Structural Changes Associated with the Pseudoelastic Response of Fe-Based Shape Memory Alloys

The pseudoelastic responses of two types of iron base shape memory alloys (SMAs) were introduced and discussed. The former was based on Fe-Mn-Si system, obtained by classical (CM) and by powder metallurgy (PM) manufacturing. The latter was based on Fe-Ni-Co system being processed by a non conventional technology comprising melt spinning and heat treatment. In the case of FeMnSibased SMAs, CM specimens obviously experienced larger ductility and a more pronounced pseudoelastic response while PM specimens were stiffer and underwent larger work-hardening. On the other hand, melt spun FeNiCo-based SMAs revealed an outstanding superelasticity in the case of micro-indentation tests. By means of scanning electron microscopy (SEM) observations, a martensitic morphology was identified in FeMnSi-based SMAs while FeNiCo-based SMAs revealed an austenitic structure. The presence of both α’ and ε martensites was confirmed in FeMnSibased SMAs by means of X-ray diffraction (XRD). In fully austenitic melt-spun and aged FeNiCo-based SMAs, no martensite was indentified on XRD patterns. These results sustain the conclusion that FeMnSi-based SMAs, that contain pre-existing martensite, experienced a pseudoelastic behavior caused by crystallographic reorientation of martensite plate variants while austenitic FeNiCo-based SMAs experienced a reversible stress-induced martensitic transformation, at room temperature

Structural-Functional Changes in a Ti50Ni45Cu5 Alloy Caused by Training Procedures Based on Free-Recovery and Work-Generating Shape Memory Effect

Active elements made of Ti50Ni45Cu5 shape memory alloy (SMA) were martensitic at room temperature (RT) after hot rolling with instant water quenching. These pristine specimens were subjected to two thermomechanical training procedures consisting of (i) free recovery shape memory effect (FR-SME) and (ii) work generating shape memory effect (WG-SME) under constant stress as well as dynamic bending and RT static tensile testing (TENS). The structural-functional changes, caused by the two training procedures as well as TENS were investigated by various experimental techniques, including differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), X-ray diffraction (XRD), and atomic force microscopy (AFM). Fragments cut from the active regions of trained specimens or from the elongated gauges of TENS specimens were analyzed by DSC, XRD, and AFM. The DSC thermograms revealed the shift in critical transformation temperatures and a diminution in specific absorbed enthalpy as an effect of training cycles. The DMA thermograms of pristine specimens emphasized a change of storage modulus variation during heating after the application of isothermal dynamical bending at RT. The XRD patterns and AMF micrographs disclosed the different evolution of martensite plate variants as an effect of FR-SME cycling and of being elongated upon convex surfaces or compressed upon concave surfaces of bent specimens. For illustrative reasons, the evolution of unit cell parameters of B19′ martensite, as a function of the number of cycles of FR-SME training, upon concave regions was discussed. AFM micrographs emphasized wider and shallower martensite plates on the convex region as compared to the concave one. With increasing the number of FR-SME training cycles, plates’ heights decreased by 84–87%. The results suggest that FR-SME training caused marked decreases in martensite plate dimensions, which engendered a decrease in specific absorbed enthalpy during martensite reversion

Scalable Silicone Composites for Thermal Management in Flexible Stretchable Electronics

Hexagonal boron nitride (hBN) has been incorporated, as an active filler, in a customized silicone matrix to obtain high thermal conductivity composites, maintaining high flexibility and low dielectric permittivity, which are of interest for heat dissipation in energy storage systems (e.g., batteries or supercapacitors) and electronics. By the proper processing of the filler (i.e., hydrophobization with octamethylcyclotetrasiloxane and ultrasonic exfoliation) and its optimal loading (i.e., 10 wt%), composites with thermal conductivity up to 3.543 W·m−1·K−1 were obtained. Conductive heat flow (−280.04 W), measured in real heating–cooling conditions, proved to be superior to that of a commercial heatsink paste (−161.92 W), which has a much higher density (2.5 g/cm3 compared to 1.05 g/cm3 of these composites). The mechanical and electrical properties are also affected in a favorable way (increased modulus and elongation, low dielectric losses, and electrical conductivity) for applications as thermal management materials