Prior Β-phase Research Articles

Effect of hot isostatic pressing processing parameters on microstructure and properties of Ti60 high temperature titanium alloy

Hot isostatic pressing parameters are critical to Ti60 high temperature titanium alloy castings which have wide application perspective in aerospace. In order to obtain optimal processing parameters, the effects of hot isostatic pressing parameters on defects, composition uniformity, microstructure and mechanical properties of Ti60 cast high temperature titanium alloy were investigated in detail. Results show that increasing temperature and pressure of hot isostatic pressing can reduce defects, especially, the internal defects are substantially eliminated when the temperature exceeds 920 °C or the pressure exceeds 125 MPa. The higher temperature and pressure can improve the microstructure uniformity. Besides, the higher pressure can promote the composition uniformity. With the temperature increases from 880 °C to 960 °C, α-laths are coarsened. But with increasing pressure, the grain size of prior-β phase, the widths of α-laths and α-colony are reduced. The tensile strength of Ti60 alloy is 949 MPa, yield strength is 827 MPa, and the elongation is 11% when the hot isostatic pressing parameters are 960 °C/125 MPa/2 h, which exhibits the best match between the strength and plasticity.

Analysis of hierarchical microstructural evolution in electron beam powder bed fusion Ti–6Al–4V alloys via time-of-flight neutron diffraction

Ti–6Al–4V alloys prepared via electron beam powder bed fusion (EB-PBF) generally exhibit a hierarchical microstructure consisting of columnar β-grains, an acicular α-matrix, and nanoscale β-precipitates at α-lath interfaces. In this study, we performed time-of-flight neutron diffraction measurements to investigate the texture and volume fraction of EB-PBF Ti–6Al–4V alloys via Rietveld texture analysis (RTA). The high-intensity neutron source enabled the analysis of the nanosized β-phase precipitates at the α-lath interfaces. The results indicate that the α- and β-textures of the as-built specimen can be ascribed to multiple Burgers orientation relationships among the prior β-phase, α-laths, and nanosized β-phase precipitates, which are not clearly understood. The as-built texture was maintained during post-processing, suggesting that texture manipulation in EB-PBF fabrication is important for controlling the crystallographic orientations of the built components, even after post-processing. RTA allowed a precise quantification of the β-phase fraction, which was found to have increased in post-processed specimens. The results would contribute to a comprehensive understanding of the microstructural evolution of such alloys, which is vital for the modeling and optimization of alloy performance.

Microstructure, mechanical and tribological properties of a Ti-5Cu alloy and a B4C/Ti-5Cu in situ composite fabricated by laser powder bed fusion

Both a novel Ti-5Cu alloy (in wt%) and a Ti-5Cu in situ composite containing 1 wt% B 4 C were fabricated by laser powder bed fusion, and their microstructures, mechanical and tribological properties were systematically investigated. The microstructure of the Ti-5Cu was mainly composed of α Ti laths, Ti 2 Cu and retained β phases, while the B 4 C/Ti-5Cu composite was composed of α Ti laths, Ti 2 Cu, TiC, TiB and TiB 2 phases. Both the Cu and B 4 C additions promoted a transition from columnar to equiaxed grains for prior-β and α phases. The B 4 C/Ti-5Cu composite was found to have a higher hardness (467 HV) than the Ti-5Cu (417 HV). Further, the composite exhibits a yield strength of 1100 MPa and an ultimate tensile strength of 1250 MPa. Values which are higher than those of the Ti-5Cu that exhibits a yield strength of 750 MPa and an ultimate tensile strength of 900 MPa. However, this strength increase comes at the expense of a reduction in elongation to failure from 6.2% to 1.5%. Even though the composite is significantly stronger, the two materials exhibited very similar wear rates ( ω B4C/Ti-5Cu = 4.95 × 10 −4 mm 3 N −1 m −1 and ω Ti-5Cu = 4.85 × 10 −4 mm 3 N −1 m −1 ). • The SLMed Ti-5Cu was mainly composed of α Ti laths, Ti 2 Cu and retained β phases. • The SLMed B 4 C/Ti-5Cu composite was composed of α Ti laths, Ti 2 Cu, TiC, TiB and TiB 2 phases. • Both the addition of Cu and B 4 C can refine the grain and promote columnar to equiaxed transition of prior-β phase. • The SLMed Ti-5Cu exhibit a strong combination of hardness, tensile properties and wear resistance. • Although the composite is significantly stronger owing to the addition of B 4 C, the two materials exhibited very similar wear rates.

Post-LOCA ductility assessment of Zr-Nb Alloy from 1100°C to 1300°C to explore variable peak cladding temperature and equivalent cladding reacted safety criteria

This study experimentally examines the variable PCT-ECR embrittlement criteria by conducting post-LOCA ductility assessments in the temperature range of 1100∼1300°C. Post-LOCA ductility assessments demonstrate the viability of variable PCT-ECR criteria, allowing either higher PCT for less oxidized specimens, or higher ECR limit for lower PCT specimens. The variable PCT-ECR criteria can be useful for high burnup fuel regulation. For potential burnup uprates that may lead to the violation of the current CP-ECR limit (18%) due to excessive steady-state oxidation, it may be possible to circumvent the regulation limit by allowing PCT less than the current limit (1204°C). This represents a new, yet more rational, safety envelop that may liberate nuclear reactor from sweeping conservatism associated with LOCA safety assessments. For the specimens oxidized at temperatures greater than 1200°C, ductile-brittle transitions take place even with substantially thicker prior-β layer (375 µm) and lower oxygen concentration compared to 1200°C specimens. As such, premature ductile to brittle transition occurs from the view point of oxygen uptake for steam annealing temperature above 1200°C. Premature embrittlement of 1250°C and 1300°C that cannot be explained in terms of the thickness of prior-β phase is relevant to promoted grain growth at elevated temperatures. Unlike lower temperature (≤1200°C), steam annealing temperature of 1250°C and 1300°C made grains continuously grow beyond the average grain size of 50 µm. The enhanced grain growth is considered to degrade cladding ductility as commonly observed in other HCP materials, in addition to solid-solution hardening of oxygen.

Grain selection and growth orientation of prior-β phase for Ti-6-4 during additive manufacturing: insights from a modeling perspective

The microstructure of Ti-6-4 components produced by additive manufacturing suffers from the coarse and elongated prior-β grain, which leads to a decrease of the tensile behavior and the occurrence of anisotropy. To understand and control the grain evolution, a multiscale simulation is applied to investigate the relationship between the grain selection, growth orientation, and the molten pool morphology with the different deposition layer numbers and processing parameters. The accuracy of the simulation is validated by experiments in both qualitative and quantitative ways. Results show that when the grain with unfavorable orientation loses the competitive growth with its neighbors, there will be a great chance that the blocked grain is eliminated in the following layer-and-layer deposition, which leads to the increase of the grain width. The size of the molten pool increases remarkably as the layer number increases, which lays a heavy burden on the stability of the molten pool. The analytical relationship between the molten pool morphology and the grain growth orientation is also deduced. The flat molten pool causes the grains with the direction close to the building direction to have greater survival potential. Besides, decreasing the line power energy shows little effect on the stability of the molten pool and the grain growth orientation, especially when the deposited layer number is large. The revealing mechanisms will help in understanding and further controlling the grain evolution.

Microstructure and Nanoindentation Studies of Hydridated Zircaloy-4 Claddings After High Temperature Oxidation

Abstract Zirconium-based alloys are one of the most significant materials in thermal-neutron reactor systems. With very low neutron capture cross section, good corrosion resistance, mechanical strength and resistance to neutron radiation damage, zirconium alloys are used for fuel claddings. Cladding materials are still improved and tested in normal as well as critical reactor conditions. Zircaloy-4 (Zr-1.5Sn-0.2Fe-0.1Cr) is used for west types of light-water reactors, Pressurized Water Reactors (PWR). In our study, Zircaloy-4 cladding tubes were high-temperature oxidized in steam at the series of temperatures from 950 up to 1425 °C to simulate PWR reaching severe accident conditions. To observe the influence of hydrogen (H) diffusing from the coolant water on oxidation process, the specimens with ∼1000 ppm H were compared to the specimens with almost no hydrogen content. Wave Dispersive Spectroscopy (WDS) and nanoindentation were performed in line profiles across the cladding wall. Both methods contributed to verify the pseudobinary Zircaloy-4/oxygen phase diagram with focus on determination of phase boundaries. The increase of oxygen concentration with increasing temperature was observed. Moreover, oxygen concentration profiles and related change in nanohardness and Young's modulus showed the effect of hydrogen on the cladding microstructure. Hydrogen dissolved in metallic matrix increases the oxygen solubility in prior β-phase, the specimens with 1000 ppm H showed the higher oxygen content at almost all temperatures. As well, material hardening was observed on specimens with 1000 ppm H with significant difference in β-phase, measured on specimens exposed to lowest and highest oxidation temperature. Thus, with increasing temperature and hydrogen content, increased oxygen solubility affects the cladding ductility.

Effect of cooling rate on the residual ductility of Post-LOCA Zircaloy-4 cladding

Effects of cooling rates on mechanical behavior of steam-oxidized Zircaloy-4 are systematically investigated with the cladding material oxidized to the current ductility-based regulatory limits (17% ECR at 1204 °C). Three different cooling rates associated with distinctively different quenching environments are tested; ambient air cooled (<7 °C/s), boiling water (∼400–500 °C/s), and room temperature water (∼1600–2000 °C/s). Cooling rates of post-LOCA Zircaloy-4 have (1) limited effects on average cladding residual ductility for non-hydrided cladding, (2) pronounced effects on statistical variation of ring compression test results, and (3) effects on fracture strength reduction on pre-hydrided cladding (765 wppm). The limited effect of cooling rate on average residual ductility of non-hydrided cladding is due to the cooling-rate insensitive resulting phase thicknesses and their oxygen contents, both of which dictate the level of cladding residual ductility. Homogenizing prior-β phase with finer lamellar structures and reduced α incursion, fast cooling (i.e., 25 °C water quenching) effectively reduces statistical variation of RCT results. The presented results support regulatory validity of past experiments conducted irrespective of cooling rates for Zircaloy-4 with no pre-transient hydrogen content. The observed insensitivity of cooling rates on the ECCS criteria for non-hydrided Zircaloy-4 may represent a gap between ductility-based regulation and integral fuel rod behavior. Cooling rate effect is shown to become increasingly important for increasing pre-transient hydrogen embrittlement, as shown by the fracture strength changes.

Implications of post-processing induced microstructural changes on the deformation and fracture response of additively manufactured Ti–6Al–4V

Ti–6Al–4V parts fabricated via electron beam melting (EBM) powder bed fusion are often subjected to hot isostatic pressing below the β-transus temperature to mitigate defects. During which the pressure aids in pore closure and the thermal exposure results in coarsening of α phase while retaining the columnar prior-β phase grain morphology present in the as-fabricated condition. The same post-processing treatment can also be carried out above β-transus temperature which is an effective way to modify both the grain morphology and the associated α textures. The objective of this work is to correlate thermally induced microstructural changes to the deformation and fracture response of the EBM processed Ti–6Al–4V. To this end, we have carried out both sub-transus and super-transus heat-treatments of the as-fabricated material. The mechanical response of the as-fabricated and all heat-treated materials are characterized by in-situ tensile tests under a high-resolution digital optical microscope. This enabled us to capture large-scale panoramic images of the deforming microstructure, and overcome the trade-off between the image resolution and the field of view during in-situ experiments. The series of images captured throughout the imposed deformation are subsequently used to perform microstructure-based digital image correlation to measure microstructural-scale strains over a large area. The results of the in-situ tests together with detailed fractographic analyses are then used to elucidate how the heterogeneous deformation spanning over multiple microstructural length-scales, for example, at the scale of lamellae, colonies of lamellae and grain boundaries, affect the overall deformation and fracture response of the post-processed materials.

Nucleation and propagation modeling of short fatigue crack in rolled bi-modal Ti–6Al–4V alloy

A numerical procedure centered around the crystal plasticity finite element method was developed for the simulation of fatigue crack initiation and propagation in metallic materials. Damage indicators were derived from theoretical dislocation-based models for the prediction of the crack initiation, crack path and crack growth rate, taking into account slip transfer across grain boundaries. They were incorporated into a numerical framework in which an explicitly growing crack was considered via remeshing technique to account for the redistribution of the micromechanical fields ahead of the crack tip. In this regard, a non-local averaging scheme was applied to account for both the spatial heterogeneity of the stress–strain fields and the grains morphology. The proposed framework was tested on a rolled bi-modal Ti-6Al-4V alloy subjected to a fully reversed uniaxial loading condition. Statistically representative synthetic microstructures were generated by a multi-scale anisotropic tessellation procedure considering the presence of multiple lamellar variants within a prior β-phase grain sharing the same orientation relationship. The predictions of crack initiation, crack path and growth rate were statistically compared with experimental data from prior study to emphasize the predictive capabilities and limitations of the proposed approach.

The Effect of Air Fraction in Steam on the Embrittlement of Zry-4 Fuel Cladding Oxidized at 1273–1573 K

This paper deals with the effect of air fraction in steam on the embrittlement of Zry-4 nuclear fuel cladding tubes exposed under steam–air atmospheres (air fractions of 10–100%) in the temperature range of 1273–1573 K. Ring compression tests were carried out in order to evaluate the embrittlement of fuel cladding. Furthermore, the microhardness of prior β-phase was measured and fractured surfaces were observed under scanning electron microscopy. The degree of the embrittlement is discussed against the results of metallographic and hydrogen analyses. The microstructure and the hydrogen pickup were substantially affected by nitride formation. Accelerated oxidation kinetics enhanced shrinking of the prior β-region. The enhanced hydrogen absorption resulted in the increased microhardness of prior β-phase. The degree of the fuel cladding embrittlement, expressed by the plastic strain at failure and the maximum load, correlated well with the microhardness and the thickness of prior β-phase.

Simulation of temperature, stress and microstructure fields during laser deposition of Ti–6Al–4V

We study the evolution of prior columnar β phase, interface L phase, and α phase during directional solidification of a Ti–6Al–4V melt pool. Finite element simulations estimate the solidification temperature and velocity fields in the melt pool and analyze the stress field and thermal distortions in the solidified part during the laser powder bed fusion process. A phase-field model uses the temperature and velocity fields to predict the formation of columnar prior-β(Ti) phase. During the solidification of the β phase from an undercooled liquid, the residual liquid below the solidus temperature within the β columns results in the α phase. The finite element simulated stress and strain fields are correlated with the length scales and volume fractions of the microstructure fields. Finally, the coalescence behavior of the β(Ti) cells during solidification is illustrated. The above analyses are important as they can be used for proactive control of the subsequent modeling of the heat treatment processes.

Fracture mechanics analysis of Zircaloy-4 tubular samples after laboratory simulated LOCA transient

This paper investigates the room temperature cladding embrittlement of Stress Relieved Annealed (SRA) Zircaloy-4 fuel cladding samples subjected to laboratory steam oxidation tests simulating Loss of Coolant Accident (LOCA) transients at 1200 °C followed by water quenching. These high temperature oxidized tubes are mechanically tested using axial tensile samples with machined gage sections. Formerly performed studies suggested that Linear Elastic Fracture Mechanics (LEFM) could provide a good understanding of the sample failure process, including crack nucleation close to the oxidized surfaces followed by crack instability and sample failure at higher applied loads. However, it was found that when the applied stress intensity was calculated using solutions for plate material given in the Tada and Paris Handbook, with the cracks in the plate corresponding to a crack formed only on the cladding’s outer surface – produced by oxidation on this surface alone – versus two opposing cracks formed on the cladding’s inner and outer surfaces – produced by oxidation on both of these surfaces – the critical stress intensity was different between these two cases. It is shown here that for both of the foregoing configurations the same critical crack intensity values are obtained when the applied LEFM stress intensity values are calculated using three dimensional finite element modeling of the axial tensile samples. The critical stress intensity factors determined by this more accurate method – and calculated as a function of degree of surface oxidation and hydrogen content in the prior-β phase – could, thus be considered to be a true materials parameter correlating well with the experimentally determined effect of hydrogen content on the failure strength of the samples.

Hydrogen motion in Zircaloy-4 cladding during a LOCA transient

Hydrogen and oxygen are key elements influencing the embrittlement of zirconium-based nuclear fuel cladding during the quench phase following a Loss Of Coolant Accident (LOCA). The understanding of the mechanisms influencing the motion of these two chemical elements in the metal is required to fully describe the material embrittlement. High temperature steam oxidation tests were performed on pre-hydrided Zircaloy-4 samples with hydrogen contents ranging between 11 and 400 wppm prior to LOCA transient. Thanks to the use of both Electron Probe Micro-Analysis (EPMA) and Elastic Recoil Detection Analysis (μ-ERDA), the chemical elements partitioning has been systematically quantified inside the prior-β phase. Image analysis and metallographic examinations were combined to provide an average oxygen profile as well as hydrogen profile within the cladding thickness after LOCA transient. The measured hydrogen profile is far from homogeneous. Experimental distributions are compared to those predicted numerically using calculations derived from a finite difference thermo-diffusion code (DIFFOX) developed at IRSN.

Proposal of new Oβ oxidation criterion for new types of the Zr1Nb alloy of fuel claddings

This paper deals with oxygen concentration measurements and mechanical testing of nonirradiated modified E110 nuclear fuel cladding tubes exposed to low- and high-temperature steam environment. The oxygen concentrations were measured in the prior β-phase region using two experimental methods (SIMS and TEA). The results of both techniques were compared with satisfactory agreement. Then ring compression tests (RCT) and fractographic analyses of failed ring specimens followed. By correlating the oxygen concentration results with the results of RCT, the oxygen concentration threshold of the β-phase embrittlement has been assessed (Cβ,threshold=0.38wt%). The tubes exceeding this oxygen concentration in the β-phase may become brittle. The threshold was compared with the results of other authors with satisfactory agreement. Eventually, the kinetics of the oxygen uptake in the prior β-phase has been assessed based on the SIMS and TEA results: Cβ=128×exp(−10,875/T)×t1/3. By implementing the oxygen concentration threshold into the kinetics equation, the new oxidation criterion Oβ has been determined and compared to the conventional 17% ECR criterion and the empirical criterion K.

Process Modeling of Ti-6Al-4V Linear Friction Welding (LFW)

A fully coupled thermomechanical finite-element analysis of the linear friction welding (LFW) process is combined with the basic physical metallurgy of Ti-6Al-4V to predict microstructure and mechanical properties within the LFW joints (as a function of the LFW process parameters). A close examination of the experimental results reported in the open literature revealed that the weld region consists of a thermomechanically affected zone (TMAZ) and a heat-affected zone (HAZ) and that the material mechanical properties are somewhat more inferior in the HAZ. Taking this observation into account, a model for microstructure-evolution during LFW was developed and parameterized for the Ti-6Al-4V material residing in the HAZ. Specifically, this model addresses the problem of temporal evolution of the prior β-phase grain size (the dominant microstructural parameter in the HAZ) during the LFW process. This model is next combined with the well-established property versus microstructure correlations in Ti-6Al-4V to predict the overall structural performance of the LFW joint. The results obtained are found to be in reasonably good agreement with their experimental counterparts suggesting that the present computational approach may be used to guide the selection of the LFW process parameters to optimize the structural performance of the LFW joints.

A study of the breakaway oxidation behavior of zirconium cladding materials

In order to investigate breakaway oxidation behavior, the high temperature oxidation test using zirconium claddings was performed at the temperature of 1000 °C for up to 5000 s in a steam environment. After the high temperature oxidation test, SEM microstructural observation and high temperature XRD analysis were applied to evaluate the oxide characteristics with alloy composition and oxidation time. From the systematic analysis of the oxide and matrix, it could be interpreted that the breakaway oxidation behavior was influenced by the hydrogen concentration at the oxide and metal interface rather than oxide phase transformation between the tetragonal phase and monoclinic phase. The hydrogen effect near the oxide metal interface was affected by the metal morphologies between the oxygen stabilized alpha phase and prior-β phase.

Computational analysis and experimental validation of the friction-stir welding behaviour of Ti—6Al—4V

A fully coupled thermomechanical finite element analysis of the friction-stir welding (FSW) process developed in the authors' previous work is combined with the basic physical metallurgy of Ti—6Al—4V to predict/assess the structural response of FSW joints. A close examination of the experimental results reported in the open literature reveals that in most cases the heat-affected zone (HAZ) of the weld possesses the most inferior properties and tends to control the overall structural performance of the weld. Taking this observation into account, a microstructure evolution model is developed and parameterized for the Ti—6Al—4V material residing in the HAZ. Specifically, this model addresses the problem of temporal evolution of the globular α-phase particles located within prior β-phase grains (the dominant microstructural parameter in the HAZ) during the FSW process. Next this model is combined with the well-established property versus microstructure correlations in Ti—6Al—4V in order to predict the overall structural performance of the weld. The results obtained are found to be in reasonably good agreement with their experimental counterparts, suggesting that the present computational approach may be used to guide the selection of FSW process parameters in order to optimize the structural performance of FSW joints (at least while they are controlled by the HAZ-material microstructure/properties).

The Origin of Microstructural Diversity, Texture, and Mechanical Properties in Electron Beam Melted Ti-6Al-4V

An additive layer manufacture (ALM) technique, electron beam melting, has been used for the production of simple geometries, from prealloyed Ti-6Al-4V powder. Microstructure, texture, and mechanical properties achieved under standard operating conditions have been investigated. Three transitional regions are observed with a change in microstructural formation dependent on the thermal mass of deposited material. Prior β-phase reconstruction, from room temperature α-phase electron backscatter diffraction (EBSD) data, reveals a strong texture perpendicular to the build axis. Variation of build temperature within the processing window of 898 K to 973 K (625 °C to 700 °C) is seen to have a significant effect on the properties and microstructure of both as-deposited and hot isostatically pressed (HIP) samples.

Investigations of the Microstructure and Mechanical Properties of Prior-β Structure as a Function of the Oxygen Content in Two Zirconium Alloys

Abstract It is now well acknowledged that, after a prototypical loss of coolant accident (LOCA) transient, the resultant mechanical properties of fuel cladding tubes depend strongly on the oxygen content of the residual prior-β layer, as this phase is the only metallic part of the high-temperature oxidized cladding that may show some residual ductility. The aim of this study is to obtain relevant information on the evolution of the mechanical properties, on the one hand, of the prior-β structure as a function of the oxygen content, assuming that there is a critical oxygen content that leads to a ductile-to-brittle failure mode transition at low testing temperatures (20–135°C); and on the other hand, of the α(O) structure as a function of the oxygen content. Sheets of Zircaloy-4, 1 to 3 mm thick, and M5®M5® is a registered trademark of AREVA-NP. advanced alloys from AREVA NP have been studied. To obtain different oxygen contents, they were oxidized at high temperature and then annealed under vacuum in order to reduce the oxide layer. Systematic post-treatment measurements of the oxygen concentration and of its homogeneity within the sheet thickness were performed. The different prior-β and α(O) structures thus obtained have homogeneous oxygen content between ∼0.14 wt. % and 0.9 wt. % and ∼2 wt. % and 7 wt. %, respectively. Such oxygen concentration ranges cover the solubility values that are expected in the β phase and in the α(O) phase at high temperatures typical of LOCA transients. Detailed microstructure investigations were subsequently performed on the prior-β structures since it is considered to be the most important layer when regarding the post-quench mechanical behavior of the material. Continuous cooling temperature (CCT) phase diagrams as a function of the oxygen content were established to correctly interpret the results. Electron backscattered diffraction (EBSD) analysis has then allowed the crystallographic orientations and the morphology of prior-β phase sub-grains to be determined. For each considered prior-β grain, it was possible to interpret the data by taking into account the “Bürgers” crystallographic relationship between the parent β phase and the resultant α phase. Complementary electron probe microanalysis (EPMA) was also used. These last experiments have shown a spatial fluctuation of the oxygen content within the microstructure that depends both on the nominal oxygen content and on the cooling rate. Nanohardness measurements were also performed and correlated with this oxygen spatial partition. These measurements proved to be useful for the understanding of the tensile macroscopic mechanical behavior. Finally, on the one hand, tensile tests were performed on prior-β phase at testing temperatures ranging from −100°C up to 260°C. The ductile-to-brittle temperature transition and the mechanical constitutive laws as a function of the oxygen content were then described. These tests show the existence of a ductile-to-brittle failure mode transition at 20°C for a critical oxygen concentration of ∼0.5 wt. %. A detailed fractographic analysis was performed to assess the failure mechanism. On the other hand, four-point bending tests were conducted on α(O) phase at 25°C and 135°C in order to obtain behavior laws. Preliminary finite element calculations were performed to simulate ring compression tests carried out on multi-layered high-temperature oxidized cladding tubes.

Effect of Cooling History on Cladding Ductility under LOCA Conditions

In a Loss-of-Coolant Accident of LWRs, heated and oxidized fuel cladding is slowly cooled and then quenched by re-flooding water. Effects of cooling rate during the slow-cooling process and quench temperature were investigated on post-quench ductility of the oxidized cladding. Unirradiated Zircaloy-4 cladding specimens were oxidized in steam at 1,373 or 1,473 K, cooled at a rate from 2 to 7 K/s, and finally quenched from temperatures ranging 1,073 to 1,373 K. Post-quench ductility was evaluated by ring-compression test, and microscopic properties were examined by metallurgical examination, hardness test, and oxygen analysis. Morphology of oxygen-rich α phase in the metallic prior-β phase layer changed depending on the cooling history. Area fraction of α phase region in the cross section obviously increased and post-quench ductility reduced with decrease in the quenching temperature. Since α phase region with low ductility can be preferential path for crack propagation, increase in area fraction of α phase region possibly decreases resistance for fracture, which results in ductility reduction. On the other hand, the area fraction was nearly constant irrespective of the rate of the slow cooling, and consequently, effect of the cooling rate on the post-quench ductility was negligible.