Fundamental insights are presented into the factors influencing the temperature-dependent strengthening of nickel by transition metal solutes. Strain rate jump tests are used using the alloying elements Ta, W and Re which lie beside one another in the d-block. At 800 degrees C, the hardening induced by Ta is significantly greater than that by W and Re. At T >= 1000 degrees C, Re becomes more potent than Ta at slow strain rates; at 1200 degrees C it is the most effective at all the strain rates employed. The results are interpreted using theory for temperature-dependent strengthening that emphasises solute dislocation interaction. It is confirmed that at low temperatures the solute strengthening is controlled by paraelastic interaction the solute with the largest size difference with the host Ni is the most potent. At higher temperatures, the evidence indicates that solutes collect on dislocations such that the slowest diffusing solutes confer maximal resistance to dislocation glide and climb; thus Re is particularly potent. The findings elucidate the role of transition metal solutes in strengthening. Moreover, they provide the necessary quantitative data for ongoing alloy design efforts. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The properties of a newly developed single crystal superalloyknown as STAL-15 -is described which is suitable for use in first stage blades of industrial gas turbines (IGTs).With 15 wt.%Cr and 4.55 wt.% Al, the alloy combines good corrosion and oxidation resistance with sufficient creep and fatigue performance.Traditionally, polycrystalline alloys such as IN792 and IN738LC or the single crystal alloy PWA-1483 have been used for this application; unfortunately they display only limited resistance to environmental degradation.The new alloy does not display this weakness and is therefore highly optimised for IGT applications.The new alloy is shown to be an alumina (Al2O3) former; the mechanisms behind the Al2O3-formation process are studied and the effects arising from changes in the chemical composition have been modelled.In addition, the mechanical properties in terms of creep and fatigue resistance are demonstrated together and the alloy stability evaluated during long term (up to 10,000 hours) exposure.For such applications, the new alloy is superior to existing nickel-based single crystal superalloys designed for aeroengine applications and which are optimised for very high creep resistance.The absence of Re contributes to a lower cost of alloy stock, and enhanced castability.

The oxidation behaviour at 950 degrees C of a prototype nickel-based single crystal superalloy containing similar to 15 wt%Cr is studied, with and without silicon alloying. It is demonstrated that the alloy is a marginal alumina former, with further alloying by Si promoting the formation of a continuous, protective alumina scale. Transmission electron microscopy reveals that out-growing alumina forms during the early stages of oxidation. The presence of silicon within the alumina is detected using atom probe tomography.

The thermomechanical fatigue (TMF) behaviour of a new high Cr-containing single crystal superalloy, known as STAL-15, is investigated. This is a candidate alloy for future industrial gas turbine (IGT) applications. TMF involves complex interactions between high and low temperature deformation mechanism, and this study highlights important factors controlling their interrelationship. Emphasis is placed particularly on the microstructural aspects which control deformation. It is demonstrated that the TMF performance of the new alloy is comparable to that of a second generation alloy such as CMSX-4, despite the absence of Re alloying. An addition of 0.25wt% Si significantly improves the resistance to TMF further; this is attributed to a greater resistance to recrystallization and cracking along the deformation bands that develops across the cross section of the specimen during testing. The TMF resistance increases with increasing primary gamma' size, but the degree of solutioning of the gamma' phase caused by the solution heat treatment seems to have no significant effect.

The mechanical behavior of a new single-crystal nickel-based superalloy for industrial gas turbine (IGT) applications is studied under creep and out-of-phase (OP) thermomechanical fatigue (TMF) conditions. Neutron diffraction methods and thermodynamic modeling are used to quantify the variation of the gamma prime (γ′) strengthening phase around the γ′ solvus temperature; these aid the design of primary aging heat treatments to develop either uniform or bimodal microstructures of the γ′ phase. Under creep conditions in the temperature range 1023 K to 1123 K (750 °C to 850 °C), with stresses between 235 to 520 MPa, the creep performance is best with a finer and uniform γ′ microstructure. On the other hand, the OP TMF performance improves when the γ′ precipitate size is larger. Thus, the micromechanical degradation mechanisms occurring during creep and TMF are distinct. During TMF, localized shear banding occurs with the γ′ phase penetrated by dislocations; however, during creep, the dislocation activity is restricted to the matrix phase. The factors controlling TMF resistance are rationalized.

The microstructural evolution of uncoated single crystal superalloys is modeled taking into account the interplay between oxide growth and substrate response. Experimental investigations demonstrate that gamma' fraction of specimens with thicknesses less than 1 mm are strongly affected by oxidation. A model based on thermodynamic and kinetic data only, is presented calculating the growth kinetics of oxide scales and the resulting influence on microstructure evolution of the substrate. The model combines models for oxide growth and substrate response. Currently the main focus is on alumina (Al2O3) scale growth as it is the most important oxide for long term behavior. A dynamic growth parameter is used to describe the growth rate of the alumina scale. The model predicts the distribution of the alloying elements as well as the evolution of the generated phases as functions of depth and oxidation time. The model has been applied to three different alloys: the strong alumina forming alloy Rene N5, the moderate alumina forming alloy M247LC SX and the weak alumina forming alloy SCA425+. Since the gamma' fraction is one of the most relevant factors for high temperature creep properties, the present work concentrates on the calculation of the time and space dependent gamma' precipitate profile, which is most important for thin wall specimens. The predictions have been verified with very good agreement at an oxidation temperature of 980 degrees C with respect to alumina scale growth and gamma' fraction distribution. Predicted and measured alumina scale growth and gamma' fraction distribution for oxidation at 980 degrees C are in very good agreement.

Thermomechanical fatigue (TMF) in superalloys is growing in importance due to the introduction of advanced cooling systems but also due to the changes in demand and competition within the power generation market; this is requiring many power plants to operate under cyclic conditions. In this paper the TMF behaviour of three different single crystal nickel-based superalloys are compared. It is demonstrated that the deformation and damage mechanisms occurring during TMF are rather different from those traditionally reported for creep or isothermal fatigue. In all cases examined, the deformation is localized within a rather small number of deformation bands. While these bands were found to consist mainly of micro-twins in some alloys, in others they might be better described as slip or shear bands. Furthermore, in some circumstances these bands are prone to recrystallization. In CMSX-4, the intersection points of twins of different orientation act as initiation sites for this process. In the SCA425 alloy – of smaller gamma’ content, lower creep resistance and less great oxidation resistance – twinning is observed infrequently; however the deformation is still very localized and in the distorted gamma-gamma’ microstructure, along the shear bands, recrystallization is observed. Furthermore the recrystallization is enhanced by oxidation due to the development of a gamma’-depleted zone. In CMSX-4, TCP phases precipitated during long term ageing cause a more dispersed deformation behaviour which prevents recrystallization. Our findings confirm the importance of an inhomogeneous microstructure for good TMF resistance.

In this paper, the factors influencing the oxidation resistance of superalloys are studied. A model is proposed by which the Al2O3-forming properties of a given composition can be estimated, based upon the thermodynamic and kinetic factors influencing scale growth. The numerical modelling is tested by experimental work on a number of compositional variants of the newly-developed SCA425+ superalloy, which contains appreciable quantities of Cr. The modelling is shown to be in broad agreement with experiment. The effects of Al, Cr and Si on the oxidation resistance of this class of alloy have been rationalised.