Papers by Keyword: Superplasticity

Abstract: The data on the occurrence of martensitic transformation in steel under the action of a magnetic field were obtained by the electric resistivity method. The obtained data indicate the possibility of stress-assisted martensite formation in the temperature range of M_s-M_d (in which superplasticity of austenite is observed). This possibility is due to the magnetic heterogeneity of austenite. Nanosized regions with a ferromagnetic order are present in the paramagnetic matrix. They can perceive the energy of the external magnetic field through the magnetostrictive stresses and change the fields of the elastic forces in the crystal lattice. All this leads to a decrease in the energy of formation of the nucleation center.

Abstract: High-entropy alloys (HEAs) are currently attracting much interest because they offer unique properties and good ductility at low temperatures. These materials are of interest primarily because they contain five or more principal elements, with each element having a concentration between 5 and 35 at. %, and yet they have very simple structures based on solid solution phases. Superplasticity is defined formally as a tensile elongation without failure of at least 400% and very recent experiments have shown that the HEAs also have a potential for exhibiting superplastic ductilities when testing at elevated temperatures. Since superplasticity requires a very small grain size, typically <10 μm, it is feasible to process HEAs using severe plastic deformation in order to introduce significant grain refinement. The objective of this review is to summarize the recent results showing superplasticity in HEAs and to compare directly the superplastic flow in HEAs and superplasticity in conventional metallic alloys.

Abstract: It is widely accepted that the dominant deformation mechanism of fine-grained superplasticity is through grain boundary sliding (GBS) that occurs in fine-grained materials. However, it has been reported that in “Class I” solid solution alloys, superplastic-like behavior controlled by trans-granular deformation occurs by solute drag creep. In this study, we have investigated superplastic behavior in a fine-grained aluminum solid solution alloy with a thermally unstable microstructure. To obtain fine-grained microstructure, friction stir processing (FSP) was applied to a commercial 5083 aluminum (Al−Mg) alloy. An equiaxial fine-grained microstructure with a grain size of 7.4 μm was obtained after FSP; however, this microstructure was unstable at high temperatures. Generally, for fine-grained superplasticity or GBS to occur or continue, the fine-grained microstructure must be smaller than 10 μm during high-temperature deformation. However, a large elongation of over 200% was observed at high temperatures despite the occurrence of grain growth. From microstructural observations, it was determined that a fine-grained microstructure is maintained in the early stage of deformation, but at strain levels greater than 100%, trans-granular deformation occurs. The microstructural feature of this trans-granular deformation is similar to the deformation microstructure of solute drag creep observed in “Class I” solid solution alloys. This indicates that a change in the deformation mechanism from GBS to solute drag creep takes place during high-temperature deformation. Here, based on our observations on our model system, which is a thermally unstable aluminum solid solution alloy, we discuss the possibility of a superplastic elongation occurring by means of a transition of the deformation mechanism.

Abstract: Recent Al7075 severe friction stir processing (FSP) data gave new insights regarding the relationship among processing, microstructure and high temperature behaviour. Grain boundary sliding, GBS, usually operates with fine, equiaxed and highly misoriented grains although, so far, the variable misorientation is missing from the constitutive equation. A collection of very fine microstructures comprising various grain size and misorientation values is employed to evidence the relative importance of grain size vs misorientation in the superplastic behaviour of the processed alloy. This relationship is included into a new GBS constitutive equation incorporating the average misorientation as a variable.

Abstract: Superplastic titanium alloy (SP-700 with nominal composition of Ti-4.5Al-3V-2Fe-2Mo) an alpha-beta alloy, with a beta-rich fine microstructure and excellent superplastic formability has wide applications in aerospace components, metal wood heads, tools, automotive components. However, very little information is available regarding friction stir processing (FSP) characteristics of this alloy. This study discusses the trials of FSP of this highly formable titanium alloy. Results are discussed in terms of hardness and temperature measurements and microstructural observations.

Abstract: Titanium Ti-6Al-4V alloys are known to exhibit interesting superplastic properties for different conditions of temperature and strain rate, depending on the initial grain size. Even if superplasticity is generally explained in terms of grain boundary sliding (GBS) accompanied by several accommodation mechanisms, it appears that the micromechanisms of superplasticity are still controversial especially at the grain scale and even more at lower scale. These micromechanisms, involving microstructural evolution, depend also on the SPF conditions (temperature, strain rate and initial microstructure). In this study, the flow stress in the Ti-6Al-4V alloy is investigated for different strain rate and for temperature in the range of the α/β transformation. The preferred orientation evolution of alpha phase grains for different percentage of deformation is studied for a non-optimal SPF regime (920°C-10^-4 s^-1) in order to highlight the microstructural evolution and so the deformation mechanisms involved. For that, mechanical interrupted test combined with Scanning Electron Microscopy (SEM) and Electron Back Scatter Diffraction (EBSD) are used.

Abstract: Mechanical behavior and microstructure evolution of the cast Ti-43.2Al-1.9V-1.1Nb-1.0Zr-0.2Gd-0.2B alloy were studied at temperatures from 1100 to 1250°С and strain rates in the range 0.001-1 s^-1. Following phase fields (α₂+γ), (α+γ), (α) and (α+β) during heating of alloy were revealed. Microstructure analysis after deformation and mechanical behavior allowed defining main processes of structure formation. Two temperature-strain rate conditions with pronounced superplastic behaviour were found: the first one corresponded to the (α₂+γ)-phase field (1100°C), where the microstructure had mainly a lamellar morphology, and the second was associated with the (α+β)-phase field (1250°C), in which the α-phase dominated. At T=1100°C and έ=0.05 s^-1 the maximum strain rate sensitivity m was of 0.40. At T=1250°C and έ=0.5 s^-1 the maximum strain rate sensitivity m was of 0.59. In the (α₂+γ)-phase field, superplastic behavior was associated with the transformation of the lamellar structure into globular one. In the (α+β)-phase field, it was due to the formation of a homogeneous refined microstructure during dynamic recrystallization. The relationship between coefficient m value and microstructure formed was discussed.

Abstract: The microstructure and superplasticity of the thermomechanically proceeded sheets of Al - 3wt.%Mg - 0.25wt.%Zr were investigated. High density of L1₂-structured fine dispersoids of Al₃Zr metastable phase was observed by TEM analysis. Alloy demonstrated high recrystallization resistance at elevated temperature due to Al₃Zr dispersoids. The tensile tests were carried out in a temperature range of 440-500 °C and a strain rate of 1×10^-3 to 1×10^-2 s^-1. The maximum elongation to failure of 370% was observed at 480 °C at the constant strain rate values of 2×10^-3 and 5×10^-3 s^-1.

Abstract: Since the development of the Ti54M titanium alloy in 2003, its application within the aerospace sector has gradually increased due to the combination of properties such as improved forgeability and machinability, low flow stress at elevated temperatures, and superplastic characteristics. However, for the successful exploitation of Ti54M a comprehensive understanding of its mechanical characteristics, microstructure stability, and superplastic behaviour is required. The superplastic forming of titanium alloys is characterised by high deformation at slow strain rates and high temperatures which influence the material microstructure, and in turn, determine the forming parameters. These mechanisms make the prediction of the material behaviour very challenging, limiting its application within the aerospace industry. Even though Ti54M has been commercially available for over 10 years, further studies of its mechanical and superplastic properties are still required with the aim of assessing its applicability within the aerospace industry as a replacement for other commercial titanium alloys. Therefore, in this work a study of the mechanical and superplastic properties of Ti54M, in comparison with other commercial titanium alloys used in the aerospace industry - i.e. Ti-6AL-4V, and Ti-6-2-4-2 - is presented. The final objective of this study, carried out at the Advanced Forming Research Centre (AFRC, University of Strathclyde, UK), is to obtain material data to calibrate and validate a model capable of estimating the behaviour and grain size evolution of titanium alloys at superplastic conditions.

Abstract: Superplastic sheet metal forming allows the production of complex parts that are not formable under normal conditions. Superplastic sheet metal forming processes are normally based on the same common principle: the sheet metal is firmly clamped between the die halves and is blow-formed by means of gas pressure. Generally superplastic forming can only be achieved in a very narrow range of strain rates and temperature. Superplastic materials are relatively stable when deformed; this behavior is related to the observation that the flow stress of a superplastic material is very sensitive to the rate of deformation. This paper aims to study the formability characteristic of Magnesium alloy by considering variable parameters, such as the sheet thickness, forming pressure and forming temperature. The forming time of 120 minutes is constant for the formability test. Keywards: Multi dome test, superplasticity, Mg – alloy, Thermomechanical processing, Formability.