New titanium alloys with a combination of high strength, strain hardening and high ductility, induced by TRIP and TWIP effects – TITWIP

The different axes developed had in common the development of approaches aiming at understanding the evolution of the microstructures of alloys during deformation (Axis 1 and 2) and the evolution of the associated mechanical properties (Axis 3). In Axis 1, we have undertaken an approach to alloy design based on the electronic parameters regulating the chemical stability of the ß-phase. The construction of electronic stability maps allowed us to define in the Ti-Cr-(Sn/Al)-Fe system, the zones of interest corresponding to the simultaneous activation of the different deformation mechanisms (TRIP and TWIP effects) and then to design a series of alloys around this zone of interest. These alloys were then successfully developed and characterized. The research axis 2 allowed to identify the order parameters allowing to describe the mechanical twinning of type {332}11-3 active in these alloys at the mesoscopic scale: shear and shuffle. The simulations performed were able to successfully account for the growth of twins under stress. The model was then tested to develop a 3D microstructure consisting of several twin variants, to understand the experimentally observed microstructures. Axis 3 allowed the development of a model to describe the particular plasticity of alloys. The microstructures were modeled from the macroscopic point of view using a polycrystal homogenization model. Slip systems and martensite were progressively introduced in the model in addition to slip systems, which allowed to trace back the evolution of the work hardening and the load/unload dissymmetry. The experimental observations on the evolution of the twinned volume fraction of the polycrystal grains (Axis 1) were then used to validate the model.

The TiTwip project has allowed to refine and complete the design strategies of the alloys by introducing additional design parameters (distortion parameter for example). As a result, it has also enabled the successful development of new, higher performance alloy grades with both a higher yield strength and a higher strain hardening rate. In parallel, it has allowed to increase significantly the knowledge on both the deformation mechanisms (mechanical twinning, in particular) and the relationship between these mechanisms and the macroscopic mechanical properties.

The continuation of the TiTwip project will be done in collaboration with partners from the aeronautical industry. Important perspectives are offered by the damage resistance (toughness) of these alloys (three times higher than commercial titanium alloys). Applications for retention parts (casings) for aeronautics are currently being considered. In relation to aeronautical specifications, the research to be carried out concerns the increase in the yield strength of the alloys, without compromising on the aspects of work hardening and ductility. The main challenge is therefore to keep the same deformation mechanisms by shifting their triggering towards high stresses.

The TiTwip project has resulted in the publication of 7 articles in international journals. The dissemination during the congresses was large (25 communications on the 3 axes of the project, 11 invited conferences). It should be noted that during the last World Congress on Titanium Alloys, a symposium on TRIP/TWIP alloys was created, which clearly shows the growing interest of the community for this particular family of alloys.

The TITWIP project deals with the development of a new family of titanium based materials so-called TRIP/TWIP titanium alloys (TRIP for Transformation Induced Plasticity and TWIP for Twinning Induced Plasticity), having a unique combination of high strength, strain hardening and ductility. These mechanical performances place this family of alloys among the most advanced and promising metals in the world of structural materials to fulfill the increasing demand of aerospace industry on mass optimization.
The work will be centered on:
- Electronic design, elaboration and characterization of new alloys with optimized combination of mechanical properties.
- Understanding and prediction of the evolutions of the observed microstructures, using a mesoscale phase field modeling.
- Prediction of the mechanical behavior accounting for the microstructures, using new constitutive laws in finite elements calculations for structural applications.