The fracture behavior of diffusion-bonded duplex gamma TiAl

Solid-state diffusion bonding in conjunction with superplastic forming is a potential candidate for producing complex structural components from gamma-based titanium aluminides for aerospace applications. Solidstate diffusion bonding of TiAl was carried out with different bonding parameters within the superplastic temperature range. Defect-free sound bonds were achieved within the temperature range of 925–1,100 °C and the pressure range of 20–40 MPa. Microtensile tests were carried out to evaluate room-temperature tensile properties of the bonds for comparison of the bonding parameters.     

Sheet gamma TiAl: Status and opportunities

Gamma TiAl alloys have attractive properties such as low density, high-temperature strength, and high modulus, oxidation, and burn resistance. As a result, these alloys have the potential to replace heavier superalloys in aircraft engine components. Gamma TiAl alloys were investigated in the 1950s but were too brittle for thermo-mechanical processing. However, interest in this class of material rekindled with several U.S. aerospace programs: the National Aerospace Plane, the Integrated High Performance Turbine Engine Technology, and Enabling Propulsion Materials/High Speed Civil Transport, as well as German hypersonic technology programs. Intense metallurgical and metal processing research during the last two decades led to significant progress in this area. As a result, gamma TiAl alloys are now available in all conventional product forms: ingots, forgings, extrusions, and sheets. This article reviews the current status of sheet gamma TiAl technology and its future opportunities. For more information, contact Gopal Das, Pratt & Whitney, 400 Main Street, E. Hartford, CT 06108 USA; (860) 557-1413; e-mail gopal.das@pw.utc.com.     

Application of cast gamma TiAl for automobiles

Studies of cast gamma TiAl applied for engine valves and turbocharger confirmed excellent performances. The durability of TiAl valves was also proven. Two cast gamma TiAl alloys were developed for the engine valves and the turbine wheels of turbochargers. New precision casting process of LEVICAST and induction brazing process of the gamma TiAl and alloy steels were proposed as advanced processing technologies.     

Impact toughness and fracture toughness of austempered ductile iron

1.IntFOdUction AustemPered Duetile Iron(ADI)15 a relatively newmaterial in east alloy family.Its mierostrueture eonsists ofaeieular ferrite in a high earbon austenite matrix ealledausferrite.This very fine mierostrueture offers     

Room-temperature fracture toughness of MoSiBTiC alloys

Room-temperature fracture toughnesses of TiC-added Mo-Si-B alloys were evaluated for samples of three different compositions prepared using a conventional Ar arc-melting technique. The first alloy (TiCp) had a primary phase during solidification of NaCl-type TiC including an amount of Mo, with a Mo solid solution (Mo_ss) volume fraction of approximately 49% and a TiC volume fraction of approximately 19%, while the volume fraction of Mo₅SiB₂ (T₂) was approximately 31% and the remaining 1% was Mo₂C including an amount of Ti. The second alloy (T2p) had a primary phase of T₂, with volume fractions of Mo_ss, TiC, Mo₅SiB₂ (T₂), and Mo₂C of approximately 38%, 4%, 45%, and 13%, respectively. The third alloy (Mop) had a primary phase of Mo_ss, with volume fractions of Mo_ss, TiC, Mo₅SiB₂ (T₂), and Mo₂C of approximately 55%, 8%, 32%, and 6%, respectively. Room-temperature fracture toughness was evaluated by three different bending tests using Chevron-notched specimens. Fracture toughness values obtained by the three methods were relatively close with good reproducibility. Consequently, the fracture toughness values of TiCp, T2p, and Mop were evaluated to be ∼15.2 MPa(m)^1/2, ∼10.5 MPa(m)^1/2, and ∼13.6 MPa(m)^1/2, respectively. Fracture surface observations indicated that the Mo_ss phase is subject to severe plastic deformation during the fracture process. The TiC phase was also noted to leave river patterns behind through crack propagation. These fractographic results suggest that not only the ductile-phase toughening by the Mo_ss phase but also an extra-toughening mechanism by the TiC phase are responsible for the goodness of the room-temperature fracture toughness of the MoSiBTiC alloys.     

Impact toughness and fracture of beryllium

Metal Science and Heat Treatment -     

On fracture toughness in tearing of sheet metal

An approximate analysis is presented for predicting the specific fracture toughness,R, in the tearing fractures of ductile sheet from the stress-strain relationships. The model assumes the fracture work to be the plastic work consumed by localized necking. Predictions are compared to experimental findings.     

Phase equilibria and microstructural control of gamma TiAl based alloys

Phase equilibria among the β (bcc or B2), (hcp) and γ (L1₀) phases in Ti---Al---M ternary systems at elevated temperatures have been studied, where the M is β stabilizing element for pure titanium. The β(B2) + + γ three-phase coexisting region exists at temperatures above 1473 K, and it moves towards a direction of high aluminum and high M concentrations with increasing temperature. The change in the phase equilibria by the addition M is associated with the ↔ β allotropic transformation temperature of pure titanium and can be thermodynamically interpreted in terms of the lattice stability ratios of M to Ti. The change in the phase equilibria results in new reaction pathways peculiar to the ternary systems, thereby opening more possibility for microstructure control of the gamma based alloys. The vertical section drawn based on these studies demonstrates that the γ phase can be in equilibrium with the β(B2) phase at relatively low temperatures in the ternary systems and create a reaction pathway of → β(B2) + γ. Upon cooling along the same pathway of → γ as in the binary alloy, the addition of third element affects the transformation rate, and the massive γ structure is found to be formed even under slow cooling rate.     

The fracture toughness of bulk metallic glasses

Stiffness, strength, and toughness are the three primary attributes of a material, in terms of its mechanical properties. Bulk metallic glasses (BMGs) are known to exhibit elastic moduli at a fraction lower than crystalline alloys and have extraordinary strength. However, the reported values of fracture toughness of BMGs are highly variable; some BMGs such as the Zr-based ones have toughness values that are comparable to some high strength steels and titanium alloys, whereas there are also BMGs that are almost as brittle as silicate glasses. Invariably, monolithic BMGs exhibit no or low crack growth resistance and tend to become brittle upon structural relaxation. Despite its critical importance for the use of BMGs as structural materials, the fracture toughness of BMGs is relatively poorly understood. In this paper, we review the available literature to summarize the current understanding of the mechanics and micromechanisms of BMG toughness and highlight the needs for future research in this important area.     

Fatigue and fracture toughness of epoxy nanocomposites

The fatigue and failure mechanisms of epoxy composites have been researched extensively because of their commercial importance in fields demanding materials with high specific strength. Particulate, sheets, short and long fibers with dimensions in the micrometer and nanometer range are the major fillers which have been studied for enhancing the fatigue resistance of epoxies. The nano and micro scale dimensions of the fillers give rise to unexpected and fascinating mechanical properties, often superior to the matrix including fracture toughness and fatigue crack propagation resistance. Such properties are dependent on each other (e.g., the fatigue properties of the polymer composites have been found to be strongly influenced by its toughness). This article is a review of the various developments in this field and the underlying mechanisms which are responsible for performance improvements in such composites.     

Effect of lamellar plates on creep resistance in near gamma TiAl alloys

Effect of interlamellar spacings on creep rate in Ti-48 at% Al alloy with fully transformed lamellar structure (FL) has been investigated at 1123 K/98 MPa. In addition, the correlation between creep rate and lamellar orientation to stress axis was elucidated by conducting creep tests at 1148 K/68.6 MPa for three single crystal (SC) specimens of Ti-48 at% Al with different lamellar orientations to stress axis. The difference in creep rate among FL specimens having different interlamellar spacings could not be defined in the early stage of transient creep. The onset of accelerating creep was slightly retarded by decreasing the inter-lamellar spacing. While the creep rate of the SC specimen whose lamellar orientation to stress axis 30–63 ° gradually decreases with increasing strain, and is larger than that of the FL specimen, the creep rate of the SC specimen with lamellar orientation nearly parallel to stress axis drastically decreases within small strain, and is 1/50 smaller than that of the FL specimen. This strong lamellar orientation dependence of creep rate is interpreted by the correlation between dislocation slip system and the lamellar plate.     

Fabrication and fracture toughness of macro-toughening-designed composite

lINTR0DUCTIONParticleReinforcedMetalMatrixC0mp0s-ites(PMMCs)havehighspecificstrength,spe-cificmodulus,elevatedtemperatureproperties,res1stancetowearandlowcost.However,com-paniedlowductilityandtoughnessisonemainobstacletotheirapplicationforengineeringL','j.ManystudiesonSiCparticlereinforcedalu-.minum.ll.y.['v']showthattheadditionofpar-ticlenotonlyrefinesmatrixgrainbutalsoresultsinhighdensitydislocationsinthematrixneartheinterface.Particlesblocklong-distance-slipofthedislocationsinthema…     

Temperature stability and fracture toughness of Nd-Fe-B magnets

Temperature stability and toughness of magnets are very important properties especially for application in motor. In this paper, it is found that temperature stability and toughness of magnets are improved when Fe is substituted with Co andheavy rare earth is substituted for Nd in part and suitable rich B grain-boundary phase is added. In addition, heavy rare earth substitution decreases the remanence temperature coefficient greatly, but has a little effect on Curie temperature of the magnets, which is beneficial to Nd-Fe-B magnets for the application in motor.     

The fracture toughness and fatigue behavior of DRA

‘In this study, the fracture toughness and elevated-temperature tensile and fatigue behavior of discontinuously reinforced aluminum (DRA) were examined. The effects of heat treatment and specimen thickness on fracture toughness were studied in a 7093/SiC_p composite. The toughness of the DRA was lowest in the peak-aged condition, but increased considerably in the overaged condition. The highest toughness was obtained at a critical value of specimen thickness; this critical value was used to fabricate a laminated composite consisting of alternating layers of DRA and unreinforced alloy. Elevated-temperature tensile and fatigue behaviors were investigated in a 2080-T6/SiC_p composite at different volume fractions and particle sizes. Increasing reinforcement volume fractions resulted in increases in tensile and fatigue strength. Exposure tests for 300 h at 150°C produced no significant reduction in ultimate tensile strength or yield strength, indicating good thermal stability of the 2080 matrix. For more information, contact A.B. Pandey, Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Ohio 45433; (937) 255-1320; fax (937) 255-3007; e-mail awadh.pandey@ml.afrl.af.mil.     

On the fracture toughness of intermetallics estimated from the work of fracture

In the present paper the regression analysis of the fracture toughness data obtained from 3 and 4 pt bending of single-edge pre-cracked beam and chevron-notched beam specimens of various intermetallic alloys calculated from the work of fracture γ_wof and the maximum load on the load–displacement curve is carried out in order to establish a relationship between both quantities.