A Study of Interdiffusion in the Fe-C/Ti System Under Equilibrium and Nonequilibrium Conditions

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UM and various Ti alloys are considered as structural materials for highly oxidizing environments in chemical and nuclear industries due to their remarkable corrosion resistance.[1] An a-b Ti alloy of nominal composition Ti-5 wt pct Ta-1.8 wt pct Nb (TiTaNb) with excellent corrosion resistance in the condensate phase of HNO3 (corrosion rate = ~0.98 mpy) is a candidate structural material for the dissolver vessel handling 11.5 M boiling HNO3 in a spent fuel fast reactor reprocessing plant.[2] The materials requirement for other components in the plant is satisfied by 304L austenitic stainless steel (SS), which necessitated welding of SS with TiTaNb alloy. Due to the difficulties involved in fusion welding of SS and TiTaNb, a solid-state welding process called ‘‘explosive cladding’’ is used for joining these dissimilar materials to satisfy the zero failure requirement of the weld interface during service.[3,4] A study on growth characteristics of the diffusion zones in the explosive clads showed enhanced diffusion kinetics as compared to that in the diffusion-bonded and friction-welded joints due to the availability of a large number of lattice defects at the explosively clad

T.N. PRASANTHI, C. SUDHA, and S. SAROJA are with the Physical Metallurgy Division, Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, Tamilnadu, India. Contact email: [email protected] Manuscript submitted January 26, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A

interface.[5] Therefore, understanding the interdiffusion phenomena in the Fe/Ti system is of technological importance, since performance of these joints is dictated by the temperature and time-dependent growth of reaction zones at the interface.[6] Several reports are available in the literature, where growth characteristics of the diffusion zones in Fe-Ti systems are observed to be governed by temperature and time-dependent interdiffusion of the alloying elements across the interface.[7–9] Formation of FeTi- and Fe2Ti-type brittle intermetallics and other secondary phases could not be completely avoided in the Fe-Ti system by solid-state joining methods even under optimized conditions.[7,8] Ghosh et al.[8] calculated the activation energy (Q) required for the growth of FeTi and Fe2Ti intermetallics as 124.9 and 125.8 kJ/mol, respectively. In contrast, Kale et al.[9] reported a low value of 73.07 kJ/mol for growth of the Fe-Ti interface zones annealed in the temperature range of 1125 K to 1322 K (852 C to 1049 C), ignoring the formation of intermetallic phases at the interface. The aim of the present study is to understand the microstructural evolution at the interface of mild steel (MS) and the Ti Grade 2 dissimilar joint under equilibrium conditions for comparison with diffusion under nonequilibrium conditions. Attempts to join the two metals by conventional diffusion couple experiments were not successful due to the formation of thick dTiO oxide layers at the interface. To circumvent this problem, friction welding is employed to obtain a good qua