  The identification and use of reversible Martensitic transformations,
typically described as shape memory transformations, as a new class of
solid-solid phase change material is experimentally demonstrated here for the
first time. To prove this claim, time-domain thermoreflectance,
frequency-domain thermoreflectance, and differential scanning calorimetry
studies were conducted on commercial NiTi alloys to quantify thermal
conductivity and latent heat. Additional Joule-heating experiments demonstrate
successful temperature leveling during transient heating and cooling in a
simulated environment. Compared to standard solid-solid materials and
solid-liquid paraffin, these experimental results show that shape memory alloys
provide up to a two order of magnitude higher Figure of Merit. Beyond these
novel experimental results, a comprehensive review of >75 binary NiTi and
NiTi-based ternary and quaternary alloys in the literature shows that shape
memory alloys can be tuned in a wide range of transformation temperatures (from
-50 to 330{deg}C), latent heats (from 9.1 to 35.1 J/g), and thermal
conductivities (from 15.6 to 28 W/mK). This can be accomplished by changing the
Ni and Ti balance, introducing trace elements, and/or by thermomechanical
processing. Combining excellent corrosion resistance, formability, high
strength and ductility, high thermal performance, and tunability, SMAs
represent an exceptional phase change material that circumvents many of the
scientific and engineering challenges hindering progress in this field.
