Materials that undergo a reversible change of crystal structure through martensitic transformation(MT)possess unusual functionalities including shape memory, superelasticity, and low/negative thermal expansion. These properties have many advanced applications, such as actuators, sensors, and energy conversion, but are limited typically in a narrow temperature range of tens of Kelvin. Here we report that, by creating a nano-scale concentration modulation via phase separation, the MT can be rendered continuous by an in-situ elastic confinement mechanism. Through a model titanium alloy, we demonstrate that the elastically confined continuous MT has unprecedented properties, such as superelasticity from below 4.2 K to 500 K, fully tunable and stable thermal expansion, from positive, through zero, to negative, from below 4.2 K to 573 K, and high strength-to-modulus ratio across a wide temperature range.The elastic tuning on the MT, together with a significant extension of the crystal stability limit, provides new opportunities to explore advanced materials.
Ferroic glasses(strain glass,relaxor and cluster spin glass)refer to frozen disordered states in ferroic systems;they are conjugate states to the long-range ordered ferroic states—the ferroic crystals.Ferroic glasses exhibit unusual properties that are absent in ferroic crystals,such as slim hysteresis and gradual property changes over a wide temperature range.In addition to ferroic glasses and ferroic crystals,a third ferroic state,a glass-ferroic(i.e.,a composite of ferroic glass and ferroic crystal),can be produced by the crystallization transition of ferroic glasses.It can have a superior property not possessed by its two components.These three classes of ferroic materials(ferroic crystal,ferroic glass and glass-ferroic)correspond to three transitions(ferroic phase transition,ferroic glass transition and crystallization transition of ferroic glass,respectively),as demonstrated in a generic temperature vs.defectconcentration phase diagram.Moreover,through constructing a phase field model,the microstructure evolution of three transitions and the phase diagram can be reproduced,which reveals the important role of point defects in the formation of ferroic glass and glass-ferroic.The phase diagram can be used to design various ferroic glasses and glass-ferroics that may exhibit unusual properties.
