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.     

Microstructures and properties of ultrafine-grained pure titanium processed by equal-channel angular pressing and cold deformation

Equal-channel angular pressing (ECAP) has been used to refine the grain size of commercially pure (CP) titanium as well as other metals and alloys. CP-Ti is usually processed at about 400 degrees C because it lacks sufficient ductility at lower temperature. The warm processing temperature limits the ability of the ECAP technique to improve the strength of CP-Ti. We have employed cold deformation following warm ECAP to further improve the strength of CP-Ti. Ti billets were first processed for eight passes via ECAP route Bc, with a clockwise rotation of 90 degrees between adjacent passes. The grain size obtained by ECAP alone is about 260 nm. The billets were further processed by cold deformation (cold rolling) to increase the crystalline defects such as dislocations. The strength of pure Ti was improved from 380 to around 1000 MPa by the two-step process. This article reports the microstructures, microhardness, tensile properties, and thermal stability of these Ti billets processed by a combination of ECAP and cold deformation.     

Effect of commercial purity levels on the mechanical properties of ultrafine-grained titanium

Two grades of commercial purity (CP) titanium (grades 2 and 4) were processed using equal-channel angular extrusion (ECAE) at 300 °C and 450 °C, respectively. The processing temperatures were the minimum temperatures at which eight pass ECAE could be performed without any shear-localization. The coarse-grained (CG) microstructures of as-received grade-2 and grade-4 CP-Ti, with average grain sizes of 110 μm and 70 μm, respectively, were refined down to sub-micron levels with a mean grain size of about 300 nm for both grades after 8 ECAE passes. The ultrafine-grained (UFG) microstructures led to substantial enhancement in strength for both grades. The grade-2 sample showed a more than two fold increase in yield strength (σ_y), from 307 MPa for the as-received one to about 620 MPa for the processed samples. The grade-4 CP-Ti exhibited a relatively smaller increase in strength due to the higher processing temperature, and it showed about 50% increase in σ_y after eight pass ECAE, from 531 to 758 MPa. These strength levels were obtained with high ductility levels of 21% and 25% for UFG grade-2 and grade-4 Ti, respectively. These improvements in mechanical properties are attributed to the substantially refined grain size and increased dislocation density. Grade-4 Ti is stronger than grade-2 because of the higher oxygen content. The higher ductility and significantly higher strain hardening capability of UFG grade-4 Ti, in spite of the similar grain size and microstructure with UFG grade-2 Ti, is also due to the higher impurity content, probably resulting in a higher dislocation storage capability during room temperature deformation, and thus, higher strain hardening capacity. Such properties make UFG grade-4 Ti comparable to the commercial Ti-6Al-4V alloy for biomedical applications without negative effects of the alloying elements on biocompatibility.     

Microstructure evolution and hardness variation during annealing of equal channel angular pressed ultra-fine grained nickel subjected to 12 passes

The microstructure, thermal stability and hardness of ultra-fine grained (UFG) Ni produced by 12 passes of equal channel angular pressing (ECAP) through the route Bc were studied. Comparing the microstructure and hardness of the as-ECAPed samples with the published data on UFG Ni obtained after 8 passes of ECAP through the route Bc reveals a smaller average grain size (230 nm in the present case compared with 270 nm in 8-pass Ni), significantly lower dislocation density (1.08 × 10¹⁴ m⁻² compared with 9 × 10¹⁴ m⁻² in 8-pass Ni) and lower hardness (2 GPa compared with 2.45 GPa for 8-pass Ni). Study of the thermal stability of the 12-pass UFG Ni revealed that recovery is dominant in the temperature range 150–250°C and recrystallisation occurred at temperatures >250 °C. The UFG microstructure is relatively stable up to about 400 °C. Due to the lower dislocation density and consequently a lower stored energy, the recrystallisation of 12-pass ECAP Ni occurred at a higher temperature (~250 °C) compared with the 8-pass Ni (~200 °C). In the 12-pass Nickel, hardness variation shows that its dependence on grain size is inversely linear rather than the common grain size^−0.5 dependence.     

Cyclic deformation behavior and fatigue lives of ultrafine-grained Ti-6AL-4V ELI alloy for medical use

It was shown that introducing an ultrafine-grained (UFG) microstructure in pure metals as well as some alloys leads to strongly enhanced fatigue properties. The cyclic deformation behavior of UFG Ti-6Al-4V ELI (extra low interstitials) alloy is studied by both strain and stress controlled fatigue tests using plastic strain amplitudes between 3 × 10^?4 and 5 × 10^?3 and stress amplitudes ranging from 550 to 670 MPa. The UFG microstructures were obtained by equal channel angular pressing (ECAP) with different number of passes followed by a subsequent thermomechanical treatment (TMT). When compared to the conventional grain (CG) size counterpart, the UFG alloy exhibited a pronounced enhancement in the fatigue life in the S–N (Wöhler) diagram. It was also shown that additional UFG processing prior to TMT did not result in any further improvement of the fatigue resistance. Furthermore, microstructural investigations revealed a high cyclic stability of the UFG microstructure.     

Microstructure and mechanical properties of Mg–Zn–Ca alloy processed by equal channel angular pressing

An ultrafine-grained (UFG) Mg–5.12 wt.% Zn–0.32 wt.% Ca alloy with an average grain size of 0.7 μm was produced by subjecting the as-extruded alloy to equal channel angular pressing (ECAP) for 4 passes at 250 °C. The fine secondary phase restricted the dynamic recrystallized (DRXed) grain growth during the ECAP processing, resulting in a remarkable grain refinement. A new texture was formed in the ECAPed Mg alloy with the {0 0 0 2} plane inclined at an angle of 58° relative to the extrusion direction. The yield stress (YS) was decreased in the as-ECAPed alloy with finer grains, indicating that the texture softening effect was dominant over the strengthening from grain refinement. The ductility of the as-ECAPed alloy was increased to 18.2%. The grain refinement caused an obvious decrease in work hardening rate in the as-ECAPed alloy during tensile deformation at room temperature.     

Thermophysical Properties of Aluminum 1060 Fabricated by Equal Channel Angular Pressing

Equal channel angular pressing (ECAP) has the advantage of enabling an ultrafine grain size. Aluminum 1060 is used as a power plant material because of its favorable electrical properties. However, the weak strength of aluminum limits its application. In this study, the thermal conductivity and electrical conductivity of Al 1060 made by ECAP was investigated. ECAP was conducted through the die having a channel angle of 90° and a corner angle of 20° at a temperature of 473 K with a strain rate of 2 mm · s⁻¹. The specimen was then processed with 1 to 8 passes by the route Bc method with 90° rotation. In the case of eight passes, the grain size was reduced to as small as 300 nm. As a result of the ECAP, the tensile strength was raised from 75 MPa to 134 MPa, while the electrical conductivity did not show a significant difference after eight passes. The thermal conductivity gradually decreased with ECAP passes, because of the decreased grain size by ECAP.     

超细晶纯钛疲劳裂纹的扩展行为

对纯钛进行2道次室温等径弯曲通道变形(ECAP)、等径弯曲通道变形加旋锻复合变形(ECAP+RS)并在旋锻后在300℃和400℃退火1 h,制备出4种具有不同组织的超细晶纯钛。对这4种超细晶纯钛进行疲劳裂纹扩展实验并观察分析超细晶纯钛的显微组织和疲劳断口的形貌,研究了裂纹的扩展行为。结果表明：显微组织对超细晶纯钛的疲劳裂纹扩展门槛值和近门槛区有显著的影响;超细晶纯钛的疲劳裂纹扩展门槛值随着塑性变形量的增大而增大,随着旋锻后退火温度的提高而降低;疲劳裂纹扩展速率曲线因超细晶纯钛晶粒尺寸和强度的影响出现转折,转折前ECAP+RS复合变形纯钛的抗疲劳裂纹扩展能力比ECAP变形强,且随着退火温度的提高而降低;转折后4种超细晶纯钛的疲劳裂纹扩展速率相差较小,呈现出相反的结果。疲劳裂纹扩展寿命中转折前近门槛区裂纹扩展寿命占绝大部分,因而转折前的门槛值与近门槛区的扩展速率对抗裂纹扩展能力更为重要。     

Dynamic ageing and the mechanical response of Al–Mg–Si alloy through equal channel angular pressing

In this paper, dynamic ageing characteristics associated with the application of equal channel angular pressing (ECAP) to Al6061 alloy at elevated temperatures was investigated. Followed by ECAP, Vickers microhardness measurement on the cross-sectional planes and microstructural observations were undertaken using transmission electron microscopy. The combination of the ECAP process with dynamic ageing at both 100 °C and 150 °C resulted in a significant increase in hardness. The grain size was measured as ∼160 nm after four passes. A comparison with the published data on the same alloy processed by ECAP at room temperature and statically aged, suggests several advantages in incorporating dynamic ageing with ECAP. These advantages consist of the ability to attain better grain refinement, increased hardness and the potential for saving time and energy.     

Microstructure and mechanical properties of ultrafine grain ZK60 alloy processed by equal channel angular pressing

The microstructure evolution and tensile properties of ZK60 magnesium alloy after equal channel angular pressing (ECAP) have been investigated. The results show that the two-step ECAP process is more effective in grain refinement than the single-step ECAP process due to the lower deformation temperature, a mean grain size of ~0.8 μm was obtained after two-step ECAP process at 513 K for four passes and 453 K for four passes. The EBSD examination reveals that ZK60 alloy after two-step ECAP process exhibits a more homogeneous grain size and misorientation distribution than single-step ECAP process. Both alloys after ECAP process present similar strong {0002} texture. The tensile strength of two-step ECAP alloy has also been improved compared with the single-step ECAP alloy. The strengthening effect was mainly ascribed to grain refinement.     

Texture and microstructure evolution in ultrafine-grained AZ31 processed by EX-ECAP

Extruded AZ31 alloy was processed by equal channel angular pressing (ECAP) up to 12 passes at 180 °C following route B_c, i.e. rotating the sample 90° between individual passes. Microstructure evolution was investigated using EBSD and TEM, as a function of strain imposed by ECAP. The first ECAP pass resulted in the formation of a new texture component which relates to the bimodal grain structure observed in this specimen. The grains larger than 10 μm show the orientation changes corresponding to the ECAP shear, which is characterised by the rotation of the basal poles by approximately 40° from the initial orientation. The fine grains with the average size of 1 μm maintain the initial orientation. The character of the bimodal grain structure and the distinct texture components between large and small grains remained unchanged up to 4 ECAP passes. Further ECAP pressing to 8 and 12 passes leads to a grain refinement through the whole sample volume and the orientation changes of all grains corresponding to the ECAP shear.     

Ultrafine-grained Ti–Nb–Ta–Zr alloy produced by ECAP at room temperature

To ascertain the influence of severe plastic deformation (SPD) on a Ti–Nb–Ta–Zr (TNTZ) alloy, we studied the room temperature mechanical behavior and microstructural evolution of an ultrafine-grained (UFG) Ti–36Nb–2Ta–3Zr (wt%) alloy prepared via equal-channel angular pressing (ECAP) of the as-hot-extruded alloy. The tensile behavior, phase composition, grain size, preferred orientation, and dislocation density of the UFG alloy, processed under different conditions, were analyzed and discussed. Compared to the as-hot-extruded alloy, the ECAP-processed TNTZ alloy (3 passes) exhibited approximately 40 and 88 % increase in average ultimate strength and yield strength, respectively. Moreover, as the number of ECAP passes increased from 3 to 6, the TNTZ alloy exhibited not only the expected increase in ultimate and yield strength values, but also a slight increase in elongation. Our results suggest that the deformation mechanisms that govern the behavior of the as-hot-extruded coarse grained (CG) TNTZ alloy during ECAP involve a combination of stress-induced martensitic transformation and dislocation activity. In the case of the ECAP-processed UFG TNTZ alloy, the deformation mechanism is proposed to involve two components: first, dislocation activity induced by the strain field imposed during ECAP; and second, the formation of α″ martensite phase during the early stages of ECAP which eventually transforms into β phase during continued deformation. We propose that the deformation mechanism governing the room temperature behavior of the TNTZ alloy strongly depends on the grain size of the β phase.     

Corrosion and ion release behavior of ultra-fine grained bulk pure copper fabricated by ECAP in Hanks solution as potential biomaterial for contraception

Ultra-fine grained (UFG) bulk pure copper has been successfully fabricated by equal-channel angular pressing (ECAP), with the grain size about 380 nm after 8 passes. The potentiodynamic polarization results of the ECAP copper specimens tested in Hanks solution revealed that the corrosion current of UFG copper is higher than that of the coarse grained copper. The cupric ion release behaviors of UFG copper immersed in Hanks solution for 30 days only displayed a burst release during the first 3 days (in comparison to the 1-2 months for the conventional Cu) from 115 μg/day to 12.5 μg/day, after which the ion release remained constant and slow. During the immersion experiments, Cu₂O was the only corrosion product found on the surface and it took 10 days or so to form a uniform Cu₂O layer. Uniform corrosive damage on the surface and few localized corrosion is observed. The above results indicate that UFG copper could have high potential as biomedical materials for contraception.     

Effect of twins and non-basal planes activated by equal channel angular rolling process on properties of AZ31 magnesium alloy

Ultrafine-grained (UFG) metallic materials because of their superior properties have received considerable research interest. Recently, severe plastic deformation (SPD) processes are widely used for refining the grain size in magnesium alloys. Equal channel angular rolling (ECAR) is a SPD process based on equal channel angular pressing (ECAP) which is carried out on large, thin sheets. After doing this process, no significant change is occurred in cross-sectional area of specimen. In this research, an AZ31 magnesium alloy was subjected to ECAR. After completing eight passes of process, significant grain refinement was occurred, and the average grain size of about 3.9 μm was achieved. The distribution of grain size becomes more limited by increasing number of passes. Rotation of basal plane and activation of non-basal and twin planes were clearly observed in X-ray diffraction (XRD) pattern results. Mechanical properties were studied via tensile and hardness tests at room temperature. Tension tests indicated that better ductility due to the rotation of basal plane was achieved. Elongation-to-failure was increased from 8% of as-received material to 19% after two passes of process. Hardness values showed an increase of about 53% at eighth pass.     

Thermal stability study on two aluminum alloys processed with equal channel angular pressing

Equal channel angular pressing was used to produce sub-micrometer size grain structures in two aluminum alloys (commercially pure 1200 and Al-Mn-Si 3103). ECAP was conducted at room temperature following the process via route C, which involves a 180° angular rotation between passes and strongly affects material microstructure by reversing the shear strain every second pass. This unique characteristic of route C induced limited build-up of new high-angle boundaries, at least for the first three passes. The equal channel angular pressing was extended to six passes for both alloys; three passes by route C throughout the die were sufficient to produce a very fine-structured material for both alloys. High-resolution electron back-scattered diffraction pattern analysis was carried out to measure boundary misorientation within the deformed structures. Measurements of subgrain and grain spacing revealed a more effective microstructure refining effect in the 3103 than the 1200 alloy. Thermal stability of the severely deformed materials was studied at temperatures of 130, 240, 330°C, corresponding to 0.2, 0.35, 0.5 of the melting temperature. The results showed considerable grain growth in both materials solely at temperatures from and above half of the melting one.     

Impact Toughness of Ultrafine‐Grained Commercially Pure Titanium for Medical Application

This study aims at achieving the best combination of strength, ductility, and impact toughness in ultrafine‐grained (UFG) Ti Grade 4 produced by equal‐channel angular pressing via Conform scheme (ECAP‐C) with subsequent cold drawing. UFG structures with various parameters (e.g., size and shape of grains, dislocation density, conditions of boundaries) are formed by varying the treatment procedures (deformation temperature and speed at drawing, annealing temperature). The tensile and impact toughness tests were performed on samples with a V‐shaped notch and different structures of commercially pure Ti Grade 4 in the coarse‐grained and UFG states. The results demonstrated that grain refinement, higher dislocation density, and their elongated shape were obtained as a result of drawing at 200 °С, which led to a decrease in both the uniform elongation at tension and the impact toughness of Ti Grade 4. Short‐term annealing at 400–450 °C could improve the impact toughness of UFG Ti with a non‐significant decrease in strength. This short‐term annealing contributes to the dislocation density decrease without considerable grain growth as a result of the recovery and redistribution of dislocations. The dependence of impact toughness on the strain hardening ability of UFG Ti was discussed.

     

Achieving superplastic properties in a Pb–Sn eutectic alloy processed by equal-channel angular pressing

Experiments were conducted on a Pb-62% Sn eutectic alloy containing 160 ppm of Sb. The alloy was processed by equal-channel angular pressing (ECAP) through 1 to 5 passes at room temperature and then tested in tension at a temperature of 423 K using initial strain rates from 1.0 × 10⁻⁴ to 1.0 × 10⁻¹ s⁻¹. Excellent superplastic elongations were achieved at intermediate strain rates with a maximum elongation to failure of 2,665%. It is shown that, for processing through similar numbers of ECAP passes, these elongations are higher than in an earlier investigation using a Pb-62% Sn alloy of higher purity. The results are presented pictorially in the form of a deformation mechanism map by plotting normalized grain size against normalized stress at a temperature of 423 K.     

Refining SiCp in reinforced Al–SiC composites using equal-channel angular pressing

Aluminum–silicon carbide composite (Al–SiC_p) is one of the most promising metal matrix composites for their enhanced mechanical properties and wear resistance. In the present study, Al–SiC (average size 55 μm) composites with 5% and 10% by volume were fabricated by stir casting technique. The equal-channel angular pressing (ECAP) was then applied on the cast composites at room temperature in order to study the effect of ECAP passes on the SiC_p size and distribution. The ECAP process was successfully carried out up to 12(8) passes for Al–5%(10%)SiC samples. Microstructure study revealed that the highest refinement by breakage of SiC_p was achieved after the first ECAP pass and that further refinement took place in the next passes. More breakage of the SiC_p was found in the composite richer in reinforcing particles so that the SiC_p reached approximately 1 μm in the Al–10%SiC after 8 passes and 4 μm in Al–5%SiC after 12 ECAP passes. The distribution of SiC reinforcement particles also improved after applying ECAP. The factors including decrease in reinforcing particle size, improvement in their distribution, decrease in porosity in addition to strain hardening and grain refining of the matrix resulted in enhancement of tensile and compressive strengths as well as hardness by more than threefold for the Al–5%SiC after 12 passes and for Al–10%SiC after 8 passes compared to the cast composites. Additionally, the composite remained ductile after the ECAP process. The fracture surface indicated good bond between the matrix and the reinforcement.     

Quantitative study of grain refinement in Al–Mg alloy processed by equal channel angular pressing at cryogenic temperature

Strain induced grain refinement of an Al–1 wt.% Mg alloy processed by equal channel angular pressing (ECAP) at cryogenic temperature is investigated quantitatively. The results show that both mean grain and subgrain sizes are reduced gradually with increasing ECAP pass. ECAP at cryogenic temperature increases the rate of grain refinement by promoting the fraction of high angle grain boundaries (HAGBs) and misorientation at each pass. The fraction of HAGBs and the misorientation of Al–1 wt.% Mg alloy during ECAP at cryogenic temperature increase continuously as a function of equivalent strain. Both {110} and {111} twins at ultrafine-grained size are observed firstly in Al–Mg alloy during ECAP. The analysis of grain boundaries and misorientation gradients demonstrates the grain refinement mechanism of continuous dynamic recrystallization.     

Stability of the ultrafine-grained microstructure in silver processed by ECAP and HPT

The high-temperature thermal stability of the ultrafine-grained (UFG) microstructures in low stacking fault energy silver was studied by differential scanning calorimetry (DSC). The UFG microstructures were achieved by equal-channel angular pressing (ECAP) and high-pressure torsion (HPT) at room temperature (RT). The defect structure in the as-processed samples was examined by electron microscopy and X-ray line profile analysis. The stored energy calculated from the defect densities was compared to the heat released during DSC. The sum of the energies stored in grain boundaries and dislocations in the ECAP-processed samples agreed with the heat released experimentally within the experimental error. The temperature of the DSC peak maximum decreased while the released heat increased with increasing numbers of ECAP passes. The released heat for the specimen processed by one revolution of HPT was much smaller than after 4–8 passes of ECAP despite the 2 times larger dislocation density measured by X-ray line profile analysis. This dichotomy was caused by the heterogeneous sandwich-like microstructure of the HPT-processed disk: about 175 μm wide surface layers on both sides of the disk exhibited a UFG microstructure while the internal part was recrystallized, thereby yielding a relatively small released heat.