The stir zone (SZ) temperature cycle was measured during the friction stir processing (FSP) of NiAl bronze plates. The FSP was conducted using a tool design with a smooth concave shoulder and a 12.7-mm step-spiral pin. Temperature sensing was accomplished using sheathed thermocouples embedded in the tool path within the plates, while simultaneous optical pyrometry measurements of surface temperatures were also obtained. Peak SZ temperatures were 990°C to 1015°C (0.90 to 0.97 T Melt ) and were not affected by preheating to 400°C, although the dwell time above 900°C was increased by the preheating. Thermocouple data suggested little variation in peak temperature across the SZ, although thermocouples initially located on the advancing sides and at the centerlines of the tool traverses were displaced to the retreating sides, precluding direct assessment of the temperature variation across the SZ. Microstructure-based estimates of local peak SZ temperatures have been made on these and on other similarly processed materials. Altogether, the peak-temperature determinations from these different measurement techniques are in close agreement.

Abstract--The steady-state creep deformation behavior of a cast two phase gamma TiA1 alloy having the composition Ti-48AI 1Nb (at.°) has been studied. Tension creep tests using the stress increment technique (a~ < cr 2 < or3) were conducted over the temperature range of 704-850°C at constant initial applied stress level of 103.4-241.3 MPa. The activation energy for creep over the temperature and stress regime of this study varied from 317.5 kJ/mol (137.8 MPa) up to 341.0kJ/mol (206.8 MPa) with an average value of 326.4 k J/tool. This is well within the range of values previously measured for gamma TiA1 alloys where creep controlled by volume diffusion has been suggested as rate controlling. The stress exponents measured were 5.0 at 704°C, 4.9 at 750°C, 4.7 at 800°C and 4.46 at 850°C. Using the activation energy of 326.4 kJ/mol, the temperature compensated steady-state creep rate was plotted against log stress with all temperatures collapsing onto a single line having a slope equal to 4.95. Using conventional creep analysis, this value of the stress exponent can be taken as suggestive of dislocation climb controlled power law creep as the operative deformation mechanism within the stress and temperature regime of the present study. The boundary separating the lamellar grains in two phase gamma TiA1 alloys having the duplex microstructure may be a very important aspect of this microstructure with respect to creep deformation resistance. The interlocking 7/~ 2 laths making up these boundaries are expected to be very resistant to grain boundary sliding which may contribute to creep deformation in the dislocation creep regime. Finally, some previous observations along with a comparison of the creep behavior of the Ti 48Al-INb alloy to that of a Ti-50.3AI binary have been discussed in terms of the pre-exponential constant A in the power law creep equation. TiA1 alloys having similar stress and temperature dependencies but differing steady-state strain rates over comparable stress-temperature regimes may be rationalized on the basis of differing power law creep constants which may reflect differences in stacking fault energies. R 6 s u m~) n &udie la d6formation par fluage en r6gime permanent d'un alliage TiA1 gamma biphas6 coul6 ayant la composition Ti-48AI 1Nb (en % atomique). Des essais de fluage, par la technique de l'incr6ment de contrainte (a t < o 2 < ~3), sont effectu6s sur une gamme de temp6ratures allant de 704 850°C pour des niveaux initiaux constants de la contrainte appliqu6e allant de 103,4 MPa 241,3 MPa. L'6nergie d'activation du fluage dans la gamme de contraintes et de temp6ratures de cette &ude varie entre 317,5 kJ/mol. (137,8 MPa) et 341,0 kJ/mol (206,8 MPa) avec une valeur moyenne de 326,4 kJ/mol. Ces r6sultats sont bien dans le domaine des valeurs d@i mesur6es pour les alliages TiAI gamma o0 l'on supposait que la vitesse de fluage 6tait contr616e par la diffusion en volume. Les exposants de contrainte mesur6s sont de 5,0 fi 704°C, 4,9 750°C, 4,7 ~i 800°C et 4,46/l 850C. En utilisant une energie d'...

Friction stir processing (FSP) produced local microstructural refinement in cast Ni Al Bronze. The refined microstructure quality was evaluated with mechanical property characterization using monotonic tension and fatigue testing as a function of FSP raster patterns. Modifying the cast NiAl bronze with FSP resulted in a 140 - 172 % increase in yield strength, and a 40 - 57% increase in tensile strength. Changing the raster pattern from a linear to a rectangular spiral raster increased the tensile elongations by 40 - 134%. This increase in elongation was attributed to increased microstructural uniformity through the depth of the FSP raster. The ability to transfer FSP technology was demonstrated with consistent tensile property data produced by three different laboratories. Fatigue characterization (both uniaxial and rotating-bending fatigue) showed that FSP improved the cast NiAl bronze fatigue resistance. Both types of fatigue testing showed differences in fatigue resistance as a function of processing parameters.

IntroductionThe ongoing search for increased aircraft engine performance has prompted the materials community to investigate intermetallic compounds as potential replacement materials for nickel-and cobalt-based superalloys. Of particular interest over the past decade has been near-gamma titanium aluminides due to their low density, high modulus and strength retention at elevated temperatures, and good environmental resistance (1,2).While many investigators have studied the microstructure/property relationships of gamma alloys, only a limited amount of work has been performed on their creep behavior (2-4). Since gamma titanium aluminides have been targeted for use at temperatures approaching 875°C, it is clearly important to understand their elevated temperature deformation characteristics.The vast majority of creep studies performed to date have concentrated on alloy systems prepared by wrought or powder metallurgy techniques (5-11).However, recent developments have identified investment casting as a viable method for producing complex, near-net shape gamma titanium aluminide components (12-15). As such, the need to examine the creep behavior of these alloys in the cast product form has prompted the current investigation. The stress dependency and activation energy for steady-state creep deformation were determined for cast, near-gamma alloy Ti-48AI-2Nb-2Cr (at.%). Post-test microscopy also was performed to characterize the microstructural evolution in these materials after prolonged high temperature exposure. The results are discussed with respect to potential creep deformation mechanisms operating within gamma titanium aluminides. Exoerimental ProcedureThe material for this study was produced by Vacuum Arc Remelting (VAR) and casting into 1.6 cm diameter x 15.2 cm test bars. Following casting, the material was hot isostatically pressed at 1260°C/172 MPa for 4 hours. The measured chemistry of the castings is given in Table I IntroductionThe ongoing search for increased aircraft engine performance has prompted the materials community to investigate intermetallic compounds as potential replacement materials for nicke1-and cobalt-based superalloys. Of particular interest over the past decade has been near-gamma titanium aluminides due to their low density, high modulus and strength retention at elevated temperatures, and good environmental resistance (l,2). While many investigators have studied the microstructure/property relationships of gamma alloys, only a limited amount of work has been performed on their creep behavior (2-4). Since gamma titanium aluminides have been targeted for use at temperatures approaching 875°C, it is clearly important to understand their elevated temperature deformation characteristics.The vast majority of creep studies performed to date have concentrated on alloy systems prepared by wrought or powder metallurgy techniques (5-1 I). However, recent developments have identified investment casting as a viable method for producing complex, near-net shape gamma titanium aluminide componen...