UNUSUAL SEGREGATION BEHAVIOR OF BORON AND ITS EFFECT ON SEGREGATION OF SULFUR IN NICKEL

The co-segregation of B and S to the surface and grain boundaries of Ni was studied by SAES and SIMS. B showed an unusual segregation behavior wich resulted in the precipitation of Ni3B in the near surface layer and a continuous monolayer type B segregation at the grain boundary. The S segregation was not influenced by the presence of B and resulted in a uniform monolayer surface and grain boundary coverage. Analysis of hydrogen charged samples fractured in situ suggests that B segregated to the boundaries has a beneficial effect on their strength and also reduces the negative effect of S segregation

ULTRASONIC INVESTIGATION OF NIOBIUM DEUTERIDES AT LOW TEMPERATURES

The ultrasonic attenuation measured in NbD0.83 and NbD0.86 deuterides using the C11 mode has been found to show two anelastic relaxation processes at low temperatures (T1 = 156 K, T2 = 250 K for f = 10 MHz). Their activation energies and limit relaxation times are W1 = 0.18 eV, W2 = 0.37 eV t01 = 3x10-14s, t02 = 5x10-16s. Such relaxations show a Curie-Weiss temperature dependence of their relaxation strength and are believed to be due to stress-induced hydrogen density fluctuations occuring within the ß and µ (or ν) forms of the deuteride phase, respectively. Anomalies, which are probably related to the ß-µ and ß-ν phase transitions, are observed both in the elastic constant C11 and in the attenuation. The mobility of D within the different phases as well as the nature of the transitions observed are discusse

ELASTIC CONSTANT SOFTENING AND SHORT-RANGE ORDERING PHENOMENA IN METAL-HYDROGEN SYSTEMS

Stress induced short-range order changes, which give rise to anelastic relaxation processes (Zener effects) have been found to occur in several metal-hydrogen systems, such as Pd-H, Pg-Ag-H, Nb-H and Zr-H. The main results obtained so far in various Me-H systems will be reviewed and the perspectives for further work examined. Particular attention will be paid to the Nb-H system where elastic constant softening effects in the α' phase have been observed on approaching the α'→β transition temperature. The composition dependent elastic moduli changes were shown to consist of a temperature independent and a temperature dependent part. The temperature independent changes were analyzed on the basis of contributions due to volume changes and to changes in the electronic structure of the alloy. The temperature dependent contributions to the elastic moduli changes were each associated with an acoustic attenuation and exhibited a significant isotopic dependence. It was shown that the results are consistent with a relaxation due to stress-induced changes in the short-range order of the H interstitials i.e. a Zener relaxation

ULTRASONIC ATTENUATION AND VELOCITY STUDIES IN THE α-α SOLID SOLUTION PHASES OF THE Nb-H SYSTEM

Data are reported on the elastic constants and ultrasonic attenuation of a single crystal of Nb determined as a function of the hydrogen content (0 ≤ H/Nb ≤ 0.71) at 528 K and as a function of temperature (380 [MATH] ≤ 528 [MATH]) for H/Nb = 0.31, 0.45, 0.53 and 0.71. A relaxation process is observed which appears to be a Zener-type of effect

RESISTANCE STUDIES OF HYDROGEN DIFFUSION IN NIOBIUM

Hydrogen diffusion in Nb was studied using thermotransport to establish concentration gradients in a periodic width modulated sample. The resistance of the sample was measured as a function of time, by a Kelvin bridge circuit, as the perturbed hydrogen distribution returned to uniform concentration. Results for a H/Nb = 0.001 sample, measured in the temperature range 300 K and 180 K show the diffusivity of H to be two to four times larger than that measured by the Gorsky effect. When fitted to the Arrhenius relation, Q = 0.078 ± .001 eV/atom and D0 = 3.0 ± 0.7 x 10-4 cm2/s

HYDROGEN SEGREGATION AND DIFFUSION AT GRAIN BOUNDARIES

The segregation of hydrogen at grain boundaries in Ni has been shown to be dependent on the nature of the boundary using SIMS techniques. This contrasts to hydrogen segregation at free surfaces which appears to be less sensitive to the crystallography of the surface. The increased H concentration due to segregation occurs over appreciable distances from the surface or grain boundaries. The diffusivity of H along grain boundaries has been determined using SIMS techniques and correlated with studies of fracture kinetics