Functional fatigue (FF) during thermal and mechanical cycling, which leads to the generation of macroscopic irrecoverable strain and the loss of dimensional stability, is a critical issue that limits the service life of shape memory alloys (SMAs). Although it has been demonstrated experimentally that such a phenomenon is related to microstructural changes, a fundamental understanding of the physical origin of FF is still lacking, especially from a crystallographic point of view. In this study, we show that in addition to the normal martensitic phase transformation pathway (PTP), there is a symmetry-dictated non-phase-transformation pathway (SDNPTP) during phase transformation cycling, whose activation could play a key role in leading to FF. By investigating crystal symmetry changes along both the PTPs and SDNPTPs, the characteristic types of defects (e.g., dislocations and grain boundaries) generated during transformation cycling can be predicted systematically, and agree well with those observed experimentally in NiTi. By analyzing key materials parameters that could suppress the SDNPTPs, strategies to develop high performance SMAs with much improved FF resistance through crystallographic design and transformation pathway engineering are suggested.

Catalytic CO2 reduction to fuels and chemicals is a major pursuit in reducing greenhouse gas emissions. One approach utilizes the reverse water‐gas shift reaction, followed by Fischer–Tropsch synthesis, and iron is a well‐known candidate for this process. Some attempts have been made to modify and improve its reactivity, but resulted in limited success. Now, using ruthenium–iron oxide colloidal heterodimers, close contact between the two phases promotes the reduction of iron oxide via a proximal hydrogen spillover effect, leading to the formation of ruthenium–iron core–shell structures active for the reaction at significantly lower temperatures than in bare iron catalysts. Furthermore, by engineering the iron oxide shell thickness, a fourfold increase in hydrocarbon yield is achieved compared to the heterodimers. This work shows how rational design of colloidal heterostructures can result in materials with significantly improved catalytic performance in CO2 conversion processes.

A new class of non-equiatomic FeNiCoAlTaB (NCATB) high entropy alloy (HEA) is introduced, which exhibits tunable properties from cryogenic/ambient superelasticity to ultra-high strength through controlling the nature or type of martensite. In the current NCATB-HEA alloy system, depending on the size of γ'-Ni 3 Al (L1 2 ) precipitates, thin-plate, lenticular, butterfly, and lath-like martensite can form. When thin-plate thermoelastic martensite is favored, a superelastic strain of about 0.025 (ambient) and %0.01 (cryogenic) is achieved with a high yield stress of %800 MPa and a high-damping effect (10 times higher than Cu-Al-Ni superelastic alloy). While for butterfly and lath-like martensite dominated NCATB-HEA, an ultra-high yield stress of around 1.1 GPa is achieved while no superelasticity is demonstrated. This current alloy system helps to expand the application domain of HEAs, for example, into high-damping applications, robust actuators, space exploration, and other structural material applications.

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The authors report on a relatively new alloy, Ni54Ti45Hf1, that exhibits strengths more than 40% greater than those of conventional NiTi‐based shape memory alloys − 2.5 GPa in compression and 1.9 GPa in torsion − and retains those strengths during cycling. Furthermore, the superelastic hysteresis is very small and stable with cycling. Aging treatments are used to induce a very high density of Ni4Ti3 precipitates, which impede plasticity during cycling yet do not impart substantial dissipation to the reversibility of the phase transformation. Pairing compression testing with high‐energy synchrotron X‐ray diffraction and aberration‐corrected electron microscopy provides an in‐depth look at the structure‐property relationships of this alloy. Specifically, it is found that a combination of small, untwinned retained martensite laths, and dislocations on the austenite‐martensite interfaces primarily strengthen the alloy as opposed to dislocation networks. Furthermore, some combination of nanoprecipitation and interface dislocations is responsible for the remarkably low mechanical hysteresis exhibited by this material.