Fernando Carreno

Shape memory alloys (SMAs) are functional materials with a wide range of applications, from the aerospace sector to the biomedical field. Nowadays, there is a worldwide interest in developing SMAs through powder metallurgy like additive manufacturing (AM), which allows innovative building processes. However, producing SMAs using AM techniques is particularly challenging because of the microstructure required to obtain optimal functional properties. This aspect is critical in the case of Cu–Al–based SMAs, due to their high elastic anisotropy, making them brittle in polycrystalline form. In this work, we approached the processing of a Cu–Al–Ni SMA following a specific powder metallurgy route: gas atomization of a pre-alloyed melt; compaction of the atomized powders through hot isostatic pressing; and a final hot rolling plus thermal treatments. Then, the microstructure of the material was characterized by electron microscopy showing a specific [001] texture in the rolling direction that improved the functional behavior. The successive processing steps produce an increase of about 40 °C in the martensitic transformation temperatures, which can be well controlled and reproduced through the developed methodology. The thermomechanical functional properties of superelasticity and shape memory were evaluated on the final SMA. Outstanding, fully recoverable superelastic behavior of 4.5% in tension, as well as a ±5% full shape memory recovery in bending, were reported for many cycles. These experiments demonstrate the enhanced mechanical and functional properties obtained in polycrystalline Cu–Al–Ni SMAs by powder metallurgy. The present results pave the road for producing this kind of SMA with the new AM technologies, which always produce polycrystalline components and can improve their processes taking the powder metallurgy SMA, here produced, as reference material.

The mechanical properties of 2024 aluminum alloy were studied after two different tempers. The T351 temper (solution heat treatment, stress relief, and natural aging) leads to high hardness and toughness. A thermal treatment consisting of heat-treating at 280 °C for 48 h and slow cooling in a furnace, named TT temper, was performed to increase the precipitate size and their separation while minimizing the amount of solutes in solid solution, which produced the minimum hardness for an overaged Al2024 alloy and a lower tensile flow stress than for the T351 temper. The flow stress strongly decreases and the elongation to failure strongly increases for both materials above 300 °C. Differences in strain rate at a given stress in the power law regime at all temperatures for both tempers and compared with pure aluminum are attributed to the influence of solutes in solid solutions, affecting both the glide and climb of dislocations. However, the stacking fault energy, SFE, alone does not account for the hot deformation behavior. Thus, it is the synergistic effect of various solutes that affects the entire deformation process, causing a decrease of three or four orders of magnitude in strain rate for a given stress with respect to the pure aluminum matrix values.

Extensive research on severe plastic deformation (SPD) of metallic materials has been performed so far in order to improve mechanical properties, both at low and high temperatures. As a result of the extensive grain refinement obtained, increased superplastic behaviour has been attained at higher strains rates and lower temperatures than usual. This is due to finer grain sizes and higher average misorientations obtained, which enhance grain boundary sliding (GBS). However, the misorientation effect on superplasticity, and the prediction of misorientation evolution during SPD has remained so far, qualitative. In this research, especial attention has been given to the quantitative misorientation evolution with increasing equal channel angular pressing (ECAP) deformation, in order to propose an expression useful to describe and predict the influence of the different processing parameters on the misorientation evolution with strain. This is exemplified for ECAPed Al-Zn-Mg-Cu and other aluminium alloys, and it could serve as a basis for predicting the misorientation evolution, and its influence on superplasticity, of other SPD processes and metallic alloys.

A commercial 2024 aluminum alloy was heat treated at 280 °C for 48 h and then slow cooled in a furnace to obtain minimum hardness. This material was then friction stir processed (FSP) using three sets of processing conditions. To study the effect of the processing on the microstructure and the high temperature mechanical properties, the materials were tested in tension at an initial strain rate of 10⁻² s⁻¹ and temperature range 200 to 450 °C. Processing severity was selected as the main factor for obtaining fine grain sizes right after FSP. The grain size was enormously reduced from about 50 µm to 1 µm. This grain reduction gave rise to very high elongations to failure of about 400%. Strain–rate-change tests showed a stress exponent close to 2 at intermediate strain rates, which was related to grain boundary sliding as the controlling deformation mechanism and to superplasticity, which is strongly grain-size dependent. A possible controlling deformation mechanism by solute-drag creep, as proposed by other authors, was disregarded since tests conducted at 450 °C, where the microstructure of the FSP materials coarsens rapidly, gave a low elongation to failure and high resistance, which showed the importance of the grain size dependence of the operative deformation mechanism at 250–400 °C, which was only compatible with grain boundary sliding.

The hot working behavior of a 38Ni-21Cr based superalloy was studied. Torsion tests at strain rates between 2 and 20 s^-1and temperatures between 1123 and 1473 K were conducted. A fine microstructure of about 6-10 lm grain size and small TiC particles were observed after torsion tests at 1473 K and 4 s^-1. The results of these tests were analyzed usingMATLAB and MS EXCEL programs to obtain the parameters of the Garofalo equation able to correlate the experimental data with a single equation. Stress exponent values much higher than 5 and a high activation energy for deformation suggest that the deformation is controlled by a slip creep mechanism characteristic of reinforced materials. The optimum forming temperatures at different strain rates were determined by means of the second Lyapunov stability equation. An optimum stability temperature of 1643 K (13708C) at 10 s^-1is determined.

Friction stir processing (FSP) was used on coarse-grained WE54 magnesium alloy plates of as-received material. These were subjected to FSP under two different cooling conditions, refrigerated and non-refrigerated, and different severe processing conditions characterized by low rotation rate and high traverse speed. After FSP, ultrafine equiaxed grains and refinement of the coarse precipitates were observed. The processed materials exhibited high resistance at room temperature and excellent superplasticity at the high strain rate of 10⁻² s⁻¹ and temperatures between 300 and 400 °C. Maximum tensile superplastic elongation of 726% was achieved at 400 °C. Beyond 400 °C, a noticeable loss of superplastic response occurred due to a loss of thermal stability of the grain size. Grain boundary sliding is the operative deformation mechanism that can explain the high-temperature flow behavior of the ultrafine grained FSP-WE54 alloy, showing increasing superplasticity with increasing processing severity.