Effect of processing and microstructure on René 95

Metallurgical and Materials Transactions A - René 95 is a high-strength, wrought nickel-base superalloy used primarily for compressor and turbine disks in advanced aircraft engines. A special...     

Ti3Ga and αTi interdiffusion between 660° and 860°C

Diffusion experiments were conducted in vacuum with bimetallic couples of the Ti₃Ga (α₂) composition and unalloyed α titanium. The gallium-composition profiles after various timetemperature exposures were determined by microprobe analyzer transverses and evaluated by established techniques. Results from this evaluation include the definition of the α to α +α₂ and the α + α₂ to α₂ phase boundaries for the Ti?Ga system and the determination of the interdiffusion coefficients for gallium in the α Ti and Ti₃Ga (α₂) phases. The interdiffusion coefficients were found to conform to the relationships: $$\tilde D_{\alpha Ti} = 4.4 \times 10^{ - 4} \exp [ - (43.4 \pm 4.7)10^3 /RT]cm^2 /\sec $$ $$\tilde D_{\alpha Ti_3 Ga} = 7.4 \times 10^{ - 5} \exp [ - (43.8 \pm 10.7)10^3 /RT]cm^2 /\sec $$      

Embrittlement of titanium alloys by long time,high temperature exposure

α stabilized titanium alloys are known to exhibit embrittlement after long-time exposures above ∼800°F. The time-temperature dependency of this embrittlement phenomenon in the Ti-6Al-2Sn-4Zr-2Mo and Ti-5Al-6Sn-2Zr-lMo-0.25Si alloys was observed using a substandard fracture mechanics test. Room temperature slow-bend tests of fatigue precracked Charpy specimens were used to monitor toughness degradation after unstressed thermal exposures in the temperature range of 800° to 1100°F for times to 5000 hr. The activation energy for the embrittlement process was found to be ∼25 to 28 kcal per g mole, which approximates that for diffusion of aluminum or tin in α-Ti. The embrittlement is attributed to the Ti₃X (X = Al, Sn) phase with the rate controlling step that of diffusion controlled growth of the Ti₃X phase domains. The embrittlement process is reversible by heat treatment at temperatures above the α + Ti₃X two phase region.     

Minimizing beta flecks in the Ti-17 alloy

Normal liquid to solid partitioning of Cr during ingot solidification for the Ti-17 alloy can result in the formation of a segregation anomaly called “β flecks.” These β flecks can exhibit a significantly lower β transus than the alloy; they may also present mechanical property issues. While the major thrust toward minimizing β flecks is through melt process control, this article addresses the capability of reducing the extent of Cr segregation using a high-temperature β field homogenization heat treatment. The interdiffusion coefficients for Cr in β phase Ti-17 were determined and used in diffusion equations to calculate the rate of dissipation of the segregated regions. Correlation is shown for the calculated, and critical experiment demonstrated, increase in the β fleck β transus temperature with the time and temperature of these homogenization heat treatments.     

Creep resistance and high-temperature metallurgical stability of titanium alloys containing gallium

Tensile properties up to 1100°F and the creep resistance at 1000°F were correlated with composition for twelve complex developmental titanium alloys with additions of Al, Ga, Sn, Mo, Zr, and Si. Creep resistance for these alloys in the β heat-treated condition was found to be strongly dependent on the totalα stabilizer content and the silicon concentration. The creep activation energy for a Ti-4.5 Al-2 Sn-3 Zr-3 Ga-1 Mo-0.5 Si alloy, established over the 900° to 1100°F temperature range, was about 100 kcal per g-mole. This high creep activation energy is hypothesized to result from dispersion strengthening within theα matrix by the Ti₃ X (X = Al, Ga, Sn) phase and pinning of the interplatelet and priorβ grain boundaries by the Zr₅Si₃ phase. Both phases were identified by transmission electron microscopy in these respective locations. Metallurgical instability, as evidenced by decreased fracture toughness, is also shown to be relatable to the totalα stabilizer content. The activation energy for the embrittlement process is about 45 kcal per g-mole. which approximates that for interdiffusion of gallium inα titanium.     

Effect of inclusions on LCF life of HIP plus heat treated powder metal rené 95

Effects of inclusions on 538 °C (1000 °F) strain control low cycle fatigue life of hot isostatically pressed and heat treated powder metal René* 95 compacts were evaluated. Size and location (surface or internal for the test bar) effects along with inclusion types and sources are categorized. Five types of inclusions were identified based on fracture initiation site appearance, although only two major types commonly contribute to significant life low cycle fatigue life degradation. Prior particle boundary decoration reactive type inclusions typically cause the most severe low cycle fatigue life degradation, and those are followed by the discrete ceramic type inclusions. Known potential contaminant seeding study evaluations were used to confirm sources for specific inclusion types. Attempts to minimize the sources for introduction of these contaminants in the argon gas atomization process facilities were only partially successful. An advanced processing approach for the manufacture of René 95 to achieve superior low cycle fatigue life has been proposed based on the improved understanding of the inclusion problem. This paper is based on a presentation made at the symposium “Physical Metallurgy of High Temperature Alloys” held at the fall meeting of the TMS-AIME in Philadelphia, PA on October 3 and 4, 1983, under the TMS-AIME High Temperature Alloys Committee.