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Scripta Materialia 68 (2013) 17–21

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Viewpoint Paper

Diffusion as a method for producing architectured materials

Robert Cicoria,a,⇑ Bechir Chehabb and Hatem Zuroba

aDepartment of Materials Science and Engineering, McMaster University, 1280 Main Street West, Hamilton, ON, Canada L8S 4L8bArcelorMittal Stainless Europe Research Center, BP15 Rue Roger Salengro, 62330 Isbergues, France

Available online 9 August 2012

Abstract—We describe the use of controlled diffusion treatments to arrange the distribution of alloying elements within a material.The Fe–C system is particularly interesting because of the strong effect of carbon on the mechanical properties and phase stability ofsteel. We demonstrate, through multi-step treatments, both one-dimensional and two-dimensional distributions of carbon withinsteel. A sample two-dimensional architecture is prepared and characterized to demonstrate feasibility. We briefly introduce a globaloptimization algorithm as a method for computationally optimizing treatment schedules.Crown Copyright � 2012 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. All rights reserved.

Keywords: Functionally graded materials (FGM); Multilayers; Bulk diffusion; Global optimization

1. Introduction

Diffusional heat-treatments have been employed as ameans of locally changing the chemistry and electronicproperties in the semiconductor industry for over40 years. In comparison, using similar treatments to cre-ate variations in mechanical properties has been largelyoverlooked and, until recently [1], has been confined tomodifying surface properties of materials. In this contri-bution, we examine the possibility of using diffusionalheat-treatments to introduce a controlled spatial distri-bution of an alloying element throughout the bulk of amaterial, so as to produce a desired adjustment in mate-rial properties. This idea of creating compositional het-erogeneity in a material on a scale larger than themicrostructure, with the express goal of optimizing spe-cific properties, is relatively new in the materials/mechanics sphere and falls under the umbrella term“architectured materials” [2]; because our material isprocessed by diffusion and has gradually varying com-position, we are preparing a specific class of architec-tured materials known as compositionally gradedmaterials (CGMs). In our materials, the architecturecomes about from the new length scale introduced bythe diffusion treatments. The addition of features on thislength scale might confer benefits on the material thatare not superficially obvious. For instance, in these

1359-6462/$ - see front matter Crown Copyright � 2012 Published by Elsevhttp://dx.doi.org/10.1016/j.scriptamat.2012.08.004

⇑Corresponding author. Tel.: +1 519 758 7832; e-mail:robert.cicoria@gmail.com

materials, the ductility has often been documented asexceeding that which is predicted by a simple mixinglaw. This behavior has been observed both in materialswith very sharp interfaces (composites) [3] and gradualinterfaces (CGMs) [4]. Furthermore, as an example thatis less well known, the resistance of a material to crackpropagation depends on both the yield stress gradientsand on the stiffness gradients in the material [5], thusgiving us the potential to engineer spatially varyingresistance to fracture. In more general terms, we canidentify other potential engineering targets for thesearchitectured materials, including: work hardening rate,kinematic hardening and macro-internal stress distribu-tions—all of which can be affected by the presence ofcomposition gradients. For these reasons, we can seethat taking into account properties that arise from struc-tures on this scale can be of significant advantage toengineers.

The approach we discuss for fabricating these archi-tectured materials is general and could be applied to awide range of material systems. In what follows, theFe–C system is used as a model system because of thedramatic effect that carbon has on the phase stabilityand mechanical properties of steel [6], as well as the easewith which the carbon can be introduced into, and re-moved from, the steel [1]. We demonstrate that it is pos-sible to control the volume fraction and distribution ofthe reinforcement phase as well as the strength and pres-ence/absence of the interface between the matrix andreinforcement. The discussion is divided into three parts.

ier Ltd. on behalf of Acta Materialia Inc. All rights reserved.

18 R. Cicoria et al. / Scripta Materialia 68 (2013) 17–21

The first is focused on possible spatial distributions ofthe alloying element which could be obtained by thermaltreatment. Next, we discuss the possibility of usingphase transformations to enhance the contrast betweenthe matrix and reinforcement phase. Finally, experimen-tal results are presented to demonstrate the feasibility ofthis approach.

2. Control of the diffusion profile

The basic diffusion treatment needed to producearchitectured steels is essentially that used for existingindustrial gas-carburizing processes. Carbon is intro-duced into, or removed from, the steel using a carriergas, which is, in the simplest case, a mixture of methaneand hydrogen, or carbon monoxide and carbon dioxide.The composition of the gas controls the chemical poten-tial of carbon at the surface of the steel. If the chemicalpotential of carbon in the gas is higher than that of thesteel, carburization takes place; if the reverse is true, thesteel is decarburized. Thus the gas ratio is a key process-ing parameter. A second processing parameter is tem-perature, which controls the rate of carbon diffusion inthe steel; this, combined with time, determines the extentof diffusion. Diffusion masks could also be introduced asa means of introducing complex spatial distributions ofcarbon. By controlling the above-mentioned parame-ters, it is possible to produce a wide range of architec-tures ranging from simple multilayer structures tocomplex two- and three-dimensional patterns. Some ofthe carbon concentration profiles that could be readilyproduced in a square-cross section are shown inFigure 1.

Figure 1. Two-dimensional distributions that can be produced by using diffusvia a succession of processing steps involving changes in mask positions, an

The use of thermal treatments to produce composi-tion gradients has several limitations. A major limitationis that the diffusion profiles produced during a given stepwill relax during subsequent processing steps. Similarly,sideway diffusion can take place below the masked re-gions and reduce the “sharpness” of the pattern. Toappreciate the seriousness of these limitations, it is suffi-cient to consider the case of a multilayer structure pro-duced by a sequence of carburizing and decarburizingsteps. The concentration profile obtained in the thick-ness direction is not a sinusoidal one; rather it is anexponentially decaying, and broadening sine function.A second practical limitation is that performing the nec-essary heat-treatments for a given structure increases indifficulty dramatically as the scale and contrast of thepatterns becomes finer; the most notable of these arethe rapid treatment times required for small featuresand the extreme gas ratios required for high contrast.

We have previously mentioned the motivations forproducing these architectured materials, namely the opti-mization of specific material properties. Additionally, thespace of possible structures has been demonstrated to bevery large. Therefore, a key requirement in the future ofthese materials is a method of optimizing a processingroute to give a desired distribution of alloying elements,while keeping the previously discussed physical limita-tions in mind. One such global optimization algorithmis called a genetic algorithm. We have chosen this typeof algorithm because of its decent performance over awide range of optimization problems [7]; however, it isof note that many different global optimization algo-rithms exist, some of which may have improved perfor-mance for this particular problem [7]. A geneticalgorithm can be most easily understood by thinking ofits real-world, analogue evolution. In essence, the algo-

ion on square cross-sections while using masks. Each pattern was maded carrier gas ratios.

Figure 2. Progress of an evolution-type global optimization algorithmin determining processing parameters to match a target one-dimen-sional carbon distribution. Adjustable parameters were temperature,time and gas carbon activity, with three discrete steps.

R. Cicoria et al. / Scripta Materialia 68 (2013) 17–21 19

rithm searches a solution space consisting of n thermaltreatment segments, each with several potential process-ing parameters, including carbon activity, temperaturesand times, as fundamental parameters. More sophisti-cated optimizations could include parameters such asmasking positions. In this optimization scheme, the initialvalues are chosen at random (or as a guess) and the genet-ic algorithm converges upon an acceptable treatment pro-cess. We have effectively managed to implement this typeof algorithm for our treatment optimization process andwe have found that it can assist considerably in findingideal processing conditions. In developing our algorithm,

Figure 3. (a) Decarburization performed in a single austenitic phase at highperformed at medium temperature (T2) where phase depends upon carbon c

we have followed an approach similar to that used by Bra-asch and Estrin [8]; for details on the implementation, wedirect the reader to that work. Figure 2 gives a depictionof the algorithms’ progress in obtaining the processingroute to produce a target one-dimensional distribution.In particular we see how the algorithm converges uponthe target distribution we want to achieve. After a suitablenumber of generations, the algorithm outputs processingparameters to produce the closest possible match. In thefuture, it is not inconceivable that instead of optimizinga fit to a spatial distribution, we could optimize a fit to aparticular mechanical property (or combination), thusgiving the processing parameters to make a previously un-known, ideal architecture for a specific application.

3. Phase transformations

One of the key advantages of using the Fe–C systemis that phase transformations can be exploited to expandthe range of possible architectures and to accentuate themechanical contrast between regions with different car-bon contents. For example, the carbon profiles shownin Figure 1 could be produced in a single-phase austen-itic stainless steel. In this case, no phase transformationstake place on cooling. The effect of the concentrationprofiles is limited to the modest effect that carbon hason the hardness and work-hardening rate of austenite.If the same profile is produced in low-alloy steel withsuitable hardenability, then, upon quenching from the

temperature (T1), resulting in a gradual interface. (b) Decarburizationontent, resulting in a sharp interface.

Figure 4. Clockwise from bottom left: computed carbon profile within the square cross-section; etched surface of a fabricated sample revealingrelative carbon contents; microhardness of a fabricated sample; treatment schedule; and mask diagram. The final step was undertaken at a lowertemperature so that low-carbon regions would exist as ferrite.

20 R. Cicoria et al. / Scripta Materialia 68 (2013) 17–21

austenite range, the high-carbon regions will transformto martensite, while the lower-carbon regions will trans-form to ferrite or bainite. The high- and low-carbon re-gions will have dramatically different strengths,ductilities and fracture toughnesses.

Phase transformations can also be used to control theinterface between the high- and low-carbon regions. Themicrostructures shown in Figure 3 were obtained bydecarburizing steel with an initial carbon content of0.4 wt.%. In one case, the surface concentration wasset to 0.1 wt.% C and decarburization was carried outexclusively within the austenite phase field at T1. Uponquenching, the surface transformed to bainite, while thecore transformed to martensite. The transition betweenthe surface and core microstructures is gradual and sois the hardness change. On the other hand, a sharp inter-face could be introduced by adding/removing carbon ata temperature for which the phase stability depends onthe carbon content. This possibility is illustrated in Fig-ure 3b; decarburization at T2 leads to the formation of aferrite layer on the surface, which is separated by a sharpinterface from the austenite (martensite) at the core. Thestrength of this interface could be manipulated furtherby introducing other fast-diffusing elements, such as P,B and S. Manipulation of the interface is of particularinterest for controlling the fracture toughness of thematerial. Diffuse interfaces could be used to increasethe effective fracture toughness by embedding the hardphase into a softer matrix. Alternatively, a large numberof weak interfaces could be introduced into the systemas a means of blunting a growing crack through thedelamination of the interface.

4. Proof of concept

As a demonstration of the potential of the present ap-proach, we have experimentally produced a two-dimen-sional carbon concentration pattern as shown inFigure 1. The pattern is produced using three heat-treat-ment steps on steel with a base chemistry of 0.03% C and1.9% Mn. The first step is a carburizing treatment usingan atmosphere of CO and CO2, the second is a decarbu-rizing step using wet hydrogen. The third heat-treatmentstep is a decarburization performed at a lower tempera-ture of 820 �C; this is done so that the higher-carbon re-gions will transform to martensite, whereas the lower-carbon regions will remain as ferrite. The heat-treat-ments were performed with masks as shown in the insertof Figure 4. Upon quenching, ferrite (soft phase) existsin the blue-shaded regions and martensite (hard phase)is formed in the red-shaded regions. The success of theheat-treatment is confirmed by the metallographic imageshown in Figure 4, where ferrite appears as the light-col-ored phase and martensite is the dark phase. The spatialdistribution of carbon in the sample was also confirmedusing microhardness measurements along the cross-sec-tion, with the knowledge that hardness is directly pro-portional to the carbon content of martensite over thecarbon range of interest [6].

5. Conclusions

We have explored the use of diffusion as a method ofproducing architectured materials in the Fe–C system.

R. Cicoria et al. / Scripta Materialia 68 (2013) 17–21 21

The versatility of the method combined with its ease ofprocessing makes this an attractive method for prepar-ing these materials. The range of possible architectures,in both one- and two-phase cases, and in one and twodimensions, gives significant freedom for optimizing thisclass of materials to obtain properties not possible in ahomogeneous structure. The use of a global search algo-rithm to define the processing route required to makespecific structures is feasible and in the future will allowfor the optimization of both architecture and physicalproperties.

References

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[5] O. Kolednik, International Journal of Solids and Structures37 (2000) 781.

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