چكيده به لاتين
Owing to various special properties such as super-elasticity, good wear resistance, and corrosion resistance, NiTi alloy has been used as a coating material for protection of Ti, Ti alloys and stainless steel. In this research, intermetallics phase formation kinetics of cold sprayed (CS) single particles and coatings of Ni onto Ti and Ti on Ni substrates and Ni-Ti composite coatings after post spray heat-treatment (PSHT) were investigated. Phase formation kinetics were simulated using a thermodynamic phase field model.
Intermetallics growth of cold sprayed single particles in combination with finite element simulations of particle deformation were employed to investigate bonding mechanism in cold spraying. Characterisation of the samples after a brief heat treatment at 700°C indicated that intermetallic formation, and hence metallurgical bonding of the pairs is more likely to occur at the particle peripheries where the interface areas are highly strained, and rarely achieved at the particle base. Results also reveal that bonding extends from peripheries toward the central part of the interfaces with increasing the impact velocity.
CS of dissimilar build-up coatings has been addressed to understand the role of the deposition sequence on the characteristics of the coating/substrate interface. Nickel and titanium coatings were sprayed onto the substrates of the opposite material. Interface morphologies of as-sprayed and heat treated dissimilar pairs showed that the bonding characteristics of the Ni/Ti pairs are significantly different from those of the Ti/Ni pairs. Mechanical interlocking and intermixing instabilities were observed only at the interfaces of the Ni/Ti pairs, which also showed a more uniform and higher fraction of intermetallics compared to the Ti/Ni pairs in similar spraying conditions. This was attributed to higher degrees of particle/substrate deformation in the Ni/Ti pair. Also, the bond strength of the Ni/Ti pair appeared to be almost three folds of the Ti/Ni pair (64.31 MPa vs. 22.8 MPa).
DSC analyses in combination with PSHT were employed to investigate interfacial intermetallic components growth at 700, 800, 900, 945 and 982 ̊C for 5, 60,180 and 360 minutes. The findings of these studies showed that Ti2Ni and Ni3Ti are the first phases respectively form at Ni-rich and Ti-rich sides of the Ni/Ti pairs while NiTi phase forms after these phases at the interface of them. β Ti phase forms at annealing temperature higher than 765 ̊C after the mentioned phases. Investigation of the phase formation kinetics showed that growth of intermetallics is controlled by diffusion. It has been found that the NiTi phase has a higher growth rate over two other intermetallics at all annealing temperature.
Dense, Ni-Ti composite coating with well-dispersed ingredients, were sprayed using physically blended elemental precursors with equiatomic and equivolume ratios of Ni-Ti powders. Microstructural investigation of heat treated composite coating showed that all three equilibrium intermetallics of binary Ni-Ti phase diagram including Ni3Ti, Ti2Ni and NiTi phases were formed at all heat-treatment experiments even at annealing temperature and time as low as 700 ̊C for 5 minutes which means CS composite coatings had fast intermetallics formation and growth kinetics. It was observed that Kirkendall porosities were formed at composite coatings after PSHT at Ni/Ni3Ti interfaces. A two-step annealing regime including a long annealing step under the eutectoid temperature of 765 ̊C and a short annealing time higher than the eutectic temperature of 942 ̊C were employed to control porosities formation during heat treatment.
Three equilibrium intermetallics evolution including NiTi, Ti2Ni and Ni3Ti phases in addition to liquid phase formation during PSHTs was modelled using the phase field model. This phase field model was successfully used in combination with experimental data to further understand the microstructural evolution and intermetallics formation of Ni-Ti system during heat treatment.
Keywords: Cold spray coating, Dissimilar bonding, Ni/Ti pair, Ti/Ni pair, NiTi Intermetallics, Bond strength, Finite Element Modelling, Deformation Enhanced Interdiffusion, Metal matrix composite, Phase field simulations, Polycrystal model.
