|Shape memory alloys (SMA) are those class of alloys that can cause shape memory, pseudoelastcity and two-way memory effects due to reversible austenite to martensite phase transformations. The NiTi shape memory alloys exhibit large recovery strains, however, the transformation temperatures of these alloys are less than 100 degree Celsius and these alloys exhibit low detwinning stresses/transformation stresses. Detwinning stress is the stress at which detwinning of martensite variants is initiated and transformation stress is the stress at which austenite to martensite transformation starts to occur at any temperature. Ternary additions such as Hf, Zr, Pt, Pd, Au etc. increase the transformation temperatures and detwinning stresses/transformation stresses of NiTi alloys. Though Hf and Zr are cheaper than the rest, NiTiHf alloys exhibit brittleness, large thermal hysteresis, poor shape memory and superelastic behaviour. The poor shape memory behaviour of Ti rich NiTiHf alloys has been attributed to the presence of (001)B19’ compound twins in its substructure, low matrix strength and occurrence of plastic deformation along with detwinning of martensite variants/stress induced martensite transformation. It has been reported that among the ternary elements Hf, Zr, Pt, Pd and Au, Ni substitution by Pt results in highest transformation temperatures in NiTi alloys. The effect of Pt additions on the microstructure and shape memory behaviour of ternary NiTiHf alloys has therefore been studied in this thesis, along with some compositions with Zr/Pd additions. The compositions chosen are based on Pt substitution for Ni in the Ni(Ti40Hf10) base composition and Ni(Ti30Hf20) base composition as well the (Ni45Pd5)(Ti30Hf20) and (Ni40Pt10)(Ti35Zr15) compositions. Microstructure and shape memory behaviour of the extensively evaluated binary Ni50Ti50 and ternary Ni50(Ti40Hf10) alloys were also included for comparison since shape memory behaviour is sensitive to alloy preparation techniques.
Chapter 1 provides an overview of the shape memory effect, characterization techniques used to determine shape recovery ratio, the crystallography of martensitic transformation in NiTi, uniaxial deformation and indentation studies performed on NiTi, and shape memory studies performed on NiTiHf, NiTiZr, and NiPdTiHf alloys.
Chapter 2 describes the characterization techniques used to study the microstructure, structure, substructure, transformation temperatures, shape memory effect and pseudoelastic effect of the compositions studied in this thesis. Microstructure was examined by optical and scanning electron microscopy. Microsegregation in cast microstructures was determined by electron probe microanalysis. X ray diffraction both at room temperature and high temperature was used to determine the lattice parameters of the major constituent phases, that is the monoclinic B19’ martensite and parent B2 phase. Differential scanning calorimetry was employed to determine transformation temperatures. Substructure of martensite was examined by transmission electron microscopy. Along with constant strain rate compression tests, Vickers and spherical microindentation studies were also performed to determine mechanical properties. Compression tests were performed to determine the detwinning stress, transformation stress above Af temperature, shape recovery ratio of alloys containing the martensite phase at room temperature, and pseudoelastic strains for alloys with the parent B2 phase at room temperature. The shape recovery ratio and depth recovery ratio of B19’ containing alloys were determined using the values of sample dimensions and indentation depths, respectively, determined before and after heating above Af temperatures. An AFM operated in contact mode was used to measure the indentation depths. Depth recovery ratio and energy recovery ratio in pseudoelasticity of B2 containing alloys at room temperature were determined from spherical and Vickers microindentations from the load-displacement profiles obtained during indentation and unloading.
Since the alloys evaluated in this thesis have not been studied earlier in the literature, a detailed study of the microstructure of cast and homogenized structures of all the alloys are described in Chapter 3. The extent of micro-segregation in cast microstructures and the composition of secondary phases was determined. Partial substitution of Ni by Pt/Pd in NiTi(Hf/Zr) resulted in substantial reduction in transformation temperatures. The maximum theoretical shear strain associated with austenite to martensite transformation, calculated using the values of lattice parameters of B19’ and B2 phases. was found to increase with increase in Hf and Pt additions. (001)B19’ compound twins were present in the substructure of martensite of ternary Ni50(Ti30Hf20) and quaternary Pt containing (NiPt)(Ti30Hf20) alloys. The chapter concludes with a discussion on factors affecting transformation temperatures.
Chapter 4 describes the shape memory behaviour of those alloys that contain the B19’ martensite at room temperature, that is Ni50Ti50, ternary Ni50(Ti40Hf10) and Ni50(Ti30Hf20) alloys, quaternary (Ni(50-x)-Ptx)(Ti30Hf20) (x = 5 and 10 at. %) alloys, and the quaternary (Ni45Pd5)(Ti30Hf20) alloy. The detwinning stress at room temperature was found to increase with Pt/Pd and Hf addition, and significant hardening was observed in the room temperature stress strain behaviour of ternary NiTiHf alloys and quaternary Ni(Pt/Pd)TiHf when compared to Ni50Ti50. The shape recovery ratio at a constant plastic strain of 2% did not change with the addition of 10 at. % Hf to Ni50Ti50 but decreased with further addition of Hf. While the addition of Pt did not alter the shape recovery ratio of the ternary Ni50Ti30Hf20 alloy, Pd addition resulted in improvement of shape recovery ratio. The Vickers indentations were made to a constant load of 300 mN and spherical indentations were made to a depth of 0.5, 1, 1.5 and 2 μm. The recovery determined by these different techniques are assessed and the factors affecting stress plateau and recovery ratios are discussed.
The pseudoelasticity behavior of alloys with parent B2 phase at room temperature, namely (Ni(50-x)-Ptx)(Ti40Hf10) (x = 5 and 10 at. %), (Ni40Pt10)(Ti35Zr15) and (Ni35Pt15)(Ti30Hf20) alloys is described in Chapter 5. The maximum stress at which pseudoelasticity is exhibited increases with increase in Hf, Pt or Zr addition. The temperature range over which these alloys show pseudoelastic behaviour has been determined through compression tests. The transformation stresses for the alloys were determined as a function of temperature up to Md and these stresses increase with temperature in a non-linear fashion. The pseudoelastic strain of the alloys was determined at two deformation stresses at room temperature in compression. Ni35Pt15Ti30Hf20 exhibited the lowest values and fractured at a stress of 2.3 GPa. Microindentation studies show that Ni35Pt15Ti30Hf20 alloy exhibited the lowest pseudoelastic recovery ratio while Ni45Pt5Ti40Hf10 exhibited the highest recovery ratio at room temperature. It was found that the trend in variation of pseudoelastic recovery with alloying additions has an inverse relationship to trend in variation of slope of stress plateau and dissipation energy (hysteresis) with alloying additions. The factors that affect transformation stresses, stress plateau, and pseudoelastic strains are discussed.
A summary of the thesis and suggestions for future work are provided in Chapter 6.