Grain growth behaviour of the Al-Cu eutectic alloy during superplastic deformation

Low Temperature Superplasticity and Its Deformation Mechanism in Grain Refinement of Al-Mg Alloy by Multi-Axial Alternative Forging

Abstract: In practical application, an appearance of low temperature superplasticity (LSTP) is one of necessaries conditions. In this paper, to estimate an appearance and deformation mechanisms of this superplasticity, the role of grain boundary sliding (GBS), intragranular deformation and the change of microstructure during superplastic deformation have been investigated for ultrafine-grained Al-Mg alloy with a grain size of less than 1 μm using Multi-Axial Alternative Forging (MAF) technique. In these materials, it shows that the elongation and strain rate sensitivity (m-value) were 340% and 0.39, respectively, at 473 K under a strain rate of 2.8 x 10 -3 s -1 . These results show that superplastic appearance is possible at 473 K. The void formed at 473 K elongated in parallel to the tensile direction, with a length of 15 μm and a width of 5 μm. The intragranular deformation contribution was estimated from the aspect ratio of the grains after deformation and its contribution ratio was about 33.5 %. Therefore, for the appearance of lower temperature superplasticity with large elongation and m-value, the role of intragranular deformation was the most important factor together with GBS under these conditions. As described above, the MAF technique is one of the most effective methods to produce ultrafine-grained material and appearance of lower temperature superplasticity.

Development of hardness homogeneity and superplastic behavior in an aluminum–copper eutectic alloy processed by high-pressure torsion

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.

Superplasticity, flow and fracture mechanism in an Al–12.7Si–0.7Mg alloy

Abstract: The superplastic behavior of an Al–12.7mass%Si–0.7mass%Mg alloy was investigated under different conditions. Reasonable superplastic elongations were achieved in the fine-grained (9.1 μm) Al–Si–Mg alloy at temperatures ranging from 733 to 793 K at initial strain rates ranging from 1.67×10 –4 to 1.67×10 –3 s −1 . A maximum elongation to failure of 379% was demonstrated with a strain rate sensitivity, m , of 0.52 and an activation energy for flow, Q , of 156.7 KJ/mol at 793 K at an initial strain rate of 1.67×10 –4 s −1 , which is close to the lattice diffusion activation energy of aluminum. The dislocation activity within Al grains indicated that intragranular slip is the accommodation mechanism of grain boundary sliding. EBSD (Electron Backscatter Diffraction) results revealed that most grain boundaries were high angle boundaries and therefore indicated that boundary sliding and grain rotation occurred during deformation. A deformation mechanism map was plotted for the Al–Si–Mg alloy at 793 K and it is shown that the experimental datum points are in excellent agreement with the predictions of the map. Most cavities were formed around silicon particles and the cavity formation mechanism was proposed. The observation on the fracture surface revealed the presence of filaments. The filament quantity or density increased with increasing testing temperature, which can be interpreted by the transition of dislocation viscous glide creep to grain boundary sliding mechanism at elevated temperatures. The formation of filaments was related to the deformation mechanisms and the lattice diffusion at elevated temperatures. The superplastic fracture in the Al–Si–Mg alloy exhibited a diffuse necking and was a pseudo-brittle fracture. The fracture mechanism was intergranular fracture.

Study of dynamic grain growth by electron microscopy and EBSD.

Abstract: The effect of hot deformation on fully recrystallized aluminium-copper alloys (Al-4wt%Cu and Al-33wt%Cu) with different volume fractions of CuAl(2) has been studied. The alloys are Zener pinned systems with different superplastic properties. Strain-induced grain growth, observed in both alloys, was quantitatively estimated by means of electron microscopy and EBSD and compared with the rate of static grain growth. Surface marker observations and in situ hot-deformation experiments combined with EBSD were aimed at clarifying the mechanisms responsible for the changes in the deformed microstructures. A sequence of secondary and backscattered electron images and EBSD maps was obtained during in situ SEM deformation with different testing conditions. Overlaying EBSD maps for the Al-4wt%Cu with channelling contrast images showed that grain boundary motion occurred during deformation, creating a layered structure and leading to an increase in size of some grains and shrinkage of others. Of a particular interest are results related to behaviour of CuAl(2) in superplastic Al-33wt%Cu during deformation, including several problems with the use of EBSD in this alloy.

Effect of hydrogen on high temperature flow behavior of near α-Ti alloy

Abstract: The effect of hydrogen on high temperature flow behavior of VT20, a near α-Ti alloy, was studied using differential strain rate compression tests. Hydrogen level was varied from 0.0015 to 0.36 wt%. Strain rate jump tests were carried out over the strain rate and temperature ranges of 10−3–10−1 s−1 and 600–947 °C, respectively. The addition of hydrogen increased the volume fraction of β phase, decreased the grain size and lowered the flow stress up to test temperature of 900 °C. The values of apparent strain rate sensitivity (m) and activation energy for deformation (Q) were found to vary from 0.03 to 0.46 and 176–382 kJ/mol, respectively, depending on strain rate, test temperature and hydrogen level. With increasing hydrogen content the peak strain rate sensitivity shifted to lower temperatures. The values obtained for m (≥0.30) and Q (~176 kJ/mol) suggested that the deformation mechanism is the grain boundary sliding accommodated by lattice diffusion. The lower values of m (0.20–0.25) suggested dislocation climb as the deformation mechanism and the further decrease in m and increase in Q suggested power law breakdown.

Diffusion-accommodated flow and superplasticity

Abstract: Polycrystalline matter can deform to large strains by grain-boundary sliding with diffusional accommodation. A new mechanism for this sort of deformation is described and modelled. It differs fundamentally from Nabarro-Herring and Coble creep in a topological sense: grains switch their neighbors and do not elongate significantly. A constitutive equation describing the mechanism is derived from the model. The strain-rate may be diffusion controlled, in which case the constitutive equation resembles the Nabarro-Herring-Coble equation but predicts strain-rates which are roughly an order of magnitude faster. Or it may be controlled by an interface reaction—roughly speaking, by the restricted ability of a boundary to act as a sink or source for point defects, or by its restricted ability to slide. The flow behavior of superplastic alloys can be explained as the superposition of this mechanism and ordinary power-law creep (“dislocation creep”). The combined mechanisms appear to be capable of explaining not only the observed relation between strain-rate and stress, but most of the microstructural and topological features of superplastic flow as well.

Diffusion of Al26 and Mn54 in Aluminum

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).

Chemically induced grain boundary migration

Abstract: Experimental evidence is presented, showing that gram boundary motion can be induced by changing the composition by means of grain boundary diffusion. The experiments were carried out by treating specimens of pure iron in an atmosphere of zinc or iron-zinc specimens in an atmosphere with a lower zinc potential. The reaction resembles the grain boundary migration in discontinuous precipitation and provides an experimental method of isolating grain boundary migration for study. The experimental results can be used to evaluate rather directly the grain boundary diffusivity and mobility. The diffusivities thus obtained are several orders of magnitude larzer than the values reported for stationary boundaries. It is concluded that the boundary diffusivity is greatly enhanced by grain boundary motion.

On the mechanism of strain-enhanced grain growth during superplastic deformation

Abstract: Superplastic deformation is often accompanied by grain growth, the rate of which depends on both strain and strain-rate, and is usually well in excess of that found in the absence of deformation. Two models for this process have been developed. In the first, which is most applicable to single phase materials, we assume that the deformation enhancement of grain growth is due to the damage created at triple junctions by grain boundary sliding. A geometrical model is used to show how the recovery of this damage by boundary migration, enhances the normal grain growth process. A second model, more suitable to microduplex alloys, is based on the postulate of Holm el al. [8, Acta metall. 25, 1191 (1971)] that superplastic flow enhances the coarsening of particles which pin grain boundaries. Experimental data for a variety of materials give excellent agreement with these models.

Isotope Effect and Divacancies for Self-Diffusion in Copper

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 ,