COMBINATORIAL EXPLORATION OF PHASE TRANSFORMATION IN NiTi-BASED THIN FILM LIBRARIES FOR SHAPE MEMORY ALLOY APPLICATIONS

Abstract

Ni-Ti based shape memory alloys (SMAs) have found widespread use in the last 70 years but improving their functional stability remains a key quest for more robust and advanced applications. Named for their ability to retain their shape via a reversible martensitic phase transformation (PT), they are sensitive to compositional variations. Tuning the SMA lattice parameters, transformation temperature and thermal hysteresis (DeltaT) by alloying with ternary and quaternary elements, therefore, is a challenging materials exploration effort. Combinatorial materials science streamlines synthesis, characterization and data management processes from multiple high-throughput techniques. In this dissertation, composition spreads of Ni-Ti-X (X = Co, Hf, Pd, V) and Ni-Ti-Cu-Y (where Y = Co, Fe, Pd, V) thin film libraries were synthesized by magnetron co-sputtering to probe a substantial composition space with different stoichiometries under identical conditions. Composition-dependent PT temperature, microstructure and thermal conductivity were investigated using high-throughput wavelength dispersive spectroscopy (WDS), temperature-dependent resistance R(T), synchrotron x-ray diffraction (XRD) and scanning hot probe (SHP) microscopy measurements.

Through case studies of ternary Ni-Ti-Co and quaternary Ni-Ti-Cu-V systems, I discuss phase determination and how functional properties correlate with composition and local microstructure using composition-structure-property maps. In the Ni-Ti-Co library, a new, expanded composition space having PT with small thermal hysteresis and c(Co) >10 at.% was identified. Of the 177 compositions, 31 had stable PT with near-zero DeltaT in four. Elemental range for SMA compositions was 25.8 at.% < c(Ni) < 70.5 at.%, 21.4 at.% < c(Ti) < 64.3 at.%, and 5.5 at.% < c(Co) < 26.4 at.%. Crystallographic evidence points to a cubic Pm3m structure present as single or mixed with hexagonal or orthorhombic structures for all these compositions.

In the Ni-Ti-Cu-V library, PT was observed in 32 compositions (21.3 at.% < c(Ni) < 30.9 at.%, 49.4 at.% < c(Ti) < 57.5 at.%, 13.8 at.% < c(Cu) < 21.6 at.% and 4.1 at.% < c(V) < 6.2 at.%), predominantly in the Ti-rich region, with zero or near-zero DeltaT in five. Increasing V up to 6 at.% stabilized the mixture of transforming cubic and tetragonal phases. These newly identified composition regions provide flexibility in and expand the operating temperature window for their application in different technologies.

Lastly, a novel application of SMAs as phase change materials is briefly investigated through high-throughput determination of their thermal conductivity using scanning hot probe microscopy. Binary, ternary and quaternary thin film libraries of Ni-Ti, Ni-Ti-V and Ni-Ti-Cu-V were evaluated as a benchmarking exercise.