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Th e Fabrication and Characterization of Ternary Boride Composites by Combustion Synthesis Method

Sevinch Rahimi Moghdaam, Bora Derin

Istanbul Technical University, Faculty of Chemical and Metallurgical Engineering, Department of Metallurgical and Materials Engineering, Istanbul, Turkey

Abstract

In this study, the influence of different stoichiometric ratios of raw materials such as metal oxides (MoO3, Fe2O3, etc), B2O3 and Al on the formation of complex ternary borides of M3B2-type structure was investigated. The simultaneous production of hard phase and matrix was achieved using simple and low-cost combustion synthesis process. The simulation of the initial mixture was done to estimate the adiabatic temperature and the possible phases by using FactSage. The composition, microstructure and mechanical properties of products were analyzed by SEM, XRD, AAS, and microhardness methods.

1. Introduction

Due to the uneven distribution of tungsten resources, some studies have focused on finding alternative materials with less or no tungsten in composition [1]. Transition�metal borides are known for their unique physical, chemical and mechanical properties such as good refractoriness, chemical inertness, high hardness, and metallic conductivity [2]. Takagi et al have synthesized composites consist of M3B2-type complex ternary borides (Mo2NiB2

or Mo2FeB2) as hard phase and a transition metal as a matrix using reaction boronizing sintering method. They found that these types of cermets have been presented excellent physical properties such as high hardness and TRS values of about 84-89 HRA and 1.7-2.4 GPa, respectively. The Mo2FeB2 based cermets have high hardness and fracture toughness values similar to cemented carbide cermets. They possess lower density and thermal expansion coefficient that found to be close to steels [3,4]. Because the sintering method has some drawbacks such as high energy consumption, choosing an inexpensive production method for these hard materials to increase their abundance has great importance. Self-propagating high temperature synthesis (SHS) is a method which has been used for the production of high temperature ceramics, intermetallics and composites in a one-step operation. This technique has the advantage of providing high purity products, low energy requirements

and simplicity of the process over the conventional high temperature methods [5]. Different types of pure refractory metals, binary and ternary alloys, ceramics and composites have been produced using thermite-type SHS process during reactions occurring in the exothermic systems [6]. The indicative parameter of self-propagating reaction is adiabatic temperature (Tad) which has to be higher than 1527°C (1800K). In this study, the formation of Mo2FeB2 ternary boride phase in composites with intermetallic (Fe-Al) matrix during SHS synthesis method was investigated. The effect of raw materials ratio on the phase composition and microstructure of products was studied.

2. Experimental Procedure Before every experiment, thermodynamical studies were

conducted by using FactSage 7.0 software[referans n koymakaleninaynisiolsun]. The effect of raw material ratios on the SHS results was determined using “Equilibrium” module of the Factsage7.0 which assumes chemical equilibrium by “Gibbs Energy Minimization” method. In the experiments, MoO3 (99.5% purity), Fe2O3 (95% purity), B2O3 (94% purity), Al (99.7% purity) and Al2O3

were used as raw materials.B2O3 was obtained by the calcination of 99.5 % pure boric acid (H3BO3, Eti Holding Inc.) in a nickel crucible at 800 °C for 2 hours followed by milling and sieving. The amounts of prepared raw materials mixture werecalculated from the following equation (1) and presented in Table (1).

1 (Fe2O3+2Al) + 2 (MoO3+Al) + 3 (B2O3 + Al) FexMoyBz -Fe+ Al2O3 (1)

The thoroughly mixed raw materials were dried at about100°C for 30 minutes and were compacted. The mixtures were placed into a copper crucible with an inner diameter of 50mm, a wall thickness of 5 mm and a height of 45mm.A tungsten filament on top of the powder mixture,

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connected to a 20VA power suppexothermic reaction. After the combustmetallic part was separated from the slbulk metallic samples were metallographically prepared for furtcrystal structure of the final product is ray diffractometer (XRD, PANalytical PCu Ka radiation), microstructure andphases were characterized by scmicroscopy and energy dispersive speJeol JSM-840) and the elemental analysby AAS spectrometry (Perkin-Elmer hardness test (Shimadzu Corporation carried out on the specimens to identifproperties.

Table 1. Amount of raw materials production of SHS composites

Experiment number MoO3/F

Sample-1 1.6/0

Sample-2 1.6/0Sample-3 1.6/1

3. Results and Discussion

In order to simulate the SHS reaction, 2 mmole of Fe2O3, 1 mole of B2O3 werdifferent amounts of Al. The reactions assumed as adiabatic ( H=0) and temperature was selected as 25 °C. Figuthe Al content in the raw materials inreduction of oxides starts. Among the odissolve in the slag phase due to temperature. MoO3 transforms to MoO2

release Mo when Al reaches to 4.2 molephase reacts with Mo to form alloys. Aamount of Al (7.5 moles), the elemental concluded from the figure that with incrreducing agent, the total gaseous phase cAlBO2(g), % 34.714 Al2O(g), % 3.34 Fe1.22 (BO)2(g), % 1.162 Al(g) and % 1system increases. The adiabatic temperFactSage at the stoichiometric ratio of rato 2700°C. The SHS product consist of % 68.17 MoB, and % 2 Al (in %wt) at the stoichiomthe system. Thermodynamically, unreduction of metal oxides and boron tridiffusion of Al in the final product comavoided due to the high percentage of g

ply, initiated the tion synthesis, the lag. The produced

weighed and ther analyses.The determined by X-

PW3040/60 with a d composition of canning electron ectroscopy (SEM, sis was performed 1100B). Vickers HMV) was also

fy the mechanical

(moles) used for

Fe2O3/B2O3/Al

0.6/0.8/6

.9/0.8/6.5

.3/0.8/7.2

moles of MoO3, 0.75 re equilibrated with of the process were the initial reaction ure shows that when ncreases, the partial oxides B2O3 starts to

its lower melting 2 and then reduces to es. Then Fe in liquid At the stoichiometric

B and Al form. It is reasing Al amount as consisting of % 44.74 e(g), % 2.73 BO(g), 1.113 B2O3(g) in the rature calculated by aw materials reaches

o, % 26.1 Fe, % 3.74 metric ratio of Al in

nless the complete oxide occurs but the

mposition couldn’t be gas formation during

According to the results of different ratio of raw materiobserve the effect of increasin

Figure 1. Thermochemicacomposition ver

The elemental analysis of thethat there is a difference betwresults of B and Al weconcentration in alloys reachethe Fe in the compositions. As it is expected, diffusion othe formation of Fe-Al intermin matrix phase of the obtainthe XRD results as shown in F

Figure 2. XRD results of comsample-2 and 3) sample-3

Mo2FeB2 and FeAl)

FactSage calculations, three ials was chosen in order to

ng Fe2O3 amount were used.

al calculation for product rsus Al additione products with AAS shows

ween estimated and observed ight percentages. The Al

es to 11-12% wt by replacing

of Al in final product causes metallics as a dominant phase ned composites according to Figure 2.

mposites with 1) sample-1, 2) 3 ( MoB, Mo0.87.5Fe0.125,

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combustion. At the other hand, the tota99.12 Al2O3, % 0.52 B2O3, % 0.366 FeO(in %wt).

By starting Fe2O3amount from 0.6 inmixture, the needle-like MoB (point1, was produced in sample-1.

Figure 3. SEM analysis of the sam

As in sample3 with higher Fe2O3 (1.3disappears and ternary boride of Moinstead. The morphology of the producematrix phases in composites were obseanalysis methods. The formation of boride is confirmed by the EDS spectromFig.4). The particles with a rectangular sdetermined to be Mo2FeB2 (M3B2-type) the tetragonal crystal structure. The dispe(point2, Fig.4) in the matrix were also deMoFe by EDS analysis. The matrix consists of Fe-Al binary composition wiMo (about 12%wt) as shown in figure4 (The average hardness values of thesobtained from the Vickers hardness testifrom 5to 10 different points at 5N indentThe average hardness value reaches 8853. The strength of composites reinforcement/matrix ratio and the percborides in the as-cast alloys.

al slag consist of % O and % 0.01 Fe2O3

n starting materials Fig.3) binary phase

mple 1 (x500)

3moles), MoB phase o2FeB2 was formed ed reinforcement and erved by SEM/EDS

Mo2FeB2 complex metry results (point1, shape (light gray) are ternary borides with ersed eutectic phases etected as iron-based (dark gray region)

ith the dissolution of (point3). se composites wereing of samples tested tation load. 5HV±170 in sample-

depends on the centage of produced

Figure 4. SEM analysis

4. Conclusion The M3B2 complex boride FeAl matrix were produced bthis study, the effect of Festructure and composition ofLarge differences were foundmatrix hardness values rangin

References [1] R. Subramanian & J. H. ScWC-based cermets by preIntermetallics 5, 401-408, 1997[2] Goldschmidt, H.J. 1967. “IButterworth & Co (Publishers) [3] Takagi, K. 2006. “Developstrength ternary boride base ceChemistry, 179, 2809–2818 [4] Takagi, K.i., Yamasaki, Atomic Ratio on the MechanicMo2NiB2 Boride Base CermetJournal of Solid State Chemistry[5] Yukhvid, V. I. “ModificatioAppl. Chem, 64, 977-988, 1992[6] A. Varma, A. S. RogachevAdvances in chemical engineeri

of the sample 2 (x500)

containing composites with by SHS synthesis method. In e2O3 amount on the phase f products was investigated. d between reinforcement and g between 700 to 1100HV.

chneibel, Intermetallic bondedessureless melt infiltration,

Interstitial alloys”, New York: Ltd

pment and application of high ermets” Journal of Solid State

Y. 2000. “Effects of Mo/B cal Properties and Structure of ts with Cr and V Additions”, y, 154, 263-268.

ons of SHS processes”, Pure & 2v, A. S. Mukasyan, S. Hwang,ing 24, 79 (1998)