Showing papers on "Superplasticity published in 1971"

The measurement of strain-rate sensitivity in superplastic alloys

Abstract: The currently used methods of determining strain-rate sensitivities,m, of superplastic alloys from strain-rate change tests are examined and found to be unsatisfactory. The stressrelaxation method is also investigated and is shown to be applicable to the superplastic Al-Cu eutectic. However, them-values derived by this method, and by a variant of the strain-rate change technique, are significantly lower than those obtained using the conventional strain-rate change tests and this is explained in terms of microstructural factors.

The variation in flow stress and microstructure during superplastic deformation of the Al-Cu eutectic

Abstract: Stress-strain curves have been obtained for the superplastically deformed Al-Cu eutectic tested in tension under constant true strain-rate conditions. It is shown that constant flow stress conditions do not obtain and that, after an initial transient, the flow stress is linearly related to natural tensile strain. Optical metallography has been employed to follow the variation of both inter-phase particle separation and α-Al grain size with strain and it is concluded that the observed strain hardening is due mainly to grain coarsening.

The influence of strain rate dependent work hardening on the necking strain in α-titanium at elevated temperatures

Abstract: Titanium has a “blue brittle” effect like that observed in steels. At the temperature of minimum ductility, the neck in the tensile specimen is very sharply defined. Above the blue brittle temperature, the tensile elongation of this metal undergoes a very significant increase that is due to the development of a diffuse neck. Diffuse necks are also observed in superplasticity. However, the conditions that cause the diffuse neck above the blue brittle temperature in titanium are different from those encountered in superplasticity. In superplasticity, the necking phenomenon is associated with a direct dependence of the flow stress on the strain rate, whereas in the present case it is primarily due to rate dependent work hardening associated with dynamic strain aging.

Application of powder metallurgy to an advanced-temperature nickel-base alloy, NASA-TRW 6-A

Abstract: Bar stock of the NASA-TRW 6-A alloy was made by prealloyed powder techniques and its properties evaluated over a range of temperatures. Room temperature ultimate tensile strength was 1894 MN/sq m (274 500 psi). The as-extruded powder product showed substantial improvements in strength over the cast alloy up to 649 C (1200 F) and superplasticity at 1093 C (2000 F). Both conventional and autoclave heat treatments were applied to the extruded powder product. The conventional heat treatment was effective in increasing rupture life at 649 and 704 C (1200 and 1300 F); the autoclave heat treatment, at 760 and 816 C (1400 and 1500 F).

New developments in superalloy powders.

Abstract: The advantages of a P/M process for Ni-based superalloys are shown in terms of better homogeneity than cast alloys. By using low interstitial-atomized powders densified by direct extrusion, superplastic structures are developed. The results of a thermo-mechanical process are presented through which large grained or even single crystal structures can be fabricated. Finally, an alloy series made specifically for P/M processes is presented. These alloys have no carbon as atomized, densified or formed. The resultant solution-annealed grain sizes are ASTM 1 to 0. These alloys are subsequently carburized to stabilize the grain boundaries.

Superplastic conditioning of ternary and quaternary zinc-aluminum alloys

Abstract: Ternary and quaternary zinc-aluminum alloys are conditioned to exhibit superplastic behaviour by hot-working thereof at temperatures between about 205*C and the eutectoid temperature of the alloy. This conditioning may be preceded by homogenization and quenching of the alloy and followed by drawing of the alloy.

A method of forming sheet or plate material

Abstract: 1,251,488. Forming metal; working at specified temperatures. PRESSED STEEL FISHER Ltd. 22 Aug., 1970, No. 40556/70. Headings B3Q and B3V. Apparatus for forming a superplastic alloy sheet 26 against a die 12 in an enclosure 11 having blank holding means 25 comprises a reservoir 14 containing a forming material 15 which is liquid at least at the forming temperature, a heater 29 for the material 15, a gas pressure supply 18 for expelling material from the reservoir into the enclosure 11, an overflow 20 for returning excess material 15 to the reservoir and a valve 19 for releasing gas pressure and allowing material 15 to flow back from the enclosure 11 to the reservoir. Thus after heating the sheet to the forming temperature, a partial vacuum set up by gravity-return of the material 15, which may be a low-melting-point alloy, oil or a salt, draws the sheet against the die. The forming action may be assisted by compressed-air supplied to a cover 27. Automatic blank loading, stripping and unloading devices may be provided and an air jet for cooling the work.

The relationship between superplasticity and formability

Abstract: Superplasticity is a high temperature deformation phenomenon in which samples exhibit extremely large tensile elongation. The key to obtaining superplasticity is producing grain sizes in the order of ten microns or less, and maintaining these fine structures for reasonable periods of time at temperatures in excess of 50% of the absolute melting point. This is most easily accomplished in two-phase alloys. In most known alloys which exhibit superplasticity, the necessary microstructure is produced by forming operations involving hot or cold working steps. The fine microstructure achieved then contributes to easier hot formability at both high and low strain rates. At high strain rates, where the superplastic effect would not be expected in the tensile test, fine-grained material requires lower working loads than coarse-grained material of identical composition. It has been shown that significant amounts of deformation can be achieved in realistically short periods of time. At low strain rates, where superplasticity is observed in tension tests, the freedom from necking, and the low stresses required for appreciable deformation will probably lead to new forming operations previously impossible in metallic systems. The forming of materials, which will deform superplastically in tension, should not be relegated to the role of a low strain rate laboratory curiosity.

Process for inducing superplasticity in zinc or zinc-aluminum alloys containing copper

Abstract: Process for imparting superplasticity to Zn or Zn-Al alloys containing 6-12 percent Cu by warm working an ingot of the metal until substantially the entire cross section has been worked and reduced to a thickness suitable for thermoforming. Stock so prepared can be thereafter formed with the usual superplastic deformation techniques, and resulting manufactures exhibit superior resistance to creep and corrosion and exhibit greater tensile strength.

The Effect of Material Properties on Tension Zone and Boundary Shear-Stress in Extrusion

Abstract: The complete solution of stress and strain were obtained for commercially pure aluminum and superplastic alloy of the eutectic of Pb-Sn in an axisymmetric extrusion process of extrusion ratio 4 and a half cone angle of 45°. The extrusion speed was 0.1/min for aluminum and.003 in/min for superplastic Pb-Sn. The stress components along and perpendicular to the flow lines were calculated for different values of workhardening and strain-rate exponents. The shear stress along the boundary was determined. The size of the tension zone was compared for different material properties.