An electron backscatter diffraction analysis of grain boundary initiated discontinuous precipitation in U–10Mo

Journal Pre-proof An electron backscatter diffraction analysis of grain boundary initiated discontinuous precipitation in U–10Mo N.R. Overman, S. Jana, D.P. Field, C. Lavender, V.V. Joshi PII:

S0022-3115(19)31053-0

DOI:

https://doi.org/10.1016/j.jnucmat.2019.151940

Reference:

NUMA 151940

To appear in:

Journal of Nuclear Materials

Received Date: 9 August 2019 Revised Date:

23 October 2019

Accepted Date: 29 November 2019

Please cite this article as: N.R. Overman, S. Jana, D.P. Field, C. Lavender, V.V. Joshi, An electron backscatter diffraction analysis of grain boundary initiated discontinuous precipitation in U–10Mo, Journal of Nuclear Materials (2019), doi: https://doi.org/10.1016/j.jnucmat.2019.151940. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

An Electron Backscatter Diffraction Analysis of Grain Boundary Initiated Discontinuous Precipitation in U-10Mo

N.R. Overman1*, S. Jana1*, D.P. Field2, C. Lavender1, V.V. Joshi 1 1

Pacific Northwest National Laboratory

P.O. Box 999 Richland, WA 99354

2

Washington State University

School of Mechanical and Materials Engineering Pullman, WA 99164

*Corresponding Author Contact Information Phone: (509) 375-1913 Fax: (509) 375-3033 [email protected]

Phone: (509) 372-6979 Fax: (509) 375-3033 [email protected]

1

ABSTRACT The effect of varied thermomechanical processing on discontinuous precipitation (DP) in U10Mo was investigated, with specific emphasis on understanding the role of grain boundary misorientation in DP. Varied prior homogenization heat treatment and thermomechanical processing resulted in differences in both the fraction of DP and the colony width, and was attributed to variations in the grain boundary misorientation distribution. Regardless of the degree of DP-based transformation, extensive growth of DP colonies was dominant on 30°–45° misorientation boundaries. Interestingly, misorientation histograms of the deformed and annealed specimens suggest the processing steps involved may have inhibited DP colony growth along a small fraction of these high angle boundaries. Large-area electron backscatter diffraction montages coupled with high resolution mapping suggest symmetric {110}-type interfaces may be important considerations for mitigating extensive DP growth in this alloy system.

GRAPHICAL ABSTRACT

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KEYWORDS: Discontinuous Precipitation; Uranium; Electron Backscatter Diffraction (EBSD); Grain Boundary; Phase Transformation 1.0 INTRODUCTION

Pure metallic uranium (α-U) is not ideal for use as a nuclear fuel, as its room temperature orthorhombic crystal structure exhibits anisotropic growth that often results in tearing or swelling along grain boundaries [1]. However, the high-temperature body-centered cubic (BCC) γ-phase is stable under low-burnup conditions and exhibits minimal swelling behavior, making it acceptable for use as a nuclear fuel [2]. Alloying additions provide further γ-phase stability, irradiation stability, mechanical properties, and corrosion resistance [3-8].

In the current study, the initiation and growth of discontinuous precipitation (DP) in a U-10.4 wt. % Mo (nominally U-10Mo) alloy has been investigated. Interest in U-10Mo alloys originates from its being considered one of the most promising materials for the fabrication of monolithic metallic fuel plates to support the conversion of nuclear research reactors from using highly enriched uranium to low-enriched uranium, to support the U.S. High Performance Research Reactor conversion program [3, 4, 9-11].

According to the equilibrium U-Mo phase diagram, the high-temperature, BCC, γ-UMo phase decomposes into orthorhombic α-U and a body-centered tetragonal γ′-phase (U2Mo) through a eutectoid transformation below 560–575° C [7, 8, 12], as indicated by the expression

γU-10Mo, BCC → αU, Orthorhombic + γ′U2Mo, BCT (Eutectoid Transformation)

(1) 3

However, as noted by the authors (and previously others), aging of the γ-UMo phase in the 400– 550 °C range leads to formation of alternating lamellae of α-U and a Mo-enriched γUMo phase, rather than the γ′ phase, occurring mostly along prior γUMo grain boundaries [4, 5, 9, 13-19]. It should be noted this is a kinetically induced metastable condition. The produced morphology is a key microstructural characteristic of a DP reaction in the U-10Mo system and is indicated by the expression

γU-10Mo, BCC → αU, Orthorhombic + γU-Mo (Mo>10wt.%), BCC

(2)

DP in U-10Mo alloys results in a lamellar microstructure that typically nucleates along the prior γUMo grain boundaries. With progression of time, the reaction front (RF) moves into grain interiors (Fig. 1). A moving grain boundary, or RF, acts as a short-circuit path for diffusion of solute atoms [20-23]. However, not all grain boundaries appear equally effective in sustaining a DP reaction. As shown in Fig. 1, nucleation of DP reaction products and subsequent growth has been observed to progress more rapidly along some grain boundaries, while others remain nearly free of transformation. Because DP involves heterogeneous nucleation and migration of boundaries, the structure and characteristics of the boundary may have a major impact on the overall DP reaction itself. It is generally accepted that high angle incoherent boundaries are the most likely sites for DP nucleation due to their inherent high mobility and diffusivity [22, 24]. Prior studies on various alloy systems, e.g., those based on Ni [25-29], Cu [30-32], Mg [23, 24], and Al [33] report preferential DP initiation and growth on random high angle grain boundaries.

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Fig. 1. (1.1) Discontinuous precipitation (DP) in an aged U-10Mo alloy illustrating preferential growth along some boundaries. (1.2) Higher magnification BSE image showing the morphology of a DP zone as the RF progresses into a grain interior. (1.3) Secondary electron image with corresponding EDS line scan illustrating local molybdenum enrichment in the interlamellar region of the DP zone.

Because formation of the α-U phase in the γ-UMo matrix is undesirable for in-reactor operations [1] significant research efforts have aimed at better understanding phase transformations [9, 12, 14, 34], reaction kinetics [7, 35], and their relation to mechanical properties [3, 11, 36] in U10Mo alloys. More recent studies have highlighted the role that grain boundary complexions, solute segregation, and alloying additions can have on the DP reaction kinetics in U-10Mo alloys 5

during sub-eutectoid annealing [4, 6]. The current effort is the first time that electron backscatter diffraction (EBSD) has been used to identify key considerations for mitigating DP in the U10Mo system. Specific emphasis is placed on understanding the effects that grain boundary misorientation and orientation relationships may have on altering DP growth kinetics in U10Mo.

EBSD mapping is a versatile analytical technique for probing microstructural aspects of a material, offering the ability to simultaneously assess grain orientation, texture, morphology, size, phase, special boundaries, misorientation, etc., from a single data set. Of particular interest to this study is the misorientation measurement. A material’s local misorientation distribution is inherently linked with its thermomechanical processing history [37-39]. Prior research on other material systems (AZ91 [24] and Cu-Be [40]) have shown that DP reaction rates and initiation are affected by thermomechanical processing. Therefore, in the current study, varied homogenization treatments and deformation were studied in a U-10Mo alloy to evaluate their effects on inhibiting or arresting DP transformation.

By convention, the grain boundary misorientation as measured by EBSD is the smallest angular misorientation about a shared misorientation axis. In other words, it is the minimum transformation that can be applied to bring two crystals into coincidence. At this point, it should be noted that the misorientation angle (occasionally termed “disorientation”) measurement alone provides only a partial solution to the three-dimensional (3D) aspects of the misorientation of a fully characterized grain boundary. The grain boundary inclination requires another two parameters to describe, and is also unknown in standard EBSD analysis, as it extends below the 6

surface into the bulk of a polished specimen. 3D EBSD through the use of serial sectioning is ideal for applications where a complete characterization of interface connectivity is needed [37, 39, 41]. Alternatively, the five-parameter grain boundary space can be statistically determined using stereological methods that are well developed [42-44]. The present study uses large-area montaging of 2D EBSD data, with an emphasis on misorientation angle, to provide preliminary insights into DP formation in U-10Mo. Specifically, we have focused on •

correlations between grain boundary misorientation angle and DP-based transformation in the U-10Mo system,

•

evaluating the effects of varied thermomechanical treatments on the misorientation of untransformed and transformed DP boundaries, and

•

observations of strain energy ahead of a DP RF,

with the ultimate goal of illustrating the ability to inhibit DP colony growth via a grain boundary engineering approach. This manuscript advances the mechanistic understanding of the relationship between thermomechanical processing and discontinuous precipitation (DP) based colony growth, which in U-10Mo is known to be detrimental to the mechanical properties. In this work, we have confirmed the observations of other researchers that high angle grain boundary (HAGB) misorientation results in preferential DP. At the same time, we have taken this analysis a step further and present cases that illustrate where DP growth has been inhibited along these typically preferred HAGB’s. Through a detailed electron backscatter diffraction analysis of the microstructure, we have highlighted additional factors (the role of grain boundary misorientation and orientation across boundaries) to elucidate potential mechanisms by which 7

thermomechanical processing influences DP nucleation and growth kinetics. Specific novel contributions of this work include observations of: • •

Coincident axial direction boundaries, or planar matching across grain boundaries exhibiting minimal DP. Direct observations of strain plumes ahead of DP reaction fronts

To the authors’ knowledge, the unique findings in this work have not been previously reported. And, in combination, these two aspects of the manuscript provide a previously undocumented link between thermomechanical processing and DP colony growth in the U-10Mo system, which may also be of consideration to other material systems. 2.0 MATERIAL AND METHODS

Characterization was performed on a depleted U-10Mo alloy (nominal composition) following two different homogenization schedules. The U-10Mo plates were cast at the Y12 National Security Complex, in Oak Ridge, Tennessee. These plates were 0.5 mm thick, 127 mm wide and 177.8 mm long and were cast in a graphite book mold along the length. The carbon concentration in these alloys was ~600 - 800 ppm with a total impurity level less than 1500 ppm. Composition was measured at A total of three “as-cast + homogenized” samples were evaluated, two of which went through additional hot and cold rolling schedules and annealing treatments to induce recrystallization. Rolled foils were prepared from a depleted U-10.4 wt. % Mo alloy. Cold rolling was performed on a Stanat model TA-215 two-high mill that was converted to a four-high mill for cold-rolling operations to attain the desired foil thickness. In the four-high mill configuration, the 22.2 mm 8

rolls were backed with 101.6 mm rolls. The rolling experiments were performed without lubrication. For rolling, the U-10Mo samples were wrapped in zirconium foils roughly 0.025 mm thick. The mill was operated at 25 revolutions per minute and had a maximum load separation force of ~45,360 kg. The samples were hot rolled from 5.08 mm to 1.02 mm, with 15% reduction per pass, and had been preheated in air at 700 °C for 20–30 min in a Thermcraft Model 1134 tube furnace before each pass. The samples were immediately rolled (within 5 s) to minimize heat loss during the hot-rolling procedure. When the desired thickness of 1.02 mm was attained, the samples were then stress-relief annealed at 700 °C for 1 h in an MTI model VBF-1200X-H8 furnace in a continuously flowing argon atmosphere, and later etched to remove any surface oxides that may have formed during the rolling and annealing operations. The samples were subsequently cold rolled in the four-high mill configuration with ∼10% reduction per pass to attain a final thickness of ~0.5 mm. Table 1 outlines the material conditions and the respective heat treatments applied prior to analysis. Heat treatments were carried out in an Ar atmosphere furnace to suppress uranium oxidation. Following homogenization, samples were furnace cooled to room temperature. Optimization of homogenization schedules has been reported in the authors’ prior works [36, 45, 46]. For the deformed samples, an intermediate annealing treatment was performed in Ar. Resultant microstructures were then aged for 10 hours at 500°C and furnace cooled, leading to DP-based transformation along prior γUMo boundaries. The specimen nomenclature shown in Table 1 is consistently referenced in Figs. 2 to 4, such that figure subsections labeled A1, A2, A3, for example, illustrate results acquired from specimen A, with data from samples B and C

9

following suit. For an analysis of the alloy material (cleanliness and second phases) after homogenization and before ageing, the readers are referred to a separate evaluation, [4].

Table 1 – Specimen Processing and Heat Treatment Overview Specimen

Homogenization

Deformation

Annealing

Aging 10 h –

A

48 h - 900°C

n/a

n/a 500°C

B

C

48 h - 900°C

16 h - 1000°C

700°C – 1

10 h –

h

500°C

700°C – 1

10 h –

h

500°C

Hot + Cold Rolled to 0.2 mm

Hot + Cold Rolled to 0.2 mm

*Hot Rolling temperature was 700°C and annealing was carried out after cold rolling.

Prior to SEM analysis, samples were mounted in Buehler Epothin 2 Epoxy Resin and Hardener. Initial surface preparation was accomplished using a Pace Nano 100T Grinder Polisher. A series of grinding steps (120, 240, 400, 600, 1200 grit) were performed using silicon carbide paper disks and water for approximately one minute, each. Initial polishing was then performed using Buehler TriDent Polishing Cloth and Buehler MetaDi Supreme 9um and 3um diamond suspension fluids in succession, for four minutes each. A Vibromet polisher with 3M OS114 lapping oil and Buehler MetaDi 2 1um diamond paste resurfaced the samples overnight (~14-16 hours) after which ultrasonic cleaning of the sample was performed in a detergent, water and ethanol bath. The final polishing stage used colloidal silica polishing fluid in a Vibromet polisher 10

for a duration of 24-36 hours. Final cleaning of the samples was accomplished using a 3 minute sonication in a solution of DI water, detergent, 1% acetic acid and ethanol followed by drying with canned air.

Scanning electron microscopy was performed using a JEOL 7600 field-emission scanning electron microscope. EBSD mapping was performed using an Oxford Instruments Nordlys detector coupled with the AZtec NanoAnalysis Software, version 3.2. Fully automated large-area mapping was performed using an accelerating voltage of 20 kV, a specimen tilt of 70.0°, and a working distance of ~24 mm. Indexing of the γUMo phase was performed using a cubic crystal system, space group (229), Laue group (Im-3m), with lattice parameters for uranium a = b = c = 3.474 Å and α = β = γ = 90°. (NOTE: the lattice parameter of γ-U-10Mo is 3.41 Å [47]; this 0.064 Å contraction is not believed to significantly affect absolute orientation measurements.) Indexing of the uranium carbide phase was performed using a cubic crystal system, space group (225), Laue group (Fm-3m) with lattice parameters a = b = c = 4.955 Å and α = β = γ = 90°. High resolution EBSD analysis of the actual DP reaction products (α-U and γU-Mo (Mo>10wt%)) was not performed, as only the γUMo orientations were of interest. Table 2 indicates EBSD conditions used for mapping each of the specimens observed. Following acquisition of the EBSD maps, post-processing was performed (wild spike elimination and a medium level of zero solution extrapolation) using the HKL Technology Channel5 Tango software. Gradients in band contrast/pattern quality (PQ) may be attributed to local lattice distortion.

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Table 2 – Typical EBSD Mapping Parameters Specimen Magnification

Step Size (µm)

Montage Area

A

250×, 400×

1.5 & 0.5

2.0 mm × 0.6 mm

B

250×

1.5

2.7 mm × 0.5 mm

C

750×

1.5, 0.75, & 0.3

2.0 mm × 0.2 mm

Low kV backscatter electron (BSE) imaging was used for imaging and energy dispersive spectroscopy (EDS) mapping. An accelerating voltage of 8–10 kV was used in combination with a probe current setting of 18 (~28 nA). EDS line scans were collected using an Oxford Instruments 80 mm2 X-Max detector coupled with the AZtec NanoAnalysis Software, version 3.2. All reported errors are indicative of one standard deviation. 3.0 RESULTS

The processing employed in this study has shown an ability to alter grain boundary misorientation distributions, grain size, and rotation axes, and reduce strain fields ahead of DP growth fronts. We have provided evidence that coincident axial direction or CAD {110}-type boundaries may play a significant role in understanding and mitigating DP transformation in U10Mo. As discussed in section 4.1, the available literature also appears to indicate the ability of these CAD {110} orientation boundaries to inhibit DP.

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3.1 MICROSTRUCTURE OVERVIEW

A comparison of the microstructures from specimens A, B, and C is shown in Fig. 2. This figure illustrates EBSD band contrast maps. The dark regions (indicative of poor band contrast) correspond to DP transformation zones. The smaller islands exhibiting bright contrast correspond to uranium carbides. Growth of the DP zones appears most extensive in specimen A. Microstructural details were quantified and are shown in Table 3. The undeformed specimen (A) has maximum grain sizes of 300 µm, while the deformed specimens following heat treatment (B and C) have maximum grain sizes less than 50 µm, clearly illustrating grain size reduction as a result of the applied processing. The rolling and annealing treatments applied to induce recrystallization resulted in successful grain refinement such that DP could be studied over a broader range of microstructural conditions and varied distribution of grain boundary types.

Fig. 2. EBSD PQ images (scaled equivalently) for specimens A, B, and C show varied grain structure and DP as a result of the different thermomechanical processing: A - 48 h Homogenization at 900°C, B - 48 h Homogenization at 900°C + Hot and Cold Rolling + Anneal 13

+ Aging, C - 16 h Homogenization at 1000°C + Hot and Cold Rolling + Anneal + Aging. Growth of DP zones is most extensive in specimen A.

Table 3 – U-10Mo Microstructural Summary Specimen

Avg. Grain

Percent DP

Typical DP Colony

Diameter

Transformation

Width*

A

70 ± 50 µm

11 ± 1

40-90 µm

B

17 ± 7 µm

6±2

13-30 µm

C

8 ± 6.3 µm

10 ± 1

9 - 15 µm

*Reported value is an approximation of the 15 largest colonies observed in each sample.

Because specimen A exhibited the most extensive DP growth (in terms of the observed colony width), this work focuses primarily on comparing DP characteristics of A to those of samples B and C, in an effort to highlight specific aspects of the microstructure that may be useful for inhibiting DP growth. Measurement of grain misorientation across transformed (DP) and untransformed boundaries was evaluated over relatively large sample areas (reported in Table 2) to isolate boundary characteristics corresponding to extensive growth of DP colonies. Grain boundary misorientation profiles were generated using the Channel5 Tango software. Profile measurement was performed relative to the first point, such that poorly indexed DP areas could be eliminated from the measurement. A rather exhaustive manual analysis of individual grain boundaries was performed, which involved numbering grain boundaries and recording misorientation, as shown in Figure 3. 14

3.2 PREVALENCE OF DP GROWTH BASED ON GRAIN BOUNDARY MISORIENTATION

An example of the manual analysis is shown in Fig. 3. The top row images compare BSE images with an EBSD map from the same area. The comparison illustrates that the unindexed areas from the EBSD map correspond to DP that has nucleated on prior γUMo boundaries and grown. While EBSD is capable of indexing the interlamellar regions within a DP zone, this analysis was intentionally performed at a step size larger than the interlamellar spacing in order to simultaneously map a large area and easily distinguish regions exhibiting DP. In the bottom row of images, varied coloring in the inverse pole figure (IPF)-Y map indicates different grain orientations, where >10° grain boundaries are indicated by black lines. When viewed alongside the band contrast map, the transformed boundaries exhibiting DP (T* red arrows) are easily distinguished from untransformed boundaries (blue arrows). Misorientation profiles were individually measured according to the locations of the arrows and are plotted at the bottom right in Fig. 3. Spurious orientation measurements within DP regions were excluded from the profiles shown, to improve clarity. As shown for this example case (measured from Sample A), boundaries exhibiting extensive DP have misorientations of ~30°-45°.

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Fig. 3. (Top row) A BSE image is cross-referenced with an EBSD PQ map, revealing that degraded PQ corresponds to regions exhibiting DP. (Bottom row) An EBSD inverse pole figure map (IPF-Y) illustrates grain structure. Degraded band contrast/PQ along transformed (red T*) grain boundaries highlights preferential DP versus grain boundaries free of transformation (blue). Misorientation profiles across transformed boundaries reveal preferential transformation of about 40° misorientation. Images shown are results from specimen A.

The first goal of this study was to determine if the observations of other researchers which correlated high angle grain boundary (HAGB) misorientation to preferential DP were applicable to the U-10Mo system. For this reason, a complete manual evaluation of the grain boundary 16

misorientation of specimen A is shown in Fig. 4 (A1). Using the Channel5 Tango software, misorientation across grain boundaries with extensive DP transformation (red lines) was measured and compared to randomly chosen boundaries devoid of DP (blue lines). Due to the increased number density of untransformed boundaries within the analyzed area, relative frequency histograms were generated. The number of transformed boundaries measured corresponded to 40, 50, and 50 measurements for specimens A, B, and C, respectively. The number of untransformed boundaries measured was 50–75 for all samples. Results of the manual analysis revealed that boundaries exhibiting extensive DP growth were normally distributed, with the highest frequency occurring at a misorientation of ~30°–45°. Using this information, the Tango software was used to highlight these specific 30°–45° boundaries (Fig. 4-A3, magenta). Auto-identification of these boundaries appears to be in agreement with the manual analysis. Grain boundary misorientation in the 5°–29° range (yellow) and >45° (aqua) are predominately devoid of extensive DP growth. To the authors’ knowledge, this is the first time such a unique result about preferential DP growth along ~30°–45° misorientation boundaries in the BCC U10Mo system is being reported. However, this result is in excellent agreement with the work of Hirth and Gottstein [33] that showed characteristic misorientation relationships (25°–40°) were found in a face-centered cubic Al alloy between grains with a propensity for DP. Experimental efforts that have investigated grain boundary misorientation with respect to DP formation have revealed that high angle boundaries transform preferentially. This work builds on these previous efforts by further investigating the small subset of these same ~30°–45° boundaries which did not exhibit extensive DP growth, in an attempt to identify DP mitigation mechanisms.

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Fig. 4. EBSD results of preferential DP in specimen A. (A1) manual analysis of individual grain boundaries, with red corresponding to transformed boundaries and blue corresponding to remaining boundaries free of transformation. (A2) histograms generated using the individual measurements from A1 reveal transformed boundaries are primarily in the 30–45° misorientation range. (A3) 30–45° grain boundaries highlighted in magenta using Tango software are in agreement with manual analysis. 18

Fig. 5 summarizes the analysis for all three samples (A, B, and C). The untransformed and transformed plots are derived from manual measurements. An automated generation of the misorientation angle distributions for “All Boundaries” is also shown for comparison. The autogenerated misorientation angle distributions show correlated misorientation angles (between neighboring map points) and uncorrelated misorientations between randomly chosen points in the data set. Because minor deviations from a typical random Mackenzie distribution [48] are only observed in the correlated data, this may indicate some dependence on the DP zone formation, potentially an artifact due to the increased DB colony width observed from specimen A. Slight differences in the full distributions between samples are weakly visible and attributed to the varied thermomechanical processing. Therefore, automated generation of complete misorientation angle distributions was not identified as an ideal methodology for coaxing out unique aspects of DP transformation boundaries. In this sense, the manual analysis that was performed was able to more clearly show these distinctions.

As anticipated, based on the prior research in this area, inspection of the transformed boundary histograms show a strong tendency for DP transformation to occur along ~30°–45° boundaries. The results of the untransformed regions appear even more interesting. For example, the transformed (red) histogram results from specimen A indicate nearly all the 30°–45° boundaries have undergone DP, due to the low frequency of untransformed (blue) boundaries observed in this same misorientation range. When comparing the untransformed (blue) grain boundary histograms for samples B and C to that of specimen A, a larger amount (~double) of these same boundaries in the ~30°–45° range were observed in B and C, devoid of significant DP growth. It 19

should be noted at this point only DP growth is believed to have been mitigated, not DP nucleation. DP nucleation is evaluated in section 3.3.

Fig. 5. Relative frequency distributions of grain boundary orientations across DP and reactionfree boundaries as measured manually (blue and red) and the auto-generated misorientation angle distribution showing all boundaries (grayscale). All specimens indicate DP reactions along a specific range (30°–45°) of high angle boundaries. Differences in the relative frequency

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histograms are attributed to the varied thermomechanical processing. (A, B, C refer to specimen ID mentioned in Table 1)

This result indicates that, while the propensity for nucleation and growth of DP regions is favorable along these ~30°–45° high energy boundaries, there is also potential for thermomechanical processing to alter grain boundary character in such a way that inhibits continued growth of DP regions. Large-area EBSD montaging and increased resolution mapping were performed in an attempt to further investigate factors that may prove useful for mitigating growth of DP colonies.

3.3 IMPACT OF BOUNDARY MISORIENTATION AND CRYSTALLOGRAPHIC RELATIONSHIPS ON DP GROWTH

One of the most striking features observed, while comparing the DP behavior between samples A, B, and C (Table 2-colony width), is the reduced DP growth following the thermomechanical processing. In addition, samples B and C both show a larger relative frequency of untransformed 30°–45° boundaries than specimen A. Therefore, an evaluation of the rotation axes (Fig. 6) coupled with an analysis of grain orientation relationships (Fig. 7) and higher resolution EBSD (Fig. 8) were performed to better elucidate microstructural differences that result in altered DP growth.

Evaluation of the rotation axes was performed over the entire region encompassed in the large area montages shown in Figure 2, including heavily transformed boundaries. This was accomplished using the same classification scheme as outlined in Fig. 4, where 30°–45° 21

transformed boundaries (magenta) were set up as one scheme and those that exhibited a lack of DP transformation in the 5°–29° range (yellow) and >45° (aqua) range were also differentiated. The goal of this evaluation was to determine if a visible change in the preferred rotation axes had occurred as a result of the applied processing schemes. As shown in Fig. 6, equal-area projections of the rotation axes from each of the samples reveal that thermomechanical processing has (1) reversed the dominant rotation axis of the 30–45° misorientation boundaries and (2) decreased the highest observed multiples of a uniform distribution (m.u.d.) value to approximately 1/3 the initial value. Rotation axes plots were also generated to compare a heavily transformed region in specimen A to a weakly transformed region identified in the sample. This result also revealed a shift away from [111] rotation axes and is included in the supplementary figure S1. At this point, it should be noted that the information presented in Figures 6 and S1 highlight the rotation axis about which the largest angular deviation is measured across a boundary. In this sense, interpretation of the shift in rotation axes is indicative of an overall alteration in the character of the grain boundary energy (in the 30°-45° range) which may be a key element in mitigating DP growth. Further investigation of the mechanism hindering DP growth is highlighted in Figures 7 and 8, with the aim of better understanding how a decreased frequency of [111] rotation axes could affect DP growth kinetics.

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Fig. 6. Equal-area projections of rotation axes shown in crystal coordinates (upper hemispheres) reveal that the 30–45° rapidly transforming boundaries present in Sample A typically coincide with [111] misorientation axes. Thermomechanical processing of Samples B and C, where DP growth was inhibited, produces a reversal of this alignment.

Fig. 7 shows a range of EBSD maps obtained from each of the samples for comparison. Band contrast (PQ) maps reveal DP zones. IPF maps show grain orientation in the x, y, and z directions, as represented by the reference frame in the upper right. All three x, y, and z directions are shown for conventional purposes to illustrate that CAD boundaries can occur along any direction and are not specific to a particular processing plane. Results shown in Figure

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7 will be discussed sample by sample, specifically focusing on boundaries where extensive DP growth was inhibited. CSL boundaries did not appear to coincide with a reduction in DP to a greater extent than the misorientation thresholds (5-29° and >45°) identified, this result can be found in the supplemental data. Inspection of the PQ map from Sample A shows a large amount of DP is present and appears to have formed all around the two central grains, yet not between them. By looking at full crystallographic orientation in the x, y and z directions, it is observed that low and high (<30° and >45°, respectively) grain boundaries that share a {110}-type orientation on both sides may offer improved resistance to DP growth. The {110}-type orientation is indicated by green coloring in the IPF legend. Grains that share an equivalent orientation on both sides of a boundary have been previously described as “plane matching” in cases where their plane normals are nearly parallel across a boundary, allowing linear matching [49]. The concept of these coincident axial direction (CAD) boundaries, or planar matching theory, is that some atomic planes from each grain (only slightly mismatched across the boundary) may relax to regions of perfect matching in the boundary plane, interspersed with lines of mismatch. The lines of mismatch are evident as arrays of intrinsic dislocations analogous to the interphase semicoherent interface case [49]. The idea of a plane matching type boundary was originally proposed by Pumphrey in the 1970s [50]. But, as others have pointed out, the important role of this type of grain boundary has not been fully recognized, especially with reference to interphase boundaries [51]. For this reason, the effects of plane matching on DP in U-10Mo was further evaluated in samples B and C.

24

The center column of images in Fig. 7 corresponds to an EBSD region acquired from Sample B. As seen in the PQ image for this sample, the top half of the image shows extensive DP while considerably less transformation is seen in the bottom half of the image (indicated by the black rectangular callout). This same callout is shown in the respective locations of the IPF-X, IPF-Y, and IPF-Z maps, where grains with boundaries sharing a {110}-type orientation on both sides of the boundary are highlighted, in contrast to grains that did not fall into this category and were grayed-out for illustration purposes. As seen from the highlighted areas, a significant portion of the rectangular region that exhibited minimal DP growth is filled with grains that exhibit plane matching. The estimated area fraction when summed from grains in the x, y, and z directions is ~75%, which is significantly higher than other orientations exhibiting plane matching in this region (as shown in supplementary figure S2). From these results, specimen B appears to confirm the lack of DP growth along grain boundaries that exhibit plane matching by sharing a {110}-type orientation on both sides.

The right column of images in Fig. 7 shows results from an area on Sample C that exhibited minimal DP growth. This region again reveals several grain clusters exhibiting plane matching of {110}-type boundaries. Sample C was investigated in further detail, as shown in Fig. 8, to study the plane matching orientation boundaries in greater detail.

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Fig. 7. EBSD results from specimen A (48 h Homogenization at 900°C) specimen B (48 h Homogenization at 900°C + Cold Rolling + Anneal + Aging) and Specimen C (16 h Homogenization at 1000°C + Cold Rolling + Anneal + Aging). Minimal growth of DP zones is shown to coincide with plane matching of {110}-type orientations.

26

The increased magnification and resolution mapping performed on specimen C provide additional insights into growth of DP regions, as shown in Fig. 8. The PQ image shown in panel C1 is matched with a backscatter contrast image (C2) that more clearly reveals regions of extensive DP growth (indicated by arrows) in contrast to boundaries showing minimal DP growth (inside the dotted rectangle). C3 shows the corresponding IPF-Z map. Data gathered from increased magnification and resolution mapping of specimen C in the area of the rectangular inset reveals two key points illustrated in C2-b and C3-b: •

DP NUCLEATION WITHOUT GROWTH: C2-b reveals that the high angle 30°–45° misorientation boundaries (identified as the most likely to exhibit DP growth) appear to have nucleated DP, which then arrested. This is especially evident when compared against the regions of extensive DP (C2). This is an important consideration because it implies that the “untransformed” (blue) 30°–45° misorientation peaks (observed in the histograms of B and C, Fig. 5) may have exhibited minimal nucleation without growth, which is not discernible in the large-area montages as a result of the length scale used in the analysis. An increased frequency of {110}-type plane matching observed in this particular location may be responsible for reduced DP growth.

•

NO DP NUCLEATION: The second key feature observed from the increased resolution mapping was that boundaries <20° were free of any DP, and also frequently corresponded to specific boundaries where a {110}-type orientation is consistent across the interface. Representative crystal elements are overlaid on the IPF-Z map shown in 27

Fig. 8, panel C3-b. Because 2D EBSD analysis is limited to the specimen surface, the exact orientation of the boundary plane is unknown, but the shared surface orientation indicates the boundary may be of a special type, likely a plane matching or a CAD boundary, which could result in a lower energy configuration and inhibit DP.

Fig. 8. EBSD PQ (C1) BSE image (C2) and EBSD IPF-Z map (C3) show an area with minimal DP (in black dotted rectangle). A higher magnification BSE image (C2-b) and IPF-Z (C3-b) show orientation relationships and DP nucleation on high angle boundaries exhibiting minimal DP growth.

As first shown in Fig. 5, samples B and C had more untransformed high angle boundaries than Sample A. High magnification EBSD mapping of the thermomechanically processed samples has revealed that these high angle (30–45°) misorientation boundaries exhibit minimal DP nucleation, without extensive growth of the DP colonies. Analysis of these boundaries has 28

indicated they frequently exhibit plane matching of {110}-type orientations, which appears to have improved their resistance to DP growth. A comparison of BSE images to EBSD maps with boundaries highlighted for specimen A is include in the supplementary data (S3).

3.4 EBSD AND EDS OBSERVATIONS OF STRAIN GRADIENTS AT THE TRANFORMATION FRONT

An overview of the microstructures of specimens A, B and C is presented in Figure 9.

Fig. 9. EBSD pattern quality maps comparing specimen microstructures.

The band contrast maps shown in Figure 9 are an indicator of the quality of the analyzed diffraction patterns, also referred to as pattern quality maps. In grayscale, higher PQ is

29

represented by brighter contrast (indicating a good fit to the reference lattice parameters), while zero solutions are represented by black coloring. EBSD pattern quality can be impacted by multiple variables, most notably surface finish, compositional variation and strain accumulation. When viewed at the macro-scale by traversing millimeters of fine grained material (Specimen B) larger regions of diffusely improved pattern quality suggests the surface finish has been improved away from more heavily transformed regions. The absence of large scale pattern quality improvements in Specimen C support this observation. Alternately, locally increased PQ near the edges of DP fronts (Specimen A) appear more indicative of changes in the stress state ahead of a DP growth front. Higher magnification and resolution EBSD mapping were performed on Specimen A in an effort to better understanding the stress state ahead of the growth front and its impact on DP reaction kinetics, as demonstrated in Figure 10. EBSD mapping at increased magnification was performed in the region of interest (A2), where locally increased PQ around the DP zone was observed, suggesting an altered stress state ahead of the growth front. The corresponding local misorientation map shown in A3 reveals two key features: •

A region of increased strain that extends 1–6 µm ahead of the transformation front in the growth direction. (Observed in A3 as yellow-green coloring, indicating a 3–4° local misorientation)

•

A zone of relatively lower strain extends ~20 µm beyond the highly strained region and coincides with the length scales of improved PQ. (Observed in A3 as a band of 0° local misorientation just past the strain front)

30

The presence of a strain field near the DP growth front was confirmed with BSE imaging in this area. A4 shows that the increased BSE contrast ahead of the transformation front coincides directly with locally increased strain at the RF. Additional backscatter imaging (A5–A7) shows that these strain plumes are present ahead of multiple transformation fronts; strain banding appears to be more prevalent as DP colony size increases.

Fig. 10. Top row illustrates an overview of a single area first identified in (A1) where a narrow region of improved PQ ahead of a DP growth front is indicated by a highlighted square.. A2: increased magnification EBSD map performed over the region of interest, also showing improved PQ around the DP zone. A3: 0–5° local misorientation map revealing that increased lattice strain ahead of the DP zone coincides with brighter contrast observed using BSE imaging. A4: BSE image of the same area. A5–A7: additional BSE images revealing increased strain ahead of DP growth fronts. 31

The cause of the strain plumes present ahead of DP growth fronts was investigated using EDS line scanning. Scanning electron microscope (SEM) BSE imaging typically highlights mass or Zcontrast, such that higher elemental mass appear brighter. Therefore, it was anticipated that the observed brightness ahead of the RF was due to an increase in the uranium concentration (decrease in Mo). However, results of the EDS line scans revealed no measurable change in the matrix composition when traversing the brightly contrasted region. Results from several different regions (A1–A3) were compared, as shown in Fig. 11. Because the compositional scans indicated this observation was not a result of lower Mo concentration, the BSE contrast is believed to arise from local compressive strain ahead of the DP zone, resulting primarily from plastic strain accommodation. This strain accommodation is believed to arise because the relative volume of a certain number of atoms within the DP structure will be different than the volume of the same number of atoms in the gamma structure. The higher magnification EDS scans showed a ~2 wt% increase in the Mo concentration when traversing from the matrix, across the growth front, and into the interlamellar region. This concentration profile was found to be a sharp stepfunction in the growth direction, occurring over a 200–300 nm distance. According to Vegard’s law, the lattice parameter of the U-10Mo matrix is 3.4 Å when traversing the RF of the DP zone; the ~2 wt% increase in the Mo concentration would correspond to further lattice contraction of 0.01 Å. To accommodate this lattice mismatch across the growth front, the nearby matrix crystal would be put in a compressive stress state, locally increasing the Z-contrast and brightness of the BSE image. Unique to this work, the EBSD pattern quality would also improve as a result of the contraction, because the difference between the lattice parameter of U (3.474 Å used for indexing) and U-10Mo (3.41 Å) is further minimized. 32

While the change in Mo concentration was observed over a 200–300 nm distance, it should be noted that, due to the comparatively larger spot size and interaction volume of SEM in contrast to transmission electron microscopy (TEM) or atom probe tomography (APT), this distance may prove to be even smaller if measured by a different technique. However, removing a TEM or APT specimen would also likely relieve contributions from any elastic strains, if present. Additionally, the most extensive strain plumes were present ahead of the DP regions exhibiting significant growth, which may contribute to increased local strain both into and out of the analysis plane. For these reasons, the bright BSE contrast is believed to be a due to an altered stress state ahead of the transformation front brought about primarily by plastic strain accommodation, while some elastic contributions may also be present. Also, when crossing the transformation front, the local increase in Mo concentration highlights the reaction shown initially in Eq. 2, where DP in the U-10Mo system progresses from γUMo, BCC to form the reaction product γUMo(Mo>10wt.%), BCC.

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Fig. 11. Three areas from specimen A showing increased BSE contrast ahead of a DP RF. EDS line scans reveal (1) no change in the matrix composition when traversing a brightly contrasted region and (2) specific to scans A2 and A3, a ~2 wt% increase in Mo, highlighting the reaction shown in Eq. 2 between the γUMo, BCC and γUMo(Mo>10wt.%), BCC.

4.0 DISCUSSION In the case of the U-10Mo system, we have used EBSD techniques to reveal two previously undocumented factors that may play prominent roles in the nucleation and growth of DP. First, thermomechanical processing was used to identify a key grain orientation relationship in which 34

plane matching (of {011}-type orientations on both sides of a grain boundary) mitigates DP growth, even on high angle boundaries where DP nucleation is typically observed. We have also shown the ability of thermomechanical processing to alter rotation axis relationships. Secondly, comparison of these specimens to an unprocessed sample has revealed distinguishing features of a rapidly progressing DP. Using this approach, we have illustrated numerous examples that indicate the presence of significant strain fields ahead of RFs from the largest (fastest growing) DP colonies, which is previously unreported. This finding suggests that strain compatibility requirements may provide a significant driving force for DP in U-10Mo.

4.1 ROLE OF GRAIN BOUNDARIES

As expected, the varied thermomechanical treatments resulted in differences in the extent of DP present, which are likely the result of an altered distribution of grain boundary types. Additionally, the applied thermomechanical treatments also reduced the colony width (inhibited DP growth). Regardless of the extent of DP, all specimens indicated that growth of DP colonies was dominant on 30°–45° misorientation boundaries. EBSD analysis revealed that •

boundaries <20° were free of DP nucleation, and

•

thermomechanical processing may play an important role in the ability to arrest or inhibit DP growth along high angle 30°–45° misorientation boundaries through introduction of CAD {110}-type boundaries and alteration of the rotation axis.

When comparing these observations from the U-10Mo system with work that has previously been reported in a variety of material systems, several trends emerge. As noted by Tu and 35

Turnbull, DP cell growth along a boundary typically proceeds on only one side of any given boundary; reduction of interfacial energy provides a driving force that leads boundary migration to replace the higher energy interface with the lower [52]. Hirth has also noted that the interface energy differential triggers a pucker mechanism that sets off DP [33]. Special boundaries that exhibit plane matching, then, may have a unique ability to lower that driving force. Additionally, as reported by Ratanaphan [53], BCC metals with {110} planes on either side of the interface are unusually low in energy, regardless of the disorientation angle. Monzen [54] also showed that DP reactions at [011] symmetric tilt boundaries exhibited a longer incubation period and slower growth rate in a Cu-Be alloy. In addition to their lower energy, these symmetric boundaries may also lower the driving force for DP by providing a mechanism for strain relaxation [22].

Finally, from a 3D perspective, it should be noted that this EBSD analysis was limited to the 2D measurement plane. This is notable because a variety of material systems show grain boundary energy is anisotropic, and as a result, grain boundaries with the same misorientation can have very different energies due to their inclination. Variations in grain boundary energy with plane inclination define the capillary force driving boundary motion [55] (in accord with the progression and kinetics of DP). In the case of this preliminary analysis, the exact orientations of boundary planes are unknown; however, our work suggests that the energies of BCC grain boundaries that share the same crystallography on both sides of an interface may be uniquely suited to mitigating DP growth kinetics in this alloy system.

36

4.2 ROLE OF STRAIN AT THE REACTION FRONT

Strain plumes ahead of RFs along the largest DP colonies (specimen A) were a common feature of the microstructure, visible by EBSD and BSE imaging. EDS mapping was employed and indicated the strain fields were not due to local changes in the solid solution Mo content of the γUMo, BCC matrix. However, when traversing the RF, a ~2 wt% shift in Mo concentration was observed, with the interior of the DP zone exhibiting a higher Mo concentration than the matrix. Because the RF delineates a compositional and structural gradient with respect to the matrix phase, strain compatibility requirements may provide a significant driving force for DP in the U10Mo system. Thermomechanical processing employed on samples B and C hindered DP growth. The reduced DP colony width seen in these samples resulted in an inability to identify the presence of any significant strain plume formation. This finding suggests that continued DP growth may proceed as a self-sustaining reaction once a DP zone width is established that provides a critical strain mismatch needed for continued DP propagation.

Review articles on DP highlight multiple theories and contributing factors that drive DP growth. Ultimately, the driving force for steady-state growth of DP (∆GDP) comprises two main components: the Gibb’s chemical (∆GChem) and interfacial (∆GInter) energies per mole [21]. The chemical differential across the interface and strain observations contained in this work suggest steady state DP growth of the unprocessed material (specimen A) was averted as a result of thermomechanical processing.

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Lastly, a prior work by the current authors has revealed cases where grain boundaries around DP zones in U-10Mo were shown to exhibit very high aspect ratio needle-like steps or ledges (possibly deformation twinning or acicular martensite) [13]. These ledges were shown to extend several microns out from the grain boundaries (the reader is referred to Fig. 11 in the referenced work). This is of interest because nucleation of the α-U phase is possible through a martensitic transformation and would be detrimental in a nuclear fuel, again highlighting that the role of strain around DP zones may be an important factor to consider with respect to mitigating DP in this alloy system.

5.0 CONCLUSION The observations discussed on DP nucleation and growth in U-10Mo are unique and provide an interesting perspective on factors controlling DP growth in this material system. The deformation processing (rolling) coupled with annealing and aging treatments employed in this study have significantly refined the microstructure and lowered the DP colony width. Investigation to the mechanisms behind these changes have shown an ability to alter grain boundary misorientation distributions, grain boundary density, rotation axes, and reduce strain fields ahead of DP growth fronts. We have provided evidence that CAD {110}-type boundaries may play a significant role in understanding and mitigating DP transformation in U-10Mo. The available literature [53, 54] also appears to indicate the ability of these CAD {110} orientation boundaries to inhibit DP. It should be noted that it is not solely the crystallography of individual interfaces alone, but also their connectivity, that drive many interfacial phenomena such as diffusion or corrosion [39]. 38

Observations of strain ahead of the RF in the U-10Mo system have illustrated that thermomechanical processing may provide a key path forward for mitigating DP initiation and growth. With respect to grain orientation and boundary analyses, this work can be used as a data set to provide insights for continued exploration of DP in U-10Mo, ideally via statistically relevant stereological methods or 3D EBSD analysis to provide a complete description of grain boundary character and connectivity. The development of synchrotron radiation techniques that can provide 2D and 3D grain-to-grain orientation and local lattice strain measurements [56, 57] are a recent area of non-destructive analysis that have been utilized to study ceramic nuclear fuels, and may be a key method for continued study of DP in U-10Mo.

6.0 ACKNOWLEDGEMENTS This work was supported by the U.S. Department of Energy National Nuclear Security Administration, for the Office of Materials Management and Minimization (M3) under Contract DE-AC05-76RL01830. Research was performed at Pacific Northwest National Laboratory (PNNL) and is operated by Battelle Memorial Institute for the United States Department of Energy under contract DE-AC05-76RL01830. The authors would like to thank PNNL staff Shelley Carlson, Jesse Lang and Ramprashad Prabhakaran for their contributions to this work.

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8.0 Supplementary Figures

Fig. S1. Data shown are from two areas of Specimen A, a weakly transformed region (top) and a heavily transformed region (bottom). Rotation axes plotted in terms of the crystal coordinate system show the 30-45° boundaries have undergone a shift away from [111] rotation axes in the case of the untransformed region.

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Fig. S2. Data shown are from the highlighted region exhibiting minimal DP from specimen B (first observed in Fig. 6). EBSD IPF maps show coincident axial direction boundaries for {100}, {111}-, and {110}-type orientations; all other orientation data has been removed. Area fractions were calculated for each of the maps and summed (Σ) according to the symmetric orientation of interest. The CAD {101}-type orientation was more frequent than the other orientations observed.

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Fig. S3. BSE Image showing DP transformation compated to EBSD Pattern quality maps with boundaries highlighted (left column) and Inverse Pole Figure maps showing grain orientation (right column) for specimen A. Sigma 7 CSL boundaries show weak correlation to DP transformation, the 30-45° definition shows better agreement.

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: