scispace - formally typeset

Journal ArticleDOI

Activation energy for superplastic deformation of an AlCu eutectic alloy

01 Dec 1985-Scripta Metallurgica (Pergamon)-Vol. 19, Iss: 12, pp 1419-1423

AbstractEssais de vitesse de deformation differentielle entre 440 et 540 C. La sensibilite a la vitesse de deformation pendant la deformation superplastique est de 0,59, elle ne depend pas de maniere significative de la temperature. Determination de l'energie d'activation

...read more


Citations
More filters
Journal ArticleDOI
Abstract: An Al–33% Cu eutectic alloy was processed by high-pressure torsion (HPT) at a pressure of 6.0 GPa for 1/4 to 10 turns. Examination after processing showed a gradual evolution to a reasonable level of hardness and microstructural homogeneity after 5 or more turns. Tensile tests were conducted at a temperature of 723 K on specimens processed through 5 and 10 turns of HPT. These specimens exhibited excellent superplastic properties with a maximum elongation of ∼1250% at strain rates lower than 10−3 s−1. The results also showed that the maximum elongation is displaced to a faster strain rate when the HPT processing is conducted to a higher number of turns. An analysis demonstrates that superplastic flow in the Al–Cu alloy processed by HPT is well described by a theoretical relationship that was developed for conventional superplastic materials.

36 citations

Journal ArticleDOI
Abstract: Department of Mechanical and Industrial Engineering, University of Manitoba, Winnipeg, Manitoba,Canada, R3T 5V6(Received March 16, 2000)(Accepted in revised form April 21, 2000)Keywords: Superplasticity; Strain rate sensitivity index; Activation energy; SuperalloyIntroductionIN718 superalloy is reported [1–5] to exhibit superplasticity in the temperature range of 1173–1273 K.Superplastic behavior is characterized by high strain rate sensitivity index (m $ 0.3) and low activationenergy (Q), which is generally comparable with that for grain boundary diffusion. However, the stress(s) 2 strain («) curves obtained under superplastic condition are known [6] to exhibit strain hardeningand/or strain softening, especially, in the early part of deformation. Under such non-steady state flowcondition, the values of m and Q can vary with strain [6–7], making them as the non-unique parametersof the constitutive relationship [8] for high temperature deformation. Fortunately, the s-« curves for alarge number of superplastic materials [7] exhibit steady state flow behavior subsequent to the largeinitial transient stage of strain hardening/softening and concomitant microstructural evolution. Hence,the stress (s)–strain rate («) data obtained [9] beyond a certain strain level represent a reasonable steadystate condition. Therefore, the aim of the present work was to evaluate the m and Q parameters, of theconstitutive relationship for superplastic deformation of IN718 superalloy after such prestraining.ExperimentalIN718 superalloy of superplastic forming (SPF) grade was obtained in the form of 1.3 mm thick sheet.The analyzed composition of the alloy is given in Table 1.Tensile specimens with gage length and width of 20 mm and 5.3 mm, respectively, were machinedand tensile tested by Instron Universal Testing Machine. Test temperatures were controlled to anaccuracy of 61 K, and the specimens were soaked at the test temperature for 30 min. prior todeformation. Metallographic samples were prepared by mechanical polishing followed by and electro-lytic etching with 10% oxalic acid in water. Grain size was measured by mean-linear intercept methodand the error in the grain size data reported here were within 65%.

35 citations

Journal ArticleDOI
Abstract: Experiments were conducted on the Al-33% Cu eutectic alloy in both an annealed and an as-extruded condition. For both conditions, the relationship between flow stress and strain rate is sigmoidal with maximum ductilities occurring at intermediate strain rates in the superplastic Region II. Specimens fail by necking at the faster strain rates in Region III, but the severity of necking is reduced with decreasing strain rate and the necks are very diffuse in Region II. There is extensive internal cavitation in the fractured specimens, especially at lower strain rates and in the vicinity of the fracture tip. It was observed that cavities form preferentially on theα-θ interphase boundaries. It is shown by calculation that the observed change from small rounded cavities to large cavities elongated along the tensile axis is reasonably consistent with the theories of cavity growth in fine-grained superplastic alloys

30 citations

Journal ArticleDOI
Abstract: Experiments were conducted to determine the mechanical properties of the superplastic Al-33 Pct Cu eutectic alloy at temperatures from 673 to 723 K. Specimens were tested in a well-annealed condition and there was no evidence for grain growth even at the lowest experimental strain rate of 6.7 × 1(10-7 s-1. It is shown that the stress-strain curves rapidly attain a steady-state value at strain rates below ′10-4 s-1, and there is a sigmoidal relationship between stress and strain rate which may be obtained using several different testing procedures. The maximum elongation to failure recorded in these experiments was 1475 Pct at an initial strain rate of 1.3 × 10-5 s-1. The true activation energy for plastic flow is 175 ±11 kJ mol-1 in the superplastic region II, but it increases to 299 ± 18 kJ mol-1 at low strain rates in region I. The exponent of the inverse grain size is 2.1 ±0.3 in region II. These results show that, when the grains size is stable, there is a genuine region I in the Al-33 Pct Cu alloy at initial strain rates below ∼10-5 s-1.

28 citations

Journal ArticleDOI
Abstract: Flow behavior and microstructural evolution in an Al-Cu eutectic alloy of equiaxed grains were investigated over e ≃ 2× 10−6 to 2 × 10−2 s−1 andT = 400° to 540 °C Depending on the test conditions, there appeared either strain hardening or strain softening predominantly in the early part of the σ-e curves The microstructural observations showed evidence for grain growth, development of zig-zag boundaries, dislocation interactions, and cavitation The grain growth adequately accounts for the observed strain hardening at higher temperatures and lower strain rates However, at lower temperatures the strain hardening can be only partly accounted for by the observed grain growth; under this condition, some dislocation interactions also contribute to the strain hardening The presence of cavitation causes strain softening predominantly at higher strain rates Therefore, to develop a proper understanding of the superplastic behavior of the Al-Cu eutectic alloy, it is necessary to take into account the influence of dislocation interactions and cavitation along with that of grain growth

25 citations


References
More filters
Journal ArticleDOI
Abstract: Diffusion coefficients of Al26 and Mn54 in aluminum have been determined between 450° and 650°C. Low specific activities of the isotopes necessitated use of a thick layer technique. The exact solution to Fick's second law for the appropriate boundary conditions was used in treating the data. Temperature dependence of the diffusion coefficients may be expressed by the following equations: DAl26=1.71 exp−(34 000/RT)DMn54=0.22 exp−(28 800/RT).

403 citations

Journal ArticleDOI
Abstract: The simultaneous diffusion of 64Cu and 67Cu has been measured in copper single crystals from 890 to 1061 °C. The strength of the isotope effect f ΔK is 0.684 ± 0.014 and is independent of temperature within the experimental error. This indicates that the contribution of divacancies to self-diffusion in copper is small. The diffusion coefficient of 67Cu in copper single crystals was measured from 700 to 1060 °C and is given by . Die gleichzeitige Diffusion von 64Cu und 67Cu wurde im Bereich von 890 bis 1061 °C an Kupfereinkristallen gemessen. Die Grose des Isotopeneffekts f ΔK ergibt sich zu 0,684 ± 0,014 und ist temperaturunabhangig innerhalb der Mesgenauigkeit. Dies bedeutet, das der Beitrag von Doppelleerstellen zur Selbstdiffusion in Kupfer gering ist. Der Diffusionskoeffizient von 67Cu in Kupfereinkristallen wurde im Bereich von 700 bis 1060 °C gemessen und ergibt sich zu ,

165 citations

Journal ArticleDOI

153 citations

Journal ArticleDOI
Abstract: The high temperature steady state deformation behaviour of PbSn eutectic alloy was investigated over wide ranges of grain size (9.7 – 32.0 μm), temperature (298 – 443 K) and strain rate (10 −7 – 10 −2 s −1 ), after stability of microstructure and strength had been achieved through prior tensile deformation. The σ-ϵ plots for these materials consisted of regions II and III. In region II the strain rate sensitivity index m is 0.6 ± 0.1 and the grain size exponent p is 3.34 ± 0.23. The activation energy Q in region II was found to depend on temperature with Q = 44.7 ± 1.1 kJ mol −1 below about 408 K and Q = 81.1 ± 3.9 kJ mol −1 above this temperature. These experimental parameters do not agree with any of the superplasticity models that have been proposed to date. However, in region III the parameters m = 0.09 ± 0.01, p = 1.00 ± 0.72 and Q = 100.0 ± 8.8 kJ mol −1 observed are in better agreement with the recovery creep model of Gifkins.

147 citations