High-strain-rate superplasticity of SiC p /8090 aluminum composite

Superplastic tensile tests of a 17 vol.% SiC _p /8090 Al-Li composite were carried out at strain rates ranging from 7.25 × 10^-4 s^-1 to 3.46 × 10^-1 s^-1 and at temperatures from 773 K to 873 K. A maximum elongation of 300% was obtained at a strain rate of 1.83 × 10^-1 s^-1 when tested at a temperature of 848 K which was slightly above the solidus temperature of the composite. The effect of a small fraction of liquid phase on high-strain-rate superplasticity was discussed. Finally, the activation energy of high-strain-rate superplastic deformation was calculated and high-strain-rate superplastic mechanism was discussed.     

High-strain-rate superplasticity in oxide ceramics

Factors limiting the strain rate of superplastic deformation in ceramic materials are discussed on the basis of existing models and experimental results concerning high-temperature plastic deformation, intergranular cavitation and dynamic grain growth. From the discussion, it is indicated that simultaneously fulfilling the following conditions is essential for attaining high-strain-rate superplasticity (HSRS) in ceramic materials: reduction in the initial grain size, enhanced diffusivity, suppressed dynamic grain growth, a homogeneous microstructure and a reduced number of residual defects. In the light of these conditions, explanations are given for HSRS attained in earlier studies on some oxide materials. It is also shown that HSRS can be intentionally attained in doped yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) and composites synthesized from ZrO₂, Al₂O₃ and MgO₂; the tensile ductility of these composites reached 300–2500% at a strain rate of 0.01–1.0 s⁻¹. The postdeformation microstructure indicates that some secondary phases may suppress cavitation damage and thereby enhance HSRS.     

High-strain-rate superplasticity in oxide ceramics

Factors limiting the strain rate of superplastic deformation in ceramic materials are discussed on the basis of existing models and experimental results concerning high-temperature plastic deformation, intergranular cavitation and dynamic grain growth. From the discussion, it is indicated that simultaneously fulfilling the following conditions is essential for attaining high-strain-rate superplasticity (HSRS) in ceramic materials: reduction in the initial grain size, enhanced diffusivity, suppressed dynamic grain growth, a homogeneous microstructure and a reduced number of residual defects. In the light of these conditions, explanations are given for HSRS attained in earlier studies on some oxide materials. It is also shown that HSRS can be intentionally attained in doped yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) and composites synthesized from ZrO₂, Al₂O₃ and MgO₂; the tensile ductility of these composites reached 300–2500% at a strain rate of 0.01–1.0 s^?1. The postdeformation microstructure indicates that some secondary phases may suppress cavitation damage and thereby enhance HSRS.     

金属基复合材料的高应变速率超塑性

综述并评论了金属基复合材料的高应变速率超塑变形机制，描述了金属基复合材料在高应变速率超塑变形中的一些理化现象，说明了变形过程中的各种影响因素，总结了具有高应变速率超塑性能的金属基复合材料及其性能，并指出了在金属基复合材料的高应变速率超塑性研究方面的不足。     

High-strain-rate superplasticity of an AL2009-SICw composite

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Deformation behavior and mechanisms of a nanocrystalline multi-phase aluminum alloy

A nanocrystalline (nc) Al–Fe–Cr–Ti alloy containing 30 vol.% nc intermetallic particles has been used to investigate deformation behavior and mechanisms of nc multi-phase alloys. High compressive strengths at room and elevated temperatures have been demonstrated. However, tensile fracture strengths below 300 °C are lower than the corresponding maximum strengths in compression. Creep flow of the nc fcc-Al grains is suppressed even though rapid dynamic recovery has occurred. It is argued that the compressive strength at ambient temperature is controlled by propagation of dislocations into nc fcc-Al grains, whereas the compressive strength at elevated temperature is determined by dislocation propagation as well as dynamic recovery. The low tensile fracture strengths and lack of ductility at temperatures below 300 °C are attributed to the limited dislocation storage capacity of nanoscale grains. Since the deformation of the nc Al-alloy is controlled by dislocation propagation into nc fcc-Al grains, the smaller the grain size, the higher the strength. This new microstructural design methodology coupled with ductility-improving approaches could present opportunities for exploiting nc materials in structural applications at both ambient and elevated temperatures.     

High strain-rate superplasticity of fine- and ultrafine-grained AA5083 aluminum alloy at intermediate temperatures

Superplastic behavior of fine and ultra fine-grained AA5083 Al alloy was examined using the shear punch test. To achieve fine- and ultra fine-grained microstructures, a relatively new severe plastic deformation (SPD) process, namely Double Equal Channel Lateral Extrusion (DECLE) was employed. The strain rate sensitivity indices (m) of samples were evaluated after 1, 2, 4, and 6 passes for shear strain rates in the range of 3 × 10^− 3 to 3 × 10^− 1 s^− 1 and temperatures in the range of 573 to 673 K. For microstructural observations, TEM images together with the corresponding SAED patterns were prepared and utilized. A considerable increase in the m-value was observed after the first pass of the operation for all testing temperatures. The best condition for achieving a good superplasticity for the alloy was found to be a single pass DECLE at 673 K in the strain rate range of 10^− 2 to 10^− 1 s^− 1. This process condition resulted in an m-value of 0.43, indicative of a high strain rate superplastic deformation behavior. Further passes of the SPD process did not show any sign of superplasticity until the last pass of the operation, during which the m-value slightly increased, compared with the previous pass.     

Microstructure and superplasticity of an Al-Zn-Mg-Cu alloy

A new superplastic process was introduced in this paper. This new process is very simple and easy to use in practical production. Continuous recrystallization occurred during superplastic deformation and very fine grained microstructure formed, which provides excellent superplasticity for the Al-Zn-Mg-Cu alloy 7075.     

High strain rate superplasticity in microcrystalline and nanocrystalline materials

Abstract

Superplasticity has evolved to become a significant industrial forming process. The phenomenon of superplasticity is explored at high strain rates where it is economically more attractive. True tensile superplasticity has been demonstrated in nanocrystalline materials. The difference in the details of superplasticity between the nanocrystalline and microcrystalline state is emphasised.     

High-strain-rate superplastic deformation behavior of a powder metallurgy-processed 2124 Al alloy

High-strain-rate superplastic behavior of a powder-metallurgy processed 2124 alloy prepared through extrusion at a high ratio of 70 : 1 was investigated. A maximum tensile elongation of 700% was obtained at 823 K and at a strain rate of 10^–2 s^–1. Deformation behavior of this alloy was similar to those reported for other many HSR superplastic materials. Incorporation of threshold stress into the constitutive equation reveals that the true stress exponent is 2 and true activation energy for plastic flow is comparable to that for lattice diffusion in pure aluminum. Comparison of the present alloy with the 2124 Al composite indicates that the composite is weaker than the unreinforced alloy in the temperature range where grain boundary sliding is rate-controlled.     

Thermodynamic stabilization of nanocrystalline aluminum

Journal of Materials Science - Nanocrystalline metals are generally unstable due to a large volume fraction of high-energy grain boundaries associated with a small grain size. Preferential dopant...     

The superplasticity and microstructure evolution of TC11 titanium alloy

The superplasticity is the capability of some metallic materials to exhibit very highly tensile elongation before failure. The superplastic tensile tests were carried out at various deformation conditions in this paper to investigate the superplastic behaviors and microstructure evolution of TC11 titanium alloy. The results indicate that the smaller the grain size, the better the superplasticity is, and the wider the superplastic temperature and strain rate is, in which the superplastic temperature is ranging from 1023 to 1223 K and the strain rate is ranging from 4.4 × 10^?5 to 1.1 × 10^?2 s^?1. The maximum tensile elongation is 1260% at the optimum deformation conditions (1173 K and 2.2 × 10^?4 s^?1). For further enhancing the superplasticity of TC11 titanium alloy, the novel tensile method of maximum m superplastic deformation is adopted in the paper. Compared with the conventional tensile methods, the excellent superplasticity of TC11 titanium alloy has been found with its maximum elongation of 2300%.     

The superplasticity and microstructure evolution of coarse-grained Ti40 alloy

The superplastic deformation characteristics of coarse-grained Ti40 alloy have been studied in the temperature and strain rate range of 760–880°C and 5?×?10^?4 to 1?×?10^?2?s^?1, respectively. The alloy exhibited good superplasticity in all test conditions except at 760°C and strain rate higher than 5?×?10^?3?s^?1, with the maximum elongation of 436% at 840°C, 1?×?10^?3?s^?1. The activation energy value was found to be close to the self-diffusion activation energy of Ti40 alloy, suggesting that the rate controlling mechanism was lattice diffusion. The coarse grain was elongated and refined which can be attributed to the occurrence of dynamic recovery and continuous dynamic recrystallisation. These processes were promoted by the subgrain formation and evolution, resulting in the good superplasticity of Ti40 alloy with coarse grains.     

纳米γ-Ni-Fe合金的磁电阻

用机械球磨Fe2O2—NiO氧化物和氢气还原原位合金化法，制备出纳米γ—Ni—xFe(x=20％-39％，质量分数)合金，并通过XRD、TEM、SEM、BET等方法研究了材料的微观结构与制备条件的关系．结果表明：当氢气还原原位合金化的温度为600-700℃时，合金化完全，Ni—Fe合金的晶粒尺寸为15-55nm，平均颗粒尺寸小于100nm；在30K至室温范围内，块体纳米Ni—Fe合金的各向异性磁电阻率（MR)随温度的降低线性地增加．室温MR＝0．98％，在50K，MR值达8．7％，高磁电阻的原因可能是纳米Ni—Fe合金强烈的磁相关界面散射．     

Developing superplasticity in a magnesium AZ31 alloy by ECAP

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.     

A new grain-refinement process for superplasticity of high-strength 7075 aluminium alloy

The superplasticity of high strength 7075 aluminium alloy has been improved to a great extent by the new thermomechanical treatment proposed. This treatment (TMPA) includes solution treatment, overageing, warm-rolling deformation, recrystallization and an artificial ageing process. The maximum elongation may be up to 2100% under deformation at an initial strain rate of 8.33×10^–4s^–1 and a temperature of 510 °C, which is much higher than reported before. Observation of the microstructure changes revealed that the excellent superplastic elongation of the alloy seems mainly to be due to a decrease in the grain growth rate of the alloy and a reduction in the number of cavities nucleated during superplastic deformation.     

铸锭铝合金Al-Cu-Mg-Ti高应变速率超塑性

采用常规熔铸、热处理、小挤压比挤压及轧制这一低成本并适合于工业化规模应用的路线,研制了一种具有高应变速率超塑性的铸锭铝合金Al-Cu-Mg-Ti.拉伸试验结果表明在温度为793K、初始应变速率为3.16×10-1s-1的拉伸变形条件下,其超塑伸长率为218%,流变应力为32.5MPa.断面及表面形貌SEM分析和初熔行为的DSC分析表明,该合金高应变速率超塑性变形来自于晶界滑动和位错滑移,与液相没有关系.