Contribution à l'étude des hétérogénéités de texture des billettes d'IMI 834

Abstract : The IMI 834 is a quasi-[alpha] titanium alloy used to design high pressure compressors for aero-engines. The microstructure of the final parts is composed of equiaxed primary [alpha]p grains in a lamellar matrix of secondary [alpha]s colonies (also called Widmanstätten microstructure). This bimodal microstructure was optimised to provide a good behaviour of the material in both creep and fatigue. However, the forged parts can contain millimetre long regions called macrozones, characterised by very sharp local textures which seem to significantly reduce the fatigue resistance of the material, in particular in dwell fatigue testing. This work is a detailed study of the texture heterogeneities and local microstructures that are damageable for the fatigue resistance of bimodal IMI 834 billets. The observed microstructures contain important indications on the formation of these heterogeneities during the billet elaboration. Their analyses allow for a better understanding of the genesis of the heterogeneities and further, guide us toward additional thermomechanical heat treatments that could lead to their reduction. The study is based on orientation maps obtained by the EBSD technique and on specifically developed tools. We first elaborated a method to distinguish the [alpha]p grains from the [alpha]s colonies, automatically on orientation maps. Thus, we determined the contribution of each population of grains to the formation of the heterogeneities. Moreover, we used the partial orientation map of the [alpha]s phase to calculate the parent [bêta] microtextures and deduced the high temperature microstructure formed by the combination of the [alpha]p and the [bêta] phases. Thus, we analysed the influence of this complex microstructure on the formation of the macrozones. In parallel, we took advantage of various compression tests to follow the development of the macrozones and suggest a deformation route that can reduce their intensity. Our work shows that the macrozones correspond to regions elongated in the axial direction of the billet, in which [alpha]p grains have an unique but dispersed texture component, so that the c axes mainly point in the radial direction. The [alpha]p colonies are oriented around several texture components but have their main component matching the unique component of the [alpha]p grains. In these zones, the local texture of the parent [bêta] phase is relatively smooth. Consequently, the macrozones cannot correspond directly to the traces of the large [bêta] grains strongly elongated in the axial direction after the forging in the [beta] field. The sharp [alpha]p texture results from an heterogeneous deformation / globularisation of the [alpha] phase that initially corresponded to the colonies of lamellae that were in Burgers orientation relation with the [bêta] matrix. The sharp texture of the [alpha]s phase is inherited from the transformation of the [bêta] phase through a variant selection mechanism occurring during the [bêta] [to] [alpha]s phase transformation in the presence of [alpha]p grains. In this variant selection, we have highlighted the role of certain [alpha]p/[bêta] phase interfaces which promote certain [alpha]s variants that minimise the [alpha]p/[alpha]s misorientations. Moreover, modelling showed that the elastic anisotropy due to the strong local textures of the [alpha]p phase tends to favour the [alpha]s orientations close to [alpha]p orientation. The analysis of the different results led us to make further compression tests to evaluate the possibility of reducing the macrozones. This work offers some new perspectives in the production of industrial parts in IMI 834.
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Lionel Germain. Contribution à l'étude des hétérogénéités de texture des billettes d'IMI 834. Autre [cond-mat.other]. Université Paul Verlaine - Metz, 2005. Français. ⟨NNT : 2005METZ018S⟩. ⟨tel-01752395⟩

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