J. Merlin

Precipitation effects on thermopower in Al-Cu alloys

Abstract The precipitation influence on the thermoelectric power (TEP) of Al-Cu alloys (with from 2 up to 5 wt% Cu) has been observed. It has been shown that incoherent θ precipitates have no influence on TEP, in agreement with previous observations in various metallic alloys where similar precipitates form. On the opposite metastable plate-like precipitates (GP zones, θ″, θ′) induce a contribution to TEP, which is the larger the higher the measuring temperature; furthermore a correlation between the sign of this contribution and the nature of the stresses induced by the particles is observed.

Coarsening of θ′ plates in Al-Cu alloys—II. influence of ledge mechanism

Abstract A theoretical study was made of the two coarsening mechanisms of θ′ plates: 1. morphological evolution and 2. coarsening at constant aspect ratio. In both of them ledge migration on the broad face of precipitates is found to be the controlling process. The analysis of the first mechanism leads to an evaluation of the mean diffusion coefficient of copper in the θ′-matrix interface. For the second mechanism, the hypothesis of an heterogeneous nucleation of ledges due to the impingement of precipitates leads to a linear theoretical relation between the mean diameter D and t 2 11 . This relation agrees rather well with the experimental results.

Experimental and theoretical determinations of the yield stress of an alloy containing plate-like precipitates: θ′ phase in an Al-4wt.%Cu alloy

Abstract The evolution of the yield stress of an Al-4wt.%Cu alloy was studied during the stage in which the precipitates became coarser. The correlation between the yield stress and the thickness of precipitates allows us, in spite of the existence of three different orientations of precipitates in the matrix, to represent the precipitate configuration by a parallel-plate model. Using this model, good agreement is found between the experimental and theoretical values of the yield stress.

Determination of copper solubility in titanium and study of TiCu solid solution stability by thermoelectric power measurements

Abstract The stability of copper in titanium as a function of temperature is determined by thermoelectric power measurements on a Ti-Cu alloy. The stability of the solid solutions which were obtained after quenching from various homogenization temperatures is studied, especially for martensitic solid solutions.

Temperature dependence of the electrical resistivity of alloys with Guinier-Preston zones

The influence of the measuring temperature TM on the residual resistivity Delta rho has been investigated in four alloys (Al-Ag, Al-Cu, Al-Zn and Cu-Be) where Guinier-Preston zone information leads to the time dependence of the resistivity showing a maximum during ageing. In the two alloys where spherical precipitates form (Al-Ag and Al-Zn), the influence of Tm on Delta rho seems to be negligible while the presence of monolayer plate-like particles (in Al-Cu and Cu-Be) induces a higher peak occurring at a later ageing time when the measuring temperature is raised. These experimental results are analysed by using the most recent theoretical models and the effects on Delta rho of the deformations induced by the precipitates are investigated.

Consolidation d'un alliage d'aluminium à 4% en poids de cuivre à l'Etat θ′. Relation entre limite elastique et dureté

Different ageing treatments have been used to produce θ′ precipitates in an AlCu 4 wt.% alloy. The tensile stress-strain curves which were obtained from the various states can be described by the equation σ = RE + Aϵm, where A and m are constants, the rate of work hardening being independent of the size and distribution of θ′ precipitates. Furthermore, a linear correlation between the conventional flow-stress RE and the Vickers hardness HV can be established. The same type of relation is also observed for the intrinsic contribution of θ′ precipitates to these two values.

Cluster influence on electrical properties of Al-Zn solid solutions

Abstract The existence of small atom clusters in metallic solid solutions leads to electrical resistivity and thermoelectric power variations. Such variations are studied in the present work in Al-Zn alloys, in a temperature range where no precipitation phenomenon occurs.

Study of the dissolution kinetics of the θ phase in AlCu alloys using thermoelectric power measurements

Abstract A comparative study of the dissolution kinetics of θ precipitates in an AlCu alloy was made using classical methods (hardness, scanning electron microscopy and differential scanning calorimetry) and thermoelectric power (TEP) measurements. It is shown that TEP measurements are the most suitable method. The advantages and limitations of this method are given.

Validity of a Lamellar Model for Yield Stress Variation of Alloy Containing Plate-like Precipitates

ABSTRACT The variation of yield stress of an Al-4 wt % Cu alloy during coalescence of θ′ plates is studied together with the geometrical parameters of precipitation. Different models of repartition for θ′ precipitates are investigated to explain these variations. The correlation between the evolution of yield stress and the thickness of 0′ plates leads to consider lamellar models.

Études des Phénomès élastiques et anélastiques associés à la précipitation des phases α et ω isothermique dans l'alliage de titane βIII

The study of elastic and anelastic phenomena with respect to the precipitation of α and ω phases has enbled us to schematize the structural evolution as follows. At first, after being reverted, the sample consists of a metastable and homogeneous solid solution β, supersaturated with titanium. The titanium is consumed by the α and ω phases until the βE equilibrium composition is reached. The ω phase precipitation is homogeneously spread and continuous whereas the α precipitation is localized and discontinuous. The study of the internal friction peak P2 provides interesting information since the height of this peak varies linearly with the square of the average titanium concentration in the supersaturated matrix and the peak position on the temperature scale is related to the matrix stability. From this study it appears that the Zener effect idea can be applied to alloys during their decomposition.