The influence of Ti addition on fracture toughness and failure of directionally solidified LaB6–ZrB2 eutectic composite with monocrystalline matrix

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The influence of Ti addition on fracture toughness and failure of directionally solidified LaB6–ZrB2 eutectic composite with monocrystalline matrix Halyna Volkova a,∗ , Vladimir Filipov b , Yurij Podrezov b a

b

National Technical University of Ukraine “Kiev Polytechnic University”, 35 Polytechnichna str., Kiev 03056, Ukraine Frantsevich Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, 3 Krzhyzhanovsky str., Kiev 03142, Ukraine

Abstract Fracture toughness and failure mechanism of directionally solidified eutectic composites LaB6 –ZrB2 and LaB6 –(Zr0,9 Ti0,1 )B2 were investigated. The addition of Ti increases the cohesion strength of the matrix–fiber interface, causes redistribution of stresses in the whole composite, and thus influences the mechanism of crack propagation. Fracture toughness of composites was determined by Brazilian test. The central crack was introduced in the plane, parallel to the axis of the eutectic rod, in direction, perpendicular to the axis. Such experimental setup enables to investigate the interaction of crack with fiber–matrix interface and eliminates the effect of other factors on K1C . The present investigation shows that Ti additions to LaB6 –ZrB2 result in up to 25% increase of fracture toughness of the composites. © 2014 Elsevier Ltd. All rights reserved. Keywords: In situ reinforced composites; Directional solidification; Fracture toughness; Fractography; Cathodes

1. Introduction LaB6 is a well-known thermoemission material. The monocrystalline LaB6 cathodes exhibit the highest stability and the longest working times, but have a major disadvantage: they are brittle. The strict technical requirements exist for fracture toughness of cathodes, because often it is necessary to produce the cathodes of complicated shapes or of small cross-sections. That is why it is very important to find the way to enhance the existing cathode materials. The very good possibility to enhance the properties of LaB6 is to create the in situ eutectic composites LaB6 –MeB2 on its basis. It is the reason why the specific features of crystallization and microstructure of directionally solidified composites with MeB2 fibers, where Me was Ti, Zr, Hf, V, Nb, Ta, Cr, Gd, Al, were studied by different authors. Especially detailed study was conducted on systems with Ti, Zr, and Hf.1–4 LaB6 –ZrB2 composite was chosen as one that had optimal properties, because strict crystallographic orientation relationship between

∗ Corresponding author at: Grenoble Institute of Technology, 46 avenue Félix Viallet, 38031 Grenoble Cedex 1, France. Tel.: +33 630735515. E-mail addresses: [email protected], [email protected] (H. Volkova).

the lattices exist in this composite and interfaces between phases are semi-coherent (Fig. 1). As a result, in the experiment where crack is introduced perpendicular to the fibers fracture toughness is up to 18.3 MPa m1/2 5 (but is lower in other directions). Moreover, thermoemissive properties of this composite are increased.6 It should be mentioned also that high melting temperature of LaB6 –ZrB2 eutectics (2740 K) and high tensile strength (200 MPa) at the temperature of 2000 ◦ C are the prerequisites for the successful use of this type of materials for ultra-high-temperature structural applications.5 This work is aimed at further enhancement of mechanical properties of LaB6 –ZrB2 composites. Many articles consider the fracture toughness dependence of composites on the properties of components and interfaces, dimensions of inclusions, density of their distribution and shape.8–11 The common conclusion of majority of works is that higher amount of energy is dissipated in the case of interface debonding, than in the case of matrix or particle cracking. But the necessary condition for increase of K1C is high enough fracture energy of the interface (cohesion energy). Although in the cited works the concept was proved experimentally only on the samples with size of inclusions ≥8 ␮m, there are no apparent reasons to alter this effect for materials with smaller fiber size. Furthermore, the effect could be even more pronounced for composites with smaller fiber size since in such

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Fig. 1. Photographs of transverse section of LaB6 –ZrB2 eutectic rod at different magnifications. (a) General appearance of structure, (b) semi-coherent fiber–matrix interface.7

case the number of objects hindering crack propagation would be even higher. As far as interface debonding by crack is observed in LaB6 –ZrB2 ,12,13 the concept that was used to influence the fracture toughness is based on the idea of increasing the cohesion energy of an interface by adding atoms that form solid solution with zirconium diboride, so that the lattice parameters of fibers are changed. Titanium is especially suitable for this purpose, because it forms Tix Zr1−x B2 solid solution, where lattice parameter decrease according to Vegard’s law14 with increase of x, and, thus, the misfit between lattices of fiber and matrix can be decreased. Earlier investigations of structure for LaB6 –(Tix Zr1−x )B2 composites with x = 0, . . ., 0.76 that were performed by Jouanny et al.14 showed that addition of significant amount of Ti causes alteration of fiber growth directions from [0001] for LaB6 –ZrB2 to <-1100> for LaB6 –(Tix Zr1−x )B2 with x≥0.32. The interfaces in both composites consist of semi-coherent ledges of different length that alternate in order to produce a fiber with rounded cross-section. The coincident planes on the most energetically favorable (frequent and long) facets after Ti addition are (0 0 0 1)-Tix Zr1−x B2 //(0 0 1)-LaB6 , while the initial facets were characterized by {1 1¯ 2 0}//{1 1 0}. It should be mentioned that in LaB6 –(Tix Zr1−x )B2 composites with x ≥ 0.32 fibers no longer have a round-shaped cross-section, as the new facets are longer than in the initial composite. As it has been established earlier, both the homogeneity of distribution in the matrix and the uniformity of the length of the fibers improves when the (Tix Zr1−x )B2 solid solution is used as a reinforcing phase as compared with the x = 0 or 1.15 Moreover, the LaB6 –TiB2 system tends to form relatively large inclusions of the diboride phase even under conditions of excessive content of hexaboride in the melt. But after substitution of 5% of Ti atoms for Zr (LaB6 –(Ti0.95 Zr0.05 )B2 composite) this effect was completely eliminated.15 Therefore, considering the specific features of the crystallization process described in Refs. 14,15 it is possible to suppose that titanium additions to the LaB6 –(Tix Zr1−x )B2 system result in some changes of interaction energy on the phase interface, which, in their turn, might effect the crack propagation process in such a material.

Fig. 2. Scheme of specimen that was used for fracture toughness test (crack in 1 0 0 direction of matrix).

Fracture toughness of two types of composites, LaB6 –ZrB2 and LaB6 –(Ti0.1 Zr0.9 )B2 , was investigated. Since both phases in eutectic rod are anisotropic, it is also essential to study the anisotropy of fracture toughness for each composite. Two types of cracks were introduced for each type of composites in different planes, that correspond to crystallographic directions of matrix 1 0 0 and 1 1 0. 2. Materials and methods Samples/rods of eutectic composition were grown by directional crystallization using floating zone method. Since LaB6 matrix is a conductive material, high-frequency heating (0.88 MHz) was used. The rods of raw material for directional solidification were produced by cold pressing of powders of zirconium diboride, titanium diboride, and lanthanum hexaboride, and subsequent sintering at 1700 ◦ C in vacuum. The starting powders were synthesized from chemically pure components. In order to accomplish the growth of a eutectic rod with the desired preferential crystallographic orientation of the matrix a seed single crystal of LaB6 was used as a seed. The process was conducted using a “Krystal-111” setup under argon atmosphere. The same parameters of crystallization were maintained for all the samples. The characterization of growth directions of phases in LaB6 –(Ti0.1 Zr0.9 )B2 eutectic rod was conducted on a X-ray diffractometer Ultima IV Rigaku. A special scheme of fracture toughness test was used. The method of Brazilian test was chosen for this purpose (Fig. 2). This method implies diametric compression of the disks with a central crack and allows to test the specimens of small dimensions, which is essential, because the diameter of eutectic rods is ∼5 mm. The central crack was introduced by electro-spark cutting in the plane parallel to the rod (growth) axis, so that

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of electro-spark cutting are atomically sharp or close to atomic sharpness). The fracture test was conducted on the setup for testing of ceramics “CeramTest”. The loading rate was 0.2 mm/min. The fracture surface of specimens after the fracture toughness test was examined by means of scanning electron microscopy. Only surfaces that formed as a result of main crack propagation were studied. 3. Results and discussion 3.1. Pole figures

Fig. 3. The appearance of the crack edge in the LaB6 –ZrB2 composite.

the direction of crack propagation should be perpendicular to the axis (Fig. 2). Such positioning of a crack enables to investigate the interaction of crack with fiber-matrix interface and insures that fracture toughness value depends only on the interface properties. The introduction of the crack to the central part of the sample ensures that the value of fracture toughness is not dependent on the properties of the outer layer of eutectic rod, which can contain chemical contaminations or irregularities of structure.3 The method has advantages of higher fracture stress, and lower dispersion of results for the same materials. Failure mode is pure mode I for vertical crack position (Fig. 2) due to the tensile stresses that appear near the edge of the crack after loading.16,17 Fracture toughness can be calculated using the following relation18 :  1.01227 · Pf λ √ K1C = · (1 − 0.60387λ · 1−λ h λR + 1.67239λ2 − 1.16988λ3 ) where 2a λ= , D 2a is the length of crack, D is the specimen diameter, Pf , h and R are respectively the fracture stress, the specimen height and radius. For each composite samples with two types of cracks were prepared. Cracks were introduced by electro-spark cutting in 1 0 0 and 1 1 0 crystallographic directions of matrix, perpendicular to the axis of macroscopic eutectic rod. Cracks in 1 1 0 direction were introduced at 45◦ angle to 1 0 0 direction, which was determined by X-ray experiment in back-reflection geometry. After preparation of samples for fracture toughness test special attention was paid to the examination of the edge of the crack that was introduced into the specimen by electro-spark cutting. As it can be seen from Fig. 3, the sharp thin microcracks form on the edge of the crack. The presence of such microcracks is very important, as it insures that the fracture toughness measurement is performed correctly (the cracks which are formed as a result

Taking into consideration the changes of the growth direction of fiber phase, that occur at high Ti content (>32% Ti), it was interesting to investigate the structural peculiarities of LaB6 –(Ti0.1 Zr0.9 )B2 composite. The analysis of obtained pole figures shows that LaB6 matrix is monocrystalline, without subgrain blocks, as only four clear peaks of high intensity are observed for {1 1 0} pole figures (Fig. 4b). On the {1 0 0} pole figure the only intensive peak is situated near the center of the figure (Fig. 4a). Thus, the growth direction of matrix is 1 0 0. On the (0 0 0 1) pole figure of Ti0.1 Zr0.9 B2 two peaks are visible near the central point with an angle of 15◦ between them (Fig. 4c). Only peaks of negligible intensity were found on the {1 1¯ 2 0} pole figure for Ti0.1 Zr0.9 B2 . This gives possibility to conclude that growth direction of fibers in the composite with Ti addition is close to [0 0 0 1], that is the growth direction stays the same as in initial composite.12,14 Despite there is no change of growth direction in LaB6 –(Ti0.1 Zr0.9 )B2 composite, there are some other facts, such as stabilization of eutectic crystallization process, indicating that the energy of the interface changes. 3.2. Fracture toughness test During the fracture test, the specimens of both composites exhibit brittle failure. The failure happens instantaneously, with the speed that is usually assumed to be close to the speed of sound in ceramic materials. As far as the deformation speed was constant for all the samples, the speed of crack propagation is also assumed to be approximately the same. In our early studies3 it has been stressed that under threepoint bending test with the stress concentrator, introduced into the sample perpendicular to the crystallization direction (i.e., perpendicular to the reinforcing fibers), the crack propagation occurs at a certain angle (40–75 degrees) to the load application direction. The loading scheme used in the present work enabled to maintain crack propagation along the plane of the initial cut/notch. The main crack develops from one of the sharp microcracks that are introduced during electro-spark cutting (Fig. 5a) and then propagates in the direction of introduced central crack independently from its type (1 0 0 or 1 1 0). For all the specimens the crack does not penetrate inside the fibers, but is deflected

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Fig. 4. Pole figures for phases in LaB6 –(Ti0.1 Zr0.9 )B2 composite. (a) {0 0 1} planes of LaB6 ; (b) {1 1 0} planes of LaB6 ; (c) (0 0 0 1) planes of (Ti0.1 Zr0.9 )B2 .

by them and crack propagation occurs exclusively through the matrix (Fig. 5b). The results of fracture toughness tests are shown in Table 1. It can be seen that the average value of fracture toughness for each type of cracks is more than 25% higher for LaB6 –(Ti0.1 Zr0.9 )B2 composites than for initial composites (LaB6 –ZrB2 ). The calculations of the confidence interval of correlation according to the Student criterion proved the existence of correlation with >98% probability. The results also show that for both composites the average values of fracture toughness are higher in 1 0 0 direction of matrix than in 1 1 0 direction. Although the confidence interval of correlation is not high, it is still possible to assume that some anisotropy exists for fracture toughness in these two directions. 3.3. Fracture surface examination The fracture surface analysis indicates the Ti effect on failure within both crack directions. At the same time, there are common features for fracture surface patterns within one direction, and some differences between directions, that signify the effect of anisotropy. The common feature for 1 0 0 cracks is that cleaved microplates of the matrix material are observed on the fracture surface (Fig. 6). For 1 1 0 cracks, the cleaved microplates are absent. This initial difference in fracture surface appearance

between two crack directions is due to the easy cleavage plane {0 0 1}.19 Thus, the intense matrix cleavage by microcracks in {0 0 1} planes plays an important role in case of 1 0 0 direction of main crack propagation. The influence of cleavage is independent from Ti/Zr ratio in fibers, because there is no solubility of these elements in matrix.14 To the contrary, in 1 1 0 case there are no additional factors that affect K1C . Thus, it is possible to suppose that the difference in K1C , introduced by Ti atoms for 1 1 0 crack, should be more significant, then for 1 0 0 cracks. Indeed, the same can be seen after comparison of K1C values in Table 1. It should be mentioned that despite the significant increase of K1C values after Ti addition, the difference in fracture surface appearance between two composites is less pronounced at the used magnifications than the difference between two crack directions. The change of K1C values in the frame of the same crack direction can be partly explained by the joint effect of the increase of interface cohesion strength, and changes of fiber characteristics after Ti addition, such as young’s modulus, shear modulus, and coefficient of thermal expansion. The decrease of the misfit in distances between the boron atoms in matrix and fiber from 3.2% to 2.8% might result in significant increase of the interfacial cohesive energy. Thus, if the crack debonds an interface, it results in fracture toughness

Fig. 5. The main crack path in the 1 0 0 direction of LaB6 matrix, visible on the face of the sample (LaB6 –ZrB2 composite). (a) Development of main crack from one of microcracks; (b) appearance of main crack at high magnification.

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H. Volkova et al. / Journal of the European Ceramic Society xxx (2014) xxx–xxx Table 1 Fracture toughness values for samples of different composites with different crack orientations. # of series

Samples of composites with different crack orientations

K1C , MPɑ m1/2

1 2 3 4

LaB6 –ZrB2 1 1 0 LaB6 –ZrB2 1 0 0 LaB6 –(Ti0.1 Zr0.9 )B2 1 1 0 LaB6 –(Ti0.1 Zr0.9 )B2 1 0 0

6.05 7.03 8.19 8.97

± ± ± ±

1.03 1.07 1.11 1.22

Number of samples

Confidence interval (1 − ␣) of correlation according to Student criterion

5 7 6 6

Correlation between %Ti and K1C : Series 1 and 3 ␣ > 98% Series 2 and 4 ␣ > 98% Correlation between plane angle and K1C : Series 1,2 80% < α < 90% Series 3,4 70% < α < 80%

Fig. 6. SEM micrographs of fracture surface of two composites, for cracks, introduced in different directions of matrix.

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Table 2 Values of Young’s modulus, Poisson’s ratio, and coefficients of thermal expansion of phases.21–26

LaB6 ZrB2 TiB2

Ex = Ey , GPa

Ez , GPa

G, GPa

αx = αy , 10−6 K−1

αz , 10−6 K−1

451.9 533.5 633.3

451.9 389.6 432.3

163 250 277

5.6 6.66 6.35

5.6 6.93 9.30

increase. The second factor, coefficient of thermal expansion, slightly decreases in XOY plane, but increases in Z direction. This should create residual compressive stress fields of complicated shapes around the fibers. Finally, the increase of relation between shear moduli Gfiber /Gmatrix , as well as decrease of difference in elastic moduli (Table 2) changes the distribution of stresses near the crack edge. It should decrease the matrix tensile stresses parallel to the interface, generated by crack).20 As a result, the overall compressive stresses, introduced by thermal expansion coefficients, should have higher value during crack propagation. However, despite all the above mentioned changes might cause some increase of K1C , it can be estimated that their joint effect can not cause a 25% elevation of the fracture toughness. Such a significant increase can be explained by the change of the electronic structure in the fiber phase, which was observed in Ref.14 and the resulting change of interactions between charges on the phase interface. In order to give a comprehensive explanation of the Ti influence on K1C for two crack directions, the detailed study of interface structure is planned in the future. 4. Conclusions The effect of Ti addition on the fracture toughness and failure of LaB6 –ZrB2 and LaB6 –(Ti0.1 Zr0.9 )B2 was studied. Investigation showed that: (1) The addition of Ti causes the 25% increase in fracture toughness (K1C ) in comparison to initial LaB6 –ZrB2 composites. This increase is, first of all, due to the change of interaction between lattices of components on the interface. In addition, the stressed state alteration due to decrease of lattice misfit, change of shear and elastic moduli, and thermal expansion coefficient also can play role in K1C increase. More complete explanation of Ti effect can be given in future after investigation of interface structure. (2) The study shows that except the evident effect of Ti on fracture toughness, there is also the effect of anisotropy. The difference in failure mechanism between two crack directions, namely presence of additional matrix cleavage for 1 0 0 crack, and it is absence for 1 1 0 crack, corresponds to more pronounced effect of Ti addition on the fracture toughness value for the second case. Acknowledgement The present research was sponsored by Science & Technology Center of Ukraine under project №. P512 (EOARD 118001).

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