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It is usual to characterize quantitatively a pearlitic structure by three parameters; (1) the ferrite and pearlite percentage, (2) the interlamellar spacing of the pearlite, and (3) nodule diameter of the pearlite. These parameters vary as a function of the transformation temperature. The conditions necessary for obtaining a fully pearlitic structure by continuous cooling have been determined for plain carbon steels containing from 0.2% to 0.8%C. When the carbon content is below 0.6%, pearlite is always degenerate, with low yield strength but good reduction in area. Pearl ites containing more than 0.6%C always present normal cementite lamellae with high yield strength but small reduction in area. For 0.6%C steel, fragmented or continuous lamellar structures can be obtained, leading to high yield strength and reduction in area values.

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-A micromechanical model describing the martensitic transformation on the grain scale has been developed, using Finite Elements. First results gained from the simulation illustrate how the morphological evolution within the grain is directly controlled by the internal stress state. The reversible and irreversible pan of transformation "plasticity" strain and their evolution with the transformation can then be obtained from these calculations. I -IntroductionA material undergoing a phase transformation and submitted to an external applied stress even lower than the yield stress of the weaker phase exhibits transfonnation "plasticity" deformation. For martensitic transformation, the so called transformation plasticity deformation is the result of two mechanisms : i) preferential orientation of the martensite plates by the stress and ii) anisotropic plastic accommodation of the transformation strains (shear strain and volumic variations) [I-41. The first contribution which is correlated to the distribution of plate orientations is reversible when the reverse transformation will be produced and is characteristic of the shape memory effect; the second one is permanent. These deformations have been mostly correlated to the external applied stress. However since the large transformation strain associated with martensitic transformation must be mechanically accommodated, the transformation process will create high internal stresses in the surrounding of the new phase. In fact, the orientation of the habit plane will be controlled by the local stress anistropy and the characteristic lattice orientation within the grain .The Finite Element model developed here aims at a better understanding of the rather complex mechanical behaviour of a material undergoing a martensitic transformation. Particularly, the effect of internal stresses on kinetics and anisotropy of plate orientation will be investigated. A first attempt to differentiate between the strain contributions of the two basic mechanisms underlying transformation "plasticity" will be shown. -Description of the micromechanical modelThe micromechanical concepts and the description of martensitic transfonnation have been discussed in [5]. We recall here the essential features of the model. The transformation strain associated to the transformation can be described as an invariant plane strain, [6,7J. It consists in a shear yo along an undistorted plane, the so-called habit plane, and a dilatation EO normal to it. The strain components yo and EO of the transfomiation strain are often quite large : typical values for a ferrous alloy are respectively 0.19 and 0.03 I l l . The basic idea in our analysis is to assume that the material consists of identical representative cells, stacked in a repetitive way, which behave in the same way, simultaneously. From a micromechanical simulation of such a cell, the macroscopic behaviour (at the scale of a specimen) can then be obtained. The need to describe the aforementioned couplings at a sufficiently fine scale leads to use...

In the Ti-6Al-4V-ELI alloy, the alpha phase is gradually transformed into the beta phase until beta-transus temperature ( 980°C) is reached, and the transformation is completed. It is important to identify the transformation kinetics to accomplish the solution heat treatments in which a phase alpha percentage remains unchanged. Kinetics and other transformation characteristics are evaluated, as well as their influence on subsequent cooling transformations, by differential and dilatometric thermal analysis, electric conductivity measurements, hardness measurements and metallographic observation, after performing controlled thermal treatments. Starting from the mill annealed condition, samples were heated at temperatures between 650-1000 °C for 1 hour, then water quenched and subsequently heated for aging, air cooled. Finally, the mechanical properties of samples heat treated were obtained.