H. Zou

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.

α-Zr self-diffusion anisotropy

Self-diffusion coefficients (D) have been measured in nominally pure (NP) α-Zr single crystals (= 50 ppm Fe) in the range 867–1107 K, in directions either parallel (Dpa) or perpendicular (Dpe) to the c-axis. Measurements were also made on high-purity (HP) α-Zr single crystals (< 1.0 ppm Fe) at 1110 K. The diffusion profiles were determined by sectioning and radio-tracer (95 Zr) counting. Sectioning was done with a sputtering device, or a microtome (some NP experiments at 1107 K). D values for NP Zr are about an order of magnitude higher than the corresponding values for HP Zr. Diffusion anisotropy is complicated. The sputter-sectioned NP Zr specimens show increasing anisotropy ratios (AR = Dpa/Dpe), from 1.0 to 3.2, with decreasing temperatures, whereas AR = 0.53 for both the microtome-sectioned NP and sputter-sectioned HP Zr: the low AR value is consistent with expectations based on intrinsic self-diffusion in hcp metals with c/a < 1.633.

Oxygen diffusion in α-Zr single crystals

Abstract Oxygen diffusion coefficients, D , have been measured in α -Zr single crystals in directions both parallel and perpendicular to the c -axisThe measurements, made in the interval 610–870 K, show that diffusion anisotropy is weak and that D is little affected by specimen impurity content. The values determined here are in good agreement with the bulk of previous literature data for the same temperature interval but they are about ten times larger than corresponding values found in a very recent AES study.

The correspondence between self-diffusion properties and melting temperatures for α-Zr and α-Ti

Abstract Determinations of hypothetical hcp (α-phase) melting temperatures (Tmα) for Zr and Ti have been updated and the results considered in terms of common correspondences between self-diffusion properties of the elements and their melting temperatures (Tm). The present Tmα values for Zr and Ti are 1990 ± 6 and 1707 ± 4 K, respectively. The value for Zr is significantly higher than comparable values found earlier. These Tmα values and current data for self- and substitutional diffusion in α-Zr and α-Ti lead to reasonable accord with a common self-diffusion/Tm relationship for metals.

A Mössbauer study of single-crystal Zr-0.065 at% 57Fe

Abstract Single crystals of Zr-0.065 at% Fe (95% 57Fe), from a common parent, have been studied by Mossbauer spectroscopy, using both transmission and conversion-electron modes. In the “as grown” state the Fe is present as the meta-stable phase, t-Zr2Fe. Vacuum annealing, in the range 795–973 K, results in gradual decomposition of t-Zr2Fe and the diffusion of Fe from the bulk to the surface, where it forms the stable Zr3Fe phase. The kinetics of Zr3Fe formation show fair accord with the known diffusion properties of Fe in α-Zr. Some results for other dilute Zr(Fe) single crystals are reported.

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.

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.