On the Influence of Alloy Composition on the Fracture Toughness of Two-Phase Ti-Al-Nb Alloys Undergoing Stress-Induced M
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INTRODUCTION
TITANIUM ALUMINIDE (Ti3Al) (a2) based Ti-Al-Nb alloys are promising materials for advanced high-temperature applications. However, these alloys lack good low-temperature ductility and fracture toughness. It is now well known that alloys containing greater than 11 at. pct Nb result in reasonable ductility at room temperature due to the introduction of additional slip systems and stabilization of the b phase along with the improvement in the fracture toughness.[1–6] Although Ti alloys containing Al and Nb in the range of 20 to 25 at. pct and 11 to 25 at. pct, respectively, have been studied extensively, very little information about mechanical properties of alloys containing lower Al and Nb, i.e., Al in the range of 15 to 20 at. pct and Nb in the range of 4 to 12 at. pct, is available in the literature. Alloys in this range assume importance, because lower Al content could further improve the fracture toughness and lower Nb content could reduce the deleterious effects of the segregation. Also, the reduction in the cost of alloy would be an added advantage. Our earlier studies have shown that the titanium alloys containing 8 to 12 at. pct Nb and 15 to 18 at. pct Al undergo stress-induced martensitic transformation (SIMT) in both single-phase b and two- phase (a2 + b) water-quenched (WQ) conditions.[7,8] The effect of grain size and composition on fracture toughness was systematically studied in these alloys[9] in the b WQ condition. The fracture toughness of these alloys was found to increase with an increase in grain size until it attained a ARCHANA PARADKAR, A.K. GOGIA, and S.V. KAMAT, Scientists, are with the Defence Metallurgical Research Laboratory, Hyderabad 500058, A.P., India. B.P. KASHYAP, Professor, is with the Indian Institute of Technology, Department of Metallurgical Engineering and Material Science, Mumbai 400076, Maharashtra, India. Contact e-mail: agparadkar@rediffmail.com Manuscript submitted September 15, 2008. Article published online May 30, 2009 1604—VOLUME 40A, JULY 2009
maximum at an intermediate grain size and subsequently decreased with further increase in grain size. The variation in fracture toughness of alloys undergoing SIMT with grain size was attributed to the effect of grain size on trigger stress.[7,9] The alloys containing higher Nb and Al exhibited a higher fracture toughness than lower Al or Nb containing alloys for all the grain sizes. The effect of change in Al or Nb content was explained on the basis of decrease in the trigger stress of alloys with an increase in Al or Nb content of the alloy for a given grain size.[10] In commercial applications, titanium alloys are often used in the a-b heat-treated condition, and the volume fraction of the second phase (a/a2) is known to influence the mechanical properties such as fracture toughness.[11] However, the influence of second phase on fracture toughness of titanium alloys undergoing SIMT has not been studied extensively. Our recent study on Ti-18Al8Nb[12] has shown that the fracture toughness decreases with an increase in vo
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