Fatigue strength of a magnesium MA2-1 alloy after equal-channel angular pressing

The fatigue strength of a magnesium MA2-1 alloy is studied after annealing and equal-channel angular pressing (ECAP). The ultrafine-grained structure formed upon ECAP is shown to increase the plasticity of the material during static tension, to decrease the cyclic life to failure, and not to decrease the fatigue limit. The mechanisms of crack nucleation and growth during cyclic deformation are investigated.     

Structure and fatigue properties of 08Kh18N10T steel after equal-channel angular pressing and heating

The structure of corrosion-resistant austenitic 08Kh18N10T steel is studied after equal-channel angular pressing (ECAP), heating, and subsequent cyclic tests. After ECAP, an oriented mainly subgrain structure with a structural element size of 100–250 nm and a high fraction of deformation twins forms in the austenite of the steel, and 42 vol % of lath martensite appears. Dynamic twinning, martensitic transformation, dynamic recovery, and even recrystallization take place in the 08Kh18N10T steel during cyclic deformation in the course of fatigue tests according to the scheme of repeated tension. The fatigue strength increases after ECAP due to the refinement and twinning of an austenite structure and the appearance of martensite. The fatigue limit is maximal after ECAP and heating at 550°C for 20 h due to a high annealing twin density in a predominantly austenitic recrystallized matrix, intense dynamic twinning, and martensitic transformation during cyclic deformation.     

ECAP of commercially pure titanium: A review

Equal Channel Angular Extrusion (ECAE) also called as Equal Channel Angular Pressing (ECAP) is an emerging mechanical or thermo mechanical method for synthesis of bulk ultra fine grained or nano materials. The uniqueness of ECAP is that the fine grains are obtained without changing any of the dimensions of the sample. The grain refinement increases the strength of CP-Ti to the strength levels of Ti-6Al-4V, a commonly used material for bio implants. Though Ti-6Al-4V alloys satisfy the biomedical requirements, the Al and V are toxic to human tissue. ECAP is an attracting technique for strengthening commercially pure titanium (CP-Ti) to a level of Ti-6Al-4V since CP-Ti has better compatibility for bio medical applications. Hence, the research is focused on ECAP of CPTi. This overview mainly focuses on the mechanical properties, corrosion resistance, wear resistance, fatigue resistance and the influence of external and internal parameters on the properties of ECAPed CP-Ti. It also highlights the methods employed for increasing the deformability of CP-Ti. Finally, the suitability of ECAP for industrial production is also discussed. The state of the art in this field is encouraging and showing positive signs of commercializing ECAP of CP-Ti in the near future.     

Fatigue-Property Enhancement of Magnesium Alloy,AZ31B,through Equal-Channel-Angular Pressing

The fatigue behavior of magnesium-alloy, AZ31B, prestrained by equal-channel-angular pressing (ECAP) was studied as a function of the accumulated plastic-strain level and the orientation of the samples (along and perpendicular to the ECAP pressing direction). The material was processed via route B _C , at 200 °C, for 1, 2, and 8 passes, with and without a back pressure (BP) applied on the billet during ECAP. The low-cycle fatigue behavior of the AZ31B alloy is shown to be anisotropic and texture dependent. Due to the initial texture orientation, the specimens loaded parallel to the ECAP pressing direction have a longer fatigue life than the samples loaded perpendicular to it. The low-cycle fatigue life of the AZ31B alloy is enhanced by ECAP. The fatigue-property improvement is discussed in light of the grain-size refinement, enhanced ductility, and texture evolution. This article is based on a presentation given in the symposium entitled “Deformation and Fracture from Nano to Macro: A Symposium Honoring W.W. Gerberich’s 70th Birthday,” which occurred during the TMS Annual Meeting, March 12–16, 2006 in San Antonio, Texas and was sponsored by the Mechanical Behavior of Materials and Nanomechanical Behavior Committees of TMS.     

Characterization of AA6061 Alloy Processed by Equal Channel Angular Pressing and Subjected to Low Cycle Fatigue

Aluminum alloy 6061 was subjected to equal channel angular pressing (ECAP) using two different processing routes B_C and C, to study the evolution of the microstructure and the effect of low cycle fatigue (LCF) on the resultant microstructure. Specimens subjected to ECAP and fatigue cycling were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD patterns of the material after each pass of ECAP and after interrupted LCF tests were analyzed. Single-line approximation method of analysis was used to obtain microstructural parameters from peak broadening observed in XRD profiles. Increase in dislocation density till saturation after pass 2 and marginal changes thereafter for successive passes were observed. From the LCF tests on specimens subjected to three ECAP passes at two strain amplitudes of 0.5 and 1.0 %, cyclic stress response up to fatigue failure were obtained. The solutionized specimens exhibited continuous strain hardening at both strain amplitudes. The ECAP processed material fatigued at 0.5 % strain amplitude exhibited stable cyclic stress response, whereas the material fatigued at 1.0 % strain amplitude exhibited cyclic softening. The LCF behaviour was the same for the material processed through both B_C and C routes. The TEM images of specimens and the associated selected area electron diffraction patterns indicated ultra fine grain structure after three passes of ECAP. However, some amount of grain coarsening was observed after LCF cycling.     

Fatigue and Tensile Behavior of Cast,Hot-Rolled,and Severely Plastically Deformed AZ31 Magnesium Alloy

The work focuses on experimental examination of the fatigue behavior of magnesium alloy AZ31 produced by three different procedures: squeeze casting (SC), hot rolling (HR), and equal-channel angular pressing (ECAP). The microstructures produced were studied by light and transmission electron microscopy (TEM). Squeeze-cast AZ31 had low porosity and coarse grains, while hot-rolled material showed microstructure with grain size of 3 to 20 μm. The finest grain structure with the average grain size of about 1 to 2 μm was found in the material pressed 4 times at 200 °C using the ECAP technique, route B _c . It was shown that low- and high-cycle fatigue behavior under symmetric loading at room temperature and with loading frequency of 20 Hz is strongly dependent on the technique employed in producing the alloy. The ECAP was shown to improve the fatigue life of the material in the low-cycle region over that of the squeeze-cast material. However, the fatigue life of AZ31 after ECAP was slightly lower than that of the hot-rolled material. In the high-cycle region, the hot-rolled material and the material that underwent ECAP exhibit the same fatigue strength, which is superior to that of the squeeze-cast alloy. Fatigue crack initiation and the character of fracture were examined by means of scanning electron microscopy. This article is based on a presentation made in the symposium entitled “Ultrafine-Grained Materials: from Basics to Application,” which occurred September 25–27, 2006 in Kloster Irsee, Germany.

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等径角挤压变形奥氏体不锈钢的组织演变

 用实验方法研究了奥氏体不锈钢在等径角挤压冷变形(路径RC)过程中组织变化。实验结果表明：当剪切方向与孪晶带方向成一定角度时，在剪切力的作用下，孪晶逐渐由大块孪晶→由剪切带分割的孪晶(楼梯状)→小块状→奥氏体亚晶或马氏体晶粒；部分孪晶在剪切力作用下，剪切带可直接碎化成具有大角度位向差的细小晶粒(奥氏体亚晶+马氏体晶粒)，可发生马氏体相变；当剪切方向与孪晶带方向相同时，孪晶带区域也可发生马氏体转变；3道次变形后，具有明显特征的孪晶已很少，此后继续进行剪切变形，孪晶碎化组织(含马氏体)和奥氏体剪切滑移带(含碎化晶粒)的变形以剪切滑移方式进行，当奥氏体的滑移遇到阻力时，可局部形成局部形变孪晶来协调变形；随变形道次的增加，马氏体转变也越多，在多次剪切以及道次中的交叉滑移作用下，马氏体板条逐渐被高密度位错墙分割而碎化成细小的晶粒；8道次变形后，可获得60~230 nm的等轴晶粒。     

Microstructure and Wear Performance of ECAP Processed Cast Al–Zn–Mg Alloys

In the present investigation, wear performance of equal channel angular pressing (ECAP) processed cast Al–Zn–Mg alloys under dry sliding wear conditions was studied against a steel disc. Initially, Al–Zn–Mg alloys (with 5, 10, 15% zinc and 2% magnesium) were ECAP processed. After ECAP, grain size was reduced and enhancement in the hardness was observed. Wear resistance of the alloys increased after ECAP processing. Wear resistance of the alloys also increased when the quantity of the zinc was increased in the alloys. But, wear resistance of all three alloys decreased with increase in the load and the sliding speed. Coefficient of friction of the alloys decreased after ECAP processing. Coefficient of friction of the alloys also decreased when the quantity of the zinc was increased in the alloys. Coefficient of friction of all three alloys increased with increase in the load and the sliding speed. Irrespective of the alloy composition and applied load, worn surfaces of the cast and homogenized samples were composed of plastic deformation, scratches and micro-ploughing. On the other hand, in ECAP processed samples, morphology of the worn surfaces depended on the applied load. Abrasive wear is the main wear mechanism perceived in cast and homogenized samples at all loads. While in ECAP processed samples, the wear mechanism shifted from adhesive and oxidation wear to abrasive wear with increase in the load. Formation of oxide layers on the surface of the sample increased with increase in the ECAP passes. In ECAP processed samples, transfer of iron content from the disc to the sample surface was identified.     

Microstructure of Equal-Channel Angular Pressed Cu and Cu-Zr Samples Studied by Different Methods

Polycrystalline samples of technical-purity Cu (99.95 wt pct) and Cu with 0.18 wt pct Zr have been processed at room temperature by equal-channel angular pressing (ECAP). The microstructure evolution and its fragmentation after ECAP were investigated by transmission electron microscopy (TEM), electron backscattered diffraction (EBSD), positron annihilation spectroscopy (PAS), and by X-ray diffraction (XRD) line-profile analysis. The first two techniques revealed an increase in the fraction of high-angle grain boundaries (HAGBs), with increasing strain reaching the value of 90 pct after eight ECAP passes. The increase was more pronounced for pure Cu samples. The following two kinds of defects were identified in ECAP specimens by PAS: (1) dislocations that represent the dominant kind of defects and (2) small vacancy clusters (so-called microvoids). A detailed XRD line-profile analysis was performed by the analysis of individual peaks and by total profile fitting. A slight increase in the dislocation density with the number of ECAP passes agreed with the PAS results. Variations in microstructural features obtained by TEM and EBSD can be related to the changes in the XRD line-broadening anisotropy and dislocation-correlation parameter.     

Densification of Al-2024 and Al-2024/Al2O3 Powders by Conventional P/M Route and ECAP: A Comparative Study

In this study, mechanically alloyed Al-2024 and Al-2024/Al₂O₃ powders are densified by conventional sintering and by equal channel angular pressing (ECAP) with and without back pressure. The powder was encapsulated in an aluminium can for consolidation through ECAP. The properties obtained in the compacts by conventional sintering route and by ECAP are compared. The effect of conventional sintering and ECAP on consolidation behaviour of powder, microstructure, density and hardness is discussed. Room temperature back pressure aided ECAP results in nearly full denser (97?% of its theoretical density) compact at room temperature. Nano Indentation technique was used to determine the modulus of the consolidated compacts.     

ECAP制备TWIP钢的力学性能研究

采用ECAP方法对TWIP钢(30Mn-3Si-3A1)试样进行一道次等径角挤压(ECAP)变形,对比研究原始态、一道次挤压态、ECAPlP+850℃×1h(空冷)处理和ECAPlP+1000℃×1h空冷处理后的微观结构及力学性能.试验结果表明:在变形过程中,形变孪晶的相互形变阻力和位错在形变孪晶界的大量塞积,使TWI...     

等通道转角挤压超细晶材料技术及其发展

通过金属的大塑性应变细化晶粒从而提高材料综合性能已在材料加工领域得到广泛应用。近年来为获得材料的亚晶组织已经出现了许多大变形技术。等通道转角挤压(ECAP)是块体材料得到超细晶粒的一种新的材料加工方法,因此受到材料研究者的广泛关注。介绍了等通道转角挤压的基本原理以及主要参数对变形的影响,分析了此加工方法对材料组织与性能的影响,讨论了其在材料加工研究中的应用。     

Microstructure and refining performance of an Al-5Ti-0.25C refiner before and after equal-channel angular pressing

Refining experiments were conducted to evaluate the grain refining performance of an Al-5Ti-0.25C refiner before and after equal-channel angular pressing (ECAP) with the use of a high-purity Al. The results show that the Al-5Ti-0.25C refiner has remarkable and stable grain refining performance when the holding times are within 5 to 30 minutes and the melt temperatures are within 1003 to 1073 K. Furthermore, some Al-5Ti-0.25C refiner samples were subjected to severe plastic deformation by using the ECAP technique at 298 K. It was found that Al₃Ti and TiC particles were significantly fragmented and their mean sizes were decreased to 10 and 1.08 μm, respectively, and the Al-5Ti-0.25C refiner appeared to have a double grain refining effect in comparison with that of before ECAP. It is also testified that the Vickers microhardness (Hv) value of the pure Al samples refined by the Al-5Ti-0.25C refiner after ECAP processing has a significant increment than that of before ECAP processing. It is concluded that the Al-5Ti-0.25C refiner with ECAP technique has a very useful practical application in refining industrial Al alloys.     

Texture of the Mg-0.49% Al-0.47% Ca alloy after equal-channel angular pressing

Equal-channel angular pressing (ECAP) is used to refine grains and to change the texture of the initial pressed Mg-0.49% Al-0.47% Ca alloy rod in order to study the possibility of increasing the low-temperature ductility of the alloy. ECAP is performed at 300°C in six passes at a total true logarithmic strain ε = 6.8 according to route B _C . As a result, an ultrafine-grained structure with a grain size of 2–5 μm forms. The initial texture of the pressed rod is characterized by the [12 11] axial orientation parallel to the pressing direction. After ECAP, the texture changes its type and is characterized by a set of preferred orientations that represent basal planes located at an angle of 40°–50° with respect to the pressing direction. An analysis of the generalized Schmid factors, which were calculated for the main operating deformation systems with allowance for the critical shear stresses in them and the volume fractions of the preferred orientations, indicates that the texture caused by ECAP affects the decrease in the strength properties of the alloy measured at room temperature and the increase in the low-temperature ductility of the alloy.     

Effect of equal-channel angular pressing and annealing conditions on the texture,microstructure, and deformability of an MA2-1 alloy

Equal-channel angular pressing (ECAP) of am MA2-1 alloy according to routes A and Bc is used to study the possibility of increasing the low-temperature deformability of the alloy due to grain refinement and a change in its texture. To separate the grain refinement effect from the effect of texture on the deformability of the alloy, samples after ECAP are subjected to recrystallization annealing that provides grain growth to the grain size characteristic of the initial state (IS) of the alloy. Upon ECAP, the average grain size is found to decrease to 2–2.4 μm and the initial sharp axial texture changes substantially (it decomposes into several scattered orientations). The type of orientations and the degree of their scattering depend on the type of ECAP routes. The detected change in the texture is accompanied by an increase in the deformability parameters (normal plastic anisotropy coefficient R, strain-hardening exponent n, relative uniform elongation δ_u) determined upon tensile tests at 20°C for the states of the alloy formed in the IS-4A-4Bc and IS-4Ao-4B_cO sequences. The experimental values of R agree with the values calculated in terms of the Taylor model of plastic deformation in the Bishop-Hill approximation using quantitative texture data in the form of orientation distribution function coefficients with allowance for the activation of prismatic slip, especially for ECAP routes 4B_c and 4B_cO. When the simulation results, the Hall-Petch relation, and the generalized Schmid factors are taken into account, a correlation is detected between the deformability parameter, the Hall-Petch coefficient, and the ratio of the critical shear stresses on prismatic and basal planes.     

Densification of Al-Y<Subscript>2</Subscript>O<Subscript>3</Subscript> composite powder by equal channel angular pressing

In this study, equal channel angular pressing (ECAP) with and without back pressure was used for consolidation of fine mechanically alloyed Al-Yttria composite powder. The effect of ECAP on consolidation behaviour of powder, microstructure, density and hardness is discussed. The powder was encapsulated in an aluminium can. Room temperature back pressure aided ECAP results in a nearly full denser (97% of its theoretical density) compact at room temperature. Nano Indentation technique was used to determine the modulus of the ECAP consolidated compact.     

Microstructure Evolution and Mechanical Properties of Al-1080 Processed by a Combination of Equal Channel Angular Pressing and High Pressure Torsion

Equal channel angular pressing (ECAP) and high pressure torsion (HPT) are the most promising severe plastic deformation (SPD) methods. Both methods impose very high strains, leading to extreme work hardening and microstructural refinement. In this paper, billets of Al-1080 were successfully processed by ECAP conducted for up to 10 passes, HPT at an applied pressure of 8 GPa for 5 revolutions, and a combination of ECAP and HPT (ECAP + HPT) at room temperature. The effects of the different SPD processes (ECAP, HPT, and ECAP + HPT) on the evolution of the microstructure and mechanical properties of Al-1080 were investigated. The HPT and ECAP + HPT processes were observed to produce finer grain sizes with greater fractions of high angle grain boundaries (HAGBs) than the ECAP alone. Although the grain sizes after HPT and ECAP + HPT were similar, the ECAP + HPT sample had more dislocations than the HPT sample. HPT after ECAP enhanced the mechanical properties (hardness, tensile strength, and ductility) of the ECAP-processed Al-1080, showing larger dimple size in the tensile fracture surfaces.     

The microstructural evolution during the equal channel angular pressing process and its relationship with the tensile behavior of oxygen-free copper

In the present study, the tensile behavior of nanograin-sized, pure copper produced by the equal channel angular pressing (ECAP) process, which is at present one of the most popular methods for producing nanograined bulk material, was examined as related to the microstructural evolution. It was found that the yield and tensile strength values of 99.99 pct pure, oxygen-free copper increased with the increasing number of ECAP cycles due to the strain hardening in the initial stage. Further ECAP process promoted the formation of equiaxed grains accompanied with the gradual decrease in dislocation density. Once the equiaxed grain formed, the decrease in dislocation density would be further accelerated due to the extremely high rate of dynamic recovery with the grain boundary area acting as a dislocation sink. The strain hardening mechanism would then stop to operate and the fine grain boundary hardening mechanism would begin to dominate after the forth cycle of the ECAP process, resulting in an increase in the tensile ductility without sacrificing the strength. This research was supported by Grant No. 03K1501-00310 from the Center for Nanostructured Materials Technology under the 21st Century Frontier R&D Programs of the Ministry of Science and Technology, Korea.     

热加工对铸造AM50镁合金显微结构和力学性能的影响

采用锻造和等通道转角挤压(ECAP)等技术研究了热加工对铸造AM50镁合金显微结构和力学性能的影响, 以改善该合金的力学性能.结果发现, ECAP对铸造AM50镁合金和锻造AM50镁合金两种显微结构的影响不同, 这是由于两种状态初始晶粒尺寸不同引起的.铸态AM50镁合金晶粒尺寸粗大, 经过ECAP工艺后, 晶界上出现大量平直滑移线;而锻态AM50镁合金经过ECAP工艺后, 晶粒进一步细化, 滑移线痕迹不明显.铸态AM50镁合金经过ECAP工艺后显微硬度从54.5提高到72.3, 锻造AM50镁合金经过ECAP工艺后显微硬度从60.3提高到81.9.铸造AM50镁合金经过锻造及ECAP工艺热加工后力学性能抗拉强度提高到320 Mpa, 同时延伸率保持在35%以上.     

Effect of Equal Channel Angular Pressing on the Fracture Behavior of Commercially Pure Titanium

This article focuses on the effect of equal channel angular pressing (ECAP) on the fracture behavior of a commercially pure (CP) Ti. Standard mechanical tensile and fracture tests are performed at room temperature and 250 °C to determine the tensile mechanical properties and the J-∆a curves. The effect of temperature on the fracture properties, namely, the fracture toughness, the slope of the J-∆a curves, the total crack growth resistance, and the stability of the crack propagation before and after ECAP is investigated. Quantitative analysis of the fracture surfaces is performed using an automatic fracture surface analysis system. The energy required to form the fracture surface is estimated. The yield stress and the ultimate tensile stress are significantly increased by ECAP. However, similar to the case of other strengthening procedures, the fracture toughness decreases after ECAP.