Anelastic relaxation processes due oxygen in Nb–3.1 at.% Ti alloys

Abstract

In the last 50 years several studies have been made to understand the relaxation mechanisms of the heavy interstitial atoms present in transition metals and their alloys. Internal friction measurements have been carried out in a Nb–Ti alloy containing 3.1 at.% of Ti produced by the Materials Department of Chemical Engineering Faculty of Lorena (Brazil), with several quantities of oxygen in solid solution using a torsion pendulum. These measurements have been performed by a torsion pendulum in the temperature range from 300 to 700 K with an oscillation frequency between 0.5 and 10 Hz. The experimental results show complex internal friction spectra that have been resolved, into a series of Debye peaks corresponding to different interactions. For each relaxation process it was possible to obtain the height and temperature of the peak, the activation energy and the relaxation time of the process.

Introduction

In bcc metals, such as niobium and niobium alloys, interstitial solute atoms cause local strain distortion of tetragonal symmetry. Thus, an interstitial atom in this lattice forms an elastic dipole that can lie along one of the three cubic axes. When an external stress is applied, differences in the free energy for different dipole orientations occurs and these interstitial atoms are redistributed among the different sites, causing energy loss, i.e. internal friction. Each species of interstitial atoms gives rise to a maximum in the internal friction spectra, which may be observed at a different temperature, depending on measuring frequency (thermally activated process). This maximum is called Snoek effect [1].

In diluted interstitials alloys (c<0.01 at.%), the Snoek peaks may be described as a single Debye peak [2]. In concentrated alloys, the peaks are broadened, obviously owing to interactions between the interstitial atoms. If more than one species of interstitial atoms is present, the analysis of internal friction spectra becomes rather complicated. The broadening was interpreted by assuming configurations of clusters of interstitial atoms, which should cause additional discrete loss maxima at higher temperatures than the Snoek maximum [3], [4], [5]. Weller et al. [6], [7], [8] questioned the clustering model by introducing a continuos distribution of relaxation times which should correspond a long-range interaction models.

The presence of substitutional solutes in bcc metals containing interstitial impurities modifies the stress-induced ordering of the interstitial atoms due, in general, to the formation of substitutional-interstitial complexes [9]. The purpose of this work was to investigate the influence of titanium on niobium containing oxygen in solid solution and the consequential effect on the mechanical properties of the alloy.