Evaluation of Ti-48Al-2Cr-2Nb Under Fretting Conditions

Kazuhisa Miyoshi, Bradley A. Lerch, Susan L. Draper, and Sai V. RajNational Aeronautics and Space AdministrationGlenn Research CenterCleveland, Ohio 44135SUMMARYAn investigation was conducted to examine the fretting behavior of 7-TiAI (Ti-48AI-2Cr-2Nb) in contact with anickel-base superalloy (Inconel 718) in air at temperatures from 23 to 550 °C. Fretting wear experiments were con-ducted with 9.4-mm-diameter hemispherical Inconel (IN) 718 pins in contact with Ti-48AI-2Cr-2Nb fiats (and thereverse) at loads from 1 to 40 N and fretting frequencies from 50 to 160 Hz with slip amplitudes from 50 to 200 gmfor 1 to 20 million fretting cycles. The results were similar for both combinations of pin and fiat. Reference frettingwear experiments were also conducted with 9.4-ram-diameter hemispherical Ti-6AI-4V pins in contact withIN718 flats.The interfacial adhesive bonds between Ti-48AI-2Cr-2Nb and IN718 in contact were generally stronger than thecohesive bonds in the cohesively weaker Ti-48AI-2Cr-2Nb. The failed Ti-48AI-2Cr-2Nb subsequently transferred tothe IN718 surface at any fretting condition. The wear scars produced on Ti-48AI-2Cr-2Nb contained metallic andoxide wear debris, scratches, plastically deformed asperities, cracks, and fracture pits. Oxide layers readily formedon the Ti-48AI-2Cr-2Nb surface at 550 °C, but cracks easily occurred in the oxide layers. Factors including frettingfrequency, temperature, slip amplitude, and load influenced the fretting behavior of Ti-48AI-2Cr-2Nb in contactwith IN718. The wear volume loss of Ti-48AI-2Cr-2Nb generally decreased with increasing fretting frequency. Theincreasing rate of oxidation at elevated temperatures up to 200 °C led to a drop in wear volume loss at 200 °C.However, the fretting wear increased as the temperature was increased from 200 to 550 °C. The highest tempera-tures of 450 and 550 °C resulted in oxide film disruption with generation of cracks, loose wear debris, and pits onthe Ti-48AI-2Cr-2Nb wear surface. The wear volume loss generally increased as the slip amplitude increased. Thewear volume loss also generally increased as the load increased. Increasing slip amplitude and increasing load bothtended to produce more metallic wear debris, causing severe abrasive wear in the contacting metals.1.0 INTRODUCTIONAdhesion, a manifestation of mechanical strength over an appreciable area, has many causes, including chemi-cal bonding, deformation, and the fracture processes involved in interface failure. A clean metal in contact with aclean metal will fail either in tension or in shear because some of the interfacial bonds are generally stronger thanthe cohesive bonds in the cohesively weaker metal (ref. 1). The failed metal subsequently transfers to the other con-tacting metal. Adhesion undoubtedly depends on the surface cleanliness, the area of real contact, the chemical,physical, and mechanical properties of the interface, and the modes of junction rupture. The environment influencesthe adhesion, deformation, and fracture behaviors of contacting materials in relative motion.Clean surfaces can be created by repeated sliding, making direct contact of the fresh, clean surfaces unavoidablein practical cases (ref. 2). This situation applies in some degree to contact sliding in air, where fresh surfaces arecontinuously produced on interacting surfaces in relative motion. Microscopically small surface-parallel relativemotion, which can be vibratory (in common fretting or false brinnelling) or creeping (in common fretting), producesfresh, clean interacting surfaces and causes junction (contact area) growth in the contact zone (refs. 3 to 5).Fretting wear produced between contacting elements is adhesive wear taking place in a nominally static contactunder normal load and repeated microscopic vibratory motion (refs. 6 to 10). The most damaging effect of frettingis the possibly significant reduction in fatigue capability of the fretted component even though the wear producedby fretting appears to be quite mild (ref. 10). It was reported that the reduction in fatigue strength by fretting ofTi-47AI-2Nb-2Mn with 0.8 vol.% TiB 2 was approximately 20 percent.Fretting fatigue is a complex problem of significant interest to aircraft engine manufacturers (refs. 11 to 14).Fretting failure can occur to a variety of engine components. Numerous approaches, depending on the componentand the operating conditions, have been taken to address the fretting problem. The components of interest in thisinvestigation were the fan and compressor blades. Many existing fan and compressor components have titaniumNASA/TM--2001-210902 i