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.

High temperature shape memory alloys RuNb and RuTa were investigated by X-ray diffraction in order to determine the structure of the low temperature β” martensite. Powder X-ray diffraction and Rietveld analysis confirm the monoclinic symmetry and allow a precise determination of the atom positions. The low temperature microstructure is the result of a martensitic transformation from a tetragonal β′ martensite. The analysis of how it is controlled by the space groups of the two phases and by their relative orientation is performed by transmission electron microscopy. A three level twinned microstructure is recognized in the β” phase. The investigation at the smallest scale twins reveals the presence of wavy contrast boundaries which are typical for translation interfaces. High resolution transmission electron microscopy images, accompanied by phase contrast imaging analysis, have been used to determine the translation vectors associated to the interfaces. These displacements are the translations of the tetragonal β′ phase which are lost during the transformation. The combination of X-ray diffraction and transmission electron microscopy investigation allows the suggestion of a global distortion mechanism that explains the atomic displacements in the β” form.

The objective of this study is the evaluation of high entropy alloys for aeroengines applications up to 1000°C. AlxNbMoTiV 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.

Separate effects of 0.3 at% Si and 0.2 at% C additions have been investigated on the Ti–43.5Al–1Mo–4Nb–0.1B alloy. Both elements appear to stabilise the γ phase. Si addition induces an increase in both γ-lamellar and γ-grain volume fractions. A heterogeneous distribution of primary and secondary silicides is observed in Al-rich regions of the microstructure, but silicon appears to be mainly in solid solution in the lamellar structure. The solid solution effect of silicon in the more plastically deformable β phase is probably very limited because of the high volume fraction of α2 in this alloy. This may explain the lack of beneficial effect of 0.3 at% of silicon on both tensile and creep properties. Carbon appears to stabilise the γ phase through an increase in γ-grain volume fraction and is found to be in solid solution in the lamellar structure. Our results suggest a very strong solid solution effect of carbon, since addition as low as 0.16 at% leads to an increase in yield strength at room and high temperature, a reduction in the secondary creep rate and a longer creep life.

In the present work we have studied the high-temperature shape memory alloys based on the Ru-Nb system by using two mechanical spectrometers working in temperature ranges from 200 to 1450ºC and -150 to 900ºC. We have studied internal friction peaks linked to the martensitic transformations in the range from 300 to 1200ºC. In addition, we have evidenced another internal friction peak at lower temperature than the transformations peaks, which apparently exhibits the behaviour of a thermally activated relaxation peak, but in fact is a strongly time-dependent peak. We have carefully studied this peak and discussed its microscopic origin, concluding that it is related to the interaction of some structural defects with martensite interfaces. Finally, we perform a complete analysis of the whole internal friction spectrum, taking into account the possible relationship between the time-dependent peak and the martensitic transformation behaviour.

In the present work we have studied a high temperature Ru-50Nb (at.%) shape memory alloy using a mechanical spectrometer able to work in the temperature range from 473 to 1723K. We have investigated two internal friction peaks, as well as the dynamic modulus variation, linked to the martensitic transformations in the range from 573K to 1473K. In addition, we have evidenced another internal friction peak PLT at lower temperature than the transformation peaks, which apparently exhibits the behaviour of a thermally activated relaxation peak. However, we show that this peak is not a relaxation one because it exhibits a strongly time-dependent behaviour. We have carefully analysed this peak and discussed its microscopic origin, concluding that it is related to the interaction of some structural defects with martensite interfaces.

Internal features and defects such as joint line remnant, kissing bond, and those induced by an initial gap between the two parent sheets were investigated in AA2198-T851 friction stir welded joints. They were compared with the parent material and to defect-free welds obtained using a seamless sheet. The cross-weld tensile strength was reduced by the defects by less than 6 pct. The fracture elongation was not significantly affected in view of experimental scatter. Fracture location, however, changed from the thermomechanically affected zone (retreating side) to the defect in the weld nugget for the welds bearing a kissing bond and for some of the gap welds. The kissing bond was shown by EBSD to be an intergranular feature; it fractured under a normal engineering stress close to 260 MPa during an in situ SEM tensile test. Synchrotron tomography after interrupted tensile testing confirmed opening of the kissing bond. For an initial gap of 23 pct of the sheet thickness, intergranular fracture of copper-enriched or oxide-bearing grain boundaries close to the nugget root was evidenced. The stress and strain state of cross-weld specimens loaded under uniaxial tension was assessed using a 3D finite element, multi-material model, determined on the basis of experimental data obtained on the same specimens using digital image correlation.

The fatigue strength and failure mechanisms of defect-free (“sound”) and flaw bearing friction stir butt-welds of 3.1mm-thick AA2198-T8 Al–Li–Cu alloy have been investigated via S–N curves at R=0.1 using cross weld specimens. The fatigue strength of sound welds is only reduced by 10–15% at the aimed lifetime of 105 cycles compared to the base material. Joint Line Remnant (JLR) bearing welds have a similar fatigue strength as sound welds and the JLR is not the crack initiation site. Kissing Bond (KB) bearing welds that have undergone a weld root polishing show a reduction in fatigue strength by 17% compared to sound welds. For specimens loaded at or above yield strength of the weld nugget the crack systematically initiates from the KB during the first cycle, which is interpreted further using fracture mechanics. The strongest reduction, about 28% in fatigue strength, is found for welds with an initial gap between the parent sheets (GAP welds) along with initiation at intergranular surface microcracks. Kahn tear tests show a reduction in tearing resistance for the flaw bearing welds with a similar ranking as for the fatigue strength.

Ru-based high temperature shape memory alloys show different structures at different temperatures and composition. Equiatomic Ru–Ta and Ru–Nb show two subsequent phase transformations, but their individual influence is still unclear because of the difficulty of comparing the high temperature cubic phase, the intermediate tetragonal phase and the low temperature monoclinic phase. It is important to find a way to compare these structures in order to better understand the microstructure of these alloys and their shape memory behaviour. In order to get the necessary information the alloys have been investigated with neutron diffraction technique during in-situ heating up to about 1100 °C.

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.

Dissimilar friction stir butt welds between the 2014-T6 and the 6061-T6 Al alloys were performed with various sets of welding parameters including a lateral shift of the tool from the initial separation between the plates to be welded and by placing the alloys, either on the advancing, or on the retreating side of the weld. Torque and temperature measurements during welding as well as macrographies and hardness profiles measurements were performed after welding. It was found that the welding torque, the temperature, the metal flow and the welds’ hardness profile depend on the proportion of each alloy included in the stirred zone. Those results are attributed to the difference between the softening temperatures of both alloys. The 6061 alloy's HAZ is the weak link in all dissimilar welds. The evolution of the hardening precipitation, the hardness and the local yield strength in the 6061 alloy are therefore modeled. The model reasonably well predicts the experimentally measured hardness of both similar and dissimilar welds. It also explains the influence of the alloys placement or tool lateral shift on the welds hardness by their influence on the precipitate radius and volume fraction.

Two intermetallic compounds, HfPd and TiAu have been analyzed as potential high temperature shape memory alloys. In spite of a reversible martensitic transformation detected at high temperature in HfPd, no meaningful shape recovery has been observed in this compound by TMA after compression pre-strain of the martensite.The mechanical properties of TiAu at high temperatures have been evaluated by thermal cycling under compression load and by compression tests. A high mobility of the dislocations is found around martensitic transformation temperatures, which limits the functional properties of this high temperature shape memory alloy. Some improvement should be obtained by precipitation hardening. (C) 2012 Elsevier B.V. All rights reserved.

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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.

The martensitic transformations in ultra-high temperature Ru-50Nb shape memory alloys have been studied by internal friction and dynamic modulus measurements. Two successive transformations from the high temperature cubic β phase to a tetragonal β′ martensite and then to another monoclinic β″ martensite have been found. Both transformations exhibit a sharp internal friction peak and a clear softening of the dynamic modulus, being a signature of the thermo-elastic martensitic transformations. In addition, a pseudo relaxation peak strongly dependent on time has been found and analyzed, concluding that it is linked to a pinning effect of martensite interfaces by point defects.

Nowadays, aeronautic and aerospace are the more demanding sectors for shape memory alloys (SMA) after the bio-medical one. In particular the interest has been recently focused on very high temperature SMA, which would be able of working as sensors and actuators in the hot areas of the engines and exaust devices. In the present work we undertook a study of the Ru-Nb SMA Intermetallics, which undergo two succesive martensitic transformations around 1050 K and 1180 K respectively, depending on composition. This study has been focused on measurements of internal friction spectra and dynamic modulus variation up to 1700 K, which have been carried out in a sub-resonant torsion mechanical spectrometer. The internal friction and dynamic modulus have been studied as a function of the heating-cooling rate and the frequency in order to compare experimental behaviour with theoretical models for martensitic transformations. In addition to the internal friction peaks linked to both martensitic transformations we have also observed a complex relaxation process around 950 K, which seems to be linked to the interaction of the martensite interfaces with structural defects. An analysis and discusion of the potential microscopic mechanisms are also presented.

High temperature shape memory alloys have received a large interest for many years but none of the systems studied so far has led to industrial applications yet. The titanium-gold system has been reported to develop a martensitic transformation at high temperature but the previous studies did not mention about shape memory effect in a quantitative way. Here are presented first elements concerning the potential of this compound as a high temperature shape memory alloy, from the functional properties and oxidation resistance point of view. A maximum recovered compression strain of 3.9% can be obtained for an initial compression strain of 5% corresponding to a 78% shape recovery, in the range of temperature 850-950 K. A two-way shape memory effect is also reported for the first time for this compound. The oxidation of the alloy at these temperatures seems to be a limitation but its behavior around 700 K is very promising. (C) 2011 Elsevier Ltd. All rights reserved.