Elsevier

Scripta Materialia

Volume 39, Issue 7, 4 September 1998, Pages 991-997
Scripta Materialia

Articles
Deformation mechanism for high temperature creep of a directionally solidified nickel-base superalloy

https://doi.org/10.1016/S1359-6462(98)00255-3Get rights and content

Introduction

Modern nickel-base superalloys, which contain a high volume fraction of hard cubical γ′ precipitates embedded coherently in a softer γ matrix, are used as turbine blade material due to their good resistance to creep deformation at high temperature. Many past investigations had found that the creep deformation mechanism in polycrystalline 1, 2, directionally solidified 3, 4, and single crystal 5, 6 superalloys were different in the high and low applied stress regimes. At high temperature, the minimum (or steady-state) creep rate is generally represented as a function of applied stress (σa) and temperature (T) by a power law [7]: ε̇=Aσanexp(−Qa/RT) where A, R is constant. For the simple alloys, the value of the apparent stress exponent (n) is between 4 and 6, and of the apparent activation energy for creep (Qa) is close to that for volume self diffusion (QSD). For particle hardened alloys, however, the much higher values (6–40) of n are found, and the compatible values of Qa are also between 2 and 3 times the value of QSD [8]. Some rationalization of these difficulties can be obtained by expressing the equation (1) in terms of an effect stress rather than the applied stress: ε̇=A(σa−σb)pexp(−Qb/RT) where σb is threshold stress (internal-stress [9], or friction stress [10]). The modified power law (eqn. 2) gives the values of p ∼ 4 and Qb ∼ QSD.

In this present study, we will analyze the relationship between the steady-state creep rate and the applied stress of DZ17G, a modern DS cast nickel-base superalloy used for producing turbine blades and vanes for aeroengine applications. On the basis of TEM observations and calculations of the threshold stress for different deformation mechanism, we finally propose the deformation mechanism for creep of DZ17G superalloy at high temperature.

Section snippets

Experimental procedure

The testing DZ17G superalloy in the present work was produced by withdrawal process at the speed of 7 mm/min in a vacuum furnace, to get round bars with 16 mm diameter and 130 mm length. The chemical composition (in wt%) is: 0.17 C, 8.96 Cr, 9.72 Co, 3.88 Mo, 0.68 V, 0.015 B, 5.38 Al, 4.79 Ti, 0.23 Fe and the balance Ni. The bars were solution treated at 1220°C for 4 hours followed by air cooling and aged at 980°C for 16 hours followed by air cooling. The characteristics of DZ17G alloy had

Results

The dependence of steady-state creep rate on applied stress in DZ17G superalloy is displayed in Figure 1. It is found that the value of the apparent stress exponent is different at the high and low stress regions. At high applied stresses (more than 280 MPa), n is between 9.4, while n is 6.8 at low stresses. It is generally thought that the value of n can represent the activation creep mechanism 1, 2, 3, 4, 5, 6. We can conclude that the transition from one creep mechanism in high stress

Discussion

With the apparent stress exponent and TEM observations, it is suggested that the dislocation climb by-pass is dominant creep deformation mechanism in the low applied stress range and the dislocation cutting is dominant in the high applied stress range. This transition of creep deformation mechanism can be interpreted in terms of the modified Dorn power law, i.e., equation (2), in which σb represents the threshold stress for different creep deformation mechanism.

For particle hardened system,

Summary

The steady-state creep behavior of a DS nickel-base superalloy (DZ17G) have been investigated in terms of a Dorn creep law. Both the apparent stress exponents measured and TEM observations indicate that the creep mechanism changes from the dislocation climb around γ′ precipitates at low applied stress to the dislocation cutting through γ′ precipitates at high applied stress. This transition can be interpreted by a modified Dorn creep law involving a threshold stress term. The theoretical values

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