PSI - Issue 2_A

L.L. Meisner et al. / Procedia Structural Integrity 2 (2016) 1465–1472 L.L. Meisner et al./ Structural Integrity Procedia 00 (2016) 000 – 000

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These results together with the data of TEM/EDS analysis of inclusions contained in the initial state (Fig. 4) suggest that microcraters of type 1 are mainly nucleated at the TiC(O) inclusions. This character of the microcrater chemistry indicates also that during pulsed melting of the inclusion-matrix system and subsequent high-rate resolidification of the melt pool, enriched in C and O impurities (accordingly TEM/EDS), in situ of comparatively course TiC(O) inclusion submicron inclusion fragments remain, elemental composition of which is similar to that of the initial inclusion. Signs of these fragments are visible in secondary electron image of microcraters (Fig. 7b). Precision TiNi alloy . The alloy in initial state contained multiple particles evenly distributed over the surface

a

b

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1

25  m

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d

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Figure 6. Optical images ( a, d ) and corresponding profilograms ( b,c ) and ( e ) of microcraters 1, 2, 3 in commercial ( a – c ) and precision ( d, e ) TiNi samples, irradiated at 1.5 J/cm 2 , n =1.

(Fig. 5a, 8a). As it was shown in Sec. 3.1 (see Fig. 5), these particles are predominantly Ti 4 Ni 2 O x oxide inclusions. Individual and overlapping microcraters, most of which relate to type (3), different from microcraters of types (1)and (2), are formed after irradiation at E =1.5 J/cm 2 (see Fig. 8a). Figs. 6 d,e show area of the surface with selected microcrater of type 3 and the corresponding profilogram. It is evident that this type of microcraters has typical “dimple - rim” shape, but unlike the microcraters of type 2, they are less deep and contain characteristic qu asi-flat bottom. There are also microcraters of types 1 and 2, but their fraction is small as compared to that of the commercial TiNi alloy. Fig. 8 shows SEM/SE images of the surface area of precision TiNi alloy before (with inclusions, a ) and after LEHCEBs irradiation (with craters, b ); the EDS data (10 keV) of the local element’s concentrations in the points {1, 2, 3} across the inclusion 1 (Fig. 8a) and in the points {1 – 5} across the microcrater of type 3 (b), and in areas 2 without inclusions and, accordingly, microcraters (Fig. 8c). It is seen that the bottom of both microcraters is depleted in Ni, but is enriched in Ti and O. The concentration of C along EDS scanline was below LOD. Hence, counting data from the TEM/EDS analysis of inclusions in the as-etched state (Figs. 2b and 3), it is possible to conclude that microcraters of type 3 are formed mainly on the oxide Ti 4 Ni 2 O x inclusions. Thus, primary cratering, induced by single-pulse irradiation of both TiNi alloys near the surface melting threshold is due to local melting of the material in the sites of inclusions. The topographical features of primary microcraters are determined by phase composition and morphology of a particular inclusion. In the case of commercial TiNi alloy, the primary microcraters are mainly formed on titanium oxycarbide TiC(O) inclusions and have their residues in the central part (type 1), or common “dimple - rim” shape (type 2). Presumably, microcraters of type 2 are formed on comparatively small TiC(O) inclusions, that facilitates its liquid-phase dissolution. In the case of precision TiNi alloy, most of primary microcraters have “dimple - rim” shape with quasi -flat bottom (type 3), and