Microstructure and Mechanical Properties of Ti Cold-Spray Splats Determined by Electron Channeling Contrast Imaging and Nanoindentation Mapping

Abstract

Cold spray is a thermo-mechanical process where the velocity of the sprayed particles affects the deformation, bonding, and mechanical properties of the deposited material, in the form of splats or coatings. At high strain rates, the impact stresses are converted into heat, a phenomenon known as adiabatic shear, which leads to grain re-crystallization. Grain re-crystallization and growth are shown to have a direct impact on the mechanical properties of the cold-sprayed material. The present study ties the microstructural features within the cold-sprayed Ti splats and the substrate to the bonding mechanism and mechanical properties. High-resolution electron channeling contrast imaging, electron backscatter diffraction mapping, and nanoindentation were used to correlate the microstructure to the mechanical properties distribution within the titanium cold-spray splats. The formation of nanograins was observed at the titanium splat/substrate interface and contributed to metallurgical bonding. An increase in grain re-crystallization within the splat and substrate materials was observed with pre-heating of the substrate. In the substrate material, the predominant mechanism of deformation was twinning. A good relationship was found between the hardness and distribution of the twins within the substrate and the size distribution of the re-crystallized grains within the splats.

Mapping viscoelastic and plastic properties of polymers and polymer-nanotube composites using instrumented indentation

“…The results presented here have demonstrated two dimensional mapping of viscoelastic and plastic properties of polymeric-based materials, extending the mapping capabilities to time-dependent deformation from those demonstrated previously 32–37 using an elastic-plastic analysis. 41 The previous studies all used indentation spacings smaller than the 100 μm used here, ranging from 0.5 μm 32 to 20 μm, 35 allowing most maps to be presented with properties as individual pixels 32,34–36 rather than as contours 33,37 as in Figs.…”

“…41 The previous studies all used indentation spacings smaller than the 100 μm used here, ranging from 0.5 μm 32 to 20 μm, 35 allowing most maps to be presented with properties as individual pixels 32,34–36 rather than as contours 33,37 as in Figs. 6, 9, and 10.…”

“…Such maps take advantage of the local probing capabilities of IIT and have been used to explore the effects of microstructure on spatial variations of mechanical properties in ceramic-metal composites, 32 tooth enamel, 33 bone, 34 cement paste and rocks, 35 and metals. 36,37 In these cases, time-dependent deformation was not considered and elastic and plastic properties were mapped. The next section develops the new VEP indentation model and expressions for h ( t ) that allow material properties to be determined from simple P ( t ) protocols.…”

Cold spraying – A materials perspective

“…Moreover, electron channelling contrast imaging (ECCI) has been used in the study of grain structure and defects in cold-sprayed deposits; see e.g. [113,114].…”

“…For instance, nanoindentation of cold-sprayed titanium has shown a correlation between grain size and hardness, following a Hall-Petch relationship [114]. Another important property of cold-spray deposits is the general lack of ductility due to severe deformation, though this can be improved by heat treatment [136][137][138].…”