Superelastic deformation of thin Ni–Ti wires containing various nanograined microstructures was investigated by tensile cyclic loading with
in situ evaluation of electric resistivity. Defects created ...by the superelastic cycling in these wires were analyzed by transmission electron microscopy. The role of dislocation slip in superelastic deformation is discussed. Ni–Ti wires having finest microstructures (grain diameter <100
nm) are highly resistant against dislocation slip, while those with fully recrystallized microstructure and grain size exceeding 200
nm are prone to dislocation slip. The density of the observed dislocation defects increases significantly with increasing grain size. The upper plateau stress of the superelastic stress–strain curves is largely grain size independent from 10 up to 1000
nm. It is hence claimed that the Hall–Petch relationship fails for the stress-induced martensitic transformation in this grain size range. It is proposed that dislocation slip taking place during superelastic cycling is responsible for the accumulated irreversible strains, cyclic instability and degradation of functional properties. No residual martensite phase was found in the microstructures of superelastically cycled wires by TEM and results of the
in situ electric resistance measurements during straining also indirectly suggest that none or very little martensite phase remains in the studied cycled superelastic wires after unloading. The accumulation of dislocation defects, however, does not prevent the superelasticity. It only affects the shape of the stress–strain response, makes it unstable upon cycling and changes the deformation mode from localized to homogeneous. The activity of dislocation slip during superelastic deformation of Ni–Ti increases with increasing test temperature and ultimately destroys the superelasticity as the plateau stress approaches the yield stress for slip. Deformation twins in the austenite phase ({1
1
4} compound twins) were frequently found in cycled wires having largest grain size. It is proposed that they formed in the highly deformed B19′ martensite phase during forward loading and are retained in austenite after unloading. Such twinning would represent an additional deformation mechanism of Ni–Ti yielding residual irrecoverable strains.
Display omitted
This paper presents a systematic study where the distribution of precipitate microstructures is mapped in the cross-section of a friction stir weld made with an AA2050 Al–Cu–Li alloy ...in the naturally aged temper, as well as the evolution of this microstructure during subsequent post-welding heat treatment (PWHT). This study is carried out using spatially resolved small-angle X-ray scattering, supported by transmission electron microscopy, differential scanning calorimetry and microhardness mapping. The as-welded microstructure is dominated by solute clusters, while very little precipitation has taken place during the welding operation. During PWHT, the precipitation kinetics in the different zones of the weld is mainly controlled by the local dislocation density inherited from welding, and by the amount of solute available for precipitation, which depends on the volume fraction of welding-induced intermetallics. Pre-deforming the weld before the PWHT results in a very effective strength recovery and a nearly homogeneous distribution of hardness.
Transmission electron microscopy, electrical resistivity measurements and mechanical testing were employed to investigate the evolution of microstructure and functional superelastic properties of 0.1
...mm diameter as-drawn Ni–Ti wires subjected to a non-conventional heat treatment by controlled electric pulse currents. This method enables a better control of the recovery and recrystallization processes taking place during the heat treatment and accordingly a better control on the final microstructure. Using a stepwise approach of millisecond pulse annealing, it is shown how the microstructure evolves from a severely deformed state with no functional properties to an optimal nanograined microstructure (20–50
nm) that is partially recovered through polygonization and partially recrystallized and that has the best functional properties. Such a microstructure is highly resistant against dislocation slip upon cycling, while microstructures annealed for longer times and showing mostly recrystallized grains were prone to dislocation slip, particularly as the grain size exceeds 200
nm.
Display omitted
•Development of half skeleton structures for silicide-based thermoelectric modules.•Fabrication of twenty-two (four legs) silicide-based thermoelectric modules.•Excellent ...repeatability of the achieved output power.•State of the art power density performance.•Use of mass-produced materials and simple fabrication tools.
Silicide-based materials are among the most promising candidates for a mass manufacturing of thermoelectric devices allowing converting waste heat into electricity in the medium temperature range (250–500 °C), as they are formed from abundant, low cost and non-toxic elements while exhibiting good thermoelectric properties. In order to manage the detrimental mismatch of thermal expansion coefficients between the n and p-type materials constituting the thermoelectric legs, inducing thus thermomechanical stresses, we propose in this paper a new design of modules having a 'half-skeleton' structure. Twenty-two modules consisting of two couples of thermoelectric legs combining n-type magnesium silicide Mg2(Si, Sn) and p-type higher manganese silicide have been fabricated according to this design, the thermoelectric materials being manufactured by kilograms. It is clearly shown that all the interfaces present in the modules are free from cracks, oxygen, and diffusion. The remarkable repeatability of the measured thermoelectric performance attests the robustness of our manufacturing process. An average power output of 0.37 W has been achieved, i.e. a power density of 0.95 W/cm2, for a temperature difference of 400 °C (hot side temperature of 450 °C), placing our modules at the state of the art level while using simple production tools and materials mass production. Comparing this performance to finite elements modelling, the performance could be even enhanced. Long term stability tests at a given temperature and under cycling conditions in different working atmospheres are underway.
Cu-lean Al-Zn-Mg alloys are known to be susceptible to hydrogen embrittlement (HE), which currently limits their use in automotive industry. Several works suggested that the resulting loss of ...mechanical properties was related to hydrogen trapping in different metallurgical sites. The present work attempts to provide a better understanding of the hydrogen-dislocations interactions to evaluate their influence on the loss of mechanical properties of hydrogen-embrittled Al-Zn-Mg alloys. Pre-strained samples of 7046 aluminium alloy (AA7046) were therefore prepared in order to increase the density of motionless dislocations. Tensile samples, pre-strained or not, were then corroded in 0.6 M NaCl and mechanically tested to evaluate their HE susceptibility and the role of dislocations on hydrogen diffusion. Results highlighted a significant improvement of the HE resistance of the alloy with the increase in the density of motionless dislocations induced by the pre-strain step. This was attributed to preferential hydrogen trapping on motionless dislocations leading to a decrease in the hydrogen amount in the grain boundaries. The measurements of hydrogen penetration depth by Scanning Kelvin Probe Force Microscopy (SKPFM) for cathodically charged samples provided further evidence to support these assumptions.
Display omitted
•The higher the density of motionless dislocations, the lower the H-induced elongation to failure loss.•H trapping by motionless dislocations limited H diffusion and decreased the H amount at grain boundaries.•H trapping by motionless dislocations promoted a brittle transgranular fracture.
Microstructural changes taking place during the heat treatment of cold-worked NiTi alloy are of key interest in shape memory alloy technology, since they are responsible for setting the austenite ...shape and functional properties of the heat-treated alloy. In this work, microstructural evolution during non-conventional electropulse heat treatment of thin NiTi filaments was investigated in a unique high-speed in situ synchrotron X-ray diffraction experiment with simultaneous evaluation of the tensile force and electrical resistivity of the treated wire. The in situ results provide direct experimental evidence on the evolution of the internal stress and density of defects during fast heating from 20°C to ∼700°C. This evidence is used to characterize a sequence of dynamic recovery and recrystallization processes responsible for the microstructure and superelastic functional property changes during the electropulse treatments.
The heterogeneous precipitate microstructure of a AA2050 Al-Li-Cu in the T8 state after friction stir welding has been mapped by small-angle X-ray scattering (SAXS). 2D resolved maps of the fraction ...and size of both T
1
platelets precipitates and clusters/GP zones formed at room temperature are provided. TEM micrographs of selected zone confirm the interpretation of SAXS intensities. This microstructure mapping is compared to microhardness mapping and a direct correlation is shown. Short duration heat treatments made in a salt bath help understanding precipitate stability and suggest that the temperature exploration alone explains to a large extent the distribution of the precipitates microstructure across the welded structure.
•Different precipitation behaviors were identified between three stainless steel grades using X-ray transmission synchrotron diffraction.•The specimens were heavily carburized in grains and at grain ...boundaries.•However, equilibrium of precipitation was not reached.•The blocking effect of carbides had to be considered and was observed to evolve with time and microstructure.•Precipitation at grain boundaries could not be simulated with the used model.
Three steels were exposed in carburizing sodium at 600 and 650 °C. The kinetics and extent of carburization were characterized. Numerical simulations using the coupled thermodynamic-kinetic modeling software DICTRA were performed. It was proposed that the observed carbon diffusion profiles were induced by the combined diffusion of carbon in the grains and at grain boundaries coupled with the slow formation of carbides. The blocking effect of carbides on the carbon diffusion was observed to evolve as a function of time and microstructure. Acceptable agreement between experimental and simulated intragranular carbon profiles was achieved by optimizing the labyrinth factor and phases.
•Evidence of absence texturing of HMS densified by Spark Plasma Sintering.•The elasticity tensor of Mn15Si26 is determined via EBSD-nanoindentation coupling.•Nanohardness and Young’s modulus ...of< 001 > orientations is higher than the< 100 > ones.•Anisotropy of the mechanicals properties of Mn15Si26 has been evidenced.•Good agreement between experimental results and Ab-initio calculation.
Display omitted
The goal of this study is to investigate the mechanical and elastic characteristics of the Mn15Si26 compound via experimental nanoindentation measurements and ab-initio calculations. The mechanical properties such as Young’s modulus (E) and nanohardness are important inputs for improving the design and mechanical reliability of thermoelectric modules. The high-energy X-ray diffraction pattern of Mn15Si26 has been indexed with the Miller indices of a tetragonal crystalline structure whose cell parameters are the following: a = b = 5.535(3) Å and c = 65.552(4) Å. Nanoindentation measurements, with a Berkovich indenter tip have been performed on higher manganese silicide (HMS) compound mainly composed of Mn15Si26 grains. For the first time ever, it has been evidenced that both elastic modulus and nanohardness of the latter varied significantly depending on their crystallographic orientations provided by electron backscatter diffraction. Nanohardness and Young’s modulus along the< 001 > orientations are higher than the< 100 > ones. The nanohardness value of Mn15Si26 ranges from 16 GPa to 20 GPa and the Young’s modulus measured varies between 234 GPa and 300 GPa. The stiffness tensor (Sij= (Cij)−1) of Mn15Si26 has been deduced from these experimental measurements as well as calculated using Ab-initio calculations. The macroscopic elastic modulus (E, G, Β) and Poisson's coefficient have been examined and discussed and their 3D-representation has been plotted. The mechanical anisotropy hereby evidenced as the existence of anisotropy of the thermoelectric properties could be a significant factor for the mechanical reliability of thermoelectric modules which consisted of Mn15Si26 legs with a possible preferred crystallographic orientation induced during their fabrication.
Understanding the thermal stability of the Mg2(Si,Sn) system is essential to define their safe temperatures of service. Despite its good thermoelectric performance, Mg2(Si,Sn) is subject to a phase ...separation during thermal cycling due to the miscibility gap, which leads to a degradation of its thermoelectric properties and affects its performance during device operation. Isothermal annealing at 500 °C and 750 °C were performed with different annealing time to investigate thermal stability of Mg2(Si,Sn). During the heat treatment, two phases were formed associated with porosity in the matrix. In addition, thickness of specimen was tracked and a significant expansion was detected. This phenomenon is attributed to the Kirkendall effect. The composition and the structure of the two forming phases were investigated by electron probe microanalysis and X-ray diffraction. Finally, the optimized thermal treatment allowed to stabilize the Mg2(Si,Sn) without porosity and the presence of two thermodynamically stabilized phase (Mg2Si0.41Sn0.59 and Mg2Si0.58Sn0.42) leading to a better reliability of the silicide thermoelectric modules.
•This article is a full length article and not a review paper.•According to the submission website and the instruction for authors the highlights are not requested only for review papers.•However, to the finalization step to build the pdf they are requested.•This is why I submit this file.
PDF
