Vakil Singh

Role of Pt content in the microstructural development and oxidation performance of Pt-aluminide coatings produced using a high-activity aluminizing process

The present study highlights the effect of Pt content on the microstructure of Pt–aluminide coatings produced using a single-step high-activity aluminizing process. The amount of Pt in the coating was varied by changing the thickness of the initial electroplated Pt layer between 1 and 15 μm. The aluminium uptake from the pack was found to be almost the same for all the coatings produced using a Pt layer of thickness 2.5 μm and above, with a somewhat lower uptake for the coating corresponding to a 1 μm thick Pt layer. The coating microstructure, which consisted of an outer two-phase (PtAl2 in a matrix of NiAl) layer, an intermediate NiAl layer and an interdiffusion layer, was also found to be independent of the Pt layer thickness when it was in the range 2.5–10 μm. In the case of the 1 μm Pt layer, however, the whole of the Pt remained in solid solution in the NiAl phase. For a Pt layer thickness exceeding 10 μm, on the other hand, a continuous surface layer of PtAl2 phase was observed. The above mentioned influence of the thickness of the Pt plated layer on the microstructure of the Pt–aluminide coatings observed in the present investigation could be explained in terms of the Pt concentration in the diffusion layer resulting from the interdiffusion between the Pt layer and the superalloy substrate during the pre-aluminizing diffusion treatment. Cyclic oxidation tests on these Pt–aluminide coatings reveal that the presence of Pt in aluminide coatings, in general, enhances oxidation resistance. However, in order to fully realize the beneficial effects of Pt on oxidation behaviour, a certain minimum Pt content in the coating was found to be necessary.

Silicide Precipitation in Alloy Ti-6AI-5Zr-0.5Mo-0.25Si

Silicide precipitation in the titanium alloy containing by wt pct 6Al-5Zr-0.5Mo-0.25Si (Alloy 685) has been investigated using electron diffraction. The solutionizing temperature for this alloy is 1323 K. It is observed that no resolvable silicide precipitates are present in the alloy as received, furnace cooled from 1323 K and aged at 823 K, or oil quenched from 1323 K and aged at 823 K. Specimen solutionized at 1323 K for 30 minutes followed by water quenching and aging at 923 K for 24 hours shows fine precipitates at boundaries of α′ platelets. Diffraction analysis shows that the fine precipitates belong to two different types of identifiable hexagonal silicides similar to those observed by Floweret al in the ternary Ti-5Zr-0.5Si alloy to be (TiZr)5Si3 and another of unknown stoichiometry. However, aging of the water quenched specimens for 24 hours at higher temperatures, 973 K and 1073 K, results in the precipitation of a silicide similar to the latter one. It is analyzed that this silicide has lattice parameters a = 0.702 nm and c = 0.368 nm. Since aging at 973 K and 1073 K gives rise to precipitation of the same silicide, it is concluded that this is the stable silicide in Alloy 685 in the temperature range investigated.

Low cycle fatigue behavior of Ti alloy IMI 834 at room temperature

Abstract Low cycle fatigue behavior of the Ti alloy IMI 834 was studied, for a bimodal microstructure with ≈14 vol.% primary α (α p ) in the matrix of transformed β, at different total strain amplitudes (Δ e t /2) from ±0.75 to ±1.7%, at room temperature. TEM examination revealed silicide particles at the interface of secondary α platelets in the transformed β, at the boundary of α p and transformed β, and extremely fine precipitates of ordered Ti 3 Al in the matrix. Cyclic softening was observed at all the above strain amplitudes. However, the degree as well as the rate of softening increased markedly at Δ e t /2⩾1%. Cyclic softening was much less at low strain amplitude (Δ e t /2⩽0.8%). The increase in the degree and the rate of softening at higher strain amplitudes is associated with increase in the number and more complete shearing of the ordered Ti 3 Al precipitates, from increase in the number of slip bands and glide dislocations. The increase in the number of glide dislocations in each cycle, with increase in strain amplitude occurs from increase in the effective strain rate to maintain the cyclic frequency at constant level. Bi-linearity was observed in the Coffin–Manson relationship and it was found to be associated with change in the deformation behavior from low to high strain amplitude.

LOW-CYCLE FATIGUE BEHAVIOR OF NIMONIC PE16 AT ROOM TEMPERATURE

The fatigue behavior of NIMONIC PE16 has been investigated at room temperature as a function of γ′ particle size (from 10 to 30 nm) and total strain amplitude (0.44 to 2.60 pct). All specimens initially harden and then soften on further deformation. The degrees of hardening and softening show a marked variation with γ′ particle size and strain amplitude. Cyclic stress-strain and Coffin-Manson plots show a bilinear behavior with a change of slope at Δεp/2, the plastic strain amplitude, of about 0.3 pct. These results are interpreted in terms of microstructural observations, namely, the number of slip systems activated and mutual interaction of dislocations on these systems, as well as their interaction with γ′ particles.

Microstructural degradation of plain and platinum aluminide coatings on superalloy CM247 during isothermal oxidation

AbstractIsothermal oxidation at 1100°C of a high activity plain aluminide coating and a platinum aluminide coating, developed by the pack cementation technique, on cast nickel base superalloy CM247 has been carried out with the primary objective of systematically understanding the coating degradation process during oxidation. While the weight gains during oxidation for both plain aluminide and platinum aluminide coatings follow parabolic kinetics from the very beginning of oxidation exposure, the bare alloy was seen to exhibit a considerably long initial transient oxidation period (∼20 h), beyond which the parabolic law was followed. The parabolic rate constant for the platinum aluminide coating was found to be nearly two orders of magnitude lower than that for the plain aluminide coating. Alumina was identified as the only oxide phase that formed on both plain aluminide and platinum aluminide coatings during most of the oxidation exposure, although NiAl2 O4 was also found in the case of the plain alumini...

Tem investigation of γ' free bands in nimonic PE16 under LCF loading at room temperature

Abstract The cyclic stress strain behavior of nickel base alloy Nimonic PE16 has been studied at room temperature under constant strain rate of 4 × 10 −3 s −1 and at a strain amplitude of 0.57% for different γ′ particle sizes. Fatigue softening is observed for all microstructures. Detailed TEM investigations reveal γ′ free bands in all specimens. Evolution of these bands is studied as a function of cumulative deformation the specimen has bee subjected to γ′ particles within deformation bands are initially sliced by movement of dislocations and on further deformation slices are fragmented by cross slip of dislocations within bands. A conceptual model for reduction in size of γ′ particles during fatigue is proposed on the basis of these results. Reaging studies of deformed samples reveal nucleation of very fine γ′ particles within bands. Comparison of γ′ particle size within these bands with those formed after ageing in specimens deformed in solution treated condition indicates that γ′ particles have completely dissolved in the specimen containing smallest γ′ precipitates. These results are explained on the basis of the concept of particle fragmentation under cyclic loading.

Oxidation behaviour of the near α-titanium alloy IMI 834

Oxidation behaviour of the near α-titanium alloy IMI 834 was investigated over a range of temperatures, from 600–800°C, in air. Specimens were solution-treated in the α + β and β phase fields for 1 h and 1/2 h, respectively and cooled in air to room temperature. The solution treated samples were subjected to stabilization treatment at 700°C for 2 h, followed by cooling in air. Oxidation behaviour of these samples was studied from 600–800°C in air, for 50 h. The morphology of the scales formed was examined by SEM and the phases present in the scales were characterized by X-ray diffraction. While there was little oxidation at 600°C, the rate of oxidation increased at higher temperatures. In general, the rate of oxidation was found to be more in the α + β treated condition than that in the β treated one. The results are discussed in terms of the characteristics of the oxide film formed under different conditions.

Effect of Al content on microstructure and cyclic oxidation performance of Pt-aluminide coatings

The effect of Al content, i.e., the amount of Al picked up during aluminizing, on the microstructure and cyclic oxidation properties of Pt-aluminide coatings has been investigated. The cast Ni-base superalloy CM-247 was used as the substrate material and a single-step, high-activity pack aluminizing process was used to produce the Pt-aluminide coatings. The Al content of these coatings was varied by using packs with different compositions of the Al source. Pt-aluminide coatings having three different Al contents, namely 6.5, 16, and 21 mg cm-2, were evaluated for their cyclic oxidation resistance at 1200°C in air. It was found that the Pt-aluminide coatings, irrespective of their Al contents, evolve in the same manner during aluminizing and result in a three-layer structure with an outer PtAl2+NiAl two-phase layer, an intermediate NiAl layer, and the inner interdiffusion layer. The stability of this three-layer coating structure over long periods of aluminizing, however, is dependent on the availability of Al from the pack during this period. Below a certain threshold Al availability, the two-phase outer layer transforms to a single-phase NiAl structure causing the coating to change from its three-layer structure to a two-layer one. Cyclic oxidation results indicate that, while a minimum Al content in Pt-aluminide coatings is essential for deriving the best oxidation performance, increasing the Al content beyond a certain level does not significantly enhance oxidation behavior. The effect of Al content on aspects, such as coating degradation and nature of coating–surface damage during cyclic oxidation, is also discussed.

Effect of silicide precipitation on tensile properties and fracture of alloy Ti-6Al-5Zr-0.5Mo-0.25Si

The effect of silicon in solid solution and in the form of suicides has been studied on the tensile properties and fracture behavior of alloy Ti-6Al-5Zr-0.5Mo-0.25Si (alloy 685). The heat treatment to hold silicon in solid solution consists of solutionizing at 1323 K for 0.5 hour under vacuum (∼10-5 MPa), followed by water quenching, and the treatment to precipitate suicides involves subsequent aging of the solutionized and water quenched specimens at 1073 K for 24 hours. There is only marginal effect of the aging treatment on strength values; however, the ductility parameters are found to be drastically reduced. There are marked differences in the fracture behavior of the alloy in the as-quenched and the quenched and aged conditions. While the fracture surface of the unaged specimen shows characteristic dimples, there is a large number of facets on the fracture surface of the aged specimen. The facets in the central region are relatively smaller in size than those in the peripheral zone. The central facets show fluted features at higher magnifications; however, the peripheral facets are usually featureless. The faceted fracture in the aged condition is attributed to enhanced tendency for heterogeneous planar slip. The fracture characteristics correlate with the observed differences in the ductility in the two conditions.

Processing map for controlling microstructure in hot working of hot isostatically pressed powder metallurgy NIMONIC AP-1 superalloy

The hot deformation behavior of hot isostatically pressed (HIP) NIMONIC AP-1 superalloy is characterized using processing maps in the temperature range 950 °C to 1200 °C and strain rate range 0.001 to 100 s•1. The dynamic materials model has been used for developing the pro-cessing maps which show the variation of the efficiency of power dissipation given by [2m/ (m + 1)] with temperature and strain rate, withm being the strain rate sensitivity of flow stress. The processing map revealed a domain of dynamic recrystallization with a peak efficiency of 40 pct at 1125 °C and 0.3 s•1, and these are the optimum parameters for hot working. The microstructure developed under these conditions is free from prior particle boundary (PPB) de-fects, cracks, or localized shear bands. At 100 s•1 and 1200 °C, the material exhibits inter-crystalline cracking, while at 0.001 s•1, the material shows wedge cracks at 1200 °C and PPB cracking at 1000 °C. Also at strain rates higher than 10 s•1, adiabatic shear bands occur; the limiting conditions for this flow instability are accurately predicted by a continuum criterion based on the principles of irreversible thermodynamics of large plastic flow.