Daniel Salas

Entropy Changes in Ferromagnetic Shape Memory Alloys

Several features of the entropy change S related to the martensitic transformation (MT) in metamagnetic alloys are discussed. In these alloys a change in magnetic order is concomitant with the MT, as it occurs between ferromagnetic austenite and non-magnetic (weakly magnetic) martensite. In this case it has been shown that S strongly decreases as the MT temperature range go far below the Curie temperature of austenite. The behavior of S can be understood considering the different signs of the lattice and magnetic contributions to the total entropy change. It has been shown that the so called kinetic arrest of the MT is directly related to the decrease of S, which in the limit S → 0 leads to the suppression of the driving force for the transformation.

Microstructural development during aging of 2014 aluminum alloy composite

The 2014 aluminum alloy reinforced with 0.1 and 0.15 volume fraction of alumina particles (VFAP) have been solutionized for a range of time from 1.5 to 20 h at 813 K. The effect of solutionizing time (ST) on the age hardening response of the composites has been studied and compared with the characteristics exhibited by the monolith. The results indicate that increasing the ST decreases the time required to get the peak hardness (TPH) values in the monolith but the composites do not show a systematic monotonic behavior. The TPH values first decrease and then increase with an increase in ST at an aging temperature of 473 K for the composite. It has been speculated that he ST influences the concentration of quenched-in vacancies and continued heating may affect the bonding between particles and matrix which can generate additional dislocations throughout the solutionizing process due to curvature effects.

The control of grain size and distribution of particles in a (6061 alloym/(Al2O3)P composite by solutionizing treatment

The grain growth during an isothermal treatment at a solutionizing temperature of 540°C has been studied in a composite containing 6061 aluminum alloy matrix with A12O3 particles. The grain growth law is generally applicable to the composites containing 0.10, 0.15, and 0.20 volume fraction of the A12O3 particles (VFAP). It has been observed that the grain growth process involves the disintegration of the agglomerated particles first and then particles coalesce at longer solutionizing times in the composite containing 0.20 VFAP. The process of coalescence has not been observed up to a heating time of 20 hours at this temperature in the composites containing 0.10 and 0.15 VFAP. The trans-mission electron microscopy (TEM) study indicates the generation of a large number of dislocations in both the matrix and the area adjacent to the particles. The dislocation densities at these two locations in the composites increase with an increase in VFAP and the particle size. The microhard-ness measurements confirm the microstructural observations, and the hardness values for the com-posite and the matrix appear to be more sensitive to the particle distribution and the particle size compared to the grain size.

The effect of stacking fault energy on the microstructural development during room temperature wire drawing in Cu, Al and their dilute alloys

The effect of stacking fault energy (SFE) on the evolution of microstructures during wire drawing at room temperature has been studied in pure aluminium, pure copper and Cu-2.2% Al andCu-4.5% Al alloys which covers a range of SFE values from 4 to 166 mJ m−2. The compositions are expressed in atomic parts per million by weight. The microstructures have been characterized from samples obtained by deforming rods of these materials to true wire drawing strain values of up to 1.47. A decrease in the SFE value changes the deformation mechanisms from the formation of cell structure and their size refinement in a high SFE material to the formation of deformation bands and deformation twins in a low SFE materials. The Cu-2.2% Al alloy deforms by deformation bands at low true strain values while deformation twins within the bands control the deformation mechanisms at higher true strain values. The alloy, Cu-4.5% Al, with the lowest SFE value deforms only by deformation twins even at low true strain values and the presence of overlapping and intersecting deformation twins are the dominating features as the rods are drawn to higher true wire drawing strains.

Strain-Glass Revisited

According to the most general interpretation, a glassy transition (transition to the glassy state) is a transition between the high temperature “liquid” state in which certain species are disordered due to thermal excitation. This disordered “liquid” state is an equilibrium state and a “glassy” state is the one in which the same species are in a “frozen” disorder, inherited from the high-temperature “liquid” state. This is a kinetically non-equilibrium state, when the equilibrium state cannot be reached due to a divergence of the relaxation time.