Enhancement of wettability in AZ91/B4C system by a mechanical purificatio of liquid alloy

Artur Kudy,Wojciech Polkowski,Grzegorz Bruzd,Adeljd Polkowsk,Dontell Giurnno

a ?ukasiewicz Research Network-Krakow Institute of Technology,Zakopia′nska 73 Str.,30-418 Kraków,Poland

b National Research Council of Italy,Institute of Condensed Matter Chemistry and Technologies for Energy,Via De Marini 6,16149,Genova, Italy

Abstract Mg-based alloys are potential candidate materials for a fabrication of lightweight boron carbide based composites through a reactive melt infiltratio approach.In this paper,the effect of a mechanical purificatio of molten AZ91 alloy’s surface on its wettability with polycrystalline B4C is experimentally evaluated for the firs time.For this purpose,sessile drop experiments were performed under the same operating conditions (700 °C/5 min;Ar atmosphere),by using both the classical contact heating (CH) and the improved capillary purificatio (CP) procedure.It was found that the evolution of contact angle values was strongly influence by the applied procedure.In particular,by using the classical CH procedure,the presence of a native oxide layer on the metal surface hinders the observations of melting process,resulting in a misleading conclusion that the system is non-wettable.Contrarily,during the wetting test performed by applying the CP procedure,the surface oxide layer was mechanically removed by squeezing the molten AZ91 alloy through a capillary.Accordingly,the oxide-free AZ91 drop with a regular and spherical shape was successfully obtained and dispensed on the B4C substrate.A reliable contact angle value of θ=83° was measured at the AZ91/B4C triple line at 700 °C,which in turn proves that B4C is wetted by the liquid AZ91 alloy.In contradiction to the literature,these good wetting conditions were assisted by a non-reactive wetting mechanism occurring at the AZ91/B4C interface.To succeed in the fabrication of AZ91/B4C composites by liquid metal infiltration such experimental observations make it reasonable to expect a spontaneous infiltratio process exclusively driven by capillarity,which in turn increases the efficien y of the process by the absence of reaction products that could be a potentially detrimental factor.

Keywords: AZ91 magnesium alloy;Magnesium matrix composites;Sessile drop;Capillary purificatio procedure;Wettability;Contact angle;MMCs;B4C.

1.Introduction

An increased interest in the optimization of processing technologies related to Mg-based ceramic matrix composites (CMCs) prioritizes experimental measurements of thermophysical,physicochemical and technological properties of liquid Mg and Mg-based alloys in both basic and applied research areas [1–11].Due to their high specifi strength and stiffness,as well as their very good wear resistance [11],Mgbased composites are attractive structural candidate materials for a wide range of applications.In particular,the high strength-to-weight Mg-based materials offer incredible capabilities in aerospace applications and are extremely interesting for various structural and functional applications in automotive,military,medicine and others industrial branches [2–4].The use of Mg-alloys has become significan owing to the required properties and low density (Mg has 30% lower density as compared to Al)[12–15].The successful fabrication of high performance Mg-based CMCs requires achieving a good thermomechanical compatibility and strong bonding between the composite’s constituents [15–18].It is worth highlighting that global mechanical response of a composite is generally determined by matrix/reinforcement interfaces formed during the fabrication process [18–19].Therefore,a better understanding of high-temperature interaction phenomena occurring in the Mg/ceramic systems is crucial for improving industrial liquid-assisted processes and the quality and longterm performance of fina products [20].

This aspect seems to be particularly important,as Mg-Al-Zn-Mn and Mg-Al-Mn alloys (AZ and AM series) have been pointed out as ideal candidates for fabricating Reaction-Bonded Boron Carbide (RBBC) based composites – a new class of ultra-hard materials for e.g.ballistic protection.Cafri et al.[21,22] have proposed to introduce the AZ91 alloy as a liquid infiltratin material instead of liquid Si that is widely applied for the reactive melt infiltratio (RMI) based production of B4C-Si/SiC composites.Beside of a lowering the composite density (ρSi= 2.33 gcm?3vs.ρAZ91=1.81 gcm?3),this kind of materials replacement would also allow strongly decreasing the processing temperature (TmSi= 1414 °C vs.TmAZ91=470–598 °C) [23].

According to Cafri et al.[24],the following conditions ought to be ensured in order to support an efficien fabrication of Mg-containing composites via the RMI approach: (i)a low chemical reactivity to avoid the formation of new interfacial compounds blocking open channels within a preform;(ii)a good wettability of the porous preform at reasonably low temperature.Furthermore,a non-oxidizing atmosphere ought to be also applied in order to hinder a massive internal oxidation of the preform resulting in a possibly shortening the infiltratio paths [25,26].

For quantifying both the wettability behavior and reactivity in metal/ceramic systems,the sessile drop (SD) method has become a widely applied tool.This method allows determining basic wetting characteristics of examined metal/ceramic systems such as a contact angle(θ),wettability kineticsθ=f(t)and the work of adhesion.However,the examination of wettability and spreading of liquid Mg-based alloys is a very challenging task,due to a high chemical affinit of Mg to oxygen,leading to an almost instant formation of a continuous and very thick oxide layer at the metal surface.The presence of such oxide fil impedes the intimate contact between the molten metal phase and a substrate.Consequently,it makes it difficul to conduct drop-assisted experiments on molten Mgbased alloys,while reliability of the results obtained is highly questionable [11].The surface oxidation of liquid Mg-based alloys is clearly observed during the wettability tests involving the SD method combined with a classical contact heating(CH) procedure (Fig.1a).Since in this attempt a native oxide fil cannot be removed from the Mg drop surface,the results obtained by the CH procedure should be discussed by taking into account the presence of the oxide layer.For solving such problem the combined SD method with a capillary purifica tion (CP) procedure (Fig.1b),has been developed.In the CP procedure both metal and substrate specimens are heated up separately up to a certain testing temperature.Specificall ,at the beginning of the experiment,a metal piece is inserted into a capillary located above the substrate and after reaching the pre-set testing temperature,it is squeezed through a hole in the capillary [11].The native oxide fil is mechanically removed during the squeezing of the liquid Mg alloy drop,which allows performing measurements of “true” contact angle values.A more detailed description of the CP procedure is provided elsewhere [20,27-31].

The present work is focused on examining hightemperature wettability and reactivity between molten AZ91 alloy and the B4C refractory substrate by using the SD method combined with the CP procedure (Fig.1b).In our previous works on a non-reactive Mg/graphite system selected as case study,we already documented about the necessity of using the CP procedure to produce oxide free molten drop and to obtain measurable and reliable wetting characteristics.Therefore,we will adopt this approach to examine the effect of mechanical purificatio on interaction phenomena occurring between of molten Mg-based alloy having a high practical importance in contact with the B4C substrate.In contrast to the non-wettable/non-reactive Mg/C system,the AZ91/B4C has been already recognized by Cafri et al.[32] as both wettable and highly reactive when tested at 850 °C/20 min.Indeed,the authors found that a very good wetting reflecte by the contact angle value ofθ～12° is governed by a chemical interaction leading to the formation of new interfacial products,including MgB2and MgB2C2phases.However,for increasing the efficien y of the RMI process,the existence of such interfacial products should be considered as detrimental,since the eventualities of blocking potential infiltratio channels and decreasing the infiltratio depth,may be expected.One way to limit the reactivity consists in reducing the process temperature.On the other hand,the temperature should be kept high enough to ensure a good wetting conditions.Hence,a trade-off between reactivity/wettability requires a proper design and selection of the process conditions.In other words,the following research question is relevant to this problem:Can we improve the wettability inAZ91/B4Csystem without increasing the processing temperature?

In order to provide an experimentally based answer/recommendation,in the present work we discuss for the firs time about the possibility of enhancing the wettability in AZ91/B4C system at relatively low temperature,through an implementation of the mechanical surface purificatio via the CP procedure.Therefore,the specifi purpose of this work is to compare the wetting behavior of molten AZ91 alloy on a polycrystalline B4C substrate by using either CH or CP procedures.

2.Experimental details

2.1.Materials and samples preparation

A commercial AZ91 Mg-based alloy (Stanchem,Poland)having the following certifie chemical composition: Mg –bal.;8.98 Al;0.55 Zn;0.22 Mn;0.025 Si;0.0038 Cu;0.0024 Fe;0.0005 Ni(wt.%)was used in the high-temperature wettability tests.Dense hot-pressed polycrystalline B4C substrates with a diameter of 17 mm and a thickness of 5 mm,were provided by KAMB Import-Export company (Warsaw,Poland).The nominal chemical composition of the B4C substrates (as declared by the supplier),is detailed in Table 1.

Fig.1.A schematic illustration of the procedures used in the investigation: (a) Contact Heating (CH);(b) Capillary Purificatio (CP);(c) experimental setup and high-temperature device for investigating properties of magnesium alloy in contact with ceramic materials [11,30].

Table 1 Nominal chemical composition of B4C substrates (wt.%).

According to theoretical and practical recommendations[33],a contact angle value must be measured on a fla and smooth surface.For this reason,the surfaces of B4C substrate were mechanically polished down to a roughness of Ra≈0.05 – 0.1 μm.

Depending on selected testing procedure,the AZ91 alloy piece had either the cubic shape with dimensions of 4×4×4 mm3for the CH experiment;or a cylindrical shape with dimensions adopted to the size of the graphite capillary(diameter ofΦ= 10 mm and height ofh= 15 mm).Before starting the experiments,each piece of the AZ91 alloy was subjected to a mechanical grinding with SiC papers followed by ultrasonic cleaning in C3H8O alcohol (isopropanol).After that,cleaned and degreased AZ91 pieces were put either on the B4C substrate (for the CH procedure) or in the graphite capillary (for the CP procedure).Graphite capillaries were selected as “inert” containers for avoiding alloy contamination by interactions between the molten material and the capillary.

2.2.Experimental setup and procedures

As-prepared samples were immediately transferred to the high-temperature devicead-hocdeveloped for investigating molten Mg alloys in contact with ceramic materials.A simplifie schematic diagram of the experimental setup is illustrated in Fig.1c.A detailed description of the testing device and available procedures are shown in [11,30].

After positioning the alloy/substrate couple inside the chamber,residual gasses were evacuated using Scroll and turbo-molecular pumps.When a total pressure inside the chamber reached the value of around 5 × 10?6mbar,heating up to test temperature of 700 °C was started at a rate of 20 °C/min (Fig.2).At temperature of 100 °C the inert gas(fl wing pure Ar N60,p= 810–830 mbar) was introduced into the chamber in order to suppress the evaporation of AZ91 alloy.

The SD method combined with two different procedures:the CH (Fig.1a) or CP (Fig.1b) was employed to examine the wetting behavior between the liquid AZ91 alloy and the B4C refractory.The CH and CP experiments were performed in a one single trial,under the same operating conditions,namely under a fl wing Ar atmosphere at the test temperature ofT=700°C.The testing temperature was kept constant fort= 5 min and then the sample was cooled down to room temperature at a cooling rate of 20°C/min.During the wetting experiments,images of the AZ91/B4C couple were recorded in real time by using a high-speed digital CCD camera (Microtron MC 1310) at 50 frames per second.A backlight was applied to enhance contrast between the sessile drop couple profil and the background.The resulting images were analyzed using the ASTRAView? computer software developed by CNR-ICMATE(Genoa,Italy)[34],and used for measuring the contact angle valueθ;and after that-the wettability kineticsθ=f(t).During the whole experiment,the following process parameters were simultaneously monitored:total pressure inside the experimental chamber;temperature on the test table;heater’s temperature and capillary temperature.After the end of experiment,the solidifie couples were removed from the chamber and subjected to a careful microstructural characterization by a scanning electron microscopy techniques of FEI SciosTM(SEM,FEI,Eindhoven,Netherlands) and Zeiss Ultra 55 (SEM,Carl Zeiss Microscopy GmbH,Jena,Germany) microscopes coupled with Energy Dispersive X-Ray Spectroscopy (EDS).

3.Results and discussion

3.1.Macroscopic analysis of AZ91/B4C couples upon wettability tests

The most relevant images of the AZ91/B4C couple recorded during the wetting tests performed by the CH or CP procedures,are shown in Figs.3 and 4,respectively.

During the wetting experiment performed by using the CH procedure,a start of melting of the AZ91 alloy piece was detected atT= 510 °C at the right site of the triple line(Fig.3b).It is in a good agreement with solidus and liquids temperatures for AZ91 alloy reported by Zhang et al.[35],namely 470 °C and 598 °C,respectively.A slight shift of the melting temperature observed in the present work,might be attributed to a relatively high heating rate (20 °C min?1)applied in our SD experiments.As it can be seen in Fig.3,the irregular shape of AZ91 alloy piece did not significantl change during the whole test,i.e.upon heating up to 700 °C and holding for 5 min.This was caused by the presence of a native surface oxide layer hindering the complete melting of AZ91 alloy sample,as shown in Figs.3e-h.Consequently,the reliability of the measured contact angle values,far from the “true” values (i.e.values not affected by any “external”factors) is highly questionable [29].

Fig.2.Temperature profil of high-temperature wetting test performed on the AZ91/B4C couple.

Fig.3.Images recorded during the test of AZ91/B4C couple by using Contact Heating (CH) procedure: (a) the couple before test;(b) the beginning of melting;(c) the beginning of the test;(d-g) various stages of the CH experiment;(h) the end of the test..

Fig.4.Images recorded by the CCD camera during the test of AZ91/B4C couple by using Capillary Purificatio (CP) procedure: (a) the start of dropping molten AZ91 alloy from a graphite capillary;(b) a deposition of molten AZ91 drop on B4C substrate;(c) the beginning of the test;(d-g) various stages of the CP experiment;(h) the end of the test..

In contrast,a different behavior of the molten AZ91 alloy drop deposited on the B4C substrate was observed upon the wetting experiment performed by the CP procedure.In particular,the squeezed AZ91 alloy drop exhibited regular and spherical shape pointing out that the complete melting of AZ91 alloy in contact with the B4C substrate was reached and maintained until the end of the experiment (Fig.4).This different behavior results from the mechanical removal of native oxide layer by squeezing the molten AZ91 alloy drop through a graphite capillary.Therefore,it might be pointed out that in the case of CP couple,a “true” triple line and contact angle values were achieved,as well as reliable wetting kinetics were obtained.

The two solidifie AZ91/B4C couples after the hightemperature wetting tests performed at 700 °C/5 min by using the two different testing procedures (CH or CP),are compared in Fig.5.In the case of AZ91 processed by the CH procedure,the developed native oxide scale is clearly distinguished at the surface,as shown in Fig.5a.The existence of a thermally stable and thick oxide scale at the surface was confirme by the results of SEM/EDS analyses performed at the top of the solidifie AZ91/B4C sample after the CH test(Fig.8a).Whereas a regular round shape of the solidifie AZ91 alloy drop on the B4C substrate was observed for the sample produced by the CP procedure.In addition,a smooth and shiny surface free of the oxide layer was achieved in the solidifie CP drop (Fig.5b).The absence of oxide layer was confirme by the results of SEM/EDS analyses carried out at the top of the solidifie CP AZ91/B4C sample,as shown in Fig.9a.

3.2.Wetting behavior of AZ91/B4C couple

The wetting behavior of AZ91 alloy in contact with the B4C ceramic observed by applying the CH or CP procedures at 700 °C for 5 min is plotted as a wetting kinetics curve (θvs.time plots) in Fig.6.The evolution of the contact angle(values were measured on the right and left side of each drop)for the AZ91/B4C system is summarized in Table 2.

Fig.5.The images of AZ91/B4C couples (top,side and cross sections view) after the wettability test at 700 °C for 5 min,by using two different testing procedures: (a) the CH;and (b) the CP.

Fig.6.Wetting kinetics of molten AZ91 alloy on the B4C substrate recorded at test temperature 700 °C for 5 min,by using two different testing procedures:(a) the CH;and (b) the CP..

Table 2 Contact angle values (average from the left and right side of the drop) for the AZ91/B4C system,at T = 700 °C for two different test procedures (CH and CP).

When the CH procedure was applied,the contact angle value recorded over the entire experiment exhibited values significantl above 90°,i.e.above the cut off value definin the wettability/non-wettability conditions.In the CH procedure,the contact angle values varied between 116 and 113°after 1 min and 5 min of the test,respectively (Table 2).The obtained results point towards a conclusion that under the selected testing conditions of the CH procedure,the B4C substrate is not wetted by the liquid AZ91 alloy.However,these finding are quite misleading since,the lack of wettability in the AZ91/B4C system was caused by the presence of a native surface oxide layer hindering the complete melting of AZ91 alloy sample (Fig.5a),as well as a “real” AZ91/B4C interface.In fact,this situation excludes a possibility to measure a“true” value of the contact angle and makes the chemical and physical meaning of the values obtained in the CH procedure as “apparent” ones [29].

By contrast to the results obtained by the CH procedure,the CP wetting kinetics curve of the AZ91/B4C couple showed a plateau at the testing temperature within the wettability regime (i.e.below 90°) (Fig.6b).In the CP procedure,the contact angle values (θave) measured for 1 min intervals(Table 2) varied between 87 and 83° (after 1 min and 5 min of the test,respectively).This proves that the AZ91/B4C system meets the wettability requirements when a“real”intimate contact between the melt and ceramic is ensured through a mechanical surface purificatio of the alloy surface from oxide.Thus,it is documented that under exactly the same test conditions (temperature,time and pressure),a change of experimental procedure (from CH to CP) resulted in a change from a non-wetting to a wetting state.

In all previous studies [5,6,10,14,36-39] a very limited work has been done for studying the wetting behavior of liquid AZ91 alloy in contact with B4C,especially by using the CP procedure.Some available examples on similar metal/ceramic systems include e.g.a work by Vyas et al.[36] who analyzed the wetting behavior of liquid AZ91 alloys on ZrSiO4and Al2O3substrates by the SD method combined with the CH procedure atT= 680 °C and under a CO2+SF6protective atmosphere.The authors reported that ZrSiO4and Al2O3substrates are not wetted by the liquid AZ91 alloy.Indeed,the average contact angle values reported wereθ=137°andθ=145° for the AZ91/ZrSiO4and AZ91/Al2O3couples,respectively (Fig.7).Analogously to the present work,the authors concluded that the wetting kinetics observed at the AZ91/ZrSiO4and AZ91/Al2O3triple lines were strongly affected by the presence of an oxide layer on the liquid alloy surface.The affecting phenomena of oxidation and evaporation of Mg upon wettability tests,have been carefully described by Contreras et al.[10].The authors studied the wetting and spreading of pure Mg on TiC by the SD method and the CH procedure in a temperature range ofT= 800–1000 °C,under a static Ar atmosphere.A high contact angle value ofθ=120°,almost independent on temperature,was measured by the authors.However,an unusual change in the molten drop diameter was observed at 1000 °C,i.e.,it was firstl increased and then decreased,due to Mg deoxidation and its massive evaporation.Moreover,the wetting and spreading of liquid Mg on B4C,TiC and graphite substrates in the temperature range ofT= 700–1000 °C in a fl wing Ar atmosphere was studied by Zhang et al.[6],using the SD method combined with the CP procedure.They observed that the initial contact angle values of molten Mg on the B4C,TiC and graphite substrates were in the ranges of 95–87°,74–60° and 142–124°,respectively,decreasing with increasing temperature (Fig.7).Accordingly,the authors found that the Mg/graphite system is non-wettable,the Mg/TiC system is partially wettable and the Mg/B4C system changes from non-wetting to wetting state with increasing temperature(i.e.the transition was observed atT= 850 °C).By taking into account thermodynamic considerations [40],at high temperatures the oxide layer at the surface of the liquid Mg or Mg-based alloys can be thermally destabilized and removed.However,at temperatures much higher than the melting point of Mg and Mg-based alloys,the substantial evaporation of the material should be carefully considered,in order to limit the contamination of the experimental environment and to protect the apparatus against a severe damage [11].

Fig.7.Comparison of the presently obtained contact angle values in the AZ91/B4C system with those reported in literature for various AZ91/ceramic and metal/B4C systems,[6,10,36].

A comparison of the presently obtained results with those reported in the literature for similar Mg-based/ceramic systems is shown in Fig.7.It is found that the values of contact angle obtained by using the CP procedure in the present work,are in most cases lower (what means a better wettability) as compared to similar examined systems,such as AZ91/ZrSiO4and AZ91/Al2O3[25] (the CH procedure) or Mg/B4C [6] (the CP procedure).It should be mentioned that the value ofθ～12° reported by Cafri [32] for AZ91/B4C at 850°C was measured on porous substrates(with 20%of open porosity).In such a case the condition of perfectly fla and rigid surface requested for an adequate measurement ofθis not fulfilled Consequently,the values obtained on porous surfaces might be strongly underestimated,due to a combination of wetting and infiltratio phenomena [20,27-31].

From the practical point of view,the most important find ing coming from the presently obtained results,is that an implementation of mechanical purificatio (via the CP procedure) allows reducing contact angle values to that lying within the wettability range (θ=83°,θ<90°).This is crucial for the successful fabrication of composites through a pressure-less infiltratio approach,because higher contact angle values (θ>90°) induce a negative capillary pressure,as it comes from the Laplace-Young equation [41].However,it should be noted that this threshold value (90°) is only valid when a cylindrical geometry of pores is assumed.In a more recent work [42] assuming the effect of pore morphology (by introducing a pore shape coefficient) a theoretical threshold value was re-calculated with a new model and established to be at least 65° Similar values were experimentally obtained by Jiang et al.[43,44] as between 70 and 80° Furthermore,Tremble [45] has evaluated the spontaneous infiltratio into a bulk samples having non-cylindrical porosity.By taking into assumption a close-packed tetragonal arrangement of pores (that better reflect a distribution of pores in a real powder compact) the author has proposed that a spontaneous penetration of the firs layer of pores requiresθ=65.5°,but in order to complete the infiltratio into neighboring layers,the critical contact angle value should be reduced down toθ=50.7°

Fig.8.The results of SEM/EDS analyses of AZ91/B4C couple after the wettability test combined with CH procedure.From the top of the solidifie AZ91 alloy drop (a) and the cross-sectioned AZ91/B4C couple (b-d).The results of quantitative EDS analyzes (e) obtained from the areas presented in (a,c,d) and a distribution of particular elements in areas marked in (a,c,d).Exemplary EDS patterns taken from selected areas of the cross-sectioned couple (f).

Therefore,it seems that the value ofθ=83° presently obtained for the AZ91/B4C couple in the CP experiment at 700 °C/5 min might be too high to ensure a complete pressure-less infiltratio into a ceramic porous material.Most probably,a further decrease of the contact angle will require an increase the processing temperature.Indeed,it has been experimentally documented by Kevorkijan and Skapin [46] that the spontaneous infiltratio in the Mg/porous B4C system was not achieved at temperatures lower than 750 °C neither by prolonging an exposure time nor by modifying the preform structure.

3.3.Microstructural analysis

Typical SEM images taken at the top of the solidifie AZ91 alloy drops and from the AZ91/B4C interfacial areas produced during wetting tests by using the CH and CP procedure are shown in Figs.8 and 9,respectively.The selected sites were subjected to local SEM/EDS analyzes and the distribution of particular elements was examined.

Both structure and morphology of the analyzed AZ91/B4C solidifie couples vary depending on the applied experimental procedure.In the case of the couple processed by the CH procedure,the developed native oxide scale is clearly distinguished on the solidifie drop of AZ91 alloy,as shown in Fig.8a.A high oxygen content (～46 at.%) was detected by the EDS analysis (area 1 in Fig.8e).As it was discussed before,the use of the CH procedure does not make it possible to remove the native surface oxide layer from the liquid AZ91 alloy.The results of SEM/EDS analysis performed at the cross-sectioned AZ91/B4C couple (Fig.8b-d) revealed a dendritic structure containing mainly Mg and Al.The local EDS chemical composition analyses performed inside the AZ91 drop (areas 2 and 5 - Figs.8c and d) pointed towards a presence of around ～87 at.% Mg and 4 at.% Al.According to the Mg-Al phase diagram [47,48] and the work of Bonnah et al.[49],this composition corresponds toα-Mg solid solution.Furthermore,another phase consisting of Mg,Al and Zn was detected inside the interdendritic areas (areas 3,6 in Figs.8c,d respectively).The phase containing ～62 at.% Mg and 29 at.% Al can be recognized asβ-Mg17Al12intermetallic phase typically observed in AZ91 alloy [49].The presence of a large amount of O(11.9 at.%)and C(10.5 at.%)detected near triple point area(point 4 in Fig.8c)probably results from a severe surface oxidation,while carbon could be dissolved from the B4C substrate.

Fig.9.The results of SEM/EDS analyses of AZ91/B4C couple after the wettability test combined with CP procedure.From the top of the solidifie AZ91 alloy drop (a) and the cross-sectioned AZ91/B4C couple (b-d).The results of quantitative EDS analyzes (e) obtained from the areas presented in (a,c,d) and a distribution of particular elements in areas marked in (a,c,d).Exemplary EDS patterns taken from selected areas of the cross-sectioned couple (f).

In the case of the AZ91/B4C couple produced by applying the CP procedure,the native surface oxide layer was not observed on the solidifie drop (no traces of oxygen were detected from the SEM/EDS inspections) (Figs.9a,e).The results of EDS local chemical composition analyzes taken on the surface of AZ91 alloy drop (area 1 in Fig.9a) revealed around 91 at.% Mg,8 at.% Al and 1 at.% Zn,which is very close to the nominal chemical composition of AZ91 alloy.The microstructural analysis performed at the cross-sectioned AZ91/B4C CP couple,showed the presence of the same two main phases (namely,α-Mg andβ-Mg17Al12),as in the case of the sample produced by the CH procedure.(Fig.9c,d).

Interestingly,in both cases Al-enriched particles were detected close to the interface lines.By comparing Fig.8c and Fig.9c,these particles were distributed either on solid/liquid and solid/gas interfaces or only at the solid/liquid interface in the CH and CP couples,respectively.

A more detailed SEM/EDS study of interfacial areas(Fig.10) revealed that such crystals mostly consist of Al and Mn,while neither B nor C coming from the substrate were detected.By referring to the Al-Mn phase diagram [50],these crystals might be recognized as Al4Mn phase which is also commonly found in the AZ91 alloy [49].It is worth to mention that the presence of Al4Mn phase at both solid/liquid and liquid/gas interfaces in the CH sample confirm that the primary surface oxide layer served as an additional nucleation site for these crystals upon the solidificatio step.Contrarily,in the oxide free CP specimen Al4Mn crystals were observed only at the solid/liquid boundary.Furthermore,neither MgB2and MgB2C2nor any other additional new phases reactively formed between AZ91 and B4C were found,what points towards a limited reactivity between these materials under the conditions imposed in the present study.

4.Conclusions and future remarks

This work was focused on studying the wettability behavior of the liquid AZ91 magnesium alloy in contact with the B4C ceramic material,with the scope to improve liquidassisted processes for fabrication of CMCs or MMCs based on Mg materials.A special attention was given to the role of mechanical purificatio of molten alloy’s surface from growing up oxide scale resulting in a substantially improved wettability in the metal/ceramic system under investigation.For this purpose,the classical CH procedure was compared with the new CP one,while the experiments were performed at relatively low temperature of 700 °C.

The main finding of the present work can be summarized as follows:

1.A native oxide layer formed at the AZ91 alloy hinders the wettability with B4C ceramic substrate.In fact,it does not allow to produce a full drop shape required for a determination of reliable contact angle.The values measured on the CH couple covered by the surface oxide layer varied betweenθ=116 and 113°These results supports a misleading interpretation that under selected testing conditions,the B4C substrate is not wetted by the liquid AZ91 alloy.

2.A very different situation was observed when the mechanical purificatio of molten alloy was applied in the CP experiment.A well-shaped and oxide-free molten drop of AZ91 alloy was produced,while an intimate contact between the alloy and B4C ceramic was preserved through the entire test.Consequently,the contact angle ofθ=83°,i.e.within the wettability regime,was obtained.

3.The SEM/EDS microstructural and chemical analyses revealed the existence of phases typically observed in the ascast AZ91 alloy,namely,α-Mg,β-Mg17Al12and Al4Mn.There were no new interfacial products formed between AZ91 and B4C materials,which confirm that at temperature of 700 °C,the AZ91/ B4C is a non-reactive system.

Above-mentioned finding allow concluding that the mechanical purificatio allows significantl decreasing the contact angle value,in other words it induces a non-wetting to wetting transition in the AZ91/B4C without involving chemical reactions between the two components.It should be noted that so far,such transition has been obtained in reported works,as the effect of increasing processing temperature (up to 850 °C) and/or by introducing additional “wetting agents”(Ti or Si).However,in both alternative cases the reactivity of system is also increased,which could negatively affects the processing efficien y via blocking infiltratio channels.

Although some firs indications/recommendations for a new design of AZ91/B4C composites technologies are provided in the present work,further efforts and deep investigations are needed especially in terms of examining the effect of temperature on an improvement of wetting (to further support pressure-less spontaneous infiltration) Finally,aiming at scaling-up the process,the capillary purificatio approach in fabricating composites should be also promoted as a valuable and competitive alternative.

Declaration of Competing Interest

The authors declare that they have no known competing financia interests or personal relationships that could have appeared to influenc the work reported in this paper.

Acknowledgments

The studies were performed within the financia support given by the National Science Centre (NCN) in Poland,under the project MINIATURA 2,No.2018/02/X/ST8/03044 in 2019 – 2020.