H.T. Pang

Fatigue crack initiation and short crack growth in nickel-base turbine disc alloys: the effects of microstructure and operating parameters

Abstract An assessment of the effects of microstructure and operating parameters on both crack initiation and propagation of short fatigue cracks is presented. The assessment was carried out on RR1000, U720Li and microstructural variants of U720Li. Fatigue tests were carried out at room temperature (20 Hz sinusoidal cycling) and at 650 °C (1–1–1–1 trapezoidal cycling). Comparisons of the performance of the different microstructures revealed that initiation occurred predominantly at pores at both temperatures. At room temperature, stage I crack growth predominated and the presence of large primary γ′ precipitates on the grain boundaries, larger grains and larger coherent γ′ sizes gave improved fatigue crack growth resistance, whereas at 650 °C, larger grains gave the most significant performance benefits.

A comparison of high temperature fatigue crack propagation in various subsolvus heat treated turbine disc alloys

Abstract The microstructure and fatigue performance of three subsolvus heat treated nickel based superalloys for turbine disc applications are reported. The alloy variants studied are RR1000, N18 and Udimet 720 low interstitial (U720Li), with the latter tested both in a standard and large grain variant (LG). Their microstructures are examined in terms of grain and gamma prime size (γ′). Fatigue crack growth (FCG) rates for all materials at 650°C show that RR1000 provides the best performance, followed by U720Li-LG, N18 and U720Li. Some of the variations in FCG rate between the alloys are due to reduction in grain boundary oxidation processes with increased grain size, but more subtle interplays between grain boundary character, alloy composition and slip character are also important.

Solution Heat Treatment Optimization of Fourth-Generation Single-Crystal Nickel-Base Superalloys

The optimization of the solution heat treatment (SHT) of fourth-generation single-crystal nickel-base superalloys LDSX-6B and LDSX-6C is presented. The methodological approach to optimizing the SHT process is particularly highlighted. Differential scanning calorimetry (DSC) measurements and electron-probe microanalysis (EPMA) mapping were carried out to investigate material properties in the as-cast condition and after SHT. The DSC equipment was also adopted as a vacuum furnace to evaluate the suitability of the SHT ramp profile and to check the safety margins with regard to incipient melting during SHT. SHT trials were carried out in a laboratory-scale vacuum furnace, after which the heat-treated samples were subjected to DSC experiments, microstructural analysis, and EPMA mapping to assess the effects of SHT peak temperatures and soak periods. From the DSC and EPMA results, as-cast LDSX-6B shows a lower degree of elemental microsegregation; hence, this alloy is relatively easier to homogenize in the SHT trials. In contrast, as-cast LDSX-6C was found to have a higher degree of elemental microsegregation; therefore, it is much more difficult to homogenize and is highly prone to incipient melting. The results of this study indicate that an increase in the SHT peak temperature and/or soak period will lead to an improved compositional homogeneity in the material as expected. After SHT, both alloys retained some residual elemental microsegregation and the LDSX-6C alloy showed precipitation of topologically close-packed (TCP) phase at the dendrite cores. The most appropriate and economic SHT process may be determined based on the methodological approach presented in this study and the requirements of the materials during service.

Detailed Analysis of the Solution Heat Treatment of a Third-Generation Single-Crystal Nickel-Based Superalloy CMSX-10K®

A detailed analysis of the response of as-cast third-generation single-crystal nickel-based superalloy CMSX-10K® to solution heat treatment (SHT) has been carried out, alongside an SHT optimization exercise. The analysis was conducted through microstructural characterization, differential scanning calorimetry, and compositional homogeneity measurements, quantifying (i) the dissolution and microstructural evolution of the inter-dendritic constituents, (ii) the shift in thermo-physical characteristics of the material, and (iii) the change in compositional homogeneity across the microstructure, in order to gain further understanding of these phenomena during the progression of the SHT. During the early stages of SHT, the coarse cellular γ′/narrow γ channel inter-dendritic constituents which were the last areas to solidify during casting, progressively dissolve; homogenization between these inter-dendritic areas and adjacent dendritic areas leads to a rapid increase in the incipient melting temperature TIM. The fine γ/γ′ morphology which were the first inter-dendritic constituents to solidify after primary γ dendrite solidification were found to progressively coarsen; however, subsequent dissolution of these coarsened γ/γ′ inter-dendritic areas did not result in significant increases in the TIM until the near-complete dissolution of these inter-dendritic areas. After the final SHT step, residual compositional micro-segregation could still be detected across the microstructure despite the near-complete dissolution of these remnant inter-dendritic areas; even so the TIM of the material approached the solidus temperature of the alloy.

Comparison of fatigue crack propagation in nickel base superalloys RR1000 and Udimet 720Li

The effects of temperature, dwell and environment on the fatigue crack growth behaviour of two nickel base turbine disc superalloys RR1000 and Udimet 720Li are presented in the present paper. Fatigue tests were carried out at room temperature in air using a 20u200aHz sinusoidal loading waveform and at elevated temperatures of 650 and 725°C in both air and vacuum environments using a trapezoidal loading waveform with dwell times at a maximum load of 1 or 20u200as. The fatigue crack propagation resistance of both materials is rationalised in terms of the influence of microstructure as well as the effects of alloy chemistry.

On effect of salt deposits on oxidation behaviour of CMSX-4 above 1000°C

Abstract Whereas the effect of contaminants in causing hot corrosion has been extensively studied at temperatures up to 1000°C, this paper describes a systematic study of the effects of various salt deposits on the cyclic oxidation of CMSX-4** in air at temperatures above 1000°C. The alloy was tested both in the bare and in the platinum aluminised (Pt-Al) coated conditions and variables included temperature, salt concentration, and sulphur level of the original alloy. It was found that salt deposition at very moderate levels, re-applied at regular intervals, significantly increased the initial weight gain and decreased the time to first spallation despite the rapid loss of the salt through evaporation in the first heating up cycle subsequent to salt deposition. The degraded microstructure of the specimens tested with salt resembled closely that of blades having experienced engine service conditions. Hence, cyclic oxidation with the addition of salt offers an accurate, flexible, reproducible and above all realistic method of assessing the life of turbine components.

A Study of the Effects of Alloying Additions on TCP Phase Formation in 4th Generation Nickel-Base Single-Crystal Superalloys

The demand for higher engine operating temperatures to improve aeroengine efficiency has meant that increasing levels of alloying additions are being added to single-crystal nickel-base superalloys for turbine blades applications. Whilst better mechanical and environmental performance may be obtained with these alloying additions, they also destabilise the alloys forming topologically closed-packed (TCP) phases. In this study, the formation of TCP phases has been studied in a series of four alloys designated LDSX1-4 which have a systematic variation in the levels of Co, Mo and W. The alloys were exposed to elevated temperatures between 900-1100°C for up to 1000 hours. This was followed by detailed analysis of the microstructures in the SEM. Identification of the TCP phases in selected alloys was also carried out. The effects of each alloying addition on TCP phase formation is discussed in light of these results.