H. Saari

Thermophysical property determination of high temperature alloys by thermal analysis

Differential scanning calorimetric measurements to determine solidus and liquidus temperatures and latent heat of fusion of two high temperature materials, PWA1484 and an experimental gamma titanium aluminide alloy, are presented. The solidus and liquidus temperatures of PWA1484 are 1340 and 1404°C. The solidus and liquidus temperatures of the titanium aluminide alloy are 1453 and 1522°C. Solidus and liquidus temperatures determined from actual heating and cooling curves, which were measured using imbedded thermocouples and analyzed by a pseudo-differential thermal analysis technique are found to be in good agreement with the differential scanning calorimetric measurements.

Oxidation behaviour of gamma titanium aluminides with or without protective coatings

Abstract The γ-TiAl alloys are promising high temperature materials for aeroengines due to their low density, high specific strength and low material cost compared with Nickel based superalloys. However, the insufficient ductility at room temperature and the limited oxidation resistance at temperatures above approximately 750°C have limited their applications. Oxidation resistance in the application temperature range of 800 and 1000°C is of particular importance. Remarkable improvement to the environmental resistance by adding ternary and quaternary elements to the γ-TiAl alloys has been reported; however, alloying additions frequently deteriorate their mechanical properties. Surface modifications or coatings, which promote the formation of highly protective alumina scales, are also viable ways to improve the environmental resistance of TiAl alloys. In this article, the research work from the last 30 years on the oxidation behaviour of bare and coated TiAl alloys will be reviewed, with special focus on the γ-TiAl alloys. The review will begin with the oxidation behaviour of TiAl substrate alloys and the oxidation behaviour of γ-TiAl alloys with high temperature coatings such as aluminide, MCrAlY, Ti–Al–X and thermal barrier coating (TBC) system will be followed.

The Effect of Heat Treatments on Microstructure and Creep Properties of Powder Metallurgy Beta Gamma Titanium Aluminide Alloys

The microstructure and creep properties of two powder metallurgy (PM) ‘beta gamma’ titanium aluminide alloys are presented. Alloy powders with nominal compositions of TiAl-4Nb-3Mn (G1) and TiAl-2Nb-2Mo (G2) were produced by gas atomization and consolidated by a two-step hot isostatic pressing (HIP) process (1250 °C/200 MPa/1 hour + 1100 °C/200 MPa/3 hours + slow cooling to room temperature). After HIP, the materials were given a step cooled heat treatment (SCHT) of 40 min at 1400 °C, furnace cooling to 1280 °C, and air cooling to room temperature. Selected specimens were aged at 900 °C for 6 or 24 hours. The SCHT yielded similar fully lamellar microstructures for both alloys, with a lamellar spacing of 0.04 m, but with different grain sizes averaging 80 m (G1) and 40 m (G2). The aging treatments generated  precipitates along lamellar colony boundaries in both alloys, but along lamellar interfaces only in alloy G2. Constant load tensile creep tests were performed at 760 °C and 276 MPa. Alloy G2 exhibited superior creep performance compared to alloy G1, due to the quantity and size of  precipitate particles at the lamellar interfaces.

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.

Constructing a Validated Deformation Mechanisms Map Using Low Temperature Creep Strain Accommodation Processes for Nickel-Base Alloy 718

A creep Deformation Mechanism Map (DMM) of an engineering alloy can be an effective tool for developing physics-based prognostics systems. Many classical diffusion based rate equations have been developed for time dependent plastic flow where dislocation glide, dislocation glide-plus-climb and vacancy diffusion driven grain boundary migration (diffusion creep) are rate controlling. These creep rate equations have been proven experimentally for simple metals and alloys and form the basis of constructing an Ashby’s DMM. Long term creep testing and analysis of complex engineering alloys has shown that power law breakdown phenomenon is related to the dominance of Grain Boundary Sliding (GBS) as opposed to diffusion creep. Rate equations are now available for GBS in complex alloys and, in this paper, a DMM is constructed for a fine grained Alloy 718 and this is validated by comparison with a collection of experimental data obtained from the literature. The GBS accommodated by wedge type cracking is considered dominant at low homologous temperatures (0.3 to 0.5Tm i. e. melting temperature in Kelvin) whereas GBS accommodated by power-law or cavitation creep dominates above 0.55Tm.Copyright

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.

A Differential Scanning Calorimetry (DSC) Study of Phase Changes in an As-Received Zr-2.5Nb Pressure Tube Material during Continuous Heating and Cooling

Differential scanning calorimetry (DSC) has been used to study the phase changes in samples of as-received Zr-2.5Nb pressure tube material by continuous heating and cooling. Two different heating rates (5 and 20°C/min) were used to heat the sample up to 1050°C. After a short time hold at 1050°C, all the samples were continuously cooled to 300°C at a rate of 20°C/min. On continuous heating, the DSC signals obtained showed two endothermic transitions. The low-temperature transition, occurring between about 500 and 650°C, is attributed to a thermal decomposition of metastable niobium-stabilized β-phase. The highertemperature transition, occurring between 600 and 950°C, is due to phase transformations of hcp α-Zr to bcc β-Zr, as previously confirmed in a companion study on the same pressure-tube material that was examined in-situ by neutron diffraction. The neutron diffraction results provided a positive identification of the two phases and also a quantification of the β-phase present in the sample at different heating temperatures, and thus provided a guide to extract the volume fraction of β-phase from the DSC signals obtained in this study. The DSC signals revealed only one exothermic transition which is correlated to the reverse transformation of β-Zr to α-Zr, as previously identified in the companion neutron diffraction study of the same pressure tube material.

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.

The Role of Interfacial Precipitates on Creep Behaviour of Power Metallurgy (PM) Ti-48Al-2Cr-2Nb+1W Alloy

Pre-alloyed powders with a nominal composition of Ti-48Al-2Cr-2Nb+1W were consolidated by hot isostatic pressing (HIP). After the HIP process, a step cooled heat treatment (SCHT) with a carefully controlled cooling rate was applied to homogenize the HIP’ed microstructure and produce a fully lamellar microstructure. Following the SCHT, various isothermal aging at 950 °C and step aging processes form interfacial precipitates at the lamellar interfaces. The morphology, size, and distribution of the precipitates are dependent on the aging condition. Creep tests were carried out in air at 760 °C and 276 MPa to investigate the effect of interfacial precipitates. Primary creep resistance and creep life of the 8 and 144 hr aged conditions are improved substantially compared to the unaged condition due to the existence of the interfacial precipitates. However, the step aging process improves the creep resistance only slightly, probably because of the size and distribution differences of the interfacial precipitates compared to the 144 hr aged condition. Microstructure control is important since it has a substantial influence on creep behavior, especially primary creep resistance.

Characterization of the Crystallographic Textures and Mechanical Anisotropy Factors in Two Modifications of Zr-2.5Nb Pressure Tube Materials

Zr-2.5Nb alloy is used for the pressure tubes in CANDU reactors. Current as-manufactured tubes are produced in a cold-worked and stress-relieved metallurgical condition. The tubes installed in reactors normally operate at ~300°C. In a hypothetical loss-of-coolant-accident (LOCA), the pressure tube may be overheated to 1000°C. During the temperature transient, a phase transformation occurs that changes the microstructure and affects the material’s high-temperature deformation behaviour. In this study, improvements to enhance the performance of pressure-tube materials are being explored by modifying the texture and microstructure of as-manufactured pressure tubes. Two modifications were carried out by high-temperature annealing, with or without subsequent cold-working. This paper presents the resulting modified textures as measured by neutron diffraction and their texture evolution during heating. The anisotropy factors calculated for the modified Zr-2.5Nb pressure-tube materials using the measured texture data are compared with those previously characterized for Zircaloy-4 fuel cladding. The resulting effect of these texture and microstructure modifications with regard to the material’s response to anisotropic deformation during heating to high temperatures is also discussed in this paper.