Showing papers on "Superplasticity published in 1975"

SP-Mylonites: Origin of some mylonites by superplastic flow

Abstract: Superplasticity in fine-grained materials is characterized by extensive grain boundary sliding. This phenomenon can take place only in special conditions. Six criteria are thus defined to determine when Superplasticity has been active. Applications of these criteria to several examples of mylonites are discussed and we conclude that superplasticity explains some types of mylonites and the tectonic banding that they exhibit.

Deformation of Ceramic Materials II

Abstract: In recent years there has been extensive progress in the research on plastic deformation of ceramic materials. This volume constitutes the proceedings of an international symposium held at Pennsylvania State University in July of 1983. It includes studies of semiconductors and minerals which are closely related to ceramic materials. This conference emphasized the deformation behavior of crystals and polycrystalline and polyphase ceramics with internationally recognized authorities as keynote lecturers on the major subtopics. The papers in this volume discuss dislocation dynamics and deformation of single crystals, including binary oxides, ternary oxides, silicates, nonstoichiometric oxides, covalent materials and halides; and the effects of point defects and twinning. Others address deformation of polycrystalline ceramics consider oxides, nonoxides, polyphase materials and superplastic deformation. There are also several papers dealing with cavity nucleation and creep crack growth, representing a major new research thrust in ceramics research.

Creep behaviour in the superplastic Pb–62% Sn eutectic

Abstract: The creep behaviour of the superplastic Pb-62% Sn eutectic was investigated for grain sizes from 5·8 to 14·5 μm and at temperatures in the range from 336 to 422 K. The results showed a sigmoidal relationship between strain rate and stress. At intermediate strain rates (∼ 10−5–10−2 sec−1), the stress exponent was ∼1·65, the exponent of the inverse grain size was ∼2·3, and the activation energy was similar to the value anticipated for grain boundary diffusion. At very low strain rates (≲10−5 sec−1), the stress exponent was ∼3·0, the exponent of the inverse grain size was ∼2·3, and the activation energy was similar to the value anticipated for lattice self-diffusion. The results are not entirely consistent with either of the two major theories of superplasticity, but suggest instead the sequential operation of two different deformation processes.

On the measurements of superplasticity in an Al-Cu alloy

Abstract: The usual method of measuring the strain rate sensitive ‘m’ values of superplastic materials through differential cross-head speed is found to result in improperly definedm values;m is found to depend strongly on the strain to which the material is subjected, especially at low strains. In this connection, the shape of the log stress-log strain rate curve is examined for the Al-33 wt pct Cu eutectic alloy. The inherent grain growth of the very fine grains which occurs during deformation, and the strain dependence ofm at low strains, are shown to be the causes of the familiarS shape of the log stress-log strain rate curves for the Al-Cu alloy. At high strains (15 to 20 pct and higher) where the stress is no longer importantly strain sensitive, the log stress-log strain rate curve is a straight line of slope near 0.5. The elongation at fracture also does not go through a maximum but continues to increase slowly to the lowest strain rate examined: 10-7 per s.

The influence of strain rate on ductility in the superplastic Zn–22% Al eutectoid

Abstract: There is an optimum range of strain rate for maximum ductility in the superplastic Zn–22% Al eutectoid, and the elongation at fracture decreases at both low and high strain rates. For a testing temperature of 473 K and a grain size of 2·5 μm, tensile specimens deform to strains in excess of 2000% over almost two orders of magnitude of strain rate.

Ultrafine grain Al-Mg alloy product

Abstract: This invention is directed to an Al-Mg alloy product containing about 0.75-5% aluminide forming transition elements, such as iron, cobalt and nickel. After cold working, the alloy product readily recrystallizes at elevated temperatures to an average grain size less than 15 microns, usually 11 microns or less to provide high mechanical properties and a degree of superplasticity.

Transformational Superplasticity in the Bi2O3‐Sm2O3 Eutectoid System

Abstract: Transformational superplasticity associated with the eutectoid reaction in the Bi2O3-Sm2O3 system was observed and characterized in hypoeutectoid and hypereutectoid compositions. Transformational strain varied linearly with applied stress and exhibited a stress-axis intercept, or threshold stress, that is related to the proeutectoid microconstituent. Results are explained quantitatively using the analysis by Greenwood and Johnson and a creep mechanism.

The Mechanical Behavior of Rapidly Solidified Al-Al2Cu and Al-Al3Ni Eutectics at Elevated Temperatures

Abstract: The mechanical behavior of semi-continuously cast Al-Al2 Cu and Al-Al3 Ni eutectics has been studied in tension at temperatures between 350–500 deg C (Al-Al2 Cu) and 500–625 deg C (Al-Al3 Ni). The microstructures of both eutectics were found to be relatively unstable at elevated temperatures, even in the absence of deformation, as a result of their fine, imperfect microstructures in the as-cast condition. During deformation spheroidization occurred rapidly, so that the fibrous/lamellar reinforcement was largely eliminated. During hot deformation of the Al-Al2 Cu eutectic, the “degenerate” lamellar structure was gradually replaced by an equiaxed microduplex structure via polygonization of both phases. This structure subsequently demonstrated superplastic flow, with an increase in the strain-rate sensitivity index from about 0.3 to about 0.5. Final failure resulted from nucleation of cavities at Al2 Cu - Al2 Cu - Al triple junctions with cavity growth along Al2 Cu-Al2 Cu grain boundaries. The Al-Al3 Ni eutectic did not exhibit superplastic flow probably due to the lower volume fraction of the Al3 Ni phase, which “spheroidized” during deformation. A process for fabricating wire by a continuous process from the melt is outlined.

Theoretical and experimental constitutive equations of superplastic behaviour: discussion

Abstract: Two basic models of superplastic deformation are considered. The stress strain-rate curves obtained by combining these two models for a material of different grain sizes at various temperatures are shown to be very different. This is due to the introduction of physically ill-defined parameters in the theoretical relations. The experimental results obtained from mechanical tests do not help in clarifying the situation since the microstructure is continually changing during such experiments. Consequently, the validity of the deformation mechanisms in dealing with models of constant grain-size cannot be demonstrated by fitting theoretical curves to the experimental results of mechanical tests.

Superplasticity in AI–17 wt.-%Cu Alloy

Abstract: Hot-rolled Al–17%Cu alloy deforms superplastically at elevated temperatures with large values of strain-rate sensitivity(> 0·30) and large, neck-free elongations (> 200%). Two mechanisms appear to be involved in the deformation process. At lower stresses and strain rates the activation energy of the deformation process is similar to that for grain-boundary diffusion while at higher stresses and strain rates the activation energy corresponds to that for volume diffusion. The room-temperature properties of hot-rolled Al–17%Cu alloy are comparable to those for conventional Al alloys but superplastic deformation at elevated temperature causes a deterioration of room temperature properties because of agglomeration and sintering of the Al2Cu intermetallic phase.

High duty ductile cast iron with superplasticity and its heat treatment methods

Abstract: A spheroidal graphite cast iron containing some carbide stabilizing elements to have the maximum strain rate sensitivity factor of more than 0.3 and having very refined grain matrix structure, so that having an improved toughness at room temperature and having a matrix which is composed of fine austenite and fine ferrite grains and superplastic properties at the temperature between the eutectoid transformation temperature range and 50° C higher than that temperature range; and the method of heat treatment to obtain the said spheroidal graphite cast iron.

The structure and properties of a cold-rolled and annealed Al-0.8 wt % Zr alloy

Abstract: The effect of annealing at 400‡ C on the microstructure of a cold-worked Al-0.8 wt % Zr alloy is reported. It is shown that the initially high dislocation density in the cold-rolled material is progressively reduced, although the grains and subgrains were exceptionally resistant to coarsening. Precipitation of the metastable cubic Al3Zr phase occurred, both discontinuously in the form of fan shaped precipitates and also on the grain boundaries and within the grains as small, nearly spherical particles. The mechanical properties of the alloy at 400‡ C are consistent with a major dislocation contribution to the overall deformation process, in contrast with most other fine grained materials which are superplastic.

Variations in activation energy for superplastic flow

Abstract: The activation energy at constant stress multiplied by a strain rate sensitivity index equals the activation energy at constant strain rate. Equations developed therefrom are checked using data from the literature for three materials, including those by Holt and Backofen for the Al-Cu eutectic. The analysis is also used for verifying the applicability of the stress function proposed for superplastic flow. A plot of activation energy at constant tensile strain rate vs tensile strain rate is linear for Al-Cu (Zn-Al shows two linear portions). The viscous boundary approach is found to assume the correct form for the dependence of activation energy on applied stress. The lowering of activation energy during superplastic deformation with stress and strain rate is considerable compared to results for high-temperature creep and hot working. Whether the cause is the ultrafine grain structure, the unique mode of deformation or some unidentified factor is not clear.

Superplastic Forming of Titanium Structures

Abstract: : This program was directed at the establishment of superplastic forming of Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo alloys as a production process. Forming studies were pursued to evaluate the process parameters of superplastic forming and to extend the process to forming aircraft structural components. A detailed study was carried out on the effect of forming parameter variations with the two titanium alloys and two configurations. It was found that the degree of forming was directly related to time at elevated temperatures. Optimum sealing techniques to eliminate atmospheric contamination of the titanium sheet material were established. Twenty-three parts were formed in six different configurations to assess the forming parameters needed to fabricate components of varying shapes and dimensions. The process was then extended to superplastic forming of Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo sheet metal alloys through the fabrication of full-scale complex structural configurations. A total of 20 full-scale formed parts was produced utilizing gas pressure forming at elevated temperatures. Structural testing was accomplished on each configuration and alloy showing the compatibility of superplastic formed parts with aircraft structural integrity requirements. Substantial cost savings were shown to be possible.

Press tools and dies made from superplastic alloys - where alloys are hot-shaped using compressed air

Abstract: A pressing die, compression mould or a press tool, are produced from a plate made of a superplastic alloy, by a process in which (1) the plate is located opposite a die-surface; (2) suction is employed to evacuate the air out of the cavity between the die-surface and the plate; (3) the plate is heated so that at least its surface becomes plastic; (4) the plate and the die surface are brought into close contact so the surface of the plate matches the die-surface; (5) the moulded plate is removed from the die surface. Air pressure is pref. used to force the plate against the die surface, and a flexible diaphragm may be used in the die assembly. Used for example, in the process given in GB 1168273, for the mfr. of embossed thermoplastic plates, or for other press tools used for processing plastics.

Superplastic aluminium base alloys

Abstract: Aluminium base alloys exhibiting superplastic deformation characteristics at elevated temperatures, having fully modified eutectic microstructures characterized by the absence of any primary phase and by the presence of at least one finely dispersed eutectic second phase.

Transformational Superplasticity in Pure Bi2O3 and the Bi2O3-Sm2O3 Eutectoid System

Abstract: Transformational superplasticity is a unique type of deformation phenomenon that is associated with an allotropic phase transition. It was first observed by Sauver in 19241. This deformation can be induced by thermally cycling a material about its phase transition while under a small externally applied stress. The superplasticity manifests itself by a large discontinuous increment of strain at the transformation temperature. If the deformation process remains active, multiple cycling about the transition yields additive strains that can lead to total strains far in excess of the strain to fracture experienced under normal conditions. This phenomenon has received considerable metallurgical attention during the past decade, and as such has been the subject of several excellent review articles2–4. It should not be confused with isothermal or structural superplasticity which depends on a fine grain size or a duplex microstructure rather than a phase transformation.