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Microstructures of titanium alloys

The formation of microstructures from the beta high temperature domain was studied for different titanium alloys [1] .Diagrams of phase transformations (TTT or TRC) were established for different titanium alloys (alpha + beta or beta metastable) by electrical resistivity and using the synchrotron XRD [2]. The figure below shows the TTT diagram obtained for the Ti 5Al5Mo5V3Cr alloy.

For titanium alloys of the alpha + beta or beta metastable families, diverse mechanisms of germination and growth were brought into play following the transformation temperatures and driving force. At weak transformation driving forces, the germination of a daughter phase was heterogeneous and was produced at the grain boundary (even triple or quadruple points) followed by a dampening along the length of the boundary (alphaGB). Lateral growth occurred, followed by the formation of lamellar alpha colonies in the same crystallographic orientation as the grain boundaries from which they issued (alphaWGB). Growth kinetics was strongly controlled by the partition of solutes between the parental and daughter phases. The figure above shows an alphaWGB lamellar colony and the parental beta phase (observed using TEM), while the composition obtained along the length of the AA' line, by EDX, illustrated the division of the solutes seen at the end of the transformation and during this phenomenon . The change in the average composition of the parental phase was also illustrated by evolution in the chemical composition characterized by the synchrotron XRD during studies in conditions of isotherm transformation.

When the driving force increased, the germination of alpha grains was intra-granular (alphaWI), with spatial arrangements evolving from several parallel platelets (entwined colonies) towards individual platelets characteristic of the different alpha variants in a single beta grain.

The characteristic sizes of colonies, as well as of intra-granular platelets, were a function of the transformation temperature. Their thickness diminished when the transformation driving force increased (when the transformation temperature dropped) and their number increased.

At low temperatures (< 500°C), metastable phases were formed (alpha', alpha''). In situ characterization by synchrotron XRD, leading to cell parameters of the parent and daughter phases, showed a weak (or even absent) partition of alloy elements at these temperatures. At the lowest temperatures studied, the alpha'' daughter phase had the same cell parameters as a martensitic phase formed under stress (beta metastable alloys). This weak division was confirmed by recent SAT measurements (Nag, etc.).

TTT Diagram of a titanium alloy
AlphaWGB Lamelle (observation by SEM)
Composition of alphaWGB lamelles obtained by EDX
Relative variation of Beta cell parameters during transformation


For beta metastable alloys, at temperatures in the region of 300-350°C, we clearly showed by synchrotron XRD characterizations that the transformation is the omega formation followed by the alpha'' formation (then when the temperature increases (and solute diffusion is more favourable) alpha'' evolves towards alpha).
(Jalcom).

 

 

The microstructures of these alloys can also be formed during the treatment of natural resource revenues, from the beta metastable phase.  We showed the influence of the speed of heating on microstructures upon ageing.

Effectively, the precipitation sequence and the function of this heating speed, has for very slow heating speeds formation of the omega phase, alpha'' then alpha, or alpha'' then alpha'. These metastable phases do not precipitate at extremely rapid speeds.

The resulting mechanical properties were very different. The TEM study is under way (collaboration with E. Sukedai Japon).

 

These studies formed an essential base for modelling of transformation kinetics (paragraph).

They have been carried out for many years with different academic and industrial partners in the frame of collaborative projects (FRAE PROMITI project, ATOME project) or direct collaborations (SAFRAN, EADS, Academics). They are continued by considering the transformations of these alloys in conditions of extreme loads….

[1] Salib, M.; Teixeira, J.; Germain, L.; Lamielle, E.; Gey, N. & Aeby-Gautier, E. Influence of transformation temperature on microtexture formation associated with alpha precipitation at beta grain boundaries in a beta metastable titanium alloy Acta Materialia, 2013, 61, 3758-3768

[2] Geandier, G.; Aeby-Gautier, E.; Settefrati, A.; Dehmas, M. & Appolaire, B. Study of diffusive transformations by high energy X-ray diffraction Comptes Rendus Physique, 2012, 13, 257-267