J. Beddoes

Lightweight materials for aircraft applications

Reducing structural weight is one of the major ways to improve aircraft performance. Lighter and/or stronger materials allow greater range and speed and may also contribute to reducing operational costs. The article reviews some recent developments in lightweight materials for airframe components (aluminum alloys, composites, and hybrid materials) and engine components (titanium aluminides and titanium-based composites). Emphasis is placed on microstructural characterization and the relationship between the microstructure and mechanical properties of specific materials systems being investigated at the National Research Councils Institute for Aerospace Research.

Heat treatment of hot isostatically processed IN-738 investment castings

Abstract An examination has been carried out of the microstructures of hot isostatically processed IN-738 turbine blades. Areas of void closure were found to be atypical of the bulk microstructure and showed recrystallization, twinning, residual gas micropores, and denudation of primary γ′. The effect of heat treatment following hot isostatic processing and solution treatment at 1200°C was investigated. Heat treatments involving controlled cooling at 140°C/hr from 1200°C to the primary aging temperature, followed by secondary aging at 845°C gave rise to the longest rupture lives in tests at 760°C and 586 MPa. Such heat treatments gave rise to finely serrated grain boundaries and a mixture of cuboidal primary γ′ and spheroidal secondary γ′. Appreciably faster or slower cooling rates gave rise to shorter rupture lives, and either planar grain boundaries, or serrated grain boundaries with overaged γ′.

Effect of fully lamellar morphology on creep of a near γ-TiAl intermetallic

Abstract Creep properties of a polycrystalline binary near γ-TiAl intermetallic in two fully lamellar microstructural conditions are presented. Creep tests (760°C/240 MPa) indicate that a lamellar structure with fine interface spacing and planar grain boundaries improves creep resistance. A lamellar structure with wide lamellar interface spacing and interlocked grain boundaries has less than half the creep life, five times higher minimum creep strain rate and a greater tertiary creep strain. The deformation substructures are presented in terms of the lamellar orientation to the stress axis and indicate that creep strain is accommodated by dislocation motion in soft oriented grains, but the creep strain rate is controlled by hard oriented grains. The extent of tertiary creep is controlled by the grain boundary morphology, with planar grain boundaries susceptible to intergranular cracking. The results suggest that to maximize the creep resistance of near γ-TiAl intermetallics with lamellar microstructures requires narrow lamellar interface spacing and interlocked lamellae along grain boundaries.

The brittle-ductile transition in HIP consolidated near γ-TiAl + W and TiAl + Cr powder alloys

Abstract The properties, deformation microstructures and fracture characteristics resulting from tensile tests between 20 °C and 850 °C are compared for Ti-48Al-2W and Ti-47.5Al-3Cr. TiAl + Cr exhibits a brittle-ductile transition with an elongation at 20 °C of 2.2% increasing to 36% at 850 °C. A much less pronounced brittle-ductile transition exists for TiAl + W, with an increase in elongation from 1.3% at 20 °C to only 4.5% at 850 °C. In both alloys fracture occurs predominantly by transgranular cleavage at low temperatures, changing to intergranular and then prior particle boundary failure with increasing temperature. High densities of deformation twins and 1 2 〈110] dislocations form in TiAl + Cr deformed at ⩾ 700 °C. For TiAl + W much less deformation twinning occurs, even during tensile deformation at 850 °C. The inability of TiAl + W to accommodate strain by deformation twinning eliminates the brittle-ductile transition that occurs for most near γ-TiAl compositions. TiAl + Cr attains higher fracture strength at 20 °C (563 MPa) than TiAl + W (505 MPa). This improved strength is related to the substructure developed during HIP consolidation. The increased 20 °C ductility of TiAl + Cr is associated with a more homogeneous as-HIPed microstructure.

Effect of aging on the tensile and creep behavior of a fully lamellar near γ-tial alloy

Abstract An investigation on the effect of aging on the tensile properties and creep behavior of a fully lamellar (FL) Ti–47Al–2Nb–1Mn–0.5W–0.5Mo–0.2Si (at.%) alloy is presented. Aging at intermediate temperatures produces precipitate particles predominantly along α 2 /γ lamellar interfaces or within α 2 lamellae. Most of the precipitates are β-particles with fewer silicides. Aging at intermediate temperatures leads to increased tensile strength as a result of precipitation of β and silicide particles. In addition, aging yields an increased primary creep resistance but a shortened creep life for the FL structure. The shortened creep life of the aged FL structures could be a result of early decomposition of α 2 lamellae during creep.

The isothermal compression response of a near γ-TiAl + W intermetallic

Abstract Hot isostatically pressed Ti-48%Al-2%W powder compacts were isothermally compressed between 900 °C and 1200 °C, and at constant strain rate in the range 10 −1 to 10 −4 s −1 . The deformation processes occurring are determined by correlation of the stress-strain response, strain rate sensitivity exponent m and activation energy, with the deformed microstructures. The resulting “deformation map” indicates that at low temperatures and high strain rates, mechanical twinning and dislocation generation cause a high flow stress, low m and high activation energy. Increasing compression temperature causes dynamic recrystallization, flow softening and increased m . Concurrent dynamic recrystallization and superplasticity can occur, with m as high as 0.44. At high temperatures and strain rates −3 s −1 , superplastic deformation is accompanied by grain coarsening. A “forging map” indicates that cracking after 120% engineering strain can be avoided if appropriate compression parameters are applied.

Microstructural evolution during heat treatments in Ti-45 and 47Al–2Nb–2Mn+0.8vol.%TiB2 XDTM alloys

Abstract Investment cast gamma XD TiAl alloys, Ti-45 and 47Al–2Nb–2Mn+0.8vol.%TiB 2 , were heat treated above and below their α transus temperatures to investigate the microstructural response. Heat treatments above T α produce fully lamellar microstructures, whereas equiaxed γ grains exist in the materials heat treated below the α transus temperature. Heat treatment time has little influence on the microstructures except for the 24 h treatment. Cooling by oil quench yields the finest lamellae in both alloys without formation of Widmanstatten and massive transformed structures. For microstructural stabilization, samples heat-treated above T α were subsequently subjected to a secondary aging treatment at lower temperatures. A single step aging treatment was applied to minimize coarsening of lamellae. The lamellar structure in XD45 alloy is relatively more stable than that in XD47. However, equaixed γ grains are nucleated and grown at the grain boundaries during aging. Multiple step aging treatment may be needed for developing stable lamellae without causing recrystallization. In general, lamellar spacing decreases as temperature and cooling rate increase. Hardness depends on microstructural features such as grain size, lamellar spacing, and volume fraction of equiaxed grains. Hardness increases as grain size, lamellar spacing, and the volume fraction of equiaxed grains decreases.

Precipitation of β particles in a fully lamellar Ti-47Al-2Nb-1Mn-0.5W-0.5Mo-0.2Si (at.%) alloy

The precipitation of β (B2) particles at intermediate temperatures, between 760°C and 1050°C, is investigated in a fully lamellar TiAl-WMoSi alloy. The β particles, having a thin-plate shape, usually precipitate on the α2 side of α2/γ interfaces at low aging temperatures in an uneven two-dimensional growth mode. While those formed at higher aging temperatures, growing extensively within the α2 plate and into the adjacent γ lamellae, have an ellipsoid shape. The growth of β particles at low aging temperatures yields an activation energy of about 366 kJ/mol. It is suggested that at low aging temperatures the growth of β particles proceeds via an α2 → β precipitation process controlled by diffusion of W and Mo along the β/α2 and β/γ interfaces.

Microstructure and mechanical properties of a PM TiAl-W alloy processed by hot isostatic pressing

Abstract A PM Ti-48at.%Al-2at.%W alloy was fully consolidated under two HIp conditions. A HIP cycle with a peak temperature of 1360 °C produced a fine-grained duplex microstructure. Increasing the peak temperature to 1395 °C developed a coarse-grained fully lamellar microstructure. The former yielded better room-temperature tensile strength and ductility, while the latter showed slightly higher ultimate tensile strength at 850 °C and superior creep resistance at 760 °C, 276 MPa. The fracture surfaces and deformation substructures of the tested specimens were analyzed and correlated with their mechanical behaviour. The effect of HIP conditions on the microstructures and the microstructure-property relationships is discussed.

Effect of HIP Conditions on the Microstructure of a Near γ-TiAl+W Powder Alloy

Abstract A Ti-48AI-2W (at%) near γ-TiAl powder alloy was hot isostatically pressed at 207 MPa and 1100°C to 1430°C, for 10 minutes to 4 hours. Densities of >99.9% were achieved in the HIP consolidated specimens. Four types of microstructure containing γ-TiAl, α2-Ti3Al and W-rich β 0 phases were produced: equiaxed, duplex, nearly lamellar and fully lamellar. HIP treatments for a short duration at≥1400°C improved the uniformity of the microstructure. A longer HIP time at 1400–1430°C was required to achieve compositional homogeneity. The HIP pressure had a significant effect on the microstructural changes, whereas the W homogenization was not as sensitive to the pressure.