Showing papers on "Superplasticity published in 2008"

Developing grain refinement and superplasticity in a magnesium alloy processed by high-pressure torsion

Abstract: Experiments were conducted on a Mg–9% Al alloy to evaluate the microstructural characteristics and the tensile properties at elevated temperatures after processing by high-pressure torsion (HPT) at room temperature and at 423 K. Ultrafine grain sizes were achieved by processing samples in both an extruded and a cast condition. The results demonstrate the viability of using HPT as a processing technique for achieving significant grain refinement in magnesium alloys which are not processed easily by equal-channel angular pressing (ECAP). Superplastic ductilities were achieved in tensile testing at a temperature of 473 K with a maximum measured elongation of 810%. In general, higher superplastic elongations were achieved after processing by HPT at 423 K because of the development of some limited internal cracking when processing at room temperature.

Record Superplastic Ductility in a Magnesium Alloy Processed by Equal‐Channel Angular Pressing

Abstract: The Mg-5.5 % Zn-0.5 % Zr alloy exhibits large superplastic elongations after processing by Equal-Channel Angular Pressing and these are strongly dependent on the number of passes through the die. Grain refinement improves the elongations up to a peak of 3050 % after 2 passes but grain growth limits ductility after larger numbers of passes.

Enhanced superplasticity utilizing dynamic globularization of Ti–6Al–4V alloy

Abstract: This study aimed to determine the optimum processing conditions for dynamic globularization of Ti–6Al–4V alloy having martensite microstructure, and to achieve enhanced superplasticity. A series of isothermal compression tests was carried out for martensite microstructure in the strain range of 0.6–1.4, the strain rate range of 10 −3 s −1 to 1 s −1 and the temperature range of 973–1223 K. At each test condition, a quantitative analysis was made to measure the fraction of dynamic globularization associated with fragmentation of initial lamellar structures. It was found that the dynamic globularized fraction increased with increasing strain and with decreasing strain rate and/or temperature tested. Based on the processing map and microstructural analysis, the optimum processing condition to obtain the finest equiaxed microstructure was determined. A very high elongation over ≈1000 pct was achieved at relatively high strain rate superplasticity regime (>10 −2 s −1 ). The present enhanced superplasticity was rationalized by examining the microstructures of the alloy associated with dynamic globularization.

Microstructural and Mechanical Characteristics of AZ61 Magnesium Alloy Processed by High-Pressure Torsion

Abstract: Experiments were conducted on an AZ61 magnesium alloy to evaluate the microstructural characteristics and the mechanical properties after processing by High-Pressure Torsion (HPT). The results show that processing by HPT produces excellent grain refinement with average grain sizes of ?0.22 and ?0.11 ?m after processing at 423 K and room temperature, respectively. Tensile testing after HPT revealed the potential for achieving superplastic elongations with a maximum recorded elongation of 620% when testing at a temperature of 473 K. Using microhardness measurements, it is demonstrated that the the microstructure gradually evolves with increasing torsional straining in HPT so that ultimately there is a reasonably homogeneous structure across the disk.

Developing superplasticity in a magnesium AZ31 alloy by ECAP

Abstract: The processing of a magnesium AZ31 alloy by equal-channel angular pressing refines the grain size to ~2.2 μm, but annealing for 30 min at 673 K coarsens the grains to ~6.0 μm. Despite this microstructural instability, the alloy is superplastic when pulled in tension at temperatures in the range of 623–723 K with elongations up to >1000% at strain rates at and below 10−4 s−1. Experiments within the superplastic regime show the strain rate sensitivity is ~0.5 and the activation energy is close to the value for grain boundary diffusion. It is demonstrated by calculation that the experimental results are in good agreement with a model for superplasticity based on grain boundary sliding.

High temperature deformation behavior of a near alpha Ti600 titanium alloy

Abstract: The high temperature deformation behavior of a near alpha Ti600 titanium alloy was investigated with isothermal compression tests at temperatures ranging from 800 to 1000 °C and strain rates ranging from 0.001 to 10.0 s−1. The apparent activation energy of deformation was calculated to be 620.0 kJ mol−1, and constitutive equation that described the flow stress as a function of the strain rate and deformation temperature was proposed for high temperature deformation of Ti600 titanium alloy in the α + β phase region. The processing map was calculated to evaluate the efficiency of the forging process in the temperatures and strain rates investigated and to recognize the instability regimes. High efficiency values of power dissipation over 55% obtained under the conditions of strain rate lower than 0.01 s−1 and temperature about 920 °C was identified to represent superplastic deformation in this region. Plasticity instability was expected in the regime of strain rate higher than 1 s−1 and the entire temperature range investigated.

High-strain creep of feldspar rocks: Implications for cavitation and ductile failure in the lower crust

Abstract: [1] Cavitation damage and ductile fracturing is a common phenomenon observed in high-temperature, ambient pressure deformation of superplastic metals and ceramics, but hardly described for geological materials. We performed high-pressure, high-temperature (400 MPa, 950°C–1200°C) torsion experiments on fine-grained (size ≈4 μm, aspect ratio ≈2.5) synthetic feldspar aggregates containing <3 vol% residual glass. Samples deformed at constant strain rates (≈2 × 10−5 – 2 × 10−4 s−1) to high strain (≈2.8–5.6) reveal strain hardening at the lower strain rates. Microstructures show pronounced cavitation and formation of porosity bands containing redistributed glass, presumably associated with grain boundary sliding and shape-preferred orientation of high-aspect ratio feldspar grains. Sudden failure by strain-induced nucleation, growth and coalescence of the cavities occurred in one-third of the samples before deformation was terminated. In natural mylonites cavitation damage may produce increased porosity enhancing fluid flow in high-temperature shear zones.

Enhanced Superplasticity in a Magnesium Alloy Processed by Equal-Channel Angular Pressing with a Back-Pressure

Abstract: An investigation was initiated to evaluate the feasibility of using equal-channel angular pressing (ECAP) to obtain high superplastic elongations in the AZ31 alloy with a back pressure producing a bimodal grain structure. Processing by ECAP was performed using a die with an angle of 90 ° between the two parts of the channel and a ram velocity of 15-20 mm/sec. Some pressing were conducted with a back-pressure by making use of a backward punch in the exit channel of the die. Molybdenum disulphide and a graphite spray were used as lubricants and billets were pressed using processing route B c in which each billet is rotated by 90 °. The pressing were conducted at temperatures in the range from 423 to 523 K and every billet was quenched in water after each pass. The significance of the bimodal microstructure is attributed to the ability of the larger grains to more easily accommodate grain boundary sliding through intragranular slip and twinning and to contribute to the strain hardening capability.

Superplastic deformation in W–0.5 wt.% TiC with approximately 0.1 μm grain size

Abstract: In order to reveal the occurrence of superplasticity in ultra-fine grained W–TiC, W–0.5 wt.% TiC consolidates were fabricated utilizing mechanical alloying (MA) in purified H2 (W–0.5TiC–H2) or Ar (W–0.5TiC–Ar) and hot isostatic pressing. The W–0.5TiC–H2 and W–0.5TiC–Ar feature equiaxed ultra-fine grains with average diameters of 150 and 70 nm, respectively, and W–0.5TiC–Ar contains a high density of nano-sized Ar bubbles. Tensile tests were conducted at 1673–1973 K (0.45–0.54Tm, Tm: melting point of W) at initial strain rates from 5 × 10−5 to 5 × 10−3 s−1. It is found that W–0.5TiC–H2 exhibits a large strain rate sensitivity of flow stress, m, of 0.5–0.6, which is a feature of superplastic materials, whereas W–0.5TiC–Ar exhibits a smaller m value of approximately 0.2. This suggests that the Ar bubbles cause an adverse effect on superplastic deformation. The activation energy for deformation is 300 kJ mol−1 for W–0.5TiC–H2 and 570 kJ mol−1 for W–0.5TiC–Ar, corresponding to those for grain boundary diffusion and lattice diffusion in W, respectively. Each deformation controlling mechanism is discussed.

Low-Temperature Coarsening and Plastic Flow Behavior of an Alpha/Beta Titanium Billet Material with an Ultrafine Microstructure

Abstract: The influence of microstructure evolution on the low-temperature superplasticity of ultrafine alpha/beta titanium alloys was established For this purpose, the static and dynamic coarsening response and plastic flow behavior of Ti-6Al-4V with a submicrocrystalline microstructure were determined via a series of heat treatments and uniaxaial compression tests at temperatures of 650 °C, 775 °C, and 815 °C At all test temperatures, static coarsening exhibited diffusion-controlled (r 3 vs time) kinetics and followed a dependence on phase composition and volume fraction qualitatively similar to previous observations at 850 °C to 950 °C Dynamic coarsening at 775 °C and 815 °C and strain rates of 10−4 and 10−3 s−1 were similar to prior higher-temperature observations as well in that the kinetics were approximately one order of magnitude faster than the corresponding static behaviors The increase in coarsening rate with superimposed deformation was attributed to the enhancement of diffusion by dislocations generated in the softer beta phase With respect to deformation response, plastic flow was superplastic with m values of ∼06 at 650 °C, 775 °C, and 815 °C and strain rates of 10−4 and 10−3 s−1 Dynamic coarsening resulted in flow hardening at both temperatures and strain rates for a short preheat time (15 minutes) but was noticeably reduced when a longer preheat time (1 hour) was used prior to testing at 10−3 s−1 The latter behavior was largely attributed to noticeable static coarsening during preheating A generalized constitutive relation based on a single stress exponent and the instantaneous alpha particle size was shown to describe the superplastic flow of ultrafine Ti-6Al-4V at low and high temperatures

Mechanical Properties of Magnesium Alloy AZ31 after Severe Plastic Deformation

Abstract: Grain refinement of magnesium alloy AZ31 was studied in multidirectional forging (MDF) under decreasing temperature conditions. MDF was carried out up to large cumulative strains of 5.6 with changing the loading direction during decrease in temperature from pass to pass. MDF can accelerate the uniform development of very fine- grained structures and an increase of the plastic workability at low temperatures. New grain structures with the minimal grain size of 0.23 μm can be developed by continuous dynamic recrystallization at a final processing temperature of 403 K. As a result, the multidirectional- forged alloy showed excellent higher strength as well as moderate ductility at room temperature, and also a superplastic elongation of over 300% at 423 K. The mechanisms of strain-induced and fine-grained structure development and of the excellent plastic deformation are discussed in detail.

Characteristics of the Transition from Grain-Boundary Sliding to Solute Drag Creep in Superplastic AA5083

Abstract: Superplastic tensile ductility has been attained when specially-processed AA5083 materials are strained in tension at relatively high strain rates, in the range of the transition from grain-boundary sliding (GBS) to solute drag creep (SDC) control of deformation. Quick plastic forming (QPF) technology involves deformation at such strain rates, and the relative contributions of GBS and SDC to the strain during deformation in this strain rate regime have been examined in this investigation. The additive, independent contributions of GBS and SDC to the elevated temperature deformation of fine-grained materials are reviewed. The transition from GBS to SDC in grain-refined AA5083 materials was evaluated by several methods, including the assessment of initial transients during straining and of transients during strain-rate change tests; the strain-rate dependence of the flow stress; the dependence of ductility on strain rate; flow localization behavior and fracture mode; cavitation growth; the evolution of microstructure and microtexture during deformation; and comparison with phenomenological models for the GBS-to-SDC transition.

Mechanical properties and texture of a superplastically deformed AZ31 magnesium alloy

Abstract: Room temperature tensile properties of a superplastic AZ31 magnesium alloy prior to and following high temperature deformation at 673 and 723 K were examined. The material behaved in a superplastic manner at low strain rates, and dislocation creep dominated deformation at high strain rates. The specimens after high temperature deformation exhibited higher ductility and lower strength as the strain rate during high temperature deformation decreased. Pre-existing basal texture changed gradually toward random during high temperature deformation at low strain rates, whereas it was strengthened at high strain rates. Both the basal texture weakening and lower cavity volume fraction brought about the higher ductility following superplastic deformation at lower strain rates.

Grain boundary sliding in a superplastic zinc-aluminum alloy processed using severe plastic deformation

Abstract: A Zn-22% Al eutectoid alloy was processed by Equal-Channel Angular Processing (ECAP) to produce a grain size of ?0.8 ?m. Tensile testing at 473 K gave a maximum elongation of ?2230% at a strain rate of 1.0 × 10-2 s-1. The significance of grain boundary sliding was evaluated by taking measurements of offsets in surface marker lines at an elongation of 30%. The highest sliding contribution was recorded under testing conditions corresponding to the maximum superplastic elongation. Detailed measurements showed that relatively large offsets occurred at the Zn-Zn and Zn-Al interfaces but there were smaller offsets at the Al-Al interfaces. It is concluded that grain boundary sliding is the dominant flow process during superplastic deformation.

Superplastic forming of friction stir welds in Titanium alloy 6Al‐4V: preliminary results

Abstract: The trend in design and fabrication of aerospace structure is moving rapidly towards the use of composite materials and the consolidation of many pieces into large monolithic assemblies. Titanium alloy 6Al-4V is more compatible with composite materials than aluminum alloys because of its superior corrosion resistance and closer match to the coefficient of thermal expansion. In addition, many components that are used for the newer composite based aircraft, and are subjected to high service temperatures, are fabricated from titanium using Superplastic Forming (SPF) and Diffusion Bonding (SPF/DB). However, the use of SPF titanium parts has been limited up until now due the size restriction of standard sheets from the titanium mills, which is generally available at a maximum size of 1.2 m x 3.6 m. The purpose of this study was to develop the Friction Stir Welding (FSW) process for both standard and fine grain titanium alloy 6Al-4V in a bid to find a process that would allow the joining of multiple pieces to fabricate much larger components. Further, the FSW process was refined such that the welds were made to have superplastic properties equal to those of the parent sheet. A secondary goal of this effort was to build full size SPF prototype parts of a generic jet engine nacelle Lipskin using one FSW titanium blank. SPF of 7475 aluminum had been reported previously in the literature by Mahoney, Barnes, Mishra and others. During this study, the FSW process for 5083 Superplastic grade aluminum was developed simultaneously along with titanium 6Al-4V. The aluminum material was used to reduce the cost of developing the SPF manufacturing process to fabricate full scale engine inlet test components. FSW blanks of both materials were used for the initial forming trials.

Plastic forming of the equal-channel angular pressing processed 6061 aluminum alloy

Abstract: Plastic formability of the ECAP processed 6061 Al alloys with ultrafine grains was examined at relatively low forming temperatures in range between 443 and 553 K. The ECAPed 6061 Al alloys after 8 and 12 passes with submicron size grains and high fractions of high angle boundaries exhibited excellent forging and micro-extrusion abilities. High quality surface was another advantageous feature of the ECAP processed samples. Compressive forging on the 8 passes 6061 Al at 523 K, however, resulted in sharp decrease in strength due to dynamic recovery accelerated in the ultra-fined (sub)grained structure. Micro-extrusion on the 12 passes 6061 Al with 2 mm in diameter was successful at 443 and 553 K. Extrusion without losing the high strength gained through using ECAP, however, could be realized only at 443 K, indicating that high strength components could be made out of the ECAPed 6061 Al using plastic working when the processing temperature was chosen to be much lower than the conventional forging temperature of ∼753 K.

Tensile deformation and fracture behaviors of high purity polycrystalline zinc

Abstract: The damage and fracture behaviors of high purity polycrystalline zinc with two grain sizes during tension were investigated experimentally at different strain rates. It was found that specimens with coarse grains (∼1 mm) showed serrated flow behavior and failed in intermittent brittle cleavage fracture, while specimens with fine grains (∼70 μm) showed no cleavage crack initiation before necking even at high strain rate. It was observed that the fracture process of the fine-grained specimen was highly related to strain rates. With the strain rate increasing, the damage mechanism transformed from formation of tearing cracks along interfaces (including grain boundaries, twin boundaries and kink band boundaries) and cavity coalescence into abrupt quasi-cleavage fracture. Based on the observation, the inter-crystalline fracture of zinc was investigated, and the damage and fracture behaviors of polycrystalline zinc during tension at room temperature were discussed in general.

The determination of material parameters from superplastic free-bulging tests at constant pressure

Abstract: Superplastic forming (SPF) of sheets is a very interesting and useful process when there is a need to produce items with complex geometry and integrated structures characterized by high specific resistance. To accurately model SPF processes it is important to know the constitutive equation of the material. With the help of the finite element method (FEM) this study proposes an innovative method for determining the characteristic parameters of superplastic materials. This method requires the material to undergo free-forming tests in which the stress state induced is biaxial (nearer the real stress conditions to which the material is subject during the SPF processes that are of interest to industry). The dimensions of the sheet and the forming die do not place any constraint on the application of this type of characterization procedure. The method adopted to characterize superplastic materials was validated by means of experimental activity carried out on an AZ31 magnesium-based alloy.

Enhanced superplasticity of magnesium alloy AZ31 obtained through equal-channel angular pressing with back-pressure

Abstract: Excellent superplastic elongations (in excess of 1,200%) were achieved in a commercial cast AZ31 alloy processed by low temperature equal-channel angular pressing (ECAP) with a back-pressure to produce a bimodal grain structure. In contrast, AZ31 alloy processed by ECAP at temperatures higher than 200 °C showed a reasonably uniform grain structure and relatively low ductility. It is suggested that a bimodal grain structure is advantageous because the larger grains contribute to strain hardening thus delaying the onset of necking, while grain boundary sliding associated with small grains provides a stabilizing effect due to enhanced strain rate sensitivity.

Evolution of Ultra-Fine Grains in AZ31 and AZ61 Mg Alloys during Multi Directional Forging and Their Properties

Abstract: AZ31 and AZ61 Mg alloys were multi directionally forged (MDFed) during decreasing temperature from 623 K to 423 K to cumulative strain of ΣΔe = 4.8 by Ae = 0.8 pass strain at a strain rate of 3 x 10 -3 s -1 . In both Mg alloys, the average grain size gradually decreased with increasing cumulative strain. After straining to ΣΔe = 3.2, i.e., after 4 passes of MDF, ultra fine grained (UFG) microstructures with average grain size of 1 μm were uniformly evolved. By prolonged straining, the grains became further finer. The AZ61 Mg alloy MDFed to ΣΔe = 4.0 showed quite high hardness over 1.2GPa, while that of the AZ31 Mg alloy was 850 MPa at ΣΔe = 4.8. The differences of UFG evolution and mechanical behaviors during MDF of AZ 31 and AZ61 Mg alloys are precisely investigated.

High temperature deformations of Mg-Y-Nd alloys fabricated by different routes

Abstract: The high temperature tensile properties and microstructural evolutions of Mg-5Y-4Nd (WE54) alloys with extrusion and extrusion followed by equal channel angular extrusion (ECAE) were investigated at elevated temperatures. The results suggested that the as-extruded specimens exhibited enhanced ductility at 400 and 450 degrees C, and a maximum elongation of 290% was obtained at 450 degrees C and 1 x 10(-3) s(-1), corresponding to the m-value of 0.38. On the other hand, ECAE following extrusion led to high strain and/or superplasticity behaviors, and a maximum elongation of 600% was achieved at 400 degrees C with the m-value of 0.49. In addition, the active energies of WE54 alloys with different fabrication routes were examined and compared. (C) 2008 Elsevier B.V. All rights reserved.

Improvement of room-temperature superplasticity in Zn–22 wt.%Al alloy

Abstract: Zn–22 wt.%Al alloy with sub-micrometer-grained microstructure was prepared by three different processing routes: (1) solid solution treatment + quenching + aging, (2) solid solution treatment + quenching + 8 passes equal channel angular pressing (ECAP) at room temperature (RT), and (3) Route 1 + cryo-rolling at 203 K (with a reduction of 75%). The samples were tested in tension at strain rates range from 1 × 10 −4 to 1 × 10 0 s −1 at room temperature. All the samples processed through these three routes exhibit good superplasticity at room temperature. At the strain rate of 4 × 10 −3 s −1 , samples processed via Routes 2 and 3 reach a high elongation of 335 and 315%, respectively. At a higher strain rate of 1 × 10 −1 s −1 , the elongation of samples processed by these two routes is still higher than 220%. Besides the high elongation, at strain rate of 4 × 10 −2 s −1 , a high value of strain-rate sensitivity (∼0.35) was observed in samples processed by Route 2. A room-temperature grain growth during ECAP was also observed in the alloy.

Achieving superplastic behavior in fcc and hcp metals processed by equal-channel angular pressing

Abstract: Processing through the application of severe plastic deformation provides an opportunity for producing polycrystalline metals with ultrafine grain sizes in the submicrometer or nanometer ranges. If these small grains are reasonably stable at elevated temperatures, the materials will exhibit large elongations to failure when tested in tension without the development of any significant necking within the gauge lengths. Examples of superplastic flow are presented for ultrafine-grained aluminum and magnesium alloys. The results available to date reveal several similarities with conventional superplastic materials in terms of both the mechanical properties and the development of internal cavities during deformation. An important difference visible especially for Al-based alloys is that, because of the exceptionally small grain size after processing, optimum superplastic flow occurs at strain rates which are significantly faster than in conventional alloys.