2016 Volume 63 Issue 7 Pages 684-687
We have investigated the orientation relationship between TiB2 particles and Ni binder phase in TiB2-15 vol.%Ni cermet. About 25 % TiB2 particles showed the six types of orientation relationships with Ni. These TiB2/Ni interfaces were delimited by prismatic or basal plane of TiB2, and the most frequently observed relationship was (0 0 0 1)TiB2 // (1 1 1)Ni, [1 1 −2 0]TiB2 // [−1 1 0]Ni. This orientation relationship comprised about 50 % of the observed orientation relationships.
Hard materials composed of TiC and Ni is well known as cermets, and their major application field is for cutting tools. In recently, the cermet is in much attention for alternative material of cemented carbides (WC-Co), because cermets are composed of common elements. However, a disadvantage of cermets for cutting tools is a low thermal conductivity, because the TiC is a low thermal conductive material (20 W / (m K)1). Hence, we have tried making a high thermal conductive cermet using higher thermal conductive TiB2 hard particles (96 W / (m K))2,3). In fact, the TiB2-Ni cermet exhibited the high thermal conductivity of 65 W / (m K)4).
In hard materials, the interface structure plays an important role for mechanical, thermal and electrical properties. Hence, the orientation relationships have been investigated in cemented carbides5,6). In this cermet, the TiB2 particles showed faceted morphology. This suggests that the particular orientation relationship between TiB2 and Ni exists as in the case of cemented carbides. Therefore, in this study, the investigation of orientation relationship between TiB2 and Ni in TiB2-Ni cermet was examined.
Titanium diboride (TiB2; 98 % up) and Ni (99.9 %) powders were used for source materials. These powders were mechanically milled using planetary mill (Fritsch, Classic line P-5/4) for 14.4 ks under Ar atmosphere. The nominal composition of the milled powder was TiB2-15 vol.% Ni. The milled powder was sintered at 1793 K for 3.6 ks under vacuum. The microstructural observation was performed using scanning electron microscopy (SEM; KEYENCE, VE-8800). The orientation relationship between TiB2 and Ni was investigated using electron back scattered diffraction (EBSD). For analyzing Kikuchi pattern, the following crystal structures were used. The TiB2 has a hexagonal cell with space group of P6/mmm (191), and the lattice parameters are a : 3.0280 Å, c : 3.2280 Å. The Ni is a face centered cubic structure and its space group is Fm-3m (225) with lattice parameter of a : 3.5805 Å. For SEM and EBSD observations, the sample surface was polished using 1 μm diamond paste, and then surface residual stress introduced by polishing was released using ion-etching technique.
Fig. 1 shows the XRD pattern and SEM image of the cermet used in this study. The XRD reveals that the cermet is mainly composed of TiB2 and Ni. In addition, the un-known peak at 2θ = 44° is seen. The SEM image shows the TiB2 particles exhibit the faceted morphology, and this morphology is due to the hexagonal structure of TiB2 particles. The several TiB2 particles show an abnormal grain growth.
(a) XRD pattern and (b) SEM image of sintered TiB2-15 vol.% Ni.
Fig. 2 shows the macroscopic orientation distribution of TiB2 and Ni obtained from the same area. The Ni inverse pole figure map reveals that the Ni has coarse oriented microstructure with a few hundred micrometers scale, even though the interconnection of Ni phase was interrupted by TiB2 particles. In contrast, the TiB2 is randomly distributed as compared with the same scale of Ni. However, the detailed EBSD analysis clarified the several orientation relationships between TiB2 and Ni existed. The most frequently observed orientation relationship is shown in Fig. 3. From the SEM image and equal area projection maps, the TiB2 and Ni show the following relationship (OR1);
Inverse pole figure maps of (a) TiB2 and (b) Ni. The key diagrams are set in respective right upper corner.
Frequently observed orientation relationship between TiB2 and Ni. (a) SEM image of the sample. (b) Equal area projection of TiB2 obtained form b′ particle in (a). (c) Equal area projection of Ni obtained from c′ in (a).
From the above relationship, the estimated atomic arrangement at the boundary is depicted in Fig. 4. In this figure, the boron atoms are not displayed for simplify. In the case of this relationship, the large lattice mismatch of 18 % along <1 1 −2 0>TiB2 exist. Hence, the actual orientation slightly tilted about 10 degree from the above relationship.
Estimated atomic position at interface from Fig. 3. The boron atoms are not displayed for simplify.
In addition to the above relationship, five types of orientation relationships (OR2–OR6) were observed as listed in Table 1, and the estimated these 6 types atomic arrangements are shown in Fig. 5. All of TiB2/Ni interfaces are delimited by basal, (0 0 0 1)TiB2, or prismatic, (1 0 −1 0)TiB2, plane. In these orientation relationships, large lattice mismatches of 10–20 % exist. Therefore, the actual orientation relationships tilted about 10 degrees from the relationships listed in Table 1.
| Relationship | |
|---|---|
| OR1 | (0 0 0 1)TiB2 // (1 1 1)Ni, [1 1 −2 0]TiB2 // [−1 1 0]Ni |
| OR2 | (0 0 0 1)TiB2 // (1 1 1)Ni, [1 1 −2 0]TiB2 // [1 2 1]Ni |
| OR3 | (0 0 0 1)TiB2 // (0 0 1)Ni, [1 1 −2 0]TiB2 // [1 0 0]Ni |
| OR4 | (1 0 −1 0)TiB2 // (1 0 0)Ni, [1 1 −2 0]TiB2 // [0 1 1]Ni |
| OR5 | (1 0 −1 0)TiB2 // (1 1 0)Ni, [1 1 −2 0]TiB2 // [0 0 1]Ni |
| OR6 | (1 0 −1 0)TiB2 // (1 1 0)Ni, [1 1 −2 0]TiB2 // [−1 1 0]Ni |
Estimated atomic positions at TiB2/Ni interface. No. 1–3 and 4–6 are delimited by basal and prismatic plane of TiB2, respectively.
Fig. 6 shows the fraction of the observed orientation relationship. About 25 % particles exhibit the particular orientation relationships with Ni. In these particles, about 50 % TiB2 particles showed the OR 1. This is a sufficient fraction to ensure that the existence of the preferable orientation relationships between TiB2 and Ni.
Fraction of the orientation relationships between TiB2 and Ni.
These orientation relationships would be formed to reduce the interfacial energy in the cermet. In the sintering process, it was expected that the TiB2 particles were surrounded by liquid Ni; thus the rotation of TiB2 particles easily occurred. Hence, during the cooling process, the TiB2 particles would rotate to reduce the interfacial energy between TiB2 and Ni, resulting that the particular orientation relationships were formed. These relationships would contribute to mechanical and thermal properties in the cermet. However, the actual contribution has not been clarified. The further investigation, such as a direct observation of interface in an atomistic scale, is desired.
The orientation relationship between TiB2 and Ni was investigated using EBSD technique. The Ni binder phase showed coarse oriented microstructure of a few hundred micrometers scale. In contrast, the TiB2 particles exhibited random orientation distribution in the same scale of Ni. However, the particular orientation relationships between TiB2 and Ni were observed. About 25 % TiB2 particles exhibited the orientation relationships with Ni. The most frequently observed orientation relationship was (0 0 0 1)TiB2 // (1 1 1)Ni, [1 1 −2 0]TiB2 // [−1 1 0]Ni, which comprised about 50 % of the observed orientation relationships.
