Cryogenic Mechanical Properties of Warm Multi-Pass Caliber-Rolled Fine-Grained Titanium Alloys: Ti-6Al-4V (Normal and EL
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TITANIUM alloys find extensive applications in aerospace industry owing to their high specific strength coupled with good toughness, corrosion resistance, and fabrication characteristics.[1,2] These alloys are especially important for applications that experience cryogenic temperatures during service. Typical examples include components for space applications like pressurization gas bottles and propellant tanks. However, the applicability of a + b titanium alloys like Ti-6Al-4V for such application is strictly limited up to 77 K, which is mainly due to their poor ductility and increased sensitivity to notches at temperatures below 77 K.[3–8] With the advances in cryogenic engineering, there is a continuous
NIRAJ NAYAN and S.V.S. NARAYANA MURTY are with the Materials and Mechanical Entity, Vikram Sarabhai Space Center, Trivandrum 695 022, India. Contact e-mail: [email protected] GAURAV SINGH and U. RAMAMURTY are with the Department of Materials Engineering, Indian Institute of Science, Bangalore 560 012, India. T. ANTONY PRABHU is with the ISRO Propulsion Complex, Mahendragiri 627 133, India. Manuscript submitted on June 13, 2017. Article published online November 21, 2017 128—VOLUME 49A, JANUARY 2018
quest to improve the cryogenic strength and ductility of titanium alloys simultaneously through alloy design, and processing and heat treatment. Severe plastic deformation (SPD) is an effective route to produce ultrafine-grained (UFG) metals with improved strength and ductility.[9,10] Several SPD techniques have been developed like equal-channel angular pressing (ECAP),[11] high-pressure torsion (HPT),[12] multi-axial forging (MAF),[13] accumulative roll bonding (ARB),[14] and multi-pass rolling (MPR).[15,16] SPD processes can be further subdivided into two groups—continuous and batch.[17,18] For example, in continuous process entire billet goes in and out of the die continuously without any dwell time until the required final dimensions are achieved. In contrast, batch process is characterized by a disconnected flow line where the billet is processed once and then held for intermittent annealing/dwell, before being subjected to the next thermo-mechanical processing step. Thus, the billet requires several passes with intermittent annealing/dwell times superposed, so as to achieve the final shape. In SPD methods, large strains are imposed on the material in consecutive deformation steps under die constraint at relatively low temperatures (usually less than 0.4Tm, where Tm is the melting temperature).[9,10] Except ECAP,
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all the above processes lead to a reduction in the specimen cross-sectional area and geometry. For highstrength materials like titanium and its alloys, SPD is often performed at elevated temperatures where the enhancement in ductility and reduction in yield strength allow for large strain deformation. However, the high temperatures employed during SPD can lead to thermal activation of recovery and recrystallization, which, in turn, results in undesirable gr
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