Young-Won Kim

Microstructural evolution and mechanical properties of a forged gamma titanium aluminide alloy

Abstract A two-phase gamma titanium aluminide alloy, Ti-47Al-1Cr-1V-2.5Nb (in at.%), was studied under forged and various subsequent heat treatment conditions, to investigate the microstructural evolution and the effect of microstructure on room temperature (RT) tensile properties and fracture toughness behavior. Four classes of microstructure and three types of lamellar formation were identified, and their formation mechanisms were analyzed using various analytical techniques including metallography, electron optics, differential thermal analysis (DTA), and crystallography. It was found that both tensile and toughness behavior were profoundly affected by the microstructural variations.

Effects of microstructure on the deformation and fracture of γ-TiAl alloys

Abstract Deformation and fracture behavior of two-phase γ-TiAl alloys were investigated under monotonic tension loading conditions for duplex and lamellar microstructural forms. The effects of microstructure on tensile properties and deformation-fracture behavior are analyzed for deformation temperatures below and above the brittle-ductile transition. The crack initiation toughness and associated strains near the crack tip are used to explain the inverse relationship between ductility and toughness observed at room temperature. Fracture resistance behavior and toughening mechanisms at room temperature are explained in terms of microstructure and deformation anisotropy. The competition between the effects of grain size and lamellar spacing or tensile and toughness properties is discussed.

Rapid solidification of lightweight metal alloys

Abstract Rapid solidification (RS) of the light metals aluminium, magnesium, and titanium can lead to enhanced mechanical properties which can be rationalized in terms of the alloying behavior of these systems. The status of RS alloy exploration for the three lightweight metals is reviewed and suggestions made on possible future directions for alloy development.

Laser melting and heat treatment of m2 tool steel: A microstructural characterization

Microstructural characterization and electron diffraction crystal structure determinations have been carried out on the tool steel M2. Detailed examination using optical microscopy and thin-film and replica transmission electron microscopy substantiate preliminary findings.2,3 Namely, that laser melted material has a two-phase σ-ferrite/austenite matrix containing a dispersion of fine M2C carbides together with a smaller amount of M23C6 carbides. Subsequent heat treatment at 560 and 1230°C increases the hardness (to a maximum value of ∼1100 VHN) by carbide precipitation. In material heat treated at 560°C there is a preponderance of M23C6 carbides together with a small amount of M2C carbides. In contrast, the effect of heat treatment at 1230°C is to produce material containing only MC1-x type carbides with diameters in the range from 20 nm to ∼1 µm.

Microstructure development in gamma alloy mill products by thermomechanical processing

Gamma titanium aluminides are emerging as a revolutionary high-temperature material. The last decade led to significant engineering advances and component demonstrations for cast gamma-alloy products. However, cast processing has yet to permit the full potential of these materials to be realized, principally as the result of the limited microstructural control available. Conversely, thermomechanically processed gamma alloys are leading to a wide spectrum of microstructures, an outstanding balance of properties within the alloy class, and prospects for improved alloy durability. The manuscript succinctly describes the general field of thermomechanical processing for gamma alloys, giving particular attention to homogenization of large ingots in wrought processing. Aspects of producing fine-grained fully-lamellar microstructures, having controlled lamellar characteristics in wrought mill products are discussed. Some influences of alloy chemistry are discussed to show the feasibility of producing high-strength alloys across the gamma alloy class. Both a β-phase forming element (1 at.% Mo) and boron in the alloys are examined as grain-size controlling agents. These alloys are compared along with traditional alloys containing neither of these elements. Thermal-treatment windows are identified and discussed for producing fully-lamellar materials. When grain-size controlling agents such as boron or beta-phase are used, the lamellar transformation kinetics may be significantly altered relative to conventional gamma alloys, thus changing the thermal process path and affecting the perfection of the lamellar microstructures. These lead to concomitant changes in alloy properties. The prospects for attaining such structures and properties in large product scales are discussed.

Recent Advances in Gamma Titanium Aluminide Alloys

Gamma titanium aluminide alloys of current interest are two-phase alloys consisting of γ-TiAl phase as the matrix and a α 2 -Ti 3 Al phase as the second phase. The properties of these alloys depend on alloy composition, processing, microstructure, and their combination. Two major microstructural constituents are gamma grains and lamellar grains, the latter of which contain alternate layers of gamma (γ) and alpha-2 (α 2 ) thin plates. The relative amounts and distribution of these two constituents are the main factors controlling mechanical properties. This paper reviews our current understanding of the composition/microstructure/property relationships. An extended discussion will be made on the fundamental aspects of the formation of lamellar structure during cooling and the evolution of microstructure occurring during thermomechanical treatments.

Effects of microstructural parameters on the fatigue crack growth of fully lamellar γ-TiAl alloys

Abstract A study has been made to investigate the effects of colony size and lamellar spacing on the fatigue crack propagation behavior of fully lamellar γ-TiAl alloys. It has been found that the overall crack growth rates of fully lamellar microstructure are not largely affected by the variation of the colony size up to ≈400 μm, though its respective roles on the intrinsic and extrinsic nature of crack growth resistance are quite different. However, in the coarse colony microstructure (≈1400 μm), the fatigue crack growth threshold (ΔKth) is markedly decreased, while the crack growth resistance remains constant. The lamellar spacing is proved to be the more important factor to control both the fatigue crack initiation and growth resistance at room temperature. The fine lamellar spacing (0.2–0.7 μm) microstructures represent superior ΔKth and fatigue crack growth resistance compared to the coarse lamellar spacing (≈5.5 μm) microstructure. This superior fatigue resistance is mainly attributed to the higher number of lamellar interfaces resistant to crack advance, as well as to the higher closure effects. The colony boundaries and the lamellar interfaces play an important role in retarding the advancing crack at room temperature, serving as barriers for the dislocation movement and as sinks for dislocation pile-ups.

Rapidly solidified microstructure of AI-8Fe-4 lanthanide alloys

AI-817e and AI-8Fe-4RE (cerium, erbium, neodymium and gadolinium) alloys were rapidly solidified by the melt-spinning technique. The microstructure and phases of alloy ribbons were studied using optical metallography, SEM, TEM and X-ray diffraction techniques. The study has indicated that Re additions to AI-8Fe: (1) result in formation of an increased amount of fine microstructure region, (2) increase the hardness and stability, and (3) generally suppress the formation of needle-type Al3Fe compounds by substituting them with globular Al3Fe2RE compounds. The addition of gadolinium appears to produce the best results.

Rapid solidification of aluminum-rich AlV alloys☆

Rapidly solidified microstructures and phases of the aluminum-rich AlV binaries were investigated in splat-quenched material. The terminal solid solubility extension of vanadium in aluminum was determined and identifications were made for the phases including the icosahedral phase present in the AlxV (0 at.% < x < 24 at.%) splats. An attempt was made to explain the formation of various phases using the phase diagram.

Texture evolution during α-forging of γ-TiAl alloys

Abstract Four γ -TiAl alloys; Ti–47.0Al–2.0Cr–2.5Nb–0.21W–0.12B, Ti–46.8Al–2.0Cr–4.0Nb–0.30B, Ti–47.0Al–1.7Cr–2.4Nb–0.16W and Ti–46.3Al–1.8Cr–3.0Nb–0.26W–0.2Si–0.10B were forged within the α -phase field ( α -forging) to study the possibility of obtaining aligned microstructures. Pole figures from two locations within each forged plate (near the center and at the rim) were measured and the crystal orientation distribution functions (CODFs) were calculated to evaluate texture evolution during α -forging. A general trend toward the formation of a {011}〈uvw〉 fiber texture was observed for all four alloys. The orientation density distribution along the {011}〈uvw〉 fiber texture was found to depend on the alloy, the forging temperature and the radial location within the forged plates. Some scattered individual texture components, which were different in each alloy, were also observed. The observed textures are correlated to the deformation responses of the alloys during α -forging.