Pack aluminization assisted enhancement of thermo-mechanical

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Pack aluminization assisted enhancement of thermomechanical properties in nickel inverse opal structures Pralav P. Shetty, Runyu Zhang, Jesse P. Angle, Paul V. Braun, and Jessica A. Krogstad Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04988 • Publication Date (Web): 15 Feb 2018 Downloaded from http://pubs.acs.org on February 18, 2018

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Chemistry of Materials

Pack aluminization assisted enhancement of thermo-mechanical properties in nickel inverse opal structures Pralav P. Shetty†,‡, Runyu Zhang†,‡,¶, Jesse P. Angle†,‡, Paul V. Braun†,‡,¶, Jessica A. Krogstad†,‡ †

University of Illinois at Urbana-Champaign, Department of Materials Science and Engineering, 1304 W Green St, Urbana, IL 61801. ‡

Frederick Seitz Materials Research Laboratory, 104 S Goodwin Ave MC-230, Urbana, IL 61801.

¶

Beckman Institute for Advanced Science and Technology, 405 N Mathews Ave, Urbana, IL 61801.

ABSTRACT: A low temperature, vapor phase approach to improve the thermal stability of non-refractory deterministically structured inverse opals is demonstrated. Specifically, pack aluminization at 550 oC was conducted on Ni inverse opals, introducing controlled amounts of Al into the structure. This enabled the formation of a strengthening Ni3Al phase, resulting in enhanced high temperature strength and stability. Following aluminization, the deterministic structures remained stable up to 1000 oC, a 500 oC increase relative to the starting Ni inverse opals. The thermal stability of the aluminized structures is comparable to much more difficult to fabricate deterministically structured refractory structures. Additionally, the pack aluminized structures exhibited a 17.6 % increase in elastic modulus and an 81.6 % increase in hardness relative to the initial Ni inverse opal. This is a promising combination of thermo-mechanical properties for very fine, deterministic structures used in high temperature, chemically harsh environments.

INTRODUCTION Inverse opals, three dimensional periodically porous structures, are typically formed via infilling of colloidal crystals (synthetic opals), followed by removal of the colloidal template. The characteristic dimensions of inverse opals are controlled by varying the diameter of the starting colloids. Through control of their characteristic dimensions, as well as their base materials, they can be designed to have unique and useful chemical, electrical, magnetic, mechanical, optical, and thermal properties. Examples can be found in literature where such structures have been used for chemical sensing1, battery and solar cell electrodes2-4, electrochemical actuators5, catalyst encapsulation6, and controlled stiffness MEMS devices7. There is now tremendous flexibility in terms of both material selection and fabrication routes for inverse opals. Material classes that have been successfully conformed into inverse opals involve metals8, 9, semiconductors10, ceramics11, and polymers12, 13, through electrochemical14, sintering15, melting16, atomic layer deposition17, chemical synthesis18, and chemical vapor deposition (CVD)19 techniques. The basic methodology used to make inverse opals, colloidal crystal templating, is illustrated in Figure 1. The starting colloidal crystals can be self-assembled from monodisperse colloids, or colloids with a distribution of diameters onto a substrate of interest. The interstitial space of this structure is then infused with the material of interest, followed by dissolution of the colloidal spheres. A significant limitation of non-refractory, metallic inverse opals is their poor thermal stability arising from their

large surface area to volume ratio that promotes sintering and a consequent loss of structure and properties at elevated temperatures. They often also display poor oxidation resistance due to formation of deleterious nonpassivating oxide scales. Low temperature chemical modification (alloying) of these structures is a potential route to make them appropriate for high temperature applications. Pack aluminization is a halide salt activated chemical vapor deposition process20, 21 that is commonly used to create Al-rich diffusion coatings on high temperature alloys. Diffusion coatings are unique in that they do not have a distinct coating-alloy interface, which may be prone to degradation and mechanical failure. In further contrast to other coating processes like atomic layer deposition and physical vapor deposition, which are only compatible with small structures, the aluminization process is easily scalable. Components of any size, and geometry may be embedded into a powder based pack and coated with aluminum through thermal activation as shown in Figure 2. For example, industrial turbine blade airfoils are frequently aluminized to improve mechanical properties and oxidation resistance22. This vapor mediated chemical modification method does not require lineof-sight and is therefore easily adaptable to porous structures. Two simultaneous thermally induced processes occur during pack aluminization of porous structures: (i) the formation of aluminum halide vapors and their infusion into the inverse opal structure (ii) reduction of the vapors at the Ni surface to form a Ni-Al diffusion couple through the consequent interdiffusion process. The inward diffusion of Al into the Ni matrix leads to the for-

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mation of nickel aluminide intermetallic phases the details of which are dependent on the activation temperature and final composition. There are many critical applications that would benefit from inverse opals with improved thermal, chemical, and mechanical properties. These include MEMS devices for deep well drilling, on-cylinder automotive sensing23, structural coatings for gas turbines and pipelines, and thermophotovoltaics24, 25 for solar energy harvesting. Various groups have attempted to make the inverse opals from refractories8, 24, 26-29 or to coat inverse opals with refractory metals 30 to improve their thermal stability. The refractory approach has been successful, but is significantly more difficult and costly than working with nonrefractory metals such as Ni. Concurrently, Hodge, et al.31 and others32, 33 have applied the pack aluminization process to large cell stochastic Ni foams to improve their thermo-mechanical properties. Through a hightemperature, two-step approach, the stochastic Ni foams were completely converted to the NiAl intermetallic phase, demonstrating that the aluminization process is not limited to coating applications. However, given the large-scale, stochastic nature of these foams, structural evolution during the aluminization and homogenization treatments could not be evaluated. The work presented here aims to preserve the fine, deterministic structure of simple Ni inverse opals, thereby achieving comparable thermal stability to refractory or refractory coated inverse opals, by low temperature adaption of the aluminization process, such that the entire inverse opal structure is converted to a more thermally stable composition and microstructure. Specifically, this paper demonstrates that low temperature pack aluminization can be used to enhance the thermo-mechanical properties of metallic Ni inverse opals. The curvature and large surface area to volume ratio of the Ni inverse opals used here limits their thermal stability to moderate temperatures (about 500 oC, see Figure 3) and thus commonly used aluminization temperatures around 1000 oC are not feasible. Using a high activity pack, controlled amounts of aluminum could be infused into the entirety of the bare Ni inverse opals during a single treatment at 550 oC, forming a Ni(Al) solid solution with strengthening Ni3Al intermetallic precipitates, thereby resulting in enhanced thermo-mechanical properties.

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spheres (1 µm diameter) in water at 55 oC for selfassembly onto the substrate during solvent evaporation resulting in a polystyrene opal structure. Post selfassembly sintering of the opals at 95 oC for 2 hours was used to prevent delamination from the W substrate while infilling the pores via electrodeposition. The electrodeposition was conducted in a commercial electroplating solution (Techni Nickel S, Technic Corp) under a -1.7 V potential against a Pt electrode. The thickness of the structured Ni layer is controlled by the deposition time. After deposition, the polystyrene templates were removed completely by immersing the Ni deposited sample in tetrahydrofuran followed by rinsing with DI water and ethanol before drying. Figure 4a shows the structure of the as deposited Ni inverse opals. Pack aluminization process A high activity pack similar to that used in previous studies21 comprised of three components was utilized: 82 wt. % inert Al2O3 powder (Baikowski, 99.9 % pure), 3 wt. % NH4Cl activator (Alfa Aesar, 99.999 % pure), and a 15 wt. % Raney-nickel aluminum source (Ni-50 wt. % Al, Acros, 99.99 % pure). This was to ensure a sufficiently high aluminum chloride vapor pressure at a sufficiently low activation temperature34 to minimize thermal damage of the Ni inverse opals during aluminization. The powders were thoroughly mixed using Al2O3 ball milling media for 48 hours to ensure homogeneity in the pack. The aluminization process is schematically depicted in Figure 2. The Ni inverse opal on a W substrate is wrapped in an Al2O3 cloth (Zircar Zirconia, ALF-50 Felt) and encased in the high activity pack. The W substrate was used to minimize interdiffusion at the Ni-substrate interface during thermal activation; however, any other electrically conductive material of choice could replace it. The protective Al2O3 cloth was used to avoid direct contact between the sample and the pack, which reduces the risk of localized melting or adhesion of pack particles. Thermal activation was carried out at 550ºC under low pO2 conditions (10-12-10-14 entering the furnace) for 1 hour in a custom rail furnace under flowing gettered Ar gas to minimize the oxidation of the Ni inverse opals. The temperature of 550 oC was a tradeoff between the thermal stability of the bare Ni inverse opals and the thermal activation required to produce aluminum chloride vapors. The 1 hour duration was selected to allow for ample Ni-Al interdiffusion. Figure 4b shows the final structure of the aluminized Ni inverse opals. Characterization

EXPERIMENTAL METHODS

i) Thermal stability tests

Fabrication of Ni inverse opals Figure 1 provides a schematic of the bare Ni inverse opal fabrication process. W substrates, 0.127 mm in thickness (Sigma Aldrich) were cleaned by sonication in acetone, isopropyl alcohol, and DI water for 15 minutes each prior to use. After drying, the pretreated substrates are placed vertically in a vial containing a suspension of polystyrene

Two environments were used to assess the thermal stability of bare and aluminized Ni inverse opals. A mildly oxidizing (low pO2) atmosphere, similar to that described for the pack aluminization process, was used to explore the combined effect of temperature and controlled oxidation between 550-700 oC. These tests were conducted in a sequential manner in 50 oC increments on the same sam-

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ple for 1 hour each, after which the sample was moved into a cool zone without changing the environment using a push-rod system where it was held until it reached room temperature. A highly reducing environment was also employed by performing the heat treatment under forming gas (N2 + 5% H2), so that the impact of temperature on the aluminized Ni inverse opals at 1000 oC could be isolated. These tests were carried out in a tube furnace and the samples were furnace cooled to room temperature after 1 hour. ii) Glancing incidence x-ray diffraction (GIXRD) GIXRD was conducted in a Philips X’Pert 2 Diffractometer to identify the phases present in the Ni inverse opals both before and after heat treatments. Cu Kα radiation with a 1 mm x 3 mm spot size and 1o angle of incidence was used for all the tests. The small incidence angle ensured maximum signal from the inverse opals while minimizing the signal from the W substrate. iii) Mechanical properties measurement Nanoindentation (Hysitron TI950 Triboindenter) was performed on both bare and aluminized Ni inverse opals. A Berkovic diamond tip with a 20 nm radius and a predefined triangular load-displacement function with a 1.5 mN maximum load was used for each indent. Care was taken to space the indents at least 50 μm away from each other by making one indent per inverse opal island to avoid any overlap or interaction. The indent depth was less than ten times the film thickness and more than twice the tip radius to avoid substrate effects and other testing artifacts. Twenty indents each were performed on a 3.5 μm thick multilayer Ni inverse opal sample before and after aluminization. The data was filtered for outliers using the modified Thompson tau technique to get representative values for both the elastic modulus and hardness of these structures from the unloading load-displacement curves.

RESULTS Thermal stability tests A series of images of a Ni inverse opal sample after aluminization and sequential heat treatments from 600700 oC on the same sample in low pO2 conditions is shown in Figures 4 b-e. Care was taken to image the same randomly chosen area of the sample after each treatment for direct comparison of the structure of the sample between treatments. The overall opalescent structure of the aluminized Ni inverse opals remained intact (Figures 4 f and g). In contrast to the bare (as deposited) Ni inverse opals in Figure 4 a, there is a loss of perfection in the structure caused by Al deposition and its consequent interdiffusion. The biggest differences in perfection occurred after the initial aluminization process at 550 oC (Figure 4 b) with no major changes observed upon subsequent heat treatments (Figures 4 c-e) at higher temperatures. The result of the thermal stability test on a different aluminized Ni inverse opal sample under reducing conditions at 1000 oC

is shown in Figure 5. Aside from some blunting of originally sharp surface features, the overall structure remains intact under these conditions. Phase and microstructural analysis Evidence of the Ni3Al phase in the as aluminized Ni inverse opals is directly observed from the GIXRD data presented in Figure 6. To compliment this, energy dispersive X-ray spectroscopy (EDX) in a scanning electron microscope (SEM) on as aluminized Ni inverse opal samples showed the presence of Al to be in the vicinity of 9 wt. % Al which puts their composition in the Ni-Ni3Al two phase region of the Ni-Al equilibrium phase diagram28. A shift in the lattice parameter of the Ni (111) peak towards higher values is also observed as expected from the Al in solid solution. Similar GIXRD tests run on the same aluminized Ni inverse opal sample after heat treatments at 550 oC and 700 oC under low pO2 conditions are also shown in Figure 6 c and d, respectively. The additional heat treatment at 550 oC caused a slight drop in the Ni3Al peak intensities, which is likely due to homogenization of the aluminized Ni inverse opals facilitated by inward diffusion of Al along the short strut thickness. Furthermore after successive controlled heat treatments up to 700 oC, this peak is too weak to detect suggesting the homogenization process is completed. The topmost pattern (Figure 6 e) was collected following heat treatment of an unmodified (as deposited) Ni inverse opal under the same low po2 conditions at 550ºC for 1 hour. In addition to the dramatic microstructural changes evident in Figure 3, this pattern also shows evidence of NiO formation, which was not observed on any of the Al-modified samples. Mechanical properties measurement Nanoindentation data from the bare and as aluminized Ni inverse opals are shown in Figure 7. Elastic modulus values of the 0.22 volume fraction Ni inverse opals show good agreement with previous literature7 using a Poisson ratio of 0.31. A clear increase in both the elastic modulus (calculated from reduced modulus using the Oliver-Pharr contact mechanics model) and hardness of the inverse opals is seen as a consequence of the aluminization process. When compared to the bare Ni inverse opals, a 17.6 % increase is observed in the elastic modulus (Figure 7 a) and an 81.6 % increase is observed in the hardness (Figure 7 b). However, as the volume fraction is not fixed in the two cases, normalizing for the increased volume fraction of the aluminized Ni inverse opals (0.2365 volume fraction, based on SEM EDX Al concentration) still results in an increase in intrinsic strength of the inverse opals by 9.4 % in the elastic modulus35. Making such a corrected comparison for hardness values is less straightforward due to the lack of published data.

DISCUSSION Previous application of aluminization to metallic foams have focused on much coarser stochastic structures,31



wherein it was impossible to assess the impact of the aluminization process on the foam structure. The thermal stability tests performed in this study demonstrate that low temperature pack aluminization of metallic foams is a viable route to stabilize the fine, deterministic structure in both mildly oxidizing and reducing conditions. This can be achieved without significant damage to the overall inverse opalescent structure but some loss in perfection is to be expected as a result of Al deposition and interdiffusion. Most of the imperfections in the structure are introduced during the initial aluminization step. Consequent heat treatments do not further degrade the structure. This improvement in thermal stability becomes more apparent on comparing the post heat treatment structure of a bare Ni inverse opal at 550 oC (Figure 3) versus an aluminized Ni inverse opal at 1000 oC (Figure 5). Such thermal stability is comparable to refractory inverse opal structures,8 and is unprecedented for common metals such as Ni. The formation of a secondary intermetallic phase during the aluminization process is key to this thermal stability and was confirmed to be the Ni3Al phase as shown in Figure 6. In contrast to previous aluminization studies on bulk nickel base superalloy structures36, 37, an additional high temperature homogenization step was not required to form the Ni3Al intermetallic phase throughout the structure. This can be attributed to the short diffusion length for Al into the ~50 nm radius struts. The diffusion profile across an individual strut could not be directly measured due to the complexity of the structure and using published diffusivity values of Al in Ni has been shown to underestimate the kinetics of pack aluminization34, 38. Instead a Ni foil, of the same purity, was subjected to the identical pack aluminization treatment in order to directly measure the diffusion kinetics under the same conditions. The average Al content in the top 50 nm of the Ni foil was found to be 9 wt% Al, identical (within the measurement error) to EDX measurements on the aluminized Ni inverse opals. This confirmed that the Al diffusion distance during the aluminization treatment was comparable or greater than the average radius of the struts, thus enabling formation of Ni3Al phase in a single step. Consequent heat treatments at higher temperatures concomitantly modify the distribution of Al over the strut cross-section and the ratio between the matrix solid solution phase (γ-Ni) and the intermetallic (γ’-Ni3Al) phase. This is evident by the drop in the Ni3Al (110) peak intensity in Figure 6. Nickel aluminides are also extensively acknowledged for their superior oxidation resistance and a comparison of GIXRD data from Figure 6 shows this improvement in the as aluminized samples compared to the heat treated bare Ni inverse opals (topmost diffraction profile). NiO peaks are detected in the heat treated bare sample even with a temperature of 550 oC under mildly oxidizing conditions. However, this NiO peak or an alternate oxide peak is not detected for the as aluminized sample even

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after multiple heat treatments at higher temperatures. This suggests that rapidly growing, deleterious oxides (e.g. NiO) on metallic inverse opals can be prevented and more protective slow growing oxides (e.g. Al2O3) may be promoted through the low temperature pack aluminization process. In addition to improved thermal stability and oxidation resistance, the Ni3Al phase also improves the intrinsic mechanical properties of the Ni inverse opals. This was shown through the use of nanoindentation on bare and as aluminized Ni inverse opals in Figure 7 and is consistent with prior knowledge on such Ni-Ni3Al (γ-γ’) two phase microstructures. Both the elastic modulus and hardness of the inverse opals show a marked increase even after being corrected for increased volume fraction from the Al inclusion. All characterization tests performed point towards the development of a more robust Ni-Ni3Al inverse opal structure through the low temperature high activity pack aluminization process. The new aluminized Ni inverse opals display a much higher oxidation resistance up to 700 oC and have been shown to maintain their structural integrity at temperatures up to 1000 oC. They also possess a higher elastic modulus and hardness compared to their bare counterpart facilitated by the second Ni3Al phase present. The electrochemical and pack cementation processes used in this study make the production of these structures highly scalable. Also, avoiding the use of any refractory and rare earth metals to improve high temperature properties of the bare Ni inverse opal leads to a relatively low production cost. All these features combined make such structures and methodology good candidates for industrial applications that require a very tight control on the surface morphology (roughness, waviness) in extreme environments.

CONCLUSION The use of low temperature pack aluminization has been demonstrated to enhance the thermo-mechanical properties of Ni inverse opal structures through formation of a secondary strengthening Ni3Al-phase during the aluminization process. The very fine, deterministic aluminized structures remained opalescent, exhibiting an unprecedented refractory-like thermal stability up to 1000 oC under reducing conditions and an enhanced oxidation resistance, elastic modulus, and hardness. This combination of thermal stability, environmental robustness and mechanical properties is promising for reliable insertion of such tunable mesostructured metals into harsh operating conditions found in, for example, deep well drilling, automotive cylinders, gas turbines, pipelines, and solar energy harvesting applications.

AUTHOR INFORMATION


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Corresponding Author Jessica A. Krogstad, Email: [email protected], Phone: (217) 244-2118.

Author Contributions The manuscript was written through contributions from all authors. All authors have given approval to the final version of the manuscript.

Funding Sources The authors would like to acknowledge the funding and technical support from BP through the BP International Centre for Advanced Materials (BP-ICAM), which made this research possible.

ACKNOWLEDGMENT The authors would like to thank the Seitz Materials Research Laboratory’s shared experimental facilities where most of the characterization tests were performed.

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FIGURE CAPTIONS

Figure 1. Outline of the colloidal crystal templating technique used to fabricate inverse opal structures. Specific to this study, (a) polystyrene spheres are selfassembled onto a W substrate and (b) electrodeposition of Ni is used to infill the voids followed by (c) the dissolution of the spheres to produce an inverse opal structure. (d) A cross-sectional scanning electron micrograph of an as deposited Ni inverse opal. Figure 2. General crucible nesting technique used in the pack aluminization process. Here a Ni inverse opal (5 mm x 5 mm x 3.5 µm) on a W substrate (0.127 mm thick) is wrapped in alumina felt cloth and embedded in a high activity pack. Thermal activation in an inert environment (gettered argon) is required in order to produce aluminum chloride vapors. Figure 3. Scanning electron micrograph of a Ni inverse opal sample heat treated at 550 oC for 1 hour under reducing conditions (forming gas). The initial plan view structure shown in the inset collapses and struts begins to sinter to each other as evident from this image. About 90% of the sample area is damaged as shown. The scale bar is common to both images.

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Vickers indents. The left column is a low magnification image and the right column is a high magnification image provided to ensure the same area of the sample is being compared. The scale bar is common to each column through (e). Digital optical images of (f) as deposited and (g) aluminized and heat treated to 700 oC samples (common scale bar for (f) and (g)) showing opalescence remains after heat treatment. Figure 5. Scanning electron micrograph of an aluminized Ni inverse opal sample heat treated under reducing conditions (forming gas) at 1000 oC for 1 hour. The periodic inverse structure remains intact even at this elevated temperature. Some blunting of sharp surface features is observed. Figure 6. GIXRD results for Ni inverse opals in the (a) as deposited, (b) as aluminized, and aluminized low po2 heat treated form at (c) 550 oC and (d) 700 oC for 1 hour. The evolution of the Ni3Al phase on aluminization of the Ni inverse opal sample and the effect of consequent heat treatments is evident from the reducing intensity of the Ni3Al (110) peak marked with a red circle. In the low po2 heat treated unmodified sample (e), the presence of the deleterious NiO scale is detected. Figure 7. Nanoindentation results for (a) the elastic modulus and (b) the hardness of the as deposited (bare) Ni inverse opals and their aluminized counterpart. The elastic modulus was normalized for the increase in volume fraction of material after Al deposition and still resulted in a higher intrinsic value compared to the bare Ni inverse opals. This improvement in properties is attributed to the secondary Ni3Al strengthening phase.

Figure 4. (a) Comparison of a Ni inverse opal sample to its (b) as aluminized structure and its progression as it is heat treated to (c) 600 oC, (d) 650 oC, and (e) 700 oC for 1 hour each under mildly oxidizing conditions (low po2). All images have been taken from the same randomly chosen area of the sample for direct comparison using fiducial


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Figure 1: Outline of the colloidal crystal templating technique used to fabricate inverse opal structures. Specific to this study, (a) polystyrene spheres are self-assembled onto a W substrate and (b) electrodeposition of Ni is used to infill the voids followed by (c) the dissolution of the spheres to produce an inverse opal structure. (d) A cross-sectional scanning electron micrograph of an as de-posited Ni inverse opal. 177x50mm (300 x 300 DPI)



Figure 2: General crucible nesting technique used in the pack aluminization process. Here a Ni inverse opal (5 mm x 5 mm x 3.5 µm) on a W substrate (0.127 mm thick) is wrapped in alumina felt cloth and embedded in a high activity pack. Thermal activation in an inert environment (gettered argon) is required in order to produce aluminum chloride vapors. 84x84mm (300 x 300 DPI)


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Figure 3: Scanning electron micrograph of a Ni inverse opal sample heat treated at 550○C for 1 hour under reducing conditions (forming gas). The initial plan view structure shown in the inset collapses and struts begins to sinter to each other as evident from this image. About 90% of the sample area is damaged as shown. The scale bar is common to both images. 84x84mm (600 x 600 DPI)



Figure 4: (a) Comparison of a Ni inverse opal sample to its (b) as aluminized structure and its progression as it is heat treated to (c) 600○C, (d) 650○C, and (e) 700○C for 1 hour each under mildly oxidizing conditions (low pO2). All images have been taken from the same randomly chosen area of the sample for direct comparison using fiducial Vickers indents. The left column is a low magnification image and the right column is a high magnification image provided to ensure the same area of the sample is being compared. The scale bar is common to each column through (e). Digital optical images of (f) as deposited and (g) aluminized and heat treated to 700○C samples (common scale bar for (f) and (g)) showing opalescence remains after heat treatment. 84x167mm (300 x 300 DPI)


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Figure 5: Scanning electron micrograph of an aluminized Ni inverse opal sample heat treated under reducing conditions (forming gas) at 1000○C for 1 hour. The periodic inverse structure remains intact even at this elevated temperature. Some blunting of sharp surface features is observed. 88x88mm (300 x 300 DPI)



Figure 6: GIXRD results for Ni inverse opals in the (a) as deposited, (b) as aluminized, and aluminized low pO2 heat treated form at (c) 550○C and (d) 700○C for 1 hour. The evolution of the Ni3Al phase on aluminization of the Ni inverse opal sample and the effect of consequent heat treatments is evident from the reducing intensity of the Ni3Al (110) peak marked with a red circle. In the low pO2 heat treated unmodified sample (e), the presence of the deleterious NiO scale is detected. 110x122mm (300 x 300 DPI)


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Figure 7: Nanoindentation results for (a) the elastic modulus and (b) the hardness of the as deposited (bare) Ni inverse opals and their aluminized counterpart. The elastic modulus was normalized for the increase in volume fraction of material after Al deposition and still resulted in a higher intrinsic value compared to the bare Ni inverse opals. This improvement in properties is attributed to the secondary Ni3Al strengthening phase. 84x169mm (300 x 300 DPI)



For Table of Contents Only. 73x47mm (300 x 300 DPI)


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Pack aluminization assisted enhancement of thermo-mechanical

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