Synthesis of niobium silicide based alloys by powder metallurgy route (atomisation and SPS) – SYNOPSIS

SYNOPSIS is divided into three phases. The first phase consists in setting up a procedure for producing pre-alloyed rods from a Nb-Ti-Cr-Al-Si type reference chemical composition, atomising theses rods in the crucible-free inert gas facility at ONERA, and then sintering the powder by flash sintering (SPS) at PNF2's facility in Toulouse as well as at ICB's facility, in order to produce sound materials (controlled porosity rate and microstructure). The project then moves on to a stage of optimising the chemical composition of this alloy, with the study of the influence of the elements Mo, B and Sn on the properties of arc-melted alloys, the selection of the most promising alloy and the production of powders with that chemical composition. Different microstructures generated by flash sintering are studied and the one that presents the best balance between the main usage properties of these materials (creep, strength, oxidation) will be selected. A study for the deep understanding of plasticity mechanisms is carried out. The last task concerns the production of technological parts with complex geometries without metallurgical defects and with the desired microstructure by flash sintering.

The analysis of the microstructure in the bars and in the powders obtained shows that the reduction of the diameter (50 mm compared to 70 mm) is beneficial in reducing the size of the blocky silicide phases (short cooling time in a larger area), without eliminating them; their maximum extension is however limited to the size of the droplets formed during atomisation. SPS sintering of 40-100 µm and 100-200 µm powders helps to homogenise the microstructure with coarsening in the initially eutectic zones. The «Powder« route is thus advantageous compared to the «Ingot« route. The sintering conditions have been established to provide dense materials: they remain within the window of parameters that do not overstress the machine and ensure a certain longevity of the graphite tools (T ~ 1400°C and pressure 50~75 MPa). The materials studied do not react with carbon (i.e. no bonding of the alloys with the graphite tools), which facilitated the manufacture of complex shaped parts by SPS by avoiding the use of papyex. The addition of Mo (2 to 4 at. %), B (2 to 8 at. %) and Sn (1 to 4 at. %) modifies the microstructure of the alloys through the nature, morphology and volume fraction of the phases present. For this series of materials elaborated by arc melting with microstructural, mechanical and oxidation resistance characterizations at 800°C and 1000°C, it was shown, in particular, that the simultaneous addition of Mo and Sn improves the properties of the reference alloy Nb-Ti-Cr-Al-Si. These additions contribute to the elimination of the Nb3Si type phase which is less mechanically resistant than the Nb5Si3 phases. However, the ductility of materials in the Nb-Ti-Mo-Cr-Al-Sn-Si range remains low and makes machining of the specimens difficult, so that the mechanical characterisation could not be completed.

The annual number of publications on Nb-Si materials is relatively constant: new alloy systems are proposed (e.g. with carbides or carbon nanotubes reinforcements), the influence of certain elements on the microstructural and mechanical properties is investigated deeper. However, none of them concern developments close to applications and do not propose compositions with acceptable characteristics for all properties of use. Industrial interest in this family of alloys seems to have waned. Other classes of materials, at the same stage of development or even more advanced than Nb-Si, are more promising, in particular self-healing ceramic matrix composites, whose density is two to three The SYNOPSIS project, with the experimental problems encountered partly due to a too low ductility of the designed alloys, times lower than that of Nb-Si and is out of reach for metallic materials. The SYNOPSIS project, with the experimental problems encountered partly due to a too low ductility of the designed alloys, did not complete the initial programme and failed to revive or maintain interest in Nb-Si for demanding applications. ONERA-initiated studies on these materials will probably stop, but a technology watch will be maintained.

The activities within the Synopsis project were presented at five conferences, on the topics (i) microstructural evolution between solid materials to be atomised, the obtained powders and SPS sintered samples and (ii) oxidation behaviour:
• Microstructure investigation of an Nb-Si alloy in different states of elaboration by powder metallurgy, V. Malard et al., Colloque PMF 2017 (3-5 May, Toulouse, FR);
• Microstructure investigation of new Nb-Si alloys, V. Malard et al., EUROMAT 2017 (17-22 Sept, Thessaloniki, GR);
• From Pre-Alloyed Rod to Gas-Atomized Powder and SPS Sintered Samples: How the Microstructure of an Nb Silicide Based Alloy Evolves, S. Drawin et a.l, THERMEC 2018 (9-13 July, Paris, FR).
• Study of the high temperature oxidation of Nb-Nb5Si3 alloys made by plasma fusion and flash sintering, M.-R. Ardigo-Besnard et al., MATERIAUX Congress 2018 (19-23 Nov., Strasbourg, FR).
• Microstructural study of an Nb-Si based alloy through the different steps of a powder metallurgy route, V. Malard et al., MRS Fall Meeting 2018 (25-30 Nov., Boston, USA);
• Use of the spark plasma sintering potential to develop Nb silicides and TiAl intermetallics, J.P. Monchoux et al. Beyond Nickel-Based Superalloys III (11-14 juin 2019, Nara, Japan).

The performances of aircraft engines are one of the main concerns of the manufacturers and one major differentiation lever with respect to the competitors. To do so, two main axes are under consideration, in agreement with the European ACARE recommendations: (i) the development of materials with enhanced high temperatures properties and (ii) the development of lightweight alloys for all engine components.
The SYNOPSIS project is focused on lightweight niobium silicide based alloy development for low pressure turbine components (800°C-1000°C service temperature range), designed for increased thermal loadings compared to the currently used superalloys (whether polycrystalline, directionally solidified or single-crystalline) and processed by the Spark Plasma Sintering (SPS) technology.
For a topical industrial application for which basic research was initiated several years ago and that still requires R & D efforts in the medium / long term, this project brings together three highly qualified partners around the development of these new refractory alloys and the manufacture of complex shaped components by way of Powder Metallurgy (SPS):
(i) ONERA (Châtillon) for the design, production and characterisation of non conventional alloys and the production of powders using its crucible-free inert gas atomisation facility,
(ii) ICB (Dijon) and
(iii) CEMES (Toulouse),
both for their complementary skills on materials development and characterisation, the processing of powders using the SPS technology and the associated simulations.
This consortium will study the integral production chain, from the production of alloy powders with complex compositions, to their shaping with well-controlled microstructures and properties, via a Powder Metallurgy route.
The project is split into three phases. The first deals with the setting up, for a reference alloy composition, of a procedure enabling, from the gas-atomisation of pre-alloyed ingots, to sinter the produced powder into bulk parts with sound metallurgical properties (porosity level and microstructure). Particular attention will be devoted to the full control, all along the production chain, of the chemical composition of the handled materials. Once this procedure is identified, the project will focus, in a second step, on alloy optimisation, starting with the definition of several alloy compositions, the production of the corresponding powders and their sintering to samples by SPS, with a range of microstructures by changing the SPS operating conditions. The evaluation criteria will be: strength, creep behaviour, oxidation resistances, toughness. These material properties will be evaluated at different temperatures. The ability of the alloy to allow production of parts with complex geometry (blade) will be the third key investigation point, as well as the possibilities of joining and surface modification using SPS. The fine understanding of deformation mechanisms during creep and more generally the plasticity mechanisms at stake will be considered in a dedicated work package.
Two PhD theses are associated with this project.
The SYNOPSIS project will use the skills developed during a previous ANR project (IRIS, 2009-2013), including the successful production using SPS of non optimised turbine blade blanks made of TiAl and niobium silicide based alloy powders.