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A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 1 7 3 8 iTSSe TSS iTSSe TSS FEATURE ARTICLE 8 information on crystal orien- tation, including splat-boun- dary misorientation that can help elucidate the bonding mechanismand complex ther- momechanical history of the metal powder particles during cold spray. Starting with the atomized powders, EBSD en- ables an understanding of the crystallite size as the lattice strain in the starting powder, by means of a Euler angle map. The as-sprayed coating (Fig. 1 c, d) reveals a nonuni- form deformation pattern, with some grains showing an equiaxed structure while oth- ers appear elongated. Regard- less of whether the particles remainequiaxedor areheavily deformed, the particle-parti- cle boundaries reveal ultrafine grains of <100nm. EBSD is also able to reveal the occurrence of nonuniform fine-grained structure, which can be at- tributed to dynamic—rather than static—recrystallization. EBSD characterization of an IN625 cold spray coating reveals several insights as to the nature of deforma- tion within the splats, as shown in Fig. 2. Figure 2 (a) is a high-magnification SEM micrograph focusing on both inter and intrasplat locations, labeled 1 to 4 in the figure. Fig- ure 2 (b) is a series of Euler images as well as image quality maps taken from all the locations to indicate the degree of de- formation that takes place in an IN625 coating. Figure 3, taken from the same coating, reveals details of the change in grain orientation in response to the deformation, byway of the Euler angle map, which shows a change from the [110] to the [111] direction on impact. Figure 3 (b) is an EBSD pattern from an AISI 4130 steel substrate showing two distinct deformation zones in the substrate, in the vicinity of the coating-substrate interface, in the case of hard particles on a hard substrate. Electron backscatter diffraction is also an indispensable tool in understanding the mechanical behavior of cold spray coatings on different substrates, such as enabling an under- standing of the fatigue response. The fatigue behavior of cold spray coated materials is not well understood. In the case of Fig. 2 — (a) Scanning electron micrograph indicating boundary between two splats in an IN625 cold spray coating; (b) series of Euler images and image quality maps showing the nature of deformation in inter and intrasplat locations. Fig. 3 — (a) End-pattern quality maps from IN625 coating showing the change in orientation of the grain with respect to powder impact direction, indicative of the nature of deformation in these coatings; (b) EBSD pattern from AISI 4130 steel substrate, indicating two distinct deformation zones in the as- sprayed condition. hard metallic coatings, such as titanium and nickel-base al- loys, fatigue debits have been observed, whereas for soft ma- terials such as aluminum and copper, it is reported that the compressive stress results in enhancing the fatigue properties. Here, EBSD enables a detailed understanding of the deforma- tion behavior in the substrate and is therefore able to substan- tiate the fatigue behavior. ~iTSSe For more information: Dheepa Srinivasan is a principal engineer at GE Power, GE India Technology Center, Bangalore, dheepa.srinivasan@ge.com, www.ge.com. This article series is adapted from Chapter 5, Cold Spray—Advanced Characteriza- tion , inHigh Pressure Cold Spray—Principles and Applications, edited by Charles M. Kay and J. Karthikeyan (ASM, 2016). Com- plete references are included in this volume.