Microstructural Evolution During the Hot Deformation of Ti-55Ni (at. pct) Intermetallic Alloy
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ODUCTION
IN the 1960s, the Naval Ordnance Laboratory invented an intermetallic compound of Nickel and Titanium, which was named ‘‘Nitinol.’’[1–7] Among the Nitinols, the so-called ‘‘Ti-55Ni (at. pct),’’ 60 wt pct Ni and 40 wt pct Ti, has been used extensively because of its superior mechanical and chemical properties.[1–8] This alloy is shown in the phase diagram (Figure 1). The foremost of these properties include, for example, hardness up to 62RC, nonmagnetic property, immune to corrosion in most common corrosive agents, low thermal conductivity,[2,7] and superelasticity properties.[7] Besides, its density is about 26 pct lower than steel, which makes this alloy desirable for weight saving in transportation applications.[3–5] Formability is one of the most import characteristics of materials to attain the required near-net shape and high quality for finished products. It is widely accepted that the final mechanical properties of materials are sensitive to the microstructural features. Therefore, controlling the microstructural evolution during the thermomechanical processing of materials is of significant importance.[9] Among the most important restoration mechanisms during the hot working of materials are dynamic recrystallization (DRX) and dynamic recovery.[10] As for the Ti-55Ni (at. pct), it lacks the sufficient workability because of its high strength even at high ALIAKBAR KHAMEI, Master Student, and KAMRAN DEHGHANI, Associate Professor, Department of Metallurgical Engineering, Amirkabir University of Technology, Tehran 15914, Iran. Contact e-mail: [email protected] Manuscript submitted November 14, 2009. Article published online June 24, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
temperatures. This observation is attributed to the high volume fraction of hard intermetallic phases. Thus, a clear understanding of the hot deformation behavior of Ti-55Ni (at. pct) alloy is of great importance in terms of industrial processes such as rolling, forging, and extrusion. The aim of the current work was, therefore, to investigate the effect of temperature and strain rate on the deformation behavior and microstructure evolution during the hot working of Ti-55Ni (at. pct) alloy.
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EXPERIMENTAL PROCEDURE
The Ti-55Ni (at. pct) alloy was prepared using a vacuum induction melting furnace under argon atmosphere. The ingot was then homogenized at 1343 K (1070 C) for 12 hours. Compression specimens, with a diameter of 8 mm and a height of 12 mm, were prepared from the homogenized ingots. The specimens were reheated to 1373 K (1100 C), held for 15 minutes, and cooled to the hot deformation temperature. The hot compression tests were carried out at the temperatures of 1173 K to 1323 K (900 C to 1050 C) and at strain rates of 0.001 to 0.35 s1. The tests were carried out using an Instron4208 (Instron Company, England, United Kingdom) machine equipped with a furnace attached to its load frame. Before deformation, the specimens were held for 3 minutes at the deformation temperature to eliminate the thermal gradients as wel
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