TEM observation of precipitates and their role to mechanical properties in Ti-5553 alloy heated to some temperatures up to 923 K

A Ti-5553 specimen was continuously heated to 923 K and simultaneously in-situ HEXRD profiles were taken. In addition, specimens heated at the same rate to several temperatures up to 923 K and further quenched were observed by transmission electron microscopy. Based on both results obtained, transformation sequence was clarified, precipitations of ω-, α”iso- and α-phases were confirmed, and size and density of these precipitates were measured. Hardness values of those specimens were also measured. The hardening mechanism was considered as shearing-mechanism for specimens aged at lower temperatures and by-pass one for specimens aged at higher temperature. An attempt of distinction between α”iso - and α-precipitates was also tried. Both precipitates were in needle-like shape and a possibility was suggested by measuring angles between two needle-shape precipitates on {110} of the matrix and comparing with each other

TEM observation of precipitates and their role to mechanical properties in Ti-5553 alloy heated to some temperatures up to 923 K

A Ti-5553 specimen was continuously heated to 923 K and simultaneously in-situ HEXRD profiles were taken. In addition, specimens heated at the same rate to several temperatures up to 923 K and further quenched were observed by transmission electron microscopy. Based on both results obtained, transformation sequence was clarified, precipitations of ω-, α”iso- and α-phases were confirmed, and size and density of these precipitates were measured. Hardness values of those specimens were also measured. The hardening mechanism was considered as shearing-mechanism for specimens aged at lower temperatures and by-pass one for specimens aged at higher temperature. An attempt of distinction between α”iso - and α-precipitates was also tried. Both precipitates were in needle-like shape and a possibility was suggested by measuring angles between two needle-shape precipitates on {110} of the matrix and comparing with each other

Toward a better understanding of chemical and microstructure heterogeneities inherited from solidification and solid state phase transformations in the Ti-48Al-2Cr-2Nb alloy

Vers une meilleure compréhension des hétérogénéités chimiques et microstructurales issues de la solidification et des transformations de phases à l'état solide dans l'alliage Ti-48Al-2Cr-2Nb. Pour réussir l'application et garantir une bonne reproductibilité des propriétés mécaniques des alliages intermétalliques de base TiAl, les motoristes aéronautiques et automobiles doivent parvenir à un meilleur contrôle de leur microstructure, dont la variabilité résulte essentiellement des hétérogénéités héritées de la solidification et des traitements réalisés à haute température. Dans ce travail, nous présentons quelques résultats expérimentaux visant à améliorer la connaissance du processus de solidification et de la genèse de microstructures au cours de traitements thermiques, dans le cas de l'alliage Ti-48Al-2Cr-2Nb (teneurs en % at.) actuellement en cours de développement industriel. Dans une première partie, par la réalisation d'expériences de solidification dirigée, nous étudions le partage des éléments d'alliage, le chemin de solidification ainsi que la formation de phases minoritaires dans l'alliage élaboré par fonderie. Dans une seconde partie, nous étudions grâce à la dilatométrie les transformations de phases intervenant à l'état solide ainsi que les relations existant entre les paramètres microstructuraux et les conditions de traitement thermique, en particulier la durée du maintien isotherme et la vitesse de refroidissement

Bainitic transformation under stress in medium alloyed steels

The effects of stress on bainitic transformation are measured in the case of isothermal transformations of three medium alloyed steels in a temperature range where the bainitic transformation is complete. The transformation kinetics variations and transformation plasticity evolutions are analysed for applied stress values ranging from zero up to about three times the yield stress of the austenite and compared to the results of the literature. Considering a quantitative analysis of the microstructures formed, the different related mechanisms are discussed

Effect of tensile stresses on bainitic isothermal transformation

The effects of tensile stresses on isothermal bainitic transformation were studied in the case of a 35MV7 steel. The modification of transformation kinetics and the presence of transformation plasticity is shown in a first step. Furthermore, the effect of stress on the morphological modifications of the ferrite laths is illustrated. The role of the stress on these changes is analysed

Phase transformations in PuGa 1 at.% alloy: New valuable insight into the isothermal martensitic δ→α′ transformation

International audienceThis paper deals with the results of an investigation of low-temperature martensitic transformation in a metastable PuGa 1 at.% alloy. The kinetics of this process were studied experimentally via in situ XRD characterizations performed during isothermal holds. This revealed a double-C Time–Temperature–Transformation diagram similar to that reported in the literature for enriched alloys. The originality of our results lies in the demonstration of the existence of different martensite morphologies corresponding to the upper and lower parts of the TTT diagram. These microstructural variations seem to be the result of different accommodation mechanisms.The experimental nucleation rate study has suggested an autocatalytic character of the isothermal martensitic nucleation process as well as a large sensitivity to elastic and plastic strain. Indeed, the incubation period and subsequent increase in nucleation rate could be related to the appearance of new embryos rather than the growth of existing plates, while the partial character of the transformation may be attributed to interactions between plates that are being formed in the two-phase delta + alpha′ alloy. These hypotheses have been confirmed by the classical heterogeneous nucleation approach by Pati and Cohen, which takes into account the autocatalytic character of the transformation. This approach has also indirectly revealed the existence of restraining forces that reflect a high sensitivity of the energy barrier for nucleation to the alpha′ phase amount

Recrystallization kinetics and associated textures of Cu-bearing Ti-IF steels

The influence of copper on recrystallization of Ti-stabilized interstitial free steels has been studied. It is shown that the recovery and the recrystallization kinetics are slowed down. Thus the final recrystallization texture is modified : whereas the γ-fiber is still present, its intensity is lower. The deformed α-fiber contribution is larger than without copper and its magnitude depends on the annealing conditions : it is all the more intense as the heating rate is small. These effects are related to the presence of copper-based precipitates. The recrystallization textures are discussed in function of the precipitates evolution