Zhongwen Yao

Heavy-ion irradiations of Fe and Fe–Cr model alloys Part 2: Damage evolution in thin-foils at higher doses

A study of heavy-ion damage in Fe and Fe–Cr alloys started in Part 1 1 was continued with an investigation of damage development in UHP Fe and Fe–8%Cr at higher doses up to 2 × 1019 ions m−2 (∼13 dpa). In thin-foil irradiations with 150 keV Fe+ ions at 300°C and room temperature (RT), more complex microstructures started to develop in thicker regions of the foils at doses greater than about 2 × 1018 ions m−2, apparently involving cooperative interaction, alignment and coalescence of smaller loops. First strings of loops all with the same ½⟨111⟩ Burgers vectors formed. In UHP Fe irradiated at 300°C the damage then developed into colonies of resolvable interstitial loops with ½⟨111⟩ Burgers vectors. By a dose of 2 × 1019 ions m−2, large (several hundred nanometre) finger-shaped loops with large shear components had developed by the growth and subsequent coalescence of smaller loops. Similar but finer-scale damage structures developed in UHP Fe irradiated at RT and in Fe–8%Cr irradiated at both RT and 300°C.

The temperature dependence of heavy-ion damage in iron: A microstructural transition at elevated temperatures

A transition is reported in the dislocation microstructure of pure Fe produced by heavy-ion irradiation of thin foils, which took place between irradiation temperatures (T irr) of 300°C and 500°C. At T irr ≤ 400°C, the microstructure was dominated by round or irregular non-edge dislocation loops of interstitial nature and with Burgers vectors b = ½ ⟨111⟩, although interstitial ⟨100⟩ loops were also present; at 500°C only rectilinear pure-edge ⟨100⟩ loops occurred. At intermediate temperatures there was a gradual transition between the two types of microstructure. At temperatures just below 500°C, mobile ½⟨111⟩ loops were seen to be subsumed by sessile ⟨100⟩ loops. A possible explanation of these observations is given.

Brittle-ductile transitions in polycrystalline tungsten

The strain rate dependence of the brittle-to-ductile transition (BDT) temperature was investigated in notched and un-notched miniature bars made of high-purity polycrystalline tungsten and in notched bars of less-pure sintered material. The activation energy, E BDT, for the process controlling the BDT in pure tungsten was equal to 1.0 eV both in un-notched and notched specimens, though the brittle–ductile transition temperature, T BDT, was ≈ 40 K lower at each strain rate for the un-notched samples, indicating that the activation energy, E BDT, is a materials parameter, independent of geometrical factors. The experimental data obtained from pure tungsten are described well by a two-dimensional dislocation-dynamics model of crack-tip plasticity, which is also discussed. For sintered tungsten, E BDT was found to be 1.45 eV; T BDT at a given strain rate was higher than in the pure tungsten by ≈ 90 K, suggesting that the BDT in tungsten is very sensitive to impurity levels.

Irradiation-induced stacking fault tetrahedra in fcc metals

Irradiation induces the formation of stacking fault tetrahedra (SFTs) in a number of fcc metals, such as stainless steel and pure copper. In order to understand the role of the materials parameters on this formation, pure Cu, Ni, Pd and Al, having a respective stacking fault energy of 45, 125, 180 and 166 mJ m−2, have been irradiated with high energy protons to a dose of about 10−2 dpa at room temperature. The irradiation-induced microstructure has been investigated using transmission electron microscopy. All irradiated metals but Al present SFTs. The proportion of perfect, truncated and grouped SFTs has been determined. The SFT energy as a function of size has been calculated using elasticity of the continuum, with respect to the energy of a number of other possible defect configurations. It appears that the key parameters are the stacking fault energy and the shear modulus. Their implication on the formation and stability of the SFTs is discussed.

The microstructure and tensile properties of pure Ni single crystal irradiated with high energy protons

The microstructure and tensile behavior of pure single crystalline Ni irradiated with 590 MeV protons are investigated. First results have been obtained for a damage level of 0.13 dpa at room temperature and 0.002 dpa at 523 K. The irradiation induced defect microstructure observed under transmission electron microscopy consists of loops and stacking fault tetrahedra in a lower density. Tensile tests were performed at room temperature and show observable radiation induced hardening already at 0.002 dpa. Dislocation channels and cells were observed and the transition from the initial channel formation to the development of the deformation cells is investigated. First results are presented here.

Spatial ordering of nano-dislocation loops in ion-irradiated materials

Defect microstructures formed in ion-irradiated metals, for example iron or tungsten, often exhibit patterns of spatially ordered nano-scale dislocation loops. We show that such ordered dislocation loop structures may form spontaneously as a result of Brownian motion of loops, biased by the angular-dependent elastic interaction between the loops. Patterns of spatially ordered loops form once the local density of loops produced by ion irradiation exceeds a critical threshold value.

Preparation of TEM samples of ferritic alloys

We describe techniques for electropolishing irradiated ferritic specimens for examination under the TEM in situations where the foil quality is of utmost importance. First, we describe some modifications to the standard technique for making plan-view specimens aimed at optimizing the foil quality. Second, we describe a technique for making plan-view specimens from a region of buried damage in a specimen irradiated with 2 MeV Fe(+) ions.

Cavity morphology in a Ni based superalloy under heavy ion irradiation with cold pre-injected helium. I

In order to understand radiation damage in the nickel based superalloy Inconel X-750 in thermal reactors, where (n, α) transmutation reaction also occurred in addition to fast neutron induced atomic displacement, heavy ion (1 MeV Kr2+) irradiation with pre-injected helium was performed under in-situ observations of an intermediate voltage electron microscope at Argonne National Laboratory. By comparing to our previous studies using 1 MeV Kr2+ irradiation solely, the pre-injected helium was found to be essential in cavity nucleation. Cavities started to be visible after Kr2+ irradiation to 2.7 dpa at ≥200 °C in samples containing 200 appm, 1000 appm, and 5000 appm helium, respectively, but not at lower temperatures. The cavity growth was observed during the continuous irradiation. Cavity formation appeared along with a reduced number density of stacking fault tetrahedra, vacancy type defects. With higher pre-injected helium amount, a higher density of smaller cavities was observed. This is considered to be the result of local trapping effect of helium which disperses vacancies. The average cavity size increases with increasing irradiation temperatures; the density reduced; and the distribution of cavities became heterogeneous at elevated temperatures. In contrast to previous characterization of in-reactor neutron irradiated Inconel X-750, no obvious cavity sink to grain boundaries and phase boundaries was found even at high doses and elevated temperatures. MC-type carbides were observed as strong sources for agglomeration of cavities due to their enhanced trapping strength of helium and vacancies.

Deformation mechanism study of a hot rolled Zr-2.5Nb alloy by transmission electron microscopy. I. Dislocation microstructures in as-received state and at different plastic strains

Thin foil dog bone samples prepared from a hot rolled Zr-2.5Nb alloy have been deformed by tensile deformation to different plastic strains. The development of slip traces during loading was observed in situ through SEM, revealing that deformation starts preferentially in certain sets of grains during the elastic-plastic transition region. TEM characterization showed that sub-grain boundaries formed during hot rolling consisted of screw ⟨a⟩ dislocations or screw ⟨c⟩ and ⟨a⟩ dislocations. Prismatic ⟨a⟩ dislocations with large screw or edge components have been identified from the sample with 0.5% plastic strain. Basal ⟨a⟩ and pyramidal ⟨c + a⟩ dislocations were found in the sample that had been deformed with 1.5% plastic strain, implying that these dislocations require larger stresses to be activated.

Deformation mechanism study of a hot rolled Zr-2.5Nb alloy by transmission electron microscopy. II. In situ transmission electron microscopy study of deformation mechanism change of a Zr-2.5Nb alloy upon heavy ion irradiation

The effect of heavy-ion irradiation on deformation mechanisms of a Zr-2.5Nb alloy was investigated by using the in situ transmission electron microscopy deformation technique. The gliding behavior of prismatic 〈a〉 dislocations has been dynamically observed before and after irradiation at room temperature and 300 °C. Irradiation induced loops were shown to strongly pin the gliding dislocations. Unpinning occurred while loops were incorporated into or eliminated by 〈a〉 dislocations. In the irradiated sample, loop depleted areas with a boundary parallel to the basal plane trace were found by post-mortem observation after room temperature deformation, supporting the possibility of basal channel formation in bulk neutron irradiated samples. Strong activity of pyramidal slip was also observed at both temperatures, which might be another important mechanism to induce plastic instability in irradiated zirconium alloys. Finally, {011¯1}⟨01¯12⟩ twinning was identified in the irradiated sample deformed at 300 °C.