J.A. Roy

The yield strength of the L12 phase Zr3Al

Abstract The yield strength of well-annealed, polycrystalline Zr3 Al has been measured versus temperature, strain rate and grain size. The results show that at temperatures below ∼- 875 K (for material of 5 μm grain size) yielding is controlled by dislocation generation and by the subsequent transmission of slip across grain boundaries. At higher temperatures, yielding is controlled by grain boundary sliding. Tests at 295 and 675 K show that the yield strength, σy, obeys the expression σ y = σ 0 + k y d −1 2 , where σ0 and ky are experimental parameters and d is a measure of grain size: σ0 increases with temperature, while ky decreases and this causes σy to increase with temperature for d >dc, but to decrease for d ϵ L = λd −1 2 , where λ is an experimental parameter which increases with increasing temperature. λ. is equivalent to k y θ where θ is the work-hardening rate at low strains. The positive temperature response of σ0 is discussed in terms of current theories of yielding in the Ll2 alloys.

The solid solubility of Fe in α-Zr : a secondary ion mass spectrometry study

Abstract Secondary ion mass spectrometry techniques have been used to determine the terminal solid solubility (TSS) of Fe in α-Zr. Single crystals of nominally pure and Fe-doped α-Zr were annealed in the temperature range 770–1100 K to promote equilibration of Fe between surface Zr 3 Fe precipitates, or β-Zr(Fe), and α-Zr. The results are fair in overall agreement with a recent investigation, based on thermoelectric power measurements, but they differ in detail. In particular this work indicates two regions of temperature dependence: above 930 K the TSS (ppma) is given by C Fe = 1.56 × 10 10 exp (−1.70 ± 0.05 eV / kT ), at lower temperatures a weaker temperature dependence is associated with extrinsic effects. In addition, the eutectoid temperature is shown to lie between 1063 and 1068 K.

Work-hardening of the L12 phase Zr3Al

Abstract Experiments have established that the work-hardening rate, ∂σ ∂ϵ . of well-annealed polycrystalline Zr3Al, when normalized with respect to the shear modulus. G, is temperature and strain rate insensitive below a critical temperature, Tc, but that it decreases with increasing temperature above Tc, Tc increases with increasing strain rate according to the expression T c = Q k ln( e 0 e ) where Q = 0.6 ± 0.1 eV, k is Boltzmanns constant, e 0 ~ 1.2 × 10 8 s −1 and e is the strain rate. This behaviour is attributed to thermally assisted jog dragging at T ⩾ Tc. The experiments have also shown that the athermal work-hardening rate is unusually high; i.e. ∂σ ∂ϵ = G 10 at ϵ = 0.1. This characteristic is attributed to low slip multiplicity resulting from planar glide. Finally, the experiments have established that, under conditions of glide-controlled deformation, the work-hardening rate is independent of grain size. This is another aspect of slip planarity and reflects the absence of a grain size effect on dislocation density and the absence of multiple slip in the vicinity of grain boundaries.

The notch-sensitivity of ordered Zr3Al

Abstract Experiments using notched tensile specimens have established that ordered Zr 3 Al is inherently notch sensitive. The notchsensitivity ratio, NSR, increases from ⋞0.7 at temperatures ⋞435 K to ∼-0.9 at temperatures ⋟575 K. Grain size and trace alloying with boron have negligible effects on the results. Neutron irradiation, on the other hand, suppresses notch-sensitivity and causes notch-strengthening. The NSR data are correlated with fracture mechanisms.

Formation and stability of fe-rich precipitates in dilute Zr(Fe) single-crystal alloys

The formation and stability of Fe-rich precipitates in two α-Zr(Fe) single-crystal alloys with nominal compositions I, 50 parts per million by atom (ppma) Fe, and II, 650 ppma Fe, have been investigated. Optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used to examine the characteristics of Fe-rich precipitates. The SEM and TEM micrographs showed that in as-grown alloy II, Zr2Fe precipitates were located at “stringers. ”Precipitates were not observed in as-grown alloy I. Annealing treatments below 700 °C, for alloy I, and 820 °C, for alloy II, resulted in the diffusion of excess Fe (above the α-phase solution limit) to the free surface with the subsequent formation of Zr3Fe precipitates in both alloys. Dissolution of Zr3Fe surface precipitates of alloy I (annealing above the solvus) left precipitate-like features on the surfaces. Zr2Fe precipitates in as-grown alloy II were readily dissolved by β-phase annealing.

A relationship between hardness and flow stress of ordered Zr3Al polycrystals

A relationship between hardness and flow stress for the ordered L1/sub 2/ phase Zr/sub 3/Al, a possible structural material for use in nuclear power reactors, is described. The test material was Zr-8.9 wt pct Al prepared from crystal-bar zirconium and aluminum of 99.999 pct purity by nonconsumable arc -melting. Buttons were sealed in Zircaloy-2 fuel sheathing and then hot-rolled at 1325/sup 0/K to 5 mm strip. To effect both the peritectoid transformation to form Zr/sub 3/Al, described previously, and grain growth, to be described separately, the rolled strip was annealed at a reduced pressure of < 2.6 x 10/sup -4/ Pa at temperatures between 1075 and 1260/sup 0/K for times from 2 to 1000 h. In this condition, the matrix was Zr/sub 3/Al which contained < 1 pct residual ..cap alpha..-Zr and Zr/sub 2/Al.

Diffusion of Hf and Nb in Zr19%Nb

Abstract Diffusion of Hf and Nb in large-grained bcc Zr19%Nb has been studied. Diffusion coefficients of Hf, D(Hf), were measured in the range 620–1173 K and D(Nb) was measured at 920 and 1167 K. The Elf diffusion profiles were determined by SIMS and the Nb profiles by microtome sectioning and radio-tracer counting. The Hf data show a smooth, temperature-dependent behaviour through the monotectoid temperature, 875 K, and may be characterised by D ≃ 10 −9 × exp(−1.4(eV/ kT )m 2 /s. D (Nb) tends to be lower than the corresponding values for D (Hf). Overall, diffusion of Hf and Nb are characteristic of diffusion in bcc Zr. Surface hold-up (oxide film) at low temperatures was overcome by using ion-implanted Hf diffusion sources. The results are compared with earlier work and discussed in terms of diffusion mechanisms and the β-phase transformation of commercial Zr2.5Nb.

The anisotropy of Hf diffusion in α-Zr

Abstract Hf diffusion coefficients (D) have been measured (∼ 870–1100 K) in directions parallel (Dpa)and perpendicular (Dpe) to the c-axis of double-faced, single-crystal specimens of both high purity (HP) and nominally pure (NP) α-Zr. The diffusion profiles were measured by secondary ion mass spectrometry. Hf diffusion in HP α-Zr is characterised by an activation energy of about 3.0 eV and a pre-exponential factor of about 10−5 m2/s The anisotropy ratio, Dpa/Dpe, is ∼ 1.0 for the NP specimens. A dependence of D on diffusion time/depth is indicated for some experiments on NP Zr.

Thermal etching of α-Zr single-crystal surfaces

Extensive thermal etching of α-Zr single crystals has been found to occur during high-temperature annealing (820°C) under ultra-high vacuum (< 1.0 × 10−7Pa). Two grades of material were examined, Z1, high purity; and Z2, nominally pure: levels of the “surface active” element, Fe, were about 1 and 50 ppma, in Z1 and Z2, respectively. In Z1, strong faceting occurred on a high-index surface (8° off the (1010) plane) and weak linear facets appeared on the (1010) plane. In Z2, etch pits and linear groove defects formed on the (1010) plane. Etch-pit formation may be promoted by Fe segregation to dislocations, Fe-rich precipitates were found at the bases of clusters of pits. Etch pit counts were consistent with dislocation densities of about 1.0 × 1011/m2. The (0002) plane remained comparatively flat, but Fe-rich, needle-shaped precipitates at 60° angles with each other were formed. An analysis of the results implies that the surface energies increase in the order (0002) < (1011) < (1010), and that the etching mechanism is surface diffusion.

Hf diffusion in dilute Fe-free Zr(Nb) alloys

Abstract Hf diffusion coefficients D have been measured in the α-phase of the Fe-free binary alloys Zr-1.0 at.% Nb and Zr-2.5 at.% Nb in the temperature range 830–1100 K. The D values for the two alloys are essentially indistinguishable and little different from extant Hf diffusion coefficients measured in Fe-free poly-crystalline α-Zr. The temperature dependence of D in the alloys is characteristic of intrinsic α-Zr bulk behaviour: The present Hf D values are much lower than corresponding values measured in commercial Zr-2.5 Nb. The difference is attributable to the influence of solid-solution Fe. In addition, the absence of a strong enhancement of Hf diffusion in α-Zr by Nb suggests that intrinsic Nb diffusion in α-Zr may not be controlled by a normal vacancy mechanism.