Primary creep of Ni₃(Al, Ta) in the anomalous flow regime

Abstract

The primary creep behavior of single-slip oriented Ni₃(Al, Ta) has been characterized at low temperatures in the anomalous flow regime. For temperatures ranging from 20 to 200 °C, transient creep leading to eventual exhaustion has been measured at all stresses. The decline in the creep rate has been quantitatively shown to occur more quickly than in common metals, as the decline in the creep rate is faster than predicted by the logarithmic creep law. In addition, the temperature dependence of the primary creep behavior is consistent with the flow stress anomaly, as the measured amount of creep strain at a fixed stress decreases with increasing temperature.

Introduction

In this paper we present the results of a series of low-temperature tension creep experiments performed on single-slip oriented Ni₃(Al, Ta) single crystals. We have tested these crystals at temperatures between 20 and 200 °C to further explore the active deformation mechanisms at low temperatures in the anomalous flow regime, where the flow stress and work-hardening rate are observed to have an anomalous temperature dependence. These test temperatures range from 0.18 to 0.28 of the melting temperature. Creep experiments are usually not performed at such low homologous temperatures, because the mechanical response of most metals is usually insensitive to time below temperatures of 0.3 T_m. However, time-dependent plastic flow does occur in all metals, even at temperatures as low as 4 K [1], [2], [3]. Low-temperature creep is often not considered as a viable failure mechanism, as creep strains at low temperatures are small and quickly terminate, so that failure due to low-temperature creep is unlikely (but for problems of dimensional stability, such creep effects can be of engineering importance). In the context of the present paper, creep experiments at low homologous temperatures provide a conceptually simple test method to investigate the active deformation mechanisms, and it is in this spirit that we explore the low-temperature creep properties of Ni₃Al.

In contrast to conventional high-temperature creep experiments in which the applied stress is typically below the yield stress, most low-temperature creep experiments are performed at stresses above the yield stress in order to produce a measurable amount of creep strain. Upon loading, a sample undergoes plastic deformation characterized by two components: an instantaneous part and a time-dependent part. In general, the time-dependent strain is small compared to the instantaneous plastic response; however, in measuring the creep properties we ignore the initial strain component and focus solely on the time-dependent plastic response.

Equations that describe the form of the creep curve have been empirically derived for low-temperature creep. Unlike high-temperature creep where the creep processes may be characterized by a single value—the steady-state creep rate—the relations that characterize low-temperature creep must reflect the continuous decline in the creep rate with time. For most metals at low temperatures, it is found that the change in the creep rate ($\text{γ}\text{̇}$) with time (t) can be represented by the following equation [4]:

$$\text{γ}\text{̇}\text{=At}{}^{\mathrm{-n}}$$

where A and n are constants, and where 0⩽n⩽1. In the special case of logarithmic creep, the constant n is equal to 1, and integration of Eq. (1) produces the logarithmic creep law, which can be expressed in the modified form shown below:

$$\text{γ=γ}{}_0\text{+α}\text{ln}\text{βt+1}\text{,}$$

where γ is the creep strain, γ₀ is the instantaneous strain, α and β are constants, and t is measured from the time at which the strain is equal to γ₀. Eq. (2) clearly demonstrates that the creep strain declines both quickly and monotonically with time.