Papers by Keyword: Metastable Phases

Abstract: In this experimental study, the mechanism and kinetic parameters of the metastable phases precipitation in the Al-Mg-Si alloy were determined thermally by differential scanning calorimetric (DSC) analysis. All samples were treated up to 550 °C at heating rates of 5, 10, 20 and 30 °C/min. The apparent activation energy (56.74 kJ/mol) and the Avrami exponent (0.99), were determined by DSC from the non-isothermal method, using the Ozawa, Boswell and Kissinger methods while those obtained by isothermal method using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model were 51.04 kJ/mol and 1.18. The activation energies values indicate that the formation of the metastable phases was mainly controlled by the migration of Mg and Si. The values of n, are characteristics of a growth of plate after saturation of nucleation. The frequency factor (k_o) calculated by the isothermal method is found to be 8.36×10⁷ s^-1.

Abstract: This work aims to study by neutron diffraction the evolution of metastable phases of CuAlBe shape memory alloy after plastic deformation. Two samples were studied: the first one deformed by cold rolling at a reduction rate of 15% and the second, deformed by cold rolling at 15% followed by hot rolling at 200°C for a reduction rate of 30% respectively. Before plastic deformation, the material is fully austenitic at ambient temperature. Its crystallographic texture is mainly characterized by a <001> partial fibber. After deformation, this partial fibber disappears and the crystallographic texture is composed by isolated orientations. At higher reduction rates, the texture of austenitic phase remaining in the material is characterized by a <111> fibber. The rolling process modifies metastable phase quantities. After deformation at a reduction rate of 15%, the volume fraction of metastable austenite remaining is close to 8%. Plastic deformation also greatly modifies the characteristic transformation temperatures and enlarges the hysteresis. The material plastically deformed after hot rolling presents large variations of intensities of diffraction peaks belonging to martensite phase during a thermal cycle at low temperature. This effect is attributed to a reorganization of variants due to an evolution of crystallographic texture of martensite.

Abstract: Many Aluminium alloys use the precipitation of metastable phases to generate optimum properties. The effect of including additional structures such as θ’ and GP zones is described in the context of a hierarchy of metastable structures. Extending a Thermodynamic data base that has been designed solely to deal with equilibrium conditions is a vital prerequisite to handling the heattreatment of aluminium alloys. It is then possible to generate TTT and CCT diagrams, using the Johnson-Mehl-Avrami treatment previously applied in to other materials providing provision is made for the presence of supersaturated quenched-in vacancies. Calculations using JMatPro are given for the expected behavior of commercial aluminium alloys of increasing complexity, including AA319, AA6061 and AA7075.

Abstract: Soft anti-friction alloys are metal-metal composites used in journal bearings as a functionally graded system. The physical processes occurring during severe wear of these products are analysed and placed within the framework of theories on tribolayer formation. A simplified analysis of the thermodynamics of the phenomena permits analysing the anti-friction properties of the alloys in terms of the activity coefficients of their constituents and indicates that during wear, a wear-resistant contact layer is formed which retards further damage.

Abstract: The lattice parameters of the bcc (b) and omega (W) phases occurring metastably in a series of Zr-rich Zr-Nb alloys have been determined at and above room temperature (TR) using neutron diffraction techniques. In the first place, the effect of temperature changes upon the lattice parameters of the b and W phases in alloys with 10 and 18 at.% Nb was monitored using neutron thermodiffraction. A method of analysis is applied to the data which involves a confrontation between the observed structural properties and an idealised -or ``reference'- behaviour (RB) which admits a simple mathematical description. A generalised form of Vegard's law is adopted as RB for the b phase, whereas a specific RB is proposed for the W structure. The experimental data are well accounted for by this interpretation scheme, leading to a picture of the isothermal reactions occurring at high temperature which involves the transfer of Nb from the W to the b phase. Finally, the neutron diffraction data on the W phase are combined with an electron microscopy study for the alloy with 10 at.% Nb aged at 773 K, which provides information on the composition of this phase and its evolution towards thermodynamic equilibrium.

Abstract: A fine microstructure stabilised by dispersed particles is an interesting option for increasing the resistance of Al-Sn tribo-alloys without sacrificing ductility. Particle stimulated and continuous recrystallisation allow achieving this goal, but require careful preconditioning of the microstructure to obtain the desired particle distribution. As-cast slabs of Al 12%Sn2%Pb3%Si contain large plates of b-AlFeSi and modified silicon in an aluminium matrix. Sn forms a reticular network which is liquid at conventional annealing temperatures. Experiments show that metastable b-AlFeSi initially coarsens due to Ostwald ripening, but later dissolves and the remainder transforms into FeSiAl12. Silicon partitions to the liquid, allowing the nucleation of numerous small Si-grains.