High Strength Titanium-based Harmonic Microstructures. Processing and Properties. – HighS-Ti

The starting coarse-grained powders (100-200 $m) will be purchased. They are produced (not exclusively) by Plasma–Rotating Electrode Process (PREP). In PREP technology, inside a chamber, plasma is irradiated onto a metal material rotating at high speed, which causes it to melt. The melted metal splashes due to centrifugal force form spheres and coagulates. Since both melting and coagulation take place in an inert gas ambiance, the resulting metal balls contain very few impurities, such as oxygen, and are uniform in quality. Then, the powders are mechanically milled (MM) using a planetary mill or by jet milling (JM) under high-pressure Ar gas flow (available at RITS). The JM is more suitable for small size particles. The resulting out of equilibrium powder is formed by a severely deformed shell with a high density of structural defects and a coarse-grained core. The powders will be subsequently consolidated using Spark Plasma Sintering (SPS).

The method of production is completely controlled and reproducible on all materials Highs-Ti program and beyond as new alloys have been introduced. We have for now chosen the SPS as consolidation method. The process is optimized for development drives up D30H5 (D = Diameter, H = height).
The microstructures developed were characterized under different conditions of stress, since the quasi-static tests (traction, simple shear), in-situ synchrotron traction or in situ TEM, and their shaping by cold rolling. The analyzes of the underlying mechanisms is ongoing.
Quasi-static mechanical tests on Ti-highs program materials demonstrate much improved mechanical performance especially the flow stress in all cases without loss of ductility. The micro-mechanisms underlying this behavior are the subject of the next deliverable of Task 3 (Deformation Mechanisms and damage) provided at T0 + 24. Further studies are underway to evaluate the behavior in cyclic shear, but also the fatigue behavior.

However, there is a technological barrier to blow, which enables the development of large disks. In the coming months we will discuss the development of D50H30 discs.

1. Dirras, G., Ota, M., Tingaud, D., Ameyama, K., & Sekiguchi, T. (2015). Microstructure evolution during direct impact loading of commercial purity a -titanium with harmonic structure design. Matériaux & Techniques, 103(3), 311. doi:10.1051/mattech/2015031
1. High Strength Titanium Alloys With Harmonic Structure For Enhanced Properties: Microstructure And Mechanical Properties. G. Dirras et al. Soft/Hard Materials 2016, 21-24 Jan. 2016, Kusatsu, Japan. Invited talk.
2. In situ X-ray diffraction study of mechanical behavior of harmonic structure design Ti-6Al-4V alloy” P.O. Renault et al. The Ninth Pacific Rim International Conference on Advanced Materials and Processing (PRICM9) Ed. by T. Furuhara, M. Nishida and S. Miura. The Japan Institute of Metals and Materials, 2016
3. Influence of thermo-mechanical treatments on the mechanical behavior under tensile and shear loadings of Ti-15-3-3-3 alloy processed by powder metallurgy route. S. Yokoyama et al. The Ninth Pacific Rim International Conference on Advanced Materials and Processing (PRICM9) Ed. by T. Furuhara, M. Nishida and S. Miura. The Japan Institute of Metals and Materials, 2016
4. Microstructure evolution and Mechanical behavior under shear loadings of Ti-25Nb-25Zr alloy. D. Ueda et al. The Ninth Pacific Rim International Conference on Advanced Materials and Processing (PRICM9) Ed. by T. Furuhara, M. Nishida and S. Miura. The Japan Institute of Metals and Materials, 2016
5. Oral presentation: P.O. Renault, “Mechanical behaviour of harmonic design structure Ti-6Al-4V” international Workshop “metallurgy with synchrotron” Nancy, mars 2016
6. Poster presentation: T. Sadat, international Workshop “metallurgy with synchrotron” Nancy, mars 2016

Most structural materials are used as metallic alloys, often multi elemental. This is the case for certain steels or titanium alloys for which the alloying elements are rare and/or available in low concentrations in the Earth crust and sometimes difficult to refine. Indeed, the number of elements that humans can use is merely around one hundred. Moreover, that number is further limited due to scarcity or toxicity.
There is a pressing need to open a path to the future with scientific technology, which makes possible to effectively utilize limited elements. This can perhaps be used to create materials for renewable societal infrastructure in severe resource conditions. In this context, structural materials are required to have superior mechanical characteristics while reducing the requirement for rare earth elements.
In the framework of the present proposal, owing to the versatility of powder metallurgy (PM) routes, namely spark plasma sintering (SPS) and hot isostatic pressing (HIP), a new concept that combines severe plastic deformation (by high energy milling) and PM routes (SPS and HIP) will be used to develop and design harmonic microstructures. The harmonic structure will have a 3D network structure of continuously connected "shell" with ultrafine grains and a dispersive structure of coarse-grained "core". This makes them special and different compared to heterogeneous “nano-micro” bimodal microstructure usually produced via various metallurgical routes.
The targeted materials are Ti-based: pure titanium (Ti), Ti-6-4 (Ti6Al4V) and ß-Ti (Ti-15-3-3 (Ti15V3Cr3Sn3Al) alloys (medical implants, aeronautic applications…). After processing by means of process encompassing PM routes (SPS and HIP) a combination of characterization techniques at both macroscopic (from quasi-static to impact loadings) and mesoscopic/microscopic (X-Ray Diffraction (XRD), in-situ XRD tensile tests and electron microscopy techniques) levels will be carried out to capture the elementary details of the deformation mechanisms and to provide the necessary input parameters into the predictive models for such complex harmonic structures. Indeed, numerical simulations and mechanical modeling will be proposed and will deal with damage nucleation and crack propagation that might result from deformation incompatibility between the fine-grained shell and the coarse-grained core. The models may help to capture the critical parameters and feedback the elaboration to improve the microstructure design.
In addition to the full understanding and prediction of the macroscopic mechanical behavior, we intend to answer the following questions by the end of the project:
• Are developed materials with harmonic microstructures more efficient in terms of mechanical properties than the same materials with conventional and/or bimodal microstructures?
• Are pure Ti harmonic microstructures better than the Ti-6-4 or T-15-3-3 conventional alloys?
• Will the expected high strength and high ductility properties allow making practical applications of structures that are light, compact and have superior reliability possible?
• …
If so, then we can save not only the rare and difficult to refine alloying elements but also costly thermo-mechanical treatments and machining used to transform them. This will contribute at some level to:
• Resources and energy savings,
• CO2 reduction,
• Recyclability,
• Uncovering new applications to provide society with the fruit of research results and contribute to the welfare of mankind.

The knowledge resulting from this project will be likely to participate in the recent initiatives taken at national level for a renewal of metallurgy, from the point of view of fundamental research, technology transfer as well as the training of young scientists of all levels.