Showing papers on "Superplasticity published in 2012"

Tensile properties of an AlCrCuNiFeCo high-entropy alloy in as-cast and wrought conditions

Abstract: Extensive multistep forging at 950 °C was applied to the cast AlCuCrFeNiCo high-entropy alloy to transform the cast coarse dendritic structure into a fine equiaxed duplex structure consisting of the mixture of BCC and FCC phases, with the average grain/particle size of ∼1.5 ± 0.9 μm. Tensile properties of the alloy in the as-cast and forged conditions were determined in the temperature range of 20–1000 °C. The hot forged alloy was stronger and more ductile during testing at room temperature, than the as-cast alloy. The yield stress (YS), ultimate tensile strength (UTS), and tensile ductility ( δ ) of the forged condition were 1040 MPa, 1170 MPa, and 1%, respectively, against 790 MPa, 790 MPa and 0.2% for the as-cast condition. In both conditions, the alloy showed brittle to ductile transition (BDT), with a noticeable increase in the tensile ductility within a narrow temperature range. In the as-cast condition, this transition occurred between 700 and 800 °C, while in the forged condition, it was observed between 600 and 700 °C. With an increase in the testing temperature above the BDT, a continuous decrease in tensile flow stress and an increase in tensile ductility were observed. In the temperature range of 800–1000 °C, the forged alloy showed superplastic behavior. The tensile elongation was above 400% and reached 860% at 1000 °C.

Flash-Sinterforging of Nanograin Zirconia: Field Assisted Sintering and Superplasticity

Abstract: We report on the influence of a uniaxial applied stress on flash-sintering and field assisted superplastic behavior of cylindrical powder preforms of 3 mol% tetragonal-stabilized zirconia. The experiments use the sinterforging method, where, in addition to pressure, a dc electrical field is applied by metal electrodes sandwiched between the push-rods and the specimen. The axial and radial strains in the experiment provide simultaneous measurement of the time-dependent densification and shear strains. Large effects of the electric field on sintering and superplasticity are observed. We see flash-sintering which is characterized by a threshold level of temperature and electric field. With higher applied fields, the sample sinters at a lower furnace temperature. Surprisingly, the applied stress further lowers this critical temperature: a sample, which sinters at 915°C under a stress of 1.5 MPa, densifies at only 850°C when the stress is raised to 12 MPa. This stress induced reduction in sintering temperature maybe related to the additional electrical fields generated within the specimen by the electro-chemo-mechanical mechanism described by Pannikkat and Raj [Acta Mater., 47 (1999) 3423]. Remarkably, we also show that the sample deforms in pure shear to 30% strain in just a few seconds at anomalously low temperatures. The specimen temperature was measured with a pyrometer, during the flash sintering, as a check on Joule heating. A reading of 1000°C–1100°C was obtained, up to 200° above the furnace temperature. This temperature is still too low to explain the sintering in just a few seconds. It is suggested that the electric field can nucleate a defect avalanche that enhances diffusion kinetics not by changing the activation energy but by increasing the pre-exponential factor for the diffusion coefficient, noting that the pre-exponential factor depends on concentration of defects, and not upon their mobility.

Tensile deformation behavior of Ti–22Al–25Nb alloy at elevated temperatures

Abstract: The deformation behavior of Ti–22Al–25Nb alloy at elevated temperatures and different stain rates was investigated using uniaxial tensile test. It was found that the tension was accompanied with the effect of hardening and softening, under which the stress–strain curve was characterized by a rise to a peak followed by a nearly linear drop in flow stress. The peak stress was strongly dependent on the temperature and strain rate. The underlying mechanism was clarified in terms of dislocation dynamics. Work hardening and strain rate hardening both contributed to the hardening mechanism, and the softening mode was dominated by dynamic recovery. The effect of work hardening was completely neutralized by dynamic recovery. Owing to the strain rate hardening, the alloy exhibited certain degree of superplasticity. The further drop in flow stress after the peak was due to the rise in temperature, which originated from the heat generated during deformation. The deformation mechanism was dominated by dislocation slip and climb. The misorientation distribution between β/B2 and α2 phase scarcely changed, implying a harmonious deformation of the two phases.

Extraordinary high-strain rate superplasticity of severely deformed Al–Mg–Sc–Zr alloy

Abstract: Superior superplastic behavior with elongations from 3000 to 4100% were found in temperature range of 400–475 °C at strain rates 10−2–10−1 s−1 for the severely deformed Al–5Mg–0.18Mn–0.2Sc–0.08Zr–0.01Fe–0.01Si alloy having homogeneous and thermally stable ultrafine-grain structure with ∼1 μm grain size and low amount of coarse excess phases.

Influence of texture on superplastic behavior of friction stir processed ZK60 magnesium alloy

Abstract: Commercial ZK60 extruded plate was subjected to friction stir processing (FSP). A fine-grained structure 2.9 mu m in grain size with uniformly distributed fine second-phase particles and predominant high-angle grain boundaries of 97% was obtained. A strong basal fiber texture with the (0002) planes roughly surrounding the tool pin surface was developed in the FSP ZK60 alloy. The FSP ZK60 alloy exhibited superplastic behavior at 225-325 degrees C in both the transverse direction (TD) and longitudinal direction (LD), with the superplastic elongation in the LD being higher than that in the TD. A maximum elongation of 1390% was obtained at 300 degrees C and 3 x 10(-4) s(-1) in the LD. The existence of texture was found to influence the superplastic flow stress and plasticity. The texture had a minor influence on the flow stress, due to the texture weakening during superplastic deformation. However, the anisotropy of plasticity existed at all temperatures and strain rates, as a result of the activation of different slip systems in different tensile directions. (c) 2012 Elsevier B.V. All rights reserved.

Ultrafine-grained Mg–Zn–Zr alloy with high strength and high-strain-rate superplasticity

Abstract: We report the feasibility of fabricating ultrafine-grained ZK60 magnesium alloy sheets with a combination of superior mechanical properties at room temperature and excellent superplasticity at high strain rates by means of high-ratio differential speed (HRDSR). The ZK60 alloy processed under the optimum HRDSR condition showed a high yield strength of 378 MPa and a reasonably high ductility (a total elongation of 12.3%). The major strengthening mechanisms could be attributed to the grain size and particle strengthening. Its microstructure was quite thermally stable up to 573 K, such that high strain rate superplasticity could be achieved at 10−2 and 10−1 s−1. The development of high-density shear bands that occurred uniformly over the entire volume of a sample and dynamic precipitation of spherical or irregular shaped particles with the sizes of 20–32 nm along the lamellar boundaries in the shear bands and on the boundaries of grains newly formed within the shear bands via continuous dynamic recrystallization, resulted in an ultrafine-grained microstructure with high thermal stability. The room-temperature mechanical properties and superplastic ability of the HRDSR-processed ZK60 were comparable to those of a magnesium alloy of a same or similar composition processed via powder metallurgy. Because the currently developed HRDSR technique is based on a rolling process and only a few processing steps are involved, its transfer to industry that demands high-performance magnesium alloy sheets is promising.

Microstructural evolution in recrystallized and unrecrystallized Al-Mg-Sc alloys during superplastic deformation

Abstract: The microstructural evolution of unrecrystallized (extruded) and recrystallized (friction stir processed, FSP)Al-Mg-Sc alloys during superplastic straining was investigated using electron backscatter diffraction (EBSD). The unrecrystallized structure gradually transformed into a recrystallized structure, characterized by equiaxed grains, random boundary misorientation distribution and a weak texture at high strains. This evolution was divided into three stages based on true stress-strain curves and EBSD maps, i.e. subgrain rotation and coalescence in the early stage, dynamic recrystallization in the middle stage, and grain boundary sliding (CBS) and dynamic grain growth in the final stage. By comparison, the recrystallized grains in the FSP Al-Mg-Sc maintained a random distribution during the whole deformation process, however the grain size increased significantly with increasing strain, indicating that the main deformation mechanism was always GBS and dynamic grain growth. A deformation model was proposed to explain the microstructural evolution during superplastic deformation. The microstructure with the random boundary misorientations reaches a dynamic balance because the transformation between high-angle grain boundaries and low-angle grain boundaries is equivalent. (C) 2012 Elsevier B.V. All rights reserved.

Superplastic tensile behavior of a fine-grained AZ91 magnesium alloy prepared by friction stir processing

Abstract: Friction stir processing (FSP) was used to refine the microstructure of cast AZ91 magnesium alloy. Superplastic tensile behavior of the fine-grained AZ91 was investigated by means of hot tensile tests at the temperature and strain rate ranges of 473–623 K and from 2×10 −2 to 1×10 −4 s −1 , respectively. Microstructure of the as-cast AZ91 alloy was mainly composed of coarse α dendrites and network-like eutectic β -Mg 17 Al 12 phase. After FSP, α grains were greatly refined to equiaxed grains with an average grain size of ∼3 μm due to dynamic recrystallization, and β -Mg 17 Al 12 networks were broken into small particles. It was found that the FSP AZ91 alloy exhibits excellent superplasticity, and a maximum elongation of 1604% is achieved at 573 K with a strain rate of 1×10 −4 s −1 . Moreover, its elongation to failure at 473 K (0.51 T m ) and 3×10 −3 s −1 is 204.4%, indicating superplasticity can be attained under low temperature and high strain rate. In this multi-phase fine-grained alloy, cavities nucleated around the second phase particles during superplastic deformation, and connected to each other due to the grain boundary sliding.

Equal channel angular pressing technique for the formation of ultra-fine grained structures

Abstract: Equal channel angular pressing is one of the techniques in metal forming processes in which an ultra-large plastic strain is imposed on a bulk material in order to make ultra-fine grained and nanocrystalline metals and alloys. The technique is a viable forming procedure to extrude materials by use of specially designed channel dies without substantially changing the geometry by imposing severe plastic deformation. This technique has the potential for high strain rate superplasticity by effective grain refinement to the level of the submicron-scale or nanoscale. Wereview recent work on new trends in equal channel angular pressing techniques and the manufacturing of die-sets used for the processing of metals and alloys. We also experimented on a copper alloy using the equal channel angular pressing technique to examine the microstructural, mechanical and hardness properties of the ultra-fine grained and nanocrystalline materials produced. After deformation, all samples were subjected to a hardness test and the results showed improved mechanical behaviour of the ultra-fine grained copper alloy that was developed. This research provides an opportunity to examine the significance of the equal channel angular pressing process for metals and alloys. That is, these ultra-fine grained materials can be used in the manufacturing of semi-finished products used in the power, aerospace, medical and automotive industries.

Superplasticity of AlCoCrCuFeNi High Entropy Alloy

Abstract: An AlCoCrCuFeNi high entropy alloy was multiaxially isothermally forged at 950°C to produce a fine equiaxed structure with the average grain/particle size of ~1.5 µm. The forged alloy exhibited superplastic behavior in the temperature range of 800-1000°C. For example, during deformation at a strain rate of 10-3 s-1, tensile ductility increased from 400% to 860% when the temperature increased from 800°C to 1000°C. An increase in strain rate from 10-4 to 10-2 s-1 at T = 1000°C did not affect ductility: elongation to failure was about 800%. The strain rate sensitivity of the flow stress was rather high, m = 0.6, which is typical to the superplastic behavior. The equiaxed morphology of grains and particles retained after the superplastic deformation, although some grain/particle growth was observed.

Shear band formation during hot compression of AZ31 Mg alloy sheets

Abstract: Shear band formation has been reported to depend strongly on micro-structural factors, such as grain size, texture, alloying elements, and also on forming processes. The formation mechanism of shear bands has thus been investigated in the present work in relation to improve the forming limit of AZ31 Mg alloy. A series of compression tests have thus been carried out using strip-cast (SC) and hot-rolled (AR) AZ31 Mg alloys to compare their texture effects. The tests were performed under the strain rate of 10 −1 /s at room temperature (RT) and 300 °C to clarify temperature dependence of shear band formation. Shear bands in SC specimens were observed to develop mainly at the boundaries of cast structures by generating compression twins and dynamic recrystallization (DRX) induced by the activation of the and slips as well as twinning at elevated temperatures. AR specimens exhibited, on the other hand, well-developed basal-fiber texture to generate shear bands through fine grains developed by DRX along the geometrically maximum shear stress directions.

Haemocompatibility of titanium and its alloys

Abstract: This review outlines the current understanding of the interactions of titanium and its alloys with blood components, and the ways in which surface modification techniques can be used to alter the s...

Superplastic behavior of Ti–6Al–4V–0.1B alloy

Abstract: The superplastic behavior of Ti–6Al–4V–0.1B sheet was evaluated. The strain rate sensitivity (m) is ≥0.47 in the temperature range 775–900 °C and at strain rate, e ˙ = 10 − 5 to 10 − 3 s − 1 . The material exhibits tensile elongations > 200% in the temperature range 725–950 °C at e ˙ = 3 × 10 − 4 s − 1 . The optimum superplastic forming temperature is 900 °C, which is similar to conventional Ti–6Al–4V. However, a lower flow stress is needed in the case of Ti–6Al–4V–0.1B. The superplastic deformation mechanism is suggested from estimates of activation energy to be grain boundary sliding (GBS) accommodated by dislocation motion along grain boundaries at e ˙ = 10 − 4 s − 1 and is diffusion-controlled dislocation climb at e ˙ = 10 − 3 s − 1 . Microstructural observations also confirm that GBS is the operating deformation mechanism at 900 °C and e ˙ = 3 × 10 − 4 s − 1 .

The microstructural evolution and superplastic behavior at low temperatures of Mg–5.00Zn–0.92Y–0.16Zr (wt.%) alloys after hot extrusion and ECAP process

Abstract: In this study, equal channel angular pressing (ECAP) was applied to a hot-extruded Mg–5.00Zn–0.92Y–0.16Zr (wt.%) alloy to produce an ultrafine-grained a -Mg structure of 0.6 μm with uniformly distributed fine quasicrystal Mg 3 YZn 6 particles. The basal planes in the as-ECAPed alloy were inclined approximately 45° to the ECAP direction (EPD); thus, a high Schmid factor of 0.36 for ( 0 0 0 1 ) 〈 1 1 2 ¯ 0 〉 basal slip along EPD was obtained. Then, the superplastic behavior at low temperatures of 150–250 °C and initial strain rates of 1.67 × 10 −3 s −1 –1.67 × 10 −1 s −1 of the as-ECAPed alloy was investigated and compared with that of the as-extruded alloy. The microstructural development, texture evolution and cracking behavior during tensile tests were systemically investigated by electron backscattered diffraction (EBSD) analysis. During the tensile test along the EPD, the basal planes in the as-ECAPed alloy specimen were tilted approximately 15° away from the original position; thus, the basal planes were still in a position favorable for the basal slip along the EPD. As a result, a maximum elongation of 865% was obtained at 200 °C and a strain rate of 1.67 × 10 −3 s −1 , as a result of this favored basal texture and the excellent thermal stability of the ultrafine-grained structure. This optimum superplastic temperature of 200 °C is much lower than that obtained for other Mg–Zn–Y–(Zr) alloys. In contrast, for the as-extruded alloy specimen, superplastic behavior did not occur until the critical condition of 250 °C and a strain rate of 1.67 × 10 −3 s −1 . Below this temperature and strain rate, the strong extrusion basal texture was maintained, and the cracks in the hot-worked coarse region (or un-DRXed region) were observed nucleating mainly associated with the formation of a 38° { 1 0 1 ¯ 1 } – { 1 0 1 ¯ 2 } double twin. At this temperature and strain rate, the hot-worked coarse regions were significantly refined by the dynamic recovery and the following recrystallization process. In the case, the formation of a double twin was prevented and the facture was delayed.

Superplastic deformation mechanism and mechanical behavior of a laser-welded Ti–6Al–4V alloy joint

Abstract: The mechanical behavior and superplastic deformation mechanism of a laser-welded Ti–6Al–4V alloy joint were investigated. Uniaxial tensile tests were performed on welded specimens at 870–920 °C temperature and 10 −3 to 10 −1 s −1 strain rate. The microstructural evolution of the weld zone was observed under the strains of 43%, 143%, 229%, and 387%. The laser-welded joint was found to have good superplasticity under a suitable strain rate; the highest joint elongation reached 397%. Superplastic deformation in the weld zone accompanied the globularization of the as-welded microstructure. Continuous globularization ensured a good superplasticity of the laser-welded joint. As a major cause of the globularization of lamellar structures in the weld zone, the stress activated the diffusion of Al atoms by changing the chemical potential at the boundaries of the α phase. Consequently, α → β phase transformation occurred. The globularization of the as-weld microstructures was considered to be governed by this transformation and the development of grain boundary sliding.

Effect of warm accumulative roll bonding on the evolution of microstructure, texture and creep properties in the 7075 aluminium alloy

Abstract: The commercial 7075 Al alloy was severely deformed at 300 °C by a 3:1 thickness reduction per pass accumulative roll bonding (ARB) process up to 5 passes. Examinations by transmission electron microscopy and electron backscatter diffraction revealed that the alloy microstructure was finer and more misoriented with increasing the number of ARB passes. The 5-passes sample exhibited a mean cell/(sub)grain diameter of 355 nm and a mean boundary misorientation angle of 33°. The texture of the processed alloy was characterized by a β-fibre, whose intensity increased with increasing the number of ARB passes. This intensification process is enhanced by the cutting and stacking steps promoting the formation of the Dillamore orientation. Uniaxial tensile tests conducted at 300 °C and at an initial strain rate of 10−2 s−1 revealed that the processed alloy exhibited superplasticity, which was very sensitive to the stability of its microstructure at the testing temperature. An elongation to failure of 202% was registered for the 4-passes sample. Therefore, the present ARB process would allow short forming times at much lower temperatures than conventional.

Hot deformation behavior of friction-stir processed strip-cast 5083 aluminum alloys with different Mn contents

Abstract: Fine grained 5083 aluminum alloy is the most common Al–Mg alloy for superplastic forming (SPF) of lightweight sheet metal parts in the automotive and aerospace industries. The fine grained sheet is industrially produced by massive cold rolling of conventionally rolled sheet stock at high cost. Friction stir processing (FSP) as a thermomechanical process is very effective in refining the microstructure of as-cast alloys such as that produced by continuous strip casting (CC). In this work, the effect of friction stir processing on the superplastic properties of three CC 5083 aluminum alloys, with different Mn content, has been investigated. The three alloys were friction stir processed. Very fine microstructures with grain sizes less than 3 μm were obtained. Tensile tests revealed elongations of over 600% at a high strain rate of 10−1 s−1 in all 3 alloys. The maximum tensile elongation of 800% was achieved in the alloy with the lowest Mn content at 490 °C and strain rate of 3 × 10−2 s−1. The stability of the microstructure was an important concern above 500 °C.

Tensile properties of multiphase Mo–Si–B refractory alloys at elevated temperatures

Abstract: Multiphase refractory alloys with nominal composition Mo–9Si–8B–3Hf and Mo–10Si–14B–3Hf (at.%) were produced by a powder metallurgy (PM) technique using elemental powders. These alloys have fine grain sizes and 3-phase, consisting of α-Mo (Mo solid solution) and the intermetallic phases Mo 3 Si and Mo 5 SiB 2 . The tensile properties of these alloys at elevated temperatures were evaluated in vacuum. The Mo–10Si–14B–3Hf alloy has a very good mechanical strength at elevated temperature and displays large tensile ductility. This alloy exhibits a tensile strength of 560 MPa at 1400 °C and a strain rate of 2 × 10 −2 s −1 . The Mo–9Si–8B–3Hf alloy displays extensive plasticity or superplasticity at temperatures ranging from 1400 to 1560 °C. The tensile elongation of 410% was measured at 1560 °C. Grain boundary sliding is the main mechanism of plastic deformation for these alloys.

Threshold stress for superplasticity in solid solution magnesium alloys

Abstract: The effect of alloying elements on the threshold stress for superplasticity was investigated using two binary solid solutions, namely, Mg–Al and Mg–Y alloys. Both alloys exhibited superplasticity, and in spite of the absence of fine particles showed threshold-stress-like behavior. Different origins were suggested for the threshold-stress-like behavior after considering grain growth during deformation. The threshold-stress-like behavior in Mg–Al alloys originates from the effects of microstructural instability (grain-growth hardening). On the other hand, analysis of grain-boundary segregation suggested that the threshold-stress-like behavior in Mg–Y alloy originates from the segregation of yttrium in grain boundaries and its interaction with grain-boundary dislocations.