The effect of thermomechanical treatments on grain size and precipitate evolution as well as their impact on the shape memory properties of cold-drawn Fe41–Ni28–Co17–Al11.5–Ti2.5–B0.05 (at.

Shape memory alloys (SMAs), such as Ni–Ti, are promising candidates for actuation and damping applications. Although processing of Ni–Ti bulk materials is challenging, well-established processing routes (i.e. casting, forging, wire drawing, laser cutting) enabled application in several niche applications, e.g. in the medical sector. Additive manufacturing, also referred to as 4D-printing in this case, is known to be highly interesting for the fabrication of SMAs in order to produce near-net-shaped actuators and dampers. The present study investigated the impact of electron beam powder bed fusion (PBF-EB/M) on the functional properties of C-rich Ni 50.9 Ti 49.1 alloy. The results revealed a significant loss of Ni during PBF-EB/M processing. Process microstructure property relationships are discussed in view of the applied master alloy and powder processing route, i.e. vacuum induction-melting inert gas atomization (VIGA). Relatively high amounts of TiC, being already present in the master alloy and powder feedstock, are finely dispersed in the matrix upon PBF-EB/M. This leads to a local change in the chemical composition (depletion of Ti) and a pronounced shift of the transformation temperatures. Despite the high TiC content, superelastic testing revealed a good shape recovery and, thus, a negligible degradation in both, the as-built and the heat-treated state.

A synchrotron X-ray diffraction study of high-temperature (HT) shape memory alloy 49Co–21Ni–30Ga (in at. pct.) was performed. The volume fraction, cell parameter and temperature evolution of the different secondary phases were analyzed. This study reports reliable experimental data on these parameters to be used as a future reference to adjust the composition of the material.

High Entropy Alloys (HEAs) are a new class of multi-component alloys with excellent tensile strength-ductility combination. Their yield strength levels, however, are still low as compared to other high strength materials. Here, Equal Channel Angular Pressing (ECAP) was employed to improve the yield strength of the most well-known HEA, CoCrFeMnNi, through microstructural refinement. The cyclic response of both coarse and ultrafine grained CoCrFeMnNi was investigated during strain-controlled low cycle fatigue tests under fully reversed push-pull loading at room temperature. The microstructural evolution during cyclic loading was compared to the microstructure under quasi-static monotonic loading. Very high yield strength levels around 1 GPa were obtained after ECAP. In addition, ECAP samples demonstrated a superior fatigue life at relatively low strain amplitudes while coarse grained samples exhibited a better fatigue life at the highest strain amplitude considered. X-ray diffraction, electron backscattered diffraction and transmission electron microscopy were performed to reveal underlying mechanisms for the superior fatigue life and the overall transient behavior upon cycling. The fatigue life and hardening behavior were governed by the refined grain size, high density dislocation walls, and the annihilation of existing dislocations resulting in the formation of cell structures in the ECAP samples. The lower fatigue life of the ECAP samples at the highest strain amplitude is attributed to the higher stress amplitudes and cyclic softening due to accelerated dislocation annihilation. Finally, the formation of dislocation cell structures and high-density dislocation walls simultaneously is rationalized by the effect of applied stress on the partial dislocation separation.

Co-Ni-Ga high-temperature shape memory alloy is additively processed by selective laser melting for the first time. Reversible martensitic transformation of the as-built material is proven by differential scanning calorimetry. Microstructural analysis reveals a columnar-grained microstructure resulting from epitaxial solidification. Columnar-grained microstructures are characterized by a very low degree of constraints being beneficial for superior functional performance in numerous shape memory alloys. However, process-induced crack formation remains a challenge towards robust realization of adequate conditions showing good mechanical properties.

This manuscript presents the singular event of pearlite occurrence in commercially produced Hadfield steel. A detailed characterization of the microstructure is performed, and its influence on the mechanical properties of the material is analyzed. The found microstructure may be interpreted as carbide formation if observed at the optical microscope. However, it consists of an extremely fine lamellae structure ranging from 40 to 130 nm of thickness. Experimental evidence of pearlite formation is supported by microhardness measurements, X-ray diffraction, and secondary electron microscopy. The pearlite is located on the austenitic grain boundaries and within by means of intragranular islands. The occurrence of this phase is detrimental for the ductility response of the material assessed by means of uniaxial tensile testing and reduction in area determination. It is observed that a pearlite fraction of 20% is responsible for a reduction of 90% in elongation at fracture and a drop of 80% in reduction in area. A short heat treatment performed at 1050 °C allows recovering the ductility response of the material keeping grain size and chemical composition unchanged.

The martensitic transformation properties (maximum strain, residual strain and transformation stability) of Ni50−x–Ti50–Cox strips produced by twin-roll casting (TRC) and standard casting (SC) were compared. A complete microstructural characterization was carried out on both samples using optical microscopy, transmission electron microscopy (TEM), electron backscattering diffraction (EBSD-SEM), energy dispersive X-ray spectroscopy (EDS-SEM), X-Ray diffraction and differential scanning calorimetry (DSC). According to the results obtained by DSC and load-biased thermal-cycling measurements, the TRC strip is more stable and has lower residual strain than the SC strip for loads below 90 MPa. Using the austenitic texture of each strip, the recoverable strain upper bounds of the martensitic transformation (Sachs' bound) were calculated. A comparison between the measured maximum recoverable strain and the Sachs' bound, allows us to discuss how the particular microstructures produced by the two production techniques affect the strip's shape memory properties.

We have investigated an Fe-30Mn-4Si shape memory alloy to clarify the effect, on the bulk texture, of the shear layers resulting from two different thermo-mechanical treatments. XR analysis has shown the existence of texture heterogeneity through the rolled sheet's thickness, due to the effect of friction between sheet and rolls. Neutron diffraction revealed that textured layers on the sheet's surface affect the whole volume. The texture found on the surface of the sheet rolled at 600 degrees C is the most favourable for the gamma ->epsilon martensitic transformation which is the origin of the shape memory effect. Comparing these results with those obtained on sheets rolled at room temperature, we found that shear deformation gradients produce changes in the bulk material texture. Tensile tests initially induce the martensitic transformation in those grains favourably oriented. As a result, these favourable orientations disappear in the remnant austenite.

Texture evolution and shape memory behaviour were investigated in an Fe-30Mn-4Si (at.%) alloy showing the gamma <->epsilon martensitic transformation. The alloy was subjected to different rolling and annealing treatments. Texture mapping and shape memory effect were performed in each case. After hot-rolling at 1000 degrees C, and for high values of accumulated strain, typical face-centered cubic (FCC) shear textures were observed for the austenitic phase. Further rolling at 600 degrees C leads to some texture reduction although the texture pattern remains unchanged. Alternatively, cold-rolling leads to stress-induced e martensite and, after annealing at 650 degrees C, a similar FCC rolling texture was observed. Thus, different thermomechanical processes allow the development of quite different texture patterns that might be exploited for attaining novel shape memory responses. Shape recovery and yield stress were measured for the different specimens to analyze the role of texture and processing parameters on the shape memory effect. (c) 2007 Elsevier B.V. All rights reserved.

Strips of Cu-13Al-5Ni-1Ti (wt.%) of about 400 mu m thickness were obtained by twin-roll casting directly from the melt. Temperature-induced transformations under constant tensile load show a larger deformation, associated with the reversible martensitic transformation, for strips annealed at 700 degrees C and 900 degrees C than for the as-cast samples. This effect is partially ascribed to relaxation of internal stresses produced by the rapid solidification, some recovery of short-range disorder and microstructural changes related to Ti-rich X-phase precipitates. On the other hand, the strips show a columnar grain morphology and annealing leads to an increase of texture intensity related with the increase in elongation rate. Columnar grains are mostly aligned with the < 0 4 0 > (R18) // < 100 > (DO3) directions. (c) 2007 Elsevier B.V. All rights reserved.