Dent Resistance and Effect of Indentation Loading Rate on Superelastic TiNi Alloy
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TiNi alloys have attracted considerable attention due to their unique superelastic behavior. Their superior wear[1,2] and corrosion resistance[3] make TiNi alloys popular in various applications. Their high ductility and high mechanical damping have also been exploited in numerous applications.[4–8] It has been established that the superelastic effect of TiNi alloys is related to a reversible martensitic transformation, i.e., stress-induced transformation. When a superelastic materials is deformed above its transition stress, it undergoes a transformation from the parent austenite phase to martensite phase followed by detwinning of martensite. This process is accompanied by large strain, which fully recovers when the load is released. The high elastic recovery of TiNi alloys is behind their high dent and wear resistance.[9–11] Tam et al.[12] found that superelastic TiNi behavior results in significantly larger recoverable deformation compared with 316L steel wire under the same testing conditions. It has also been suggested that the improvement in wear and dent resistance is related to the high hardness-to-elastic modulus ratio (H/E) of superelastic TiNi leading to significant increase in contact area between surfaces under load.[13] This reduces the average contact stress and, thus, reduces damages due to wear and dent. There are currently extensive efforts to develop bearings and gears using superelastic TiNi alloys.[14] ZOHEIR FARHAT and GEORGE JARJOURA, Professors, and MEISAM SHAHIRNIA, Graduate Student, are with the Materials Engineering Program, Dalhousie University, Halifax, NS, Canada. Contact e-mail: [email protected] Manuscript submitted July 17, 2012. Article published online April 12, 2013 3544—VOLUME 44A, AUGUST 2013
Conventionally, bearings are made from AISI 52100, AISI M-50, and AISI 440C steels.[15] For high-performance applications, silicon nitride has been the bearing material of choice.[16] Though these materials provide good performance for many applications, such materials are susceptible to denting and wear (and chipping in the case of silicon nitride) during high impact loading. The new high precision superelastic TiNi bearing is expected to provide high dent resistance for demanding applications to resist high impact overloading. Thus, the importance of loading rate becomes evident in applications where dent resistance is expected. The influence of loading rate, during ‘‘indentation’’ on superelasticity has not been established in the open literature. In contrast, there is a substantial amount of study on the effect of strain rate under ‘‘uniaxial’’ loading. It has been shown in several studies that varying the strain rate in loading and unloading tests can have substantial effects on the mechanical behavior of superelastic alloys. For instance, increasing the strain rate leads to shortening of the stress plateau and raising the transformation stress in tensile loading.[17,18] Quasistatic (strain rate of 0.001/s) and dynamic (strain rate of 1200/s) deformation in compression tests have
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