led to further refinement of structure, dramatically intensifying (0 0 0 2) peak, and the preferred orientation along the (0 0 0 2) crystal plane is formed at the expense of other crystal plane. After eight passes ECAE and CR at LNT, the ultimate tensile strength of UFG CP-Ti (grade 3) is 1218 MPa, and an elongation of 12.6%. Strain hardening behaviors of UFG CP-Ti (grade 3) during tensile deformation at RT have been analyzed.     

Plastic deformation characteristics of cross-equal channel angular pressing

For the first time, the plastic deformation characteristics of cross-equal channel pressing (cross-ECAP), a modified equal channel angular pressing, using a cross-shaped channel instead of a conventional l-shaped channel, was analyzed by using finite element analysis. The deformation in the cross-ECAP is more complicated and the strain induced is much more severe than that in the conventional ECAP. However, the plastic strain is localized in the central linear region of the workpiece and very small strain is developed in the edge regions, which is in good agreement with experimental results in the literature showing nonuniformity in microstructure and hardness distribution. The load requirements of cross-ECAP are much higher in comparison to conventional ECAP and T-ECAP processes.     

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.     

Dependencies of grain refinement on processing route and die angle in equal channel angular extrusion of bcc materials

The efficiency of grain refinement in equal channel angular extrusion of body-centered cubic (bcc) materials is investigated based on slip activities from crystal plasticity simulations, which account for both the macroscopic and crystallographic features of deformation. It is shown that the characteristics of slip activities, especially the relative contributions of slip systems newly activated or reversed at the transitions between successive passes, vary significantly with the processing routes (A, B and C) and die angles ( = 90° and 120°). The simulations assuming {1 1 0}111 slip suggest that routes B and A lead to the most significant contributions of newly activated slip systems and hence are most efficient for grain refinement with  = 90° and 120°, respectively. Further incorporation of {1 1 2}111 slip systems leads to the highest efficiency by route B for both die angles. These predictions are in partial agreement with experimental observations in the literature. Comparison of these results with those of face-centered cubic materials reveals the relevance of crystal structure and deformation mechanism during grain refinement.     

Microstructural and mechanical properties of commercially pure aluminum subjected to Dual Equal Channel Lateral Extrusion

This paper aims to provide an introductory insight about “Dual Equal Channel Lateral Extrusion”, a counterpart of “Equal Channel Angular Extrusion”. The process is implemented to severely refine the microstructure of aluminum slabs. Comparisons of macroscopic parameters as average straining and large scale distribution of strain, as well as process loads reveal the supremacies and short comings of DECLE with respect to ECAE. DECLE shares a relatively similar geometry of deformation with that of ECAE. The advantages of this process with respect to ECAE are: (i) more intensive strains attainable per pass, and (ii) less extruding power needed for a given sample size. Nonetheless, less homogeneous strain per pass is seen in case of DECLE. TEM inspections revealed remarkable refinement of the microstructure through out the process and also some recrystallization at the final passes. Hardness tends to increase through successive passes to a limiting value beyond which there appeared a decline associated with intense recovery and the recrystallization observed. Compression tests exhibit the same trend, viz. a general rise in strength followed by a decrease in work hardening with increasing number of passes, leading to uniform microstructure and hardness after 9 passes.     

Effect of initial microstructure on the processing of titanium using equal channel angular pressing

Equal channel angular pressing (ECAP) is a metal processing technique that is used to produce materials with ultrafine (<1 μm) grain sizes. In this work, the effect of the initial microstructure on ECAP of commercially pure titanium (CP Ti), a material used in many industrial applications, was investigated. To produce different initial microstructures, samples of CP Ti were exposed to different annealing conditions: no annealing (Material 1), annealed at 1033 K for 2 hr (Material 2), or annealed at 1173 K for 4 hr (Material 3). Each material was subjected to one pass of ECAP and the resulting microstructures were analyzed using XRD, SEM, and TEM, and compared to the microstructures before ECAP. It was found that each material developed a unique microstructure after one pass of ECAP, which was attributed to the varying microstructural characteristics before ECAP. Microhardness values before and after ECAP varied with each microstructure.     

Microstructure,mechanical and degradation properties of equal channel angular pressed pure magnesium for biomedical application

Abstract

Magnesium is a biocompatible and biodegradable metal, which has attracted much interest in biomedical engineering. Pure magnesium shows the low strength and plasticity at ambient temperature. Microstructure, mechanical properties and degradation properties of the equal channel angular pressed pure magnesium have been investigated for biomedical application in detail by optical microscopes, mechanical properties testing and corrosion testing. The results have revealed that the processing temperature and routes are important factors that affect the properties of pure Mg by equal channel angular pressing. The two-step equal channel angular pressing processing (one pass at 360°C and three passes at 200°C) has been successfully applied to control the microstructure, mechanical and degradation properties of the pure Mg. Optical microscopy observation has indicated that the grain size of the as cast pure magnesium has been significantly decreased after equal channel angular extrusion, which has mainly contributed to the high tensile strength and good elongation. Equal channel angular pressed pure magnesium has provided moderate corrosion resistance, which has opened a new window for materials design, especially for biomedical.     

Finite element analysis of the effect of the inner corner angle in equal channel angular pressing

The inner corner angle (ICA) is one of the major factors affecting deformation homogeneity in workpieces during equal channel angular pressing (ECAP). In this study, the effect of the ICA on the plastic deformation behavior in ECAP was investigated using the finite element method. A round ICA induces highly inhomogeneous deformation in the head, tail, top and bottom regions of the workpiece due to increasing compressive and decreasing shear deformation components. It was found that a round inner corner with an angle up to 9° is acceptable in finite element simulations for reproducing a sharp inner corner. These results can serve as a design guide for processing and dies of ECAP.     

ECAPT过程中载荷变化规律的数值模拟

为了掌握纯铝在ECAPT过程中的力学变化规律,利用刚塑性有限元技术对纯铝1100的ECAPT变形行为进行数值模拟,重点分析了载荷在挤压过程中的变化规律及产生变化的原因。结果表明:ECAPT过程大致分为载荷骤升、2次小幅度升高和载荷下降4个阶段;载荷峰值随着摩擦因数的增大迅速增大,应采取有效的润滑措施减小摩擦从而降低载荷峰值,提高模具寿命;随着摩擦因数的增大,载荷在达到各阶段峰值后下降趋势越来越明显。     

Effect of friction model in numerical analysis of equal channel angular pressing process

During the last decade or so there has been a tremendous growth in the research and development of equal channel angular pressing (ECAP) process which was originally proposed by Segal et al. Numerical analyses are being used extensively to evaluate the effect of various die design and process parameters in ECAP. Friction is one such important parameter. Coulomb and shear friction models have been used in the numerical analysis of ECAP process and contradicting results have been reported. This study evaluates the effect of coulomb and shear friction models on the deformation pattern, strain distribution and load requirement during ECAP process and suggests which friction model should be used in the numerical analysis of ECAP process.     

Effect of different processings on mechanical property and corrosion behavior in simulated body fluid of Mg-Zn-Y-Nd alloy for cardiovascular stent application

The biomagnesium alloys have been considered to be one of the most potential biodegradable metal materials due to its good mechanical compatibility, biological compatibility, biological security and biodegradable characteristics. However, the two major problems of high degradation rates in physiological environment and low mechanical properties prevent the development of biomagnesium alloys. In the present work, the samples of Mg-Zn-Y-Nd alloy were prepared by cyclic extrusion compression （CEC） and equal channel angular pressing （ECAP）. The microstructures, mechanical properties of alloy and its corrosion behavior in simulated body fluid （SBF） were evaluated. The results reveal that Mg-Zn-Y-Nd alloy consists of equiaxial fine grain structure with the homogeneous distribution of micrometer size and nano-sized second phase, which was caused by the dynamic recrystallization during the ECAP and CEC. The corrosion resistance of alloy was improved. The tensile and corrosion resistance were improved, especially the processed alloy exhibit uniform corrosion performances and decreased corrosion rate. This will provide theoretical ground for Mg-Zn-Y-Nd alloy as vascular stent application.     

Microstructure and mechanical properties of ZA62 Mg alloy by equal-channel angular pressing

The Mg-6Zn-2Al alloy was processed by ECAP and microstructure and mechanical properties of the alloy before and after ECAP were studied. The results revealed that the microstructure of the ZA62 alloy was successfully refined after two-step ECAP (2 passes at 473 K and 2-8 passes at 423 K). The course bulk interphase of Mg₅₁Zn₂₀ was crushed into fine particles and mixed with fine matrix grains forming “stripes” in the microstructure after the second step of ECAP extrusion. A bimodal microstructure of small grains of the matrix with size of ∼0.5 μm in the stripes and large grains of the matrix with size of ∼2 μm out of stripes was observed in the microstructure of samples after 4-8 passes of ECAP extrusion at the second step. The mechanical properties of the alloy studied were significantly improved after ECAP and the highest yield strength and elongation at room temperature were obtained at the samples after 4 and 8 ECAP passes at the second step, respectively. Tensile tests carried out at temperature of 473 K to 573 K and strain rate of 1 × 10⁻³ s⁻¹ to 3 × 10⁻² s⁻¹ revealed that the alloy after 8 ECAP passes at the second step showed superplasticity and the highest elongation and strain rate sensitivity (m-value) reached 520% and 0.45, respectively.     

Evolution of texture during equal channel angular extrusion of commercially pure aluminum: Experiments and simulations

The evolution of crystallographic texture has been comprehensively studied for commercially pure Al as a function of amount of ECAE deformation for the three major routes of ECAE processing. It has been observed that processing through different routes leads to different type of texture, in both qualitative as well as quantitative sense. The results have been analyzed on the basis of existing concepts on ECAE deformation and simulations have been carried out using the simple shear model of ECAE implemented into the Viscoplastic Self Consistent model of polycrystal plasticity. The simulations revealed that non-octahedral slip is needed to reproduce the experimental texture development.     

Improved properties of Ti-6Al-4V by combining isothermal equal-channel angular processing and hot-extrusion

The aim of this work was to study effects of hot extrusion on the microstructure of Ti-6Al-4V (wt-%) alloy processed by ECAP. Firstly, an isothermally Ti–6Al–4V alloy processed by Equal channel angular pressing(ECAP) was preheated at 950°C for 6?min and then hot extruded at 900°C. The hot extrusion minimised the grain size and maximised the mechanical strength. Therefore, it was demonstrated that hot extrusion of Ti-6Al-4V alloys that processed by ECAP could be performed without compromising any mechanical properties. Therefore, it is possible to use the ability to apply a reduced cross-section in hot extrusion for an Ti-6Al-4V processed by ECAP without concern about the reduction of properties.     

Strength and pore morphology of porous aluminum and porous copper with directional pores deformed by equal channel angular extrusion

Porous aluminum with a porosity of 17.6% and porous copper with a porosity of 39.7% (the pores of both aluminum and copper were cylindrical and oriented in one direction) were deformed by equal channel angular extrusion using a 150° die with sequential 180° rotations (route C), and the mechanical strength and pore morphology after the extrusions were investigated. In the case of porous aluminum with low porosity, the pores were collapsed by the extrusions that were both parallel and perpendicular to the orientation direction of the pores. In contrast, the porosity of porous copper decreased slightly after extrusions that were parallel to the orientation direction of the pores, and the pores thus remained even after four extrusions. The yield strength after the second extrusion was 7.3 times greater than it was before the extrusion, even though there was a decrease in porosity of only 8%. On the other hand, almost all the pores of the porous copper collapsed after the fourth extrusion, when the extrusion direction was perpendicular to the orientation direction of the pores. Thus, the yield stress cannot be enhanced without being accompanied by progressive densification.