Metastable Phase Transformation in Ti-5Ta-2Nb Alloy and 304L Austenitic Stainless Steel under Explosive Cladding Conditi
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INTRODUCTION
A titanium-based alloy of nominal composition, Ti-5Ta-2Nb, is preferred as the structural material for the process vessels handling concentrated boiling nitric acid owing to its enhanced corrosion resistance (corrosion rate = ~0.1 g/m2Æh).[1] Due to the high cost of titanium (commercial pure (cp) titanium is about 10 times more expensive than stainless steel (SS)), austenitic steels are chosen for components used in nitric acid–free environments.[2] Hence, there is a necessity to join Ti-5Ta-2Nb (TiTaNb) with 304L austenitic stainless steel (304L SS). A sound metallurgical joint between Ti- and Fe-based alloys is difficult to achieve by fusion welding processes for the following reasons. 1. As inferred from the Fe-Ti binary phase diagram, the mutual solubility of Fe and Ti is much less.[3] Hence, the Fe-Ti metal combination has a strong tendency for attractive chemical bond formation and interfacial alloying resulting in the formation of Fe- and Ti-rich FeTi, Fe2Ti-type intermetallic phases at the weld interface.[4] 2. Segregation of chemical species such as carbon (present in Fe-based alloys) results in the formation of a carbon-enriched zone near the fusion line.
C. SUDHA and V. THOMAS PAUL, Scientific Officers ‘‘E,’’ T.N. PRASANTHI, Scientific Officer ‘‘C,’’ S. SAROJA, Head, Nuclear Materials Microscopy Section, and M. VIJAYALAKSHMI, Associate Director, are with the Physical Metallurgy Group, Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamilnadu, India. Contact e-mail: sudha@igcar. gov.in Manuscript submitted April 18, 2011. Article published online June 22, 2012 3596—VOLUME 43A, OCTOBER 2012
3. Concentration of stresses in the bonded region due to thermal expansion mismatch (Ti: 7.6 lm/m/K, SS: 17 to 18 lm/m/K) lead to the formation of microcracks.[5] To overcome these difficulties, solid-state welding techniques such as diffusion bonding, friction welding, and explosive cladding were used to obtain reliable, high-strength joints between Ti and SS.[5–7] However, undesirable intermetallic phases were detected even in diffusion-bonded[6,8] and friction-welded Ti/SS systems.[5,9] Hence, in this study, the explosive cladding process, which was successfully demonstrated for the cp Ti/SS system,[10,11] is chosen for joining TiTaNb with 304L SS. In the explosive cladding process, there is no melting of the base materials and coalescence is achieved due to the high pressure developed under controlled detonation of the explosive force.[12] In all the investigations on explosive clads of titanium and SS, the main focus was to get good quality joints having intermetallicfree interface.[7,10,11,13,14] Hence, detailed study of the deformation-induced structural changes in titanium and stainless steel base materials is scarce even though limited information on mild steel and titanium-based explosive clads is available in the literature.[15,16] At ambient pressure and temperature, titanium exists as a phase having hexagonal close-packed structure. Increasing the temperature
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