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International audience; The a phase nucleated dynamically during the thermo-mechanical coupling process in titanium 8 alloy is really interesting but difficult to sufficiently ascertain. In the present work, the a phase 9 nucleation behavior, the orientation relationship between a/b as well as the phase transformation 10 kinetics during hot deformation of Ti-5553 alloy were investigated in-depth. Results reveal that the 11 "necklace" microstructure formed. The Burgers orientation relationship between a/b phases has 12 been destroyed gradually. The b ! a phase transformation is obviously retarded during the hot 13 compression due to the competitive effect of softening process (dynamic recovery/recrystallization). 14 These results could provide valuable reference for process optimization and the microstructural 15 evolution controlling. 16 1. Introduction Metastable b titanium alloys, such as Ti-10V-2Fe-3Al (Ti-17 1023), Ti-5Al-5Mo-5V-1Cr-1Fe (VT22), and Ti-5Al-5Mo-5V-18 3Cr (Ti-5553), are increasingly employed in the aerospace 19 industry because of their higher yield strength, better harden-20 ability, better fatigue, and crack propagation properties than a þ b 21 titanium alloys. [1,2] Microstructural development in these alloys 22 depends on the processing parameters, such as the type of 1 thermo-mechanical process, temperature, strain, strain rates, and 2 cooling rates. Moreover, metastable b titanium alloys are usually 3 sensitive to process parameters, even during isothermal process-4 ing. [3] Hence, significant effort to understand the microstructural 5 evolution of metastable b titanium alloys related to thermo-6 mechanical process have been done in the past decade. [4-7] 7 The b ! a phase transformation happens during the 8 thermo-mechanical processing below the b transus tempera-9 ture (T b). Generally, heterogeneous nucleation mechanisms 10 include grain boundary nucleation, dislocation nucleation, 11 and vacancy nucleation. Although much more defects are 12 introduced in microstructure during deformation, the a 13 precipitates nucleated mainly at the interface of two adjacent 14 b grains, triple junctions or quadruple points of b grains. The 15 reasons could be summarized as the following aspects: (i) the 16 higher interface energy could decrease the critical nucleation 17 energy; (ii) only part of interface of second phase need to be 18 reconstructed; (iii) the composition segregation is more 19 significant at the grain boundaries (GBs). In Ti-5553 alloy, 20 the dominant microstructural evolution mechanism during 21 hot deformation close to the T b is dynamic recovery (DRV) of 22 the b phase, [6,8] with a certain degree of the dynamic 23 recrystallization (DRX). [9] The a precipitates might nucleate 24 at these GBs preferentially including high angle grain 25 boundaries (HAGBs) and low angle grain boundaries 26 (LAGBs). Hence, dynamic stress induced phase transforma-27 tions (DSIPT) might happen during the thermo-mechanical 28 processing due to the more nucleation sites and accelerating

Metastable β titanium alloys are important structural materials for aeronautical applications due to their high strength to density ratio, good ductility and workability and excellent hardenability. Despite the efforts in resolving the complex microstructural evolution related to thermomechanical processes and in gaining knowledge on the produced phases and their contribution to the resultant mechanical properties, there are still some controversial and unresolved issues. The aim of the present PhD work is to determine precisely the metastable nature of β phase and to characterize finely the characteristics of the β→α transformation during high and low temperature thermomechanical treatments. Investigations were performed on a Ti-5553 alloy with the single β phase initial microstructure obtained by solution treatment followed by quenching using scanning and transmission electron microscopy (SEM/TEM) coupled to crystallographic orientation measurements and chemical analyses. It was demonstrated experimentally that the structure of the β phase in the metastable titanium alloy is not “pure” body centered cubic. Diffraction diagrams presents streaking of the β diffraction spots and additional spots at the 1/2, the 1/3 and 2/3 diffraction positions. Also, striations are observed in TEM images. From this experimental evidence and crystallographic calculations, it was proved that atomic displacements on the {110}β and {112}β planes formed a structure between that of the parent β phase and that of the α or ω phase, demonstrating pre-phase transformation tendency. The study of the precipitation during thermomechanical processing at higher temperature in the α+β region revealed that discontinuous equiaxed or short rod shaped α precipitates (1~2μm) mainly form on the high angle and low angle β grain boundaries but seldom in β grain interiors, forming the “necklace” microstructure. The Burgers orientation relationship (BOR) between the α and β phases is destroyed gradually by the deformation. The BOR deviation of grain boundary α is larger than that of intragranular α. The deviation from the BOR increases both with the increasing strain and decreasing strain rate. During the deformation at the lower temperature in the α+β region, the α precipitates exhibit different morphologies: such as lamellar α, equiaxed α and irregular α depending on their localization. Within the slip bands, equiaxed α/β grains which do not respect the BOR are present. However, between the bands, lamellar α and β phases maintaining the BOR are distributed alternately. In that last case a strong variant selection is observed as only the two or three variants that form are those which can accommodate the macroscopic deformation. Comparatively, in absence of compression all 12 variants are formed. The β→α phase transformation is retarded during the hot compression at higher temperature region, which is attributed to the competition between softening and phase transformation. On the contrary, it is promoted during compression at lower temperature region due to the more inducted deformation defects acting as α phase nucleation sites and due to accelerating growth of α precipitates and retarded softening. Dislocation slip is the leading deformation mechanism for the Ti-5553 alloy. Under the lower temperature deformation condition, single or multiple-slip bands with two or three different activated slip systems would form during the hot deformation process. Identification of these slip systems have been done by trace analysis. These results provide new insights into the structural nature of β metastable phase and valuable reference for β→α phase transformation during thermo-mechanical treatment in metastable β titanium alloys; Les alliages de titane β métastables sont des matériaux de structure essentiels pour les applications aéronautiques de part leurs très bonnes propriétés mécaniques. En effet, ils présentent une résistance spécifique élevée, une bonne ductilité et forgeabilité et une excellente réponse aux traitements thermiques. Toutefois, il existe encore aujourd'hui à leur sujet des controverses et des questions ouvertes et ce, malgré les efforts pour comprendre les mécanismes d'évolution microstructurale au cours de traitements thermo-mécaniques et pour déterminer les phases en présence et leur contribution aux les propriétés mécaniques résultantes. Ce travail de thèse a pour objectif de déterminer la nature de la phase β et de caractériser la transformation β→α à haute et basse températures par des caractérisations fines en microscopie électronique à balayage et à transmission couplées à des mesures d'orientations cristallographiques et de composition chimique. L'alliage étudié est un Ti-5553 avec une microstructure initiale 100% β obtenue par mise en solution et trempe. Il a été démontré expérimentalement que la structure de la phase β métastable n'est pas purement cubique centrée. Les points de la phase β dans les clichés de diffraction présentent un allongement (streaking) et des points supplémentaires sont visibles aux positions de diffraction 1/2, 1/3 et 2/3. Par ailleurs, les images MET ont un aspect en moiré. A partir de ces résultats et de calculs crystallographiques, il a été prouvé que des déplacements atomiques sur les plans {110}β et {112}β forment une structure intermédiaire entre celle de la phase β parente et celles des phases α et ω, prouvant que la phase β a intrinsèquement initié une transformation. L'étude de la précipitation au cours du procédé thermomécanique dans le domaine α+β a révélé que des précipités α discontinus, équiaxes ou légèrement allongés (1~2μm) se forment aux joints β de forte et de faible désorientation mais rarement au coeur des grains β produisant ainsi une microstructure en "collier". La relation d'orientation de Burgers (ROB) entre les phases α et β est progressivement détruite par la déformation. L'écart à la ROB est plus marqué pour les précipités α qui se forment au joint de grains qu'à l'intérieur des grains. L'écart à la ROB augmente aussi avec la déformation, mais diminue avec la vitesse de déformation. Au cours des déformations en bas du domaine α+β, les précipités α ont une morphologie qui dépend de leur position. Au coeur des bandes de glissement, les grains α/β sont équiaxes et ne respectent pas la ROB. Entre les bandes de glissement, la microstructure est lamellaire où les phases α/β alternent et respectent la ROB. Dans ce dernier cas, une forte sélection de variantes a été observée: Seuls les deux ou trois variants favorisant l'accommodation de la déformation se sont formés. A titre de comparaison, dans l'état non déformé, les 12 précipités sont présents. La transformation β→α est retardée en cours de compression à haute température. Ceci est attribué à une compétition entre adoucissement et transformation de phase. Au contraire, celle-ci est favorisée au cours de la compression à plus basse température du fait que les défauts cristallins induits par la déformation jouent le role de sites de germination et que la croissance des précipités soit accéléré alors que l'adoucissement soit ralenti. Dans le Ti-5553, le mécanisme de déformation dominant est le glissement des dislocations. Dans les déformations en bas du domaine α+β, du glissement simple ou multiple avec deux ou trois systèmes de glissement activés. L'identification de ces systèmes a pu être effectuée par des analyses de traces. Cette thèse a résolu la nature de la phase β métastable et constitue un travail de référence pour l'étude de la transformation β→α au cours de traitement thermomécanique