A high temperature deformation mechanism map for the high performance Ni-base superalloy GTD-111

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Abstract

GTD-111, used for gas turbine first stage blades, is a new, high performance and less investigated Ni-based superalloy. This alloy, with high volume fraction of γ′ phase, has excellent creep resistance. It is believed that several creep deformation mechanisms operate at various combinations of temperature and stress. The regions of the controlling mechanisms can be described in terms of a stress-temperature deformation mechanism map. To construct a creep mechanism map for GTD-111 superalloy, single-specimen creep tests at constant stress/variable temperature and constant temperature/variable stress were conducted. The map consists of two separate dislocation controlling mechanism fields; a stacking fault and anti-phase boundary region, and a dislocation climb region. Also, it was clear that a diffusional creep mechanism operated at lower stresses. Microstructural analysis using TEM on creep tested specimens in the two dislocation creep fields confirmed the deformation mechanism map for GTD-111 superalloy.

Introduction

The nickel-base superalloy GTD-111, which was designed in the mid-1970s by the General Electric company, is used as a blade material in the first row high pressure stage of gas turbines. It is reported [1] that this alloy is superior by about 20°C in creep-rupture strength in comparison with another Ni-base superalloy, IN-738LC. IN-738LC has a chemical composition and microstructure similar to that of GTD-111 as indicated in Table 1. In addition, the low-cycle fatigue resistance of GTD-111 is better than that of IN-738LC. Gaudenzi et al. [2] studied coating resistance on several alloys using the Dean test and holding at high temperatures for long periods and showed that hot oxidation and corrosion resistance of GTD-111 is much higher than that of IN-738LC. The alloy contains refractory elements such as Mo, W, Ta, Cr and Co to prevent local hot corrosion [3]. GTD-111 superalloy, in fact, is a modification of Rene 80 and has a multi-phase microstructure consisting fcc γ matrix, bimodal γ′ precipitates (primary and secondary), γγ′ eutectic, carbides and a small amount of deleterious phases such as σ, δ, η and Laves [4].

In spite of the importance of GTD-111 in manufacturing of hot components such as the first stage blades of gas turbines, unfortunately there is limited investigation and data about this high performance superalloy.

The Ni-base superalloy GTD-111 expends almost all of its life in creep deformation. Therefore, it is worth of studying its creep deformation mechanisms at various combinations of stress and temperature. Several papers related to creep in superalloys indicate an increase in the slope of the log ε–log σ at high stresses and a decrease in the slope of the log ε−1/T with decreasing temperature [5], [6], [7], [8], [9].

This work used single-specimen creep tests and transmission electron microscopy of specimens crept at different conditions to construct a deformation mechanism map for the alloy. Deformation mechanism maps provide a powerful tool to develop high temperature alloys, to achieve more resistant alloys and to rationalize the creep behavior of these alloys. The mechanical data and the corresponding microstructures in the superalloy GTD-111 obtained after deformation under various conditions are presented.

Section snippets

Experimental procedure

Chemical composition of as received Ni-base superalloy GTD-111 have been determined by means of X-ray fluorescent (XRF), Optical Emission Spectroscopy (OES) and Atomic Absorption. Grain size measurement was performed using Mean Lineal Intercept method. The primary γ′ volume percent was determined using SEM, image analyzer and a stereo microscope.

Seven millimeter diameter rods were cut from a standard heat-treated gas turbine blade made from GTD-111 with equiaxed grains between 0.67 and 1.56 mm

Results and discussion

The chemical composition of the superalloy GTD-111 is presented in Table 1. Study of microstructure of the alloy, at standard heat treated condition, by SEM shows that the structure consists of primary large cube γ′ (with edge dimension of 0.8 μm), secondary fine spherical γ′ (with 0.1 μm diameter), γγ′ eutectics and carbides distributed almost uniformly in the γ matrix. The γ′ phase is a superlattice possessing the Ll2-type structure with a nominal composition of Ni3(Al, Ti). Its long range

Conclusions

From the study of creep behavior of the cast nickel base superalloy GTD-111 the variation of the steady-state creep rate with stress and temperature could be expressed as an Arrhenius type relationship.

The creep behavior is in agreement with the behavior of precipitation strengthened alloys which on decreasing stress show a transition from more stress-sensitive cutting of γ′ precipitates to less stress-sensitive climb and diffusion. The deformation mechanism map constructed by the data obtained

Acknowledgements

The authors wish to express appreciation to Mavadkaran Eng. Co. for supporting of this project. Also, Tavanir, Deputy of Research and Technology, is gratefully acknowledged for providing the material.

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