Continuous Dynamic Recrystallization Modeling in Ti ...

Proceedings of the 13th World Conference on Titanium Edited by: Vasisht Venkatesh, Adam L. Pilchak, John E. Allison, Sreeramamurthy Ankem, Rodney Boyer, Julie Christodoulou, Hamish L. Fraser, M. Ashraf Imam, Yoji Kosaka, Henry J. Rack, Amit Chatterjee, and Andy Woodfield TMS (The Minerals, Metals & Materials Society), 2016

CONTINUOUS DYNAMIC RECRYSTALLIZATION MODELING IN Ti-17 ALLOY: APPLICATION TO THE FORGING OPERATIONS IN E AND ED FIELDS M. Semblanet 1), L. Pallot 1), D. Piot 1), F. Montheillet 1), M. Derrien 1), Y. Millet 3),C. Poletti4) and C. Desrayaud1) 1) 2) 3) 4)

Material Science and Structure department, Laboratoire Georges Friedel UMR CNRS 5307. Ecole des Mines de SaintEtienne, 158, Cours Fauriel 42 023 Saint-Etienne cedex 2, France SNECMA, SAFRAN Snecma, Site de Villaroche, Rond-Point René Ravaud – Réau, F-77550 Moissy-Cramayel, France. TIMET SAVOIE, 62 Avenue Paul Girod, 73400 Ugine, France Institute of Materials Science and Welding, Tu Graz. Kopernikusgasse 24/I 8010, Graz, Austria Keywords: thermomechanical treatments, continuous dynamic recrystallization, forge, hot compression, rheology, microstructure evolution. Abstract

recrystallization of metal alloys. Note that in many materials a combination of these mechanisms can be observed. However, for high values of stacking fault energy (SFE), materials undergo dynamic recovery phenomena due to dislocation motion and reorganization into subgrain boundaries. The subsequent deformation results in the increment of the misorientation, leading in the formation of new high angle grain boundaries (HAGB). See Gourdet [1]. These new grain boundaries generated at large strain values, illustrate the consequence of continuous dynamic recrystallization (CDRX). According to the literature the E phase is clearly subjected to this mechanism, while the D phase is not clearly understood till now. D substructure appears during the deformation of the alpha lamellae (i.e. formation of subgrain boundaries) at high strain rate and low temperature in the DE domain. Additional diffusion mechanisms are present at high temperature and low strain rate, and allow the globularization of the structure. Geometric dynamic recrystallization (McQueen [6]) may occur when high strains promotes large aspects ratios of the initial grains and make interactions between the opposite grain boundaries of a grain possible (pinching effect). The interaction between D and E phases presents a challenge makes to understand and describe the microstructure evolution. The respective flow stresses of D and E separately, are not easy to measure (especially D phase). Furthermore the local plastic flow is governed by the crystallography of D and E offering a low compatibility between the slip systems excepted {0001} and {110} . Finally, in the hot-processing route, after dual phase forging, the Ti-17 alloy is cooled down to room temperature; then it is heated into the region to be forged again but in the domain. The material recrystallizes quickly then grain growth occurs. Its kinetics during long holding was presented in the last Titanium Conference [7]. Nevertheless, it is of interest to describe the detailed mechanisms involved during heating and the phase beginning of holding, i.e. nucleation due to both transformation and recrystallization, just before the forging occurs. For these reasons the present work is focused on compression tests carried out at high temperature to identify the thermo-mechanical and microstructure evolution during forging operations that are achieved successively in the DE and then in the E domain.. The CDRX model developed by Gourdet is adapted to model the substructure generation and evolution accounting only for the E phase evolution. The relevance of the CDRX model, very suitable for the beta forging, is also discussed in the DE domain. Finally, as the structure evolves statically

Forging operations of Ti-17 alloy are carried out by successive stages taking place in the E and D + E ranges. These operations aim at homogenizing the material and decrease the grain size. The CDRX model developed by Gourdet and Montheillet [1] was used to capture the refinement observed in the Ti-17 products and can be used for microstructure optimization through the forging process. Experiments were carried out by hot torsion and compression tests. The crystallite size was characterized by EBSD on the deformed specimens. Some of them, compressed in the DE domain, were heat treated in the beta range to study the Egrain recrystallization and growth. The microstructure evolution is guided by the substructure generation in the E phase due to the combination of hardening phenomena, dynamic recovery and continuous dynamic recrystallization. Finally this work highlights the role of the thermomechanical parameters on the final microstructure. Introduction During the forging operations of Ti-17 titanium alloy, the material undergoes a complex succession of deformation stages in the DE and in the E ranges. These operations aim at optimizing the final microstructure according to mechanical properties thresholds defined by industrial specifications. In this context the material is subjected to high temperature deformation during which the D phase transforms into the E one, reaching the single E domain above the E transus temperature. Although not many elements are available in the literature concerning this aspect for the Ti 17 or similar grades, some papers detail the phase transformation during thermomechanical treatments in various titanium alloys, e.g. Warchomicka [2], Miller [3], Semiatin [4] and Ding [5]. Note that the kinetics of transformation may be increased by the deformation leading sometimes to a higher realization level of the DE equilibrium (i.e. a precipitation or dissolution induced by straining). The main mechanism occurring during thermomechanical treatments is dynamic recrystallization the type of which depends on the characteristics of the phases subjected to deformation. Depending on the value of the stacking fault energy, two types of behavior (continuous or discontinuous dynamic recrystallization) are reported in the literature for the

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during heating steps from, for example, forging in the beta field after alpha-beta forging, we investigated this to better apprehend the starting microstructure prior to beta forging operation.

The material considered for this study is a forged billet provided by TIMET. The E transus temperature is 880°C. The chemical composition is reported in Table 1:

Material and experimental procedure

wt%

Sn 2.02

Mo 4.14

Table 1. Chemical composition of the Ti-17 grade used for the present work Zr Cu Fe Mn Al C O N Cr 2.02