Papers by Author: Anne Denquin

Abstract: This work investigates the evolution of the microstructure of an Nb-23Ti-20Si (at.%) based alloy, from the primary plasma-melted material that is gas-atomized towards the consolidated material (here using SPS). The nature, morphology and size of the solid solution and the various silicides are followed by SEM, EDS and EBSD. Homogenous and fine microstructures are obtained after the SPS step and are improved by a subsequent heat treatment (1500°C, 100 h). However blocky silicides, already present in the powder particles, cannot be eliminated. A better control of the primary material’s microstructure would improve the microstructure of the final material.

Abstract: The objective of this study is the evaluation of high entropy alloys for aeroengines applications up to 1000°C. Al_xNbMoTiV alloys with 10 and 20 at.% Al have been produced by arc melting. As-cast microstructure and phase transformations during heat treatments have been investigated through SEM, DRX and TEM, revealing the possibility of homogenization at 1400°C and the stability of the BCC structure at 1000°C and 800°C for both alloys. Mechanical properties have been evaluated through micro-hardness and compression tests up to 800°C. It appears that, although both alloys show a similar microstructure and hardness evolution with heat treatment, the Alloy containing 10 at.% of Al show a higher yield strength at room temperature and 800°C, related to the brittle character of the alloy containing 20 at.% of Al.

Abstract: Nb-base refractory intermetallic materials have potential interest for high temperature applications thanks to their low density and high temperature strength. While advanced intermetallics in monolithic form have limited prospects for providing the required balance of properties for use at high temperatures, two-phase or multicomponent intermetallic systems composed of a ductile, Nb-base refractory phase in equilibrium with one or more silicide intermetallics show promise for further development as structural materials. In the present paper, Nb-base refractory alloys based on Nb-35Ti-15Al (at.%) were doped with small amount of Si (1 and 2 at% of silicon) addition to improve its high temperature strength by keeping an acceptable ductility at room temperature. The samples were prepared by arc-melting starting from pure elements (99.99%). The silicon addition effects on the microstructural features were investigated by using X Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) techniques. Its effects on the mechanical properties were assessed by compression tests at ambient and high temperatures. Compression tests show the beneficial effect of the Si addition on strength.

Abstract: Among the different systems for high temperature shape memory alloys (SMA’s), equiatomic RuNb and RuTa alloys demonstrate both shape memory effect (SME) and MT temperatures above 800°C. Equiatomic compounds undergo two successive martensitic transformations, β (B2) → β’ (tetragonal) → β’’ (monoclinic), whereas out of stoechiometry alloys exhibit a single transition from cubic to tetragonal. In the case of two successive martensitic transformations, we expect to have a finer microstructure of the second martensite because it is supposed to develop inside the smallest twin elements of the former one. In equiatomic Ru-based alloys, if the first martensitic transformation is “normal”, the second one gives different unexpected microstructures with, for instance, twins with a thickness which is larger than the smallest spacing between twin variants of the first martensite. In fact, the reason for this unexpected hierarchy of the twins size is that the second martensitic transformation takes place in special conditions: geometrically, elastically and crystallographically constrained.

Abstract: Welding cheap and ductile 6xxx Al alloys with high strength 2xxx Al alloys is desirable for instance in specific aeronautical applications. These alloys present different rheological behaviors and melting temperatures which affect the ability to produce sound dissimilar friction stir welds. Dissimilar friction stir butt welds made of 2014-T6 and 6061-T6 Al alloys were performed with various welding parameters including shifts of the tool from the initial separation between the plates to be welded and placing one alloy either on the advancing, or on the retreating side of the weld. Temperature measurements during welding, mechanical characterization (transverse tensile tests and hardness profiles) and macrographic observations were performed. Macrographies on sections perpendicular to the welding direction reveal different metal flow patterns in the weld nugget. If the 2014 alloy is placed on the advancing side of the weld, an abrupt transition between the weld nugget and the 6061 alloy is observed on macrographs leading to premature fracture in tension. Dissimilar welds are cooler on the 6061 side of the weld, i.e. the weakest side of the weld, than the corresponding 6061 similar weld, limiting the growth of the hardening precipitates. This leads thus to higher strength of the dissimilar welds. Dissimilar welds with the weld center shifted towards the 2014 alloy present lower temperatures than unshifted welds on the 6061 side of the weld, also leading to higher strength.

Abstract: Among the different systems for high temperature shape memory alloys (SMA’s), equiatomic RuNb and RuTa alloys demonstrate both shape memory effect (SME) and MT temperatures above 800°C. For both systems, it is interesting to find a way to control the transformation temperatures while keeping the shape memory effect. One way to change the transformation temperatures is to change the composition in the binary alloys; another is to add a ternary element like Fe. The eight investigated alloys show two different space groups at room temperature. The monoclinic alloys undergo two successive displacive transformations on cooling, starting from the high temperature β phase field: β (B2) à β’ (tetragonal) à β’’ (monoclinic). The tetragonal alloys exhibit a single transition from cubic to tetragonal. A multiple twinned microstructure can be found in all alloys. Transformation temperatures decrease with lower Ru content and with the addition of Fe. The β’ à β transformation seems to be the main responsible for the SME. Compression tests performed in the martensitic phase give a quantitative result of the shape memory effect. In the binary alloys, the SME decreases with decreasing Ru content, which is in accordance with the evolution of the lattice parameters of martensites. A lower SME in the ternary alloys can also be linked to the lattice parameters and seems to be quite reliable to predict the evolution of the shape memory effect.

Abstract: High temperature shape memory alloys offer numerous potential applications in industrial domains like aeronautics. Even if up to now, none of the studied alloys have found a place in airplane turbines, research in this field is still active. Starting from the well-known “room temperature” shape memory alloy NiTi, it has been demonstrated that the addition of a ternary element such as gold in substitution of nickel greatly enhances the temperatures of the martensitic transformation. In the binary TiAu compound, the martensite start temperature can attain 875 K with satisfying reversibility and cycling stability. From lattice parameters measurements, it has been shown that the maximum transformation strain can reach 10.75 % for Ti₄₇Au₅₃ alloy, which is comparable to that of the NiTi alloy. However, to the best of our knowledge, quantitative measurements of the recoverable strain by shape memory effect are not available in the literature. We present here some quantitative results of shape memory effect associated to this phase transformation in Titanium-Gold alloys measured after compression tests.