Peter Au

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.

Erosion Testing of Coatings for Aero Engine Compressor Components

The results from a comparative study of erosion resistance of various coatings and some material modification treatments for protecting compressor airfoils made of 17-4 PH stainless steel against sand erosion, are presented. Coatings based on either aluminum, nickel or titanium nitride and material modifications by ion implantation and cryogenic treatment were evaluated by dry particle erosion testing in accordance with the ASTM G76-83 Standard Practice, at different angles of impingement (30°, 60° and 90°). The TiN coatings applied by PVD techniques, were found to be the most erosion resistant. All the PVD deposition techniques appeared capable of producing durable coatings, but in degrees that varied with the deposition method and vendor. All the TiN coatings investigated proved more resistant to erosion at a 30° impingement angle. The implications for compressor airfoil applications are discussed.

Interfacial strengthening by soft phase in lamellar microstructure of TiAl alloys

The authors report a mechanism of interfacial strengthening during creep deformation of fully lamellar TiAl alloys. In the lamellar microstructure of TiAl alloys, the α2 phase or the β phase is softer than the γ phase at elevated temperature. Conventionally, the creep resistance increases with decreasing the volume fraction of the soft phases. However, the lamellar interfaces of γ∕α2 or γ∕β retard the motion of dislocations during creep, causing a great strengthening, and thus the creep resistance increases with increasing the amount of the phase interfaces. In this study, a physically based model is developed to explain the strengthening behavior of the soft phases in lamellar TiAl alloys.

Interfacial strengthening of β phase in a fully lamellar structure of TiAl alloy containing W

The strengthening of the β phase in two fully lamellar structures with different lamellar spacing of a Ti–48 at.% Al–2 W at.% alloy during creep deformation has been investigated. It was found that the β phase precipitates during aging treatments only within regions of α2 lamellae following their dissolution. The strengthening of the β phase is more effective in the coarser lamellar structure, because the wider β precipitation zones, which replace the prior coarser α2 lamellae, hinders dislocation motion stronger than the narrower ones.

Evaluation of Hard TiN Coatings by Depth Sensing Indentation and Scratch Testing Methods

The IAR Nanomechanical Probe (NMP) was used in the depth sensing indentation mode to evaluate the Vickers hardness (HV) and elastic modulus E of five commercial TiN coatings applied to a 17-4 PH stainless steel substrate. The HV values of the TiN layers at depth to thickness (DTT) ratios of less than 0.1 varied between about 28 and 39 GPa, depending on the deposition process. The elastic modulus was within the range of 300 to 400 GPa, corresponding to published values, at DTT ratios of less than or equal to 0.05. At higher DTT values, the elastic modulus decreased with increasing DTT ratio due to larger and larger interference from the less rigid stainless steel substrate. Depth sensing scratch tests were also performed on the samples to determine the critical load crvalues needed to cause spallation of the coatings. For each coating, the interfacial fracture toughness Kic was calculated from crand used to describe the adhesive strength. Distinct differences between the Kic values of the different samples were observed, reflecting differences in adhesive strength. The significance of these results is discussed in terms of the application of titanium nitride coatings to gas turbine engines.

Evaluations of P/M Gamma Titanium Aluminides

For the last five years, the Structures, Materials and Propulsion Laboratory of the Institute for Aerospace Research, National Research Council of Canada, has devoted a significant amount of effort investigating powder metallurgy gamma titanium aluminides. The main results of the studies on microstructural evolution, hot workability, creep behaviour and microstructure-mechanical property relationships for P/M gamma alloys are reviewed in this article. Work is in progress to apply the knowledge gained to cast gamma alloys for gas turbine applications.

PM matures as route to aerospace alloys

Abstract Powder metallurgy processing of aerospace alloys is an important activity at the National Research Council of Canada. Inert gas atomized, nickel base superalloys using hot isostatic pressing and forging has matured to production status, while basic studies are also underway with titanium aluminides to prepare for the likely introduction of intermetallics. The PM research team reviews its findings in both these areas.

Microstructure and Creep of γ-TiAl Containing β-Stabilizer

Fully lamellar structures of powder metallurgy (PM), investment cast, and directionally solidified (DS) TiAl alloys containing β stabilizer were produced after stepped cool heat treatment, and interface β precipitates were formed after aging at 950°C. In addition, a columnar grain structure combined with a fully lamellar structure aligned with the load direction and interface β precipitates were formed by directional solidification and subsequent heat treatments. Creep test results of PM TiAl indicate that controlling the initial microstructures is also critical for balancing the primary and steady-state creep resistance during short and long-term tests. DS TiAl alloy exhibits a significant reduction of the primary strain and creep rate compared to polycrystalline TiAl due to the unique DS microstructure. Therefore, a DS microstructure with proper lamellar orientation and controlled interface β precipitation is the ideal if maximum time to a relatively small (<0.5%) strain is the design criterion of merit.

Microstructures and Mechanical Properties of Ti-48Al-2Cr-2Nb-xW

The microstructures and mechanical properties of three powder metallurgy Ti-48Al-2Cr- 2Nb-xW alloys (where x=0, 0.5, and 1 atomic percent (at.%)) are presented. The results indicate that a solution heat treatment combined with controlled cooling generate a fully lamellar (FL) microstructure without the formation of detrimental Widmanstätten or massively transformed γ phases. Aging causes coarsening of the FL microstructure in the alloys containing 0%W and 0.5%W, while almost no coarsening occurs in the 1%W sample. The addition of W to the base composition results in the formation of precipitates at the lamellar interfaces and grain boundaries during aging which helps stabilize the FL microstructure. The amount of W and the aging time affect the room temperature hardness values and tensile properties.

The Effects of Heat Treatments and Tungsten Additions on Microstructures and Tensile Properties of Powder Metallurgy Ti-48Al-2Nb-2Cr

The microstructures and tensile properties of a fully lamellar Ti-48Al-2Cr-2Nb, and two tungsten-modified versions, Ti-48Al-2Cr-2Nb-0.5W and Ti-48Al-2Cr-2Nb-1.0W (atomic percent) are investigated. Gas atomized powders are consolidated by hot isostatic pressing followed by solution treatment and aging. The microstructures are characterized by optical, scanning electron, and transmission electron microscopy and mechanical properties are characterized by room temperature tensile testing. The solution heat treatment, combined with controlled cooling, generates relatively fine, fully lamellar grains. Tungsten reduces the propensity for martensitic gamma formation during cooling, and slows down lamellar coarsening as well as the formation of equiaxed gamma phase during aging. The aging treatment stabilizes the microstructure and, in the tungsten-modified alloys, causes beta phase precipitation at lamellar interfaces and grain boundaries. Both aging and tungsten additions increase the alloy strength and reduce ductility. The fracture morphologies of the alloys are similar and exhibit mixed-mode fracture consisting of inter- and intra-granular cracking, as well as inter-lamellar cracking.