Wear, 61 (1980) 69 - 76 @ Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
WEAR OF TWO TITANIUM COMPOUND IMPACT
STEPHEN L. RICE and STEVEN
Mechanical Engineering Department and Institute Connecticut, Storrs, Conn. 06268 (U.S.A.) (Received
of Materials Science,
July 3, 1979)
Summary This paper presents results obtained in testing two titanium alloys in compound impact wear. Relative transverse sliding velocity is found to be a significant parameter and distinctive near-surface microstructural features are noted for the two materials. For one alloy preliminary transmission electron microscope (TEM) foil studies indicate the surface layer to be crystalline. TEM replica studies suggest a continuum of deformation in the substrate for both materials.
1. Introduction The background for wear studies under controlled repetitive impact loading has recently been reviewed [ 11. To date work has been reported on polymeric materials [ 21, graphite epoxy composites [ 31, aluminum alloys [l] and a high strength steel [ 41. The present work concerns two titanium alloys. Selection of these materials for study was based on their extensive use in advanced technology gas turbine engines. These alloys possess high strength and very good elevated temperature creep resistance.
2. Materials and methods Alloy IMI 685 is a product of Imperial Metals Industry Ltd. and was obtained in a mill diameter of 2.54 cm which was subsequently swaged to 1.27 cm diameter rod. The material was heat treated by solutionizing at 1050 “C, oil quenched, aged at 550 “C for 24 h and air cooled. This treatment produces the characteristic “basket weave” microstructure shown in Fig. 1. The composition of the alloy is Ti-6Al-5Zr-0.5Mo-0.25Si. The alloy is typically processed and forged above the beta transus. When quenched from above the beta transus, the beta phase partially transforms into alpha platelets which are separated by thin films of retained beta as shown in Fig. 2.
Fig. 1. Titanium IMI 685 basket weave microstructure Fig. 2. Titanium IMI 685 alpha-beta microstructure
Alloy 55225 is a product of Reactive Metals Incorporated. This material was received as swaged rod of diameter 1.27 cm and was not subsequently heat treated. The resulting microstructure, shown in Fig. 3, is identified as continuous elongated primary alpha. The preferred orientation results from swaging. The composition of the alloy is Ti-GA1-5Sn-2Zr-2Mo-0.25Si. This alloy is typically utilized in a solution plus stabilized annealed condition which results in optimum high temperature creep resistance. However, the continuous elongated primary alpha in the present micros~cture represented an unknown in terms of wear resistance. Retails concerning specimen preparation and testing procedures have been reported earlier [I, 21. Compound impact wear testing was conduced at a variety of relative sliding velocities and at a variety of peak normal impulsive stress levels.
Fig. 3. Titanium RMI 65228 microscope).
continuous elongated primary alpha microstructure
3. Results In high transverse velocity compound impact testing (V > 3 m s-l) the IMI 685 alloy showed a relatively minor velocity dependence while the RMI 55225 demonstrated a relatively greater dependence as seen in Figs. 4 and 5. For both alloys, at the peak normal stress levels urnax noted, more wear occurs at the lower transverse sliding velocity than at the higher. While this finding conflicts with results obtained in ballistic impact wear studies [ 51, it agrees with recent results in pure sliding wear (no impact) obtained with commercially pure titanium at high sliding speeds . In this latter study wear rates were found to exhibit both a minimum and a maximum within a range of sliding velocities. Between 0.4 and 1.0 m s-l the wear rate decreased, between 1.0 and 5.0 m s-l the wear rate increased and as the sliding speed was increased from 5.0 to 10.0 m s-l the wear rate again decreased. HIGH VELOCITY SPECIMEN WEAR
HIGH VELOCITY SPECIMEN WEAR
TITANIUM TITANIUM mmore
umax = 10.6~Po
so CYCLES x IO3
CYCLES x IO3
Fig. 4. Weight loss us. number of impact cycles (IMI 685). Fig. 5. Weight loss us. number of impact cycles (RMI 55225).
It must be appreciated that in this regime of high speed sliding significantly high surface temperatures are generated both in sliding and compound impact wear. Both in the work reported in ref. 6 and in the present work with titanium alloys sparks were occasionally observed during testing. Thus high surface temperatures are thought to lead to a local softening of the specimen surface with a concomitant reduction in frictional force at the interface. Then at the higher sliding velocities reduced friction is believed to account for the lesser degree of wear observed. However, this reasoning cannot be extrapolated to a low velocity test series where thermal effects are less significant. Figure 6 is representative of the surface appearance of both titanium alloys following high speed sliding impact. While the flat sheets are suggestive
Fig. 6. Titanium IMI 685 specimen surface morphology: transverse sliding velocity, 5.3 m s-l; peak normal stress, 10.6 MPa; number of impact cycles, 50 000.
of delamination, a plowing groove network is also superposed. It is possible that the flat particles are those which have been back-transferred from the counterface and have been pounded to shape during impact. The subsurface photomicrographs from the high speed sliding tests are more interesting. Figure 7 is at relatively low magnification and shows the specimen sectioned parallel to the sliding direction. The presence of a leading and a trailing edge of material indicates the high degree of plastic deformation which occurs. Figure 8, which is a higher magnification view of the same material (IMI 685), shows a plastically deformed surface layer which contains a swatch of material which is almost broken free. Figure 9 shows a subsurface section from RMI 55225 and demonstrates the three zones typically found in such examinations. The deep substrate consists of the unaffected microstructure (zone 1). The intermediate layer is plastically
Fig. 7. Titanium IMI 685 specimen subsurface morphology: 5.3 m s-l ; peak normal stress, 10.6 MPa; number of impact Fig. 8. As Fig. 7 but with a higher magnification.
transverse sliding velocity, cycles, 500 000.
deformed, and this deformation is typically observable in the microstructure (zone 2). Often a refinement in this microstructure can be observed as the near-surface layer (zone 3) is approached. Zone 3 is thought to be a severely plastically deformed layer. In this case obvious voids and cracks are visible in zone 3. The differences in the subsurface microstructure of the alloys following impact wear prompted further investigation. Through the use of transmission electron microscopy (TEM) utilizing replication techniques the subsurface zones were examined at higher magnification than is possible in the scanning electron microscope (SEM). The subsurfaces were replicated with cellulose acetate, shadowed with pla~num-c~bon and finally coated with a thin film of carbon. Figure 10 shows the 55225 alloy, and the continuous nature of the refinement of the alpha beta microstructure as the wear surface is approached is clearly visible. This figure should be compared with the SEM photomicro~ph of Fig. 9.
Fig. 9. Titanium RMI 55225 specimen subsurface morphology: transverse sliding velocity, 0.4 m s-l ; peak normal stress, 6.8 MPa; number of impact cycles, 200 000. Fig. IO. Titanium
RMI 55223 specimen
Figure 11 shows the replicated section from the IMI 685 alloy, and is of particular interest since the continuum refinement of microstructure is not observable in the SEM (see Fig. 8). The replica, however, shows the refinement of the beta phase platelets as the wear surface is approached. Thus there are three characteristic subsurface zones with both alloys. In addition to replica studies, thin foil TEM techniques are employed. Foils are made from the wear surfa,ce layer per se and from additional layers parallel to the plane of wear extending into the substrate. One foil is taken from the deep substrate and serves as a micros~ct~ datum (Fig. 2). A micrograph of the foil from the wear surface itself is shown in Fig. 12 and is
Fig. 11. Titanium IMI 685 specimen subsurface (replicated) (TEM). Fig. 12. Titanium IMI 685 specimen wear surface layer (thin foil) (TEM).
representative of the branched tree-like structure observed throughout the entire foil. Close examination of the micrograph shows two of the striated branches and an extinction contour passing through the branches. The shift in the contour is indicative of.a substantial shear displacement within the material bounded by the two branches. TEM diffraction patterns from this surface layer foil indicate the material to be crystalline in agreement with recent results obtained by Bill and Wisander [ 71 and Van Dijck [ 81 for surface layers produced in sliding wear.
4. Discussion The present work complements earlier work [ 1 - 41 in which the peak normal impulsive stress umpx and the relative transverse sliding velocity u are shown to be significant external variables in compound impact wear. In so far as sliding velocity is concerned, the significance is probably most strongly manifested in temperatures generated during sliding. The formation of distinctive subsurface zones of transformed material structures is undoubtedly due to unique combinations of effects arising from stress and thermal loading. With the two titanium alloys examined no voids or cracks are observed in the substrate, with the possible exception of zone 3 in 55225. This is in
contrast to work with aluminum alloys in which significant subsurface void and crack features are observed in some cases [l] . Clearly the characterization of near-surface zones in wear research is most important in the understanding of basic mechanisms. The work of Suh [ 91 and Rigney et al. [ 10,111 has emphasized this point. As Rigney [ 121 notes, with correlations of temperature, stress, strain rate and microstructure it should be possible to predict wear modes under given sets of operating conditions.
5. Conclusions (1) Relative transverse sliding velocity is a significant variable in the compound impact wear of the two titanium alloys tested. (2) SEM subsurface photomicrographs reveal distinctive deformation zones. The characterization of these zones is complemented by TEM utilizing replicas from the subsurface sections. (3) Preliminary TEM foil studies of the wear surface layer indicate this near-surface layer to be crystalline. (4) The further characterization of subsurface morphology is an important goal for future wear research. Acknowledgments The authors are grateful for helpful discussion with Professors Hans Nowotny and Peter Strutt and to Mr. James Ruppen of the Metallurgy Department of The University of Connecticut. This research is sponsored by the Air Force Office of Scientific Research, Air Force Systems Command, USAF, under Grant No. AFOSR-77-3087; The United States Government is authorized to reproduce and distribute reprints for governmental purposes notwithstanding any copyright notation hereon. The support of AFOSR is gratefully acknowledged, as is that of Joseph D. Morgan of that office.
References 1 Stephen L. Rice, The role of microstructure in the impact wear of two aluminum alloys, Wear, 54 (1) (1979) 291 - 301. 2 Stephen L. Rice, Reciprocating impact wear testing apparatus, Wear, 45 (1) (1977) 85 95. 3 Stephen L. Rice, Impact wear of graphite epoxy composites, Proc. 2nd Znt. Conf. on Solid Lubrication, ASLE, London, 1978. 4 Stephen L. Rice, Variations in wear resistance due to microstructural condition in high strength steel under repetitive impact, Tribol. ht., 12 (1) (1979) 26 - 29. 5 P. A. Engel, Impact Wear of Materials, Elsevier, Amsterdam, 1976. 6 N. Saka, A. M. Eleiche and N. P. Suh, Wear of metals at high sliding speeds, Wear, 44 (1977) 109 - 126.
16 7 R. C. Bill and D. W. Wisander, Recrystallization as a controlling process in the wear of some f.c.c. metals, Wear, 41 (1977) 351 - 363. 8 J. A. B. Van Dijck, The direct observation in the transmission electron microscope of the heavily deformed surface layer of a copper pin after dry’sliding against a steel ring, Wear, 42 (1977) 109 - 117. 9 N. P. Suh, An overview of the delamination theory of wear, Wear, 44 (1977) 1 - 16. 10 D. A. Rigney and W. A. Glaeser, The significance of near surface microstructure in the wear process, Wear, 46 (1978) 241 - 250. 11 J. P. Hirth and D. A. Rigney, Crystal plasticity and the delamination theory of wear, Wear, 39 (1976) 133 - 141. 12 D. A. Rigney, Invited discussion of mechanical properties of near-surface materials in friction and wear by A.S. Argon, to be published.