Preparation And Property Characterization Of Ni-Ti Shape Memory Alloys And Composite With Light Weight And High Strength And High Damping Capacity

NiTi shape memory alloys （SMAs） have attracted significant interests in applications dueto their unique shape memory effect, superelasticity and excellent mechanical performance.These outstanding properties allow commercial applications of NiTi SMAs in the fields ofaerospace, mechanical engineering, biomedical engineering and instrumentation engineeringetc. However, for the materials used to make aerospace/aeronautical structures andcomponents as well as high-speed rotating machines, in which both manufacturing andoperating costs are very high, there is a rigorous requirement for them to be of lightweightand high-performance. In recent decades, porous NiTi SMAs have been extensivelydeveloped through several methods, and they have advantages of low density （or lightweight）and high specific strength, thus show great potential for commercial applications. Thus far,the strength, storage modulus and damping capacity of porous NiTi SMAs fabricated byconventional methods are not satisfactory, and their applications in aerospace/aeronautical andcivil engineering fields have been limited. Therefore, aiming at the above engineeingapplications, there is a great demand in development of porous NiTi shape memory alloyswith enhanced mechanical properties and damping capacity.In this thesis study, the pore pre-forming technique by using low-temperature-decomposable NH₄HCO₃powder and the nano-sized SiC reinforcement particle were used tofabricate the porous SiC/NiTi composites by means of the step powder-sintering method. Thefabricated porous SiC/NiTi composites show unique characteristics of lightweight and highstrength, yet still exhibiting the shape memory effect and a steady superelasticity, this hasmade a big step forward in solving the problem of low compressive strength and storagemodulus of porous NiTi alloys. The results show that the porous SiC/NiTi composites exhibitthe porosity ratio in the range of25.4～49.7%and consist of B19′and B2as the main phasestogether with some minor phases such as NiTi₂, Ni₂Ti and Ni₄Ti₃. Use of NH₄HCO₃powderas the space-holder has no effect on phase constituents of porous SiC/NiTi composites, but theaddition of nano-sized SiC particles has some influence on phase constituents of thecomposites. The porous SiC/NiTi composites show steady linear superelasticity after several pre-strain training compression cycles, the shape recovery rate of the composites decreaseswith increasing the fraction of SiC particles. The porous SiC/NiTi composites show highcompressive strength and equivalent compression strength, which increase with increasing thefraction of SiC particles. Moreover, it has been shown that the porous SiC/NiTi compositesstill have martensitic transformation characteristics, yet exhibiting high damping capacity andstorage modulus on a par with that of porous NiTi alloys. The internal friction （or dampingcapacity） of the porous SiC/NiTi composites decreases with increasing the fraction of SiCparticles; in particular the composite with SiC fraction of3wt.%still shows high values ofinternal friction and equivalent internal friction as well storage modulus.Furthermore, porous TiNiCu shape memory alloys were successfully fabricated bymeans of the step powder-sintering method, which exhibit much higher damping capcity overthe porous NiTi binary alloys. The results clearly manifest that the porous TiNiCu alloys showthe porosity ratio in the range of35%to40%, and both the porosity and pore-size of thealloys increase while the alloys’ apparent density decreases with increasing Cu content; inparticular, the apparent density of porous Ti₅₀Ni₃₀Cu₂₀alloy is38%lower than that of thedense Ti₅₀Ni₃₀Cu₂₀alloy. The compressiver strength of porous TiNiCu alloys decreases withincreasing Cu content, the alloys’ residual deformation increases with increasing Cu content,the alloys show steady linear superelasticity after several pre-strain training compressioncycles. After adding Cu element into NiTi alloy, the phase constituents of TiNi alloy changedsignificantly, but the addition amount of Cu element in TiNi alloys and the aging time have noinfluence on phase constituents of the alloys. Both on cooling and heating, the porousTi₅₀Ni₄₀Cu₁₀alloy has a wide temperature range of martensitic transformation raised from thehysteresis of both B2-B19′and B2-B19-B19′phase transformations, leading to a decrease inthe shape recovery rate, while the porous Ti₅₀Ni₃₀Cu₂₀alloy shows high shape recovery ratecorresponding to a narrow temperature range of martensitic transformation which is B2-B19one-step transformation. It has also been shown that the internal friction of porous TiNiCualloys increases with increasing Cu content. The porous Ti₅₀Ni₃₀Cu₂₀alloy shows highinternal friction and equivalent internal friction, with the maximum internal friction valuebeing five times higher than that of porous Ti₅₀Ni₅₀alloy and the maximum equivalentinternal friction value being37%higher than that of the dense TiNiCu alloy. Moreover, it has been shown that with increasing thermal aging time, the compressive strength, shape recoveryrate and internal friction value of porous TiNiCu alloys increase initially and then decreasegradually.In addition, the near-equiatomic NiTi shape memory alloy was fabricated by rapidsolidification process through vacuum arc melting followed by vacuum suction casting inwater cooled thick copper-mould. The alloy’s microstructure, martensitic transformationbehavior, mechanical properties and damping performance were characterized systematically.The results show that the rapidly solidified （or suction cast） NiTi alloy shows much finergrains and homogeneous microstructure, in particular a uniform distribution of various fineprecipitates, compared to the conventional cast one. In the differential scanning calorimetry（DSC） curves both on cooling and heating, the rapidly solidified NiTi alloy exhibits muchstronger and narrower endothermic and exothermic peaks during the martensitic transformation.Moreover, under a strain level of8%the suction cast NiTi alloy shows a significantimprovement over the conventional cast one, in terms of possessing higher deformationrecovery rates and displaying the increased compressive strength and damping capacity by4%and20%respectively.