PSI - Issue 14

P. Rama Rao et al. / Procedia Structural Integrity 14 (2019) 322–329 P R Rao et al/ Structural Integrity Procedia 00 (2018) 000–000

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Thus, on the basis of the experimental data displayed in figures 2, figure3, figure 4 and figure5 and Table 2, it appears to suggest the following processes to have occurred during the brazed joint formation: During the isothermal solidification of the molten brazed and had eventually formed primary β-Ti, the residual melt was solidified via eutectic reaction upon the cooling cycle of brazing. Based on the data of Table 2, the eutectic consisted most probably of (Ti, Zr) 2 Cu, α-Ti and Ti-rich as shown in figures 4(a), figures 4(b), figures 4(c) figures 4(d) and figures 5(a), figures 5(b), figures 5(c) figures 5(d). The β-Ti is totally soluble with Zr according to related binary alloy phase diagrams G.Wang et al (1995). The maximum solubilities of Cu and Ni in the β-Ti phase are 33.5 and 8 at. %, respectively. In contrast, Cu and Ni are dissolved in α-Ti phase up to 1.6 and 0.2 at. %, respectively R.R. Kapoor et al (1989). These values are significantly lower than their respective solubilities in β-Ti phase. Both Cu and Ni are hence well known to be stabilizers of the β-phase in CP-Ti. On cooling to room temperature, formation of both β-Ti phase and intermetallic compounds may take place. Accordingly, decomposition of the β-Ti phase might have most likely proceeded via eutectoid solid-state transformation upon the cooling cycle of brazing. The eutectoid of (Ti,Zr) 2 Cu, α-Ti and Ti-rich might therefore have had formed at room temperature in the earlier β-Ti grains of the brazed joint, M. Naka and I. Okamoto (1985), Yu. V. Zhernovenkova (2007) and J.P. Davis (2000). Higher brazing temperature resulted possibly in higher volume fraction of the β-Ti in the joint. This process have greatly enhances the depletion rates of Cu, Ni, and Zr from the brazed melt into CP-Ti substrate during brazing. Because Cu, Ni, and Zr are all dissolved in the β-Ti, isothermal solidification of brazed melt during brazing resulted in formation of only β-Ti in the joint. Thus, the eutectic of (Ti, Zr) 2 Cu, α-Ti and Ti-rich had most likely disappeared from the brazed zone. The β-Ti alloyed with Cu, Ni, and Zr was therefore most likely to have had transformed to fine eutectoid of (Ti, Zr) 2 Cu, α-Ti and Ti-rich upon the subsequent cooling cycle of brazing. It is expected that the disappearance of coarse eutectic intermetallic compounds from the brazed zone is beneficial for enhancing the bond strength of the joint, Zhang, Q.P. and Zhang Y.S. (2005).

Fig.6. The annealed ribbon of Ti 20 Zr 20 Cu 50 Ni 10 metallic glass at 753 K for a period of 30min and the corresponding TEM images: (a)-(c): BF image, DF image and selected area of electron diffraction pattern (SAED), respectively. The figures 6(a), figures 6(b) and figures 6(c) represent the bright field (BF), dark field (DF) and corresponding SAED, respectively for theTi 20 Zr 20 Cu 50 Ni 10 metallic glass ribbon annealed at 753 K for 30 min. Table 3 presents the corresponding data on composition analysis of the annealed Ti 20 Zr 20 Cu 50 Ni 10 metallic glass ribbon. Table 3. The analytical composition of the Ti 20 Zr 20 Cu 50 Ni 10 metallic glass ribbon (annealed at 753 K for a period of 30 min).

Elements (at %)

Ti Phases 19.52 20.82 50.36 11.09 NiTi2 Zr Cu Ni