There are three intermetallic compounds in the Ni-Ti system, Ni₃Ti, NiTi and NiTi₂, among which NiTi has received much interest for its shape memory effect [1]. NiTi₂ has favorable properties such as high corrosion resistance and wear resistance and Coating of Ti base alloys by NiTi₂ films has been examined [2]-[4]. On the other hand, bulk application of NiTi₂ has not been reported probably because of its poor ductility caused by the complicated E93-type lattice structure which contains 96 atoms in a unit cell [5].

The effective method to improve the mechanical properties is adding reinforcement components. Titanium boride (TiB) has many excellent properties such as high hardness (Vickers hardness, Hv, 16 GPa), high strength, high Young modulus (E = 371 GPa), low density (4.54 × 103 kg/m³) and good corrosion resistance [6] [7]. It has needle like shape which is favorable for reinforcement materials [8]-[10].

In the present study we have fabricated monolithic NiTi₂ and NiTi₂-TiB composite by using Spark Plasma Sintering (SPS) method and investigated the Vickers hardness and bending strength. It has been shown that NiTi₂ is brittle and has poor fracture toughness which can be improved by the addition of TiB.

The raw materials were commercially available TiB₂, Ti and Ni powders of 99.9% purity (all powders by Furuuchi Chemical Corp.). The diameters of TiB₂, Ni, and Ti powder particles were 2 microns, 10 microns and 30 microns, respectively. TiB is formed from TiB₂ and Ti during the sintering by the reaction,

A graphite die having an internal diameter of 20 mm and a wall thickness of 10 mm was filled with 5 g of the mixed powders of TiB₂, Ni and Ti and sealed by two graphite punches and mounted on the equipment, LABOX- 630, fabricated by Sinter Land Ltd. The sintering temperature is determined as 950˚C since NiTi₂ melts peritectically at 984˚C. The mixed powders were heated to 950˚C with the rate of 50˚C/min, kept for 20 minutes, and then furnace cooled to room temperature. The temperature was measured using the infrared radiation thermometer IR-AHS0 fabricated by Chino Corporation. Sintering was performed in a vacuum with a residual pressure of 5 Pa. A uniaxial pressure of 20 MPa was applied during the sintering.

Scanning electron microscope (SEM) observation was performed using a Hitachi H-3300 instrument. X-ray diffraction patterns were measured using Rigaku X-ray diffract meter with Cu-kα radiation source. Vickers hardness was measured using a Shimadzu HMV-2 micro hardness tester with the load of 9.8 N and pressing time of 15 seconds. Five indentations are made to measure the length of indentation diagonals. To perform the bending test, rectangular shape samples with the size of 2 × 3 × 20 mm³ were cut by the electric discharge. Three point bending tests were performed using a Shimadzu AGS-J test machine with the crosshead speed of 0.5 mm/min and the span of 15 mm. Four specimens ware measured for each composition.

Figure 1 shows the X-ray diffraction pattern of NiTi₂, NiTi₂-20vol%TiB, NiTi₂-40vol%TiB and NiTi₂-60volt% TiB specimens sintered at 950˚C. In the X-ray diffraction patterns of NiTi₂, peaks from cubic E93 type structure are observed. In the X-ray diffraction pattern of NiTi₂-TiB composites, peaks from TiB have been observed as indicated by arrows in addition to peaks from NiTi₂ showing that NiTi₂ and TiB co-exist in equilibrium at 950˚C. It should be noticed that no peaks from TiB2 is observed indicating that the chemical reaction of TiB2 and Ti have completed during the sintering process at 950˚C.

In Figure 2 is shown the SEM images of the polished surface of (a) NiTi₂-40vol%TiB and (b) NiTi₂-70vol%TiB. In Figure 2(a), it has been observed that dense samples are obtained with TiB grains of needle like shape dispersed in NiTi₂ matrix by the sintering at 950˚C. On the other hand, in Figure 2(b), some cracks have been observed as indicated by arrows probably because of the thermal mismatch between TiB and NiTi₂.

We have performed the 3-point bending test and the Vickers hardness test to measure the mechanical properties of NiTi₂-TiB composites. In Figure 3 is shown the dependence of the bending strength of NiTi₂-TiB com- posites on the amount of TiB. The bending strength of NiTi₂-10vol%TiB is 290 MPa, which increases as the amount of TiB increases up to 60% and the highest bending strength of 730 MPa has been obtained. The bending strength decreases as the amount of TiB increases above 60vol%. We could not measure the bending

strength of monolithic NiTi₂ because NiTi₂ specimens have been cracked during the cutting procedure by using electric discharge. This decrease of the bending strength may be caused by the cracks as observed in Figure 2(b).

Figure 4 shows the dependence of the Vickers hardness of NiTi₂-TiB composites on the amount of TiB. The Vickers hardness of NiTi₂ is 650 Hv. The Vickers hardness of NiTi₂ increases as the amount of TiB increases up to 70 vol% and decreases above 70%. The largest Hy value is 1620 Hv for NiTi₂-70vol%TiB specimen. .

Zhang et al. have fabricated Ti-TiB composite and investigated the bending strength and the Vickers hardness [11]. They have obtained the bending strength of 608 MPa and Vickers hardness of 18.3 GPa. The bending strength of NiTi2-TiB, 730 MPa, is higher than that of Ti-TiB composite while the Vickers hardness of NiTi2-TiB, 1620 Hv (15.9 GPa) is lower than that of Ti-TiB composite. This fact indicates that composites of intermetallics and ceramics are promising as well as metal-ceramics composites.

In Figure 5 is shown the SEM images of the fracture surface of (a) NiTi₂, (b) NiTi₂-20vol%TiB and (c) NiTi₂-60vol%TiB and (d) NiTi₂-80vol%TiB. In Figure 5(a) it is shown that the fracture surface of NiTi₂ is composed by a few flat steps indicating that NiTi₂ has low fracture toughness. The fracture surface of NiTi₂-20wt%TiB contains several flat surface separated by TiB grains. It seems that the crack propagation is impeded by TiB grains. The dimension of the flat surface becomes smaller in the NiTi₂-60wt%TiB specimen in comparison with that of NiTi₂-20wt%TiB specimen, which may cause the improvement of the bending strength. In the fracture surface of NiTi₂-80vol% TiB specimen is observed granular regions where bonding between grains seems to be weak. The amount of NiTi₂ is not sufficient to fill the space between TiB grains in NiTi₂-80vol%TiB specimen.

Dense NiTi₂ and NiTi₂-TiB composite specimens have been fabricated using spark plasma sintering at 950˚C. The bending strength of NiTi₂-TiB increases as the amount of TiB increases up to 60% and the highest value of

730 MPa is obtained for NiTi₂-60vol%TiB. The Vickers hardness of monolithic NiTi₂ is 650 Hv, which increases up to 1620 for NiTi₂-60%TiB. In NiTi₂-TiB composite, crack propagation is observed to be impeded by TiB grains. Those values are nearly the same as those obtained for Ti-TiB composite and it has been shown that composites of intermetallics and ceramics are promising as well as metal-ceramics composites.

This work is supported by the Grant-in-Aid for Science Research from Japan Society for the promotion of Science (Kakenhi 25420739).

Masashi Yoshida,Hideo Shiraishi,Naoya Ikki, (2015) Microstructure and Mechanical Properties of NiTi₂-TiB Composite Fabricated by Spark Plasma Sintering. World Journal of Engineering and Technology,03,84-88. doi: 10.4236/wjet.2015.33C013