Fabrication Of Porous NiTi Alloys And Composites By Optimized Pore-forming Technique And Sintering Process, And Characterization Of Their Properties

In the present PhD dissertation research, aiming at accurately adjusting the pore characteristics and quantitatively tailoring the mechanical properties of the porous alloys, the porous NiTi alloys with expected pore features and low density as well as high strength have been successfully fabricated by optimized pore-forming technique and sintering process. The problems in fabrication of porous NiTi alloys by the previous process using space-holder particles with non-uniform sizes and powder sintering technique, such as irregular pore shape, uncontrollable porosity ratio, poor mechanical properties and low processing stability, have been effectively solved by using the space-holder particles with appropriate shape and size; and an optimized method for fabricating porous NiTi alloys has been developed, which has paved the way for fabricating the porous NiTi alloys by the design requirement of the materials. Furthermore, in order to improve the mechanical properties and damping capacity of the porous NiTi alloys, an innovative method for fabricating NiTi alloy based composites with optimized damping capacity and mechanical properties using micro-sized low density particles (Al₂O₃ and Si particles) or light metal and alloy phases (Mg and Mg alloy-AZ91D) has been developed. Based on successful fabrication of the porous NiTi alloys and NiTi based composites, sophisticated apparatus for phase analysis (XRD), microstructural analysis (SEM), thermal analysis and thermomechanical approaches (DSC and DMA) as well as mechanical compression test were employed to characterize systematically the microstructures, phase transformation behaviors of the fabricated materials and their mechanical properties including superelasticity under cycling compression loading, and their damping performance at different testing frequencies and amplitudes. A new parameter, equivalent compressive strength (σeq), was for the first time proposed to evaluate quantitatively the mechanical properties of the porous NiTi alloys with various porosities by using different processes. Also, the origins of internal friction peaks of the fabricated composites were studied and the strengthening mechanisms of the engineered phases on both damping capacity and mechanical properties of the composites were investigated. The"internal friction-modulus"(EIF) was defined to evaluate synthetically the damping capacity of the fabricated composites by considering the synergistic effect of the internal friction and modulus of the composites.We for the first time used sieved NH4HCO3 powder, near-spherical urea and stearic acid particles, which had optimized shapes and sizes, as the space-holder to have improved the pore-forming technique for fabricating porous NiTi alloys and realized effective tailoring of pore characteristics, and finally fabricated the porous NiTi alloys with regular pore shape and uniform size distribution as well as controllable porosity. The fabricated porous NiTi alloys show the porosity ratio in the range of 31-64% and pore size of 127-810μm respectively, and consist of B2 and B19′NiTi as the main phases, without residuals of space-holders. It has also been proved that the shape and size of space-holder particles have a crucial influence on the pore characteristics of the porous NiTi alloys and show a geometrical heredity effect. Although the porous NiTi alloys fabricated by using different types of space-holders exhibit the same martensitic and reverse transformation features, their transformation temperatures show the different porosity-dependency responses. The porous NiTi alloys with high porosity fabricated by the optimized pore-forming technique still show high compressive strength, stable superelasticity and exhibit the steady mechanical properties after cycling compression training. However, the alloys'mechanical properties deteriorate and shape recovery ratio decreases with increasing porosity. Furthermore, it has been shown that the spherical urea particles as space-holder are superior to stearic acid and ammonium bicarbonate in controlling the pore morphology and porosity of the porous NiTi alloys, consequently the porous NiTi alloys fabricated using urea have the highest equivalent compressive strength mainly owing to the optimal pore characteristics and less impurities.Moreover, we proposed an innovative material design and fabricated the novel NiTi alloy based composites with their damping capacity and mechanical properties tailored by low density Si or Al₂O₃ particles, i.e., Al₂O₃/NiTi or Si/NiTi composites. The porosity ratios of the fabricated composites are in the range of 35-49.1%, and the dominant phases in the composites are NiTi and engineered particle phases (Al₂O₃ or Si), as well as minor silicide phases in the composites containing Si particles. The results clearly manifest that the addition of engineered particles (Al₂O₃ or Si) does not change the martensite transformed features of the dominant phase NiTi in the composites, while having some influence on the transformation sequences of the composites. It has been shown that Al₂O₃/NiTi and Si/NiTi composites exhibit satisfactory mechanical properties under cyclic compression load with the superelastic recovery strain as high as 2.2%, and Al₂O₃/NiTi composite has better mechanical properties than Si/NiTi composite. It is worth indicating that the compressive strength of both composites decreases with increasing porosity and they both fracture in ductile mode. The results also show that Al₂O₃/NiTi composite possesses higher internal friction value than Si/NiTi composite, and both of them exhibit superior damping capacity to the porous NiTi alloys. Generally, the internal friction value of both Al₂O₃(Si)/NiTi composites increases with the addition amount of Al₂O₃ and Si particles, and increase of the amplitude used, while decreasing with the testing frequency.In addition, an innovative method combining pore-forming technique, powder sintering and pressureless metal infiltration process was developed to have successfully fabricated the novel NiTi based composites engineered by Mg or Mg alloy(AZ91D). The novel composites have excellent overall mechanical and physical properties, in terms of high compressive strength, suitable damping capacity and low density. The composites mainly consist of NiTi phase and Mg (or Mg alloy) phases, with minor NixTiy type phases as well as non-NixTiy intermetallics owing to the reaction among infiltrated phases and NiTi matrix phase. Mg(AZ91D)/NiTi composites show much higher compressive strength than the porous NiTi alloys and exhibit excellent linear superelasticity up to 2.2%. It has also been shown that several damping mechanisms operate in Mg(AZ91D)/NiTi composites, that is, damping from martensitic transformation and the reverse transformation in the NiTi matrix, interface damping and dislocation damping owing to the existence of Mg or Mg alloy phase, etc. Consequently, the several damping mechanisms make the damping capacity of Mg(AZ91D)/NiTi composites increase with decreasing testing frequency and increasing amplitude. Mg/NiTi composite precedes AZ91D/NiTi composite in internal friction property under the same testing conditions.