Susan L. Draper

Microstructure and tensile properties of Fe-40 at. pct Al alloys with C, Zr, Hf, and B additions

The influence of small additions of C, Zr, and Hf, alone or in combination with B, on the microstructure and tensile behavior of substoichiometric FeAl was investigated. Tensile prop-erties were determined from 300 to 1100 K on powder which was consolidated by hot extrusion. All materials possessed some ductility at room temperature, although ternary additions generally reduced ductility compared to the binary alloy. Adding B to the C- and Zr-containing alloys changed the fracture mode from intergranular to transgranular and restored the ductility to ap-proximately 5 pct elongation. Additions of Zr and Hf increased strength up to about 900 K, which was related to a combination of grain refinement and precipitation hardening. Fe6Al6Zr and Fe6Al6Hf precipitates, both with identical body-centered tetragonal structures, were iden-tified as the principal second phases in these alloys. Strength decreased steadily as temperature increased above 700 K, as diffusion-assisted mechanisms, including grain boundary sliding and cavitation, became operative. Although all alloys had similar strengths at 1100 K, Hf additions significantly improved high-temperature ductility by suppressing cavitation.

The effect of temperature on the

The tensile stress-strain behavior and failure mechanisms of Ti-24Al-11Nb and a SiC/ Ti-24Al-11Nb composite with continuous SCS-6 fibers oriented parallel to the loading direction have been examined over a range of temperatures from 23 °C to 815°C in air. Failure in Ti- 24Al-11Nb occurred at strains of approximately 4 pct soon after crack initiation at low tem- peratures. Ductility increased with temperature up to a maximum of 20 pct elongation at 600 °C, as surface-initiated cracks did not propagate readily at intermediate temperatures. At higher temperatures, the onset of grain boundary and interfacial void nucleation limited ductility. Com- posite failure appeared to be controlled by fiber fracture at all temperatures; for practical en- gineering purposes, composite failure occurred at 0.8 pct strain at all temperatures. At temperatures of 425 °C and less, fiber fractures occurred at intervals along the lengths of the fibers and appeared to be cumulative, while at temperatures of 650 °C and greater, fiber fractures were only observed locally to the fracture surfaces. The decreased radial residual stresses, interfacial strengths, and matrix properties at 650 °C and 815 °C allowed the composite to unload at 0.8 pct strain, due to fiber fractures, followed by a reloading in which fibers pulled out and the matrix failed, resulting in composite failure. The decreasing residual stresses with increasing temper- ature determined from an elastic-plastic concentric cylinder model were shown to affect the stress-strain response of the composite and were consistent with the measured decreasing inter- facial shear stresses, the increased fiber pullout with temperature, and the circumferential de- bonding observed around the fibers at higher temperatures.

Room temperature flow and fracture of Fe-40at.%Al alloys

Abstract The room temperature tensile behavior of Fe-40at.%Al alloys was investigated. Extrusion of both prealloyed powders and castings was performed to produce a wide range of grain sizes for characterization of mechanical properties. In addition, directionally solidified single crystals were also studied. It was found that the influence of processing variables such as extrusion temperature or the form of the starting material (powder vs. cast ingot) on mechanical properties could be explained primarily by their effects on grain size. Grain refinement improved both ductility and strength, whereas rapid quenching after annealing resulted in increases in yield strength and decreases in ductility. The effects of quenching were explained by the evidence of large numbers of quenched-in vacancies. In the binary alloy, fracture was primarily intergranular irrespective of cooling rate, while alloys containing boron or Zr+B failed transgranularly and maintained their ductility in the rapidly quenched condition. Single crystals oriented in the [100] direction showed evidence of slip behavior yet did not exhibit significant tensile ductility. This lack of ductility was attributed to an early onset of cleavage failure.

Fretting Wear of Ti-48Al-2Cr-2Nb

Abstract An investigation was conducted to examine the wear behavior of gamma titanium aluminide (Ti-48Al-2Cr-2Nb in atomic percent) in contact with a typical nickel-base superalloy under repeated microscopic vibratory motion in air at temperatures from 296–823 K. The surface damage observed on the interacting surfaces of both Ti-48Al-2Cr-2Nb and superalloy consisted of fracture pits, oxides, metallic debris, scratches, craters, plastic deformation, and cracks. The Ti-48Al-2Cr-2Nb transferred to the superalloy at all fretting conditions and caused scuffing or galling. The increasing rate of oxidation at elevated temperatures led to a drop in Ti-48Al-2Cr-2Nb wear at 473 K. Mild oxidative wear was observed at 473 K. However, fretting wear increased as the temperature was increased from 473–823 K. At 723 and 823 K, oxide disruption generated cracks, loose wear debris, and pits on the Ti-48Al-2Cr-2Nb wear surface. Ti-48Al-2Cr-2Nb wear generally decreased with increasing fretting frequency. Both increasing slip amplitude and increasing load tended to produce more metallic wear debris, causing severe abrasive wear in the contacting metals.

Observations of directional gamma prime coarsening during engine operation

Two alloys, NASAIR 100 and a modified NASAIR 100 called Alloy 3, were run as turbine blades in an experimental ground-based Garrett TFE731 engine for up to 200 hours. The stress induced directional coarsening of γ′ (rafting) that developed during engine testing was analyzed and compared to previous research from laboratory tests. The blades were found to have formed a lamellar structure, the lamellae being normal to the centrifugal stress axis over much of the span. However, near the surfaces, the blades were found to have formed lamellae parallel to the centrifugal stress axis for certain cycles. Representative photomicrographs of the blades and the effects of stress and temperature on lamellae formation are shown.

Effect of fiber strength on the room temperature

To increase understanding of what controls SCS-6 SiC/Ti-24Al-11Nb (at. pct) composite strength, fibers of known strength were incorporated into composites and the effect of fiber strength variability on room temperature composite strength was investigated. Fiber was also etched out of a composite fabricated by the powder cloth technique, and the effect of the fabrication process on fiber strength was assessed. The strength of the composite was directly correlated with the strength of the as-received fiber. Fabrication by the powder cloth technique resulted in only a slight degradation of fiber strength. Examination of failed tensile specimens revealed periodic fiber cracks, and the failure mode was concluded to be cumulative.

A comparison of the mechanical properties and microstructures of intermetallic matrix composites fabricated by two different methods

The tensile properties of SCS-6 SiC fiber-reinforced Ti-24Al-11Nb (at. pct) have been measured over the past several years by a number of investigators. These composites have been fabricated by different techniques and tend to exhibit a large amount of scatter in the longitudinal tensile properties. To date, it is not known if one optimized fabrication method provides composites with improved mechanical properties over those produced by other optimized methods, since carefully controlled experiments have not been performed to determine this. Thus, the purpose of the present study was to compare the longitudinal tensile strengths of SCS-6 SiC/ Ti-24Al-11Nb composites that had been fabricated by the powder-cloth method and the lowpressure plasma spray (LPPS) method. In this study, the same lots of matrix powder and reinforcing fiber were used for fabricating the composites. It was determined that the powder-cloth and plasma spray methods produced composites having very similar tensile properties. Both fabrication methods induced damage in a small percentage of fibers, which manifested itself in the form of bimodal Weibull distributions of extracted fiber strengths. It appeared that the particular lot of SCS-6 fiber used in fabricating both types of composites was more susceptible to fabrication damage than those used in previous studies. This article also shows the dramatic effect that different handling and testing techniques can have on measured fiber strengths.

1000-1300 K SLOW STRAIN RATE PROPERTIES OF NIAL CONTAINING DISPERSED TIB2 AND HFB2

Abstract Rapid solidification technology was used to produce NiAl containing dispersions of fine TiB 2 or HfB 2 particles in an effort to improve the elevated temperature strength of this intermetallic. As-melt spun ribbons of NiAl2TiB 2 were pulverized, and the resulting powders were densified by extrusion at 1420 K with a 16:1 reduction ratio, while NiAl2HfB 2 was initiall;y consolidated by hot isostatic pressing at 1505 K and 207 MPa for 4 h followed by forging 65% at 1535 K. Both materials were compression tested in air between 1000 and 1300 K under constant velocity conditions at nominal strain rates ranging from 2 × 10 −3 to 2 × 10 −7 s −1 . NiAl2HfB 2 displayed steady state behavior under all test conditions while NiAl2TiB 2 exhibited diffuse yielding at 1000 K and generally slow, continuous work hardening at the higher temperatures. Analysis of the flow stress-strain rate data indicated that both materials behaved normally, and deformation could be described by temperature compensated power laws. HfB 2 was found to be a more effective strengthening addition than TiB 2 .

Tensile Behavior of Al2o3/feal + B and Al2o3/fecraly Composites

The feasibility of Al2O3/FeAl + B and Al2O3/FeCrAlY composites for high-temperature applications was assessed. The major emphasis was on tensile behavior of both the monolithics and composites from 298 to 1100 K. However, the study also included determining the chemical compatibility of the composites, measuring the interfacial shear strengths, and investigating the effect of processing on the strength of the single-crystal A12O3 fibers. The interfacial shear strengths were low for Al2O3/FeAl + B and moderate to high for Al2O3/FeCrAlY. The difference in interfacial bond strengths between the two systems affected the tensile behavior of the composites. The strength of the A12O3 fiber was significantly degraded after composite processing for both composite systems and resulted in poor composite tensile properties. The ultimate tensile strength (UTS) values of the composites could generally be predicted with either rule of mixtures (ROM) calculations or existing models when using the strength of the etched-out fiber. The Al2O3/FeAl + B composite system was determined to be unfeasible due to poor interfacial shear strengths and a large mismatch in coefficient of thermal expansion (CTE). Development of the Al2O3/FeCrAlY system would require an effective diffusion barrier to minimize the fiber strength degradation during processing and elevated temperature service.

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