Microstructural characteristics of NiAl/TiC composites with high TiC content prepared by pressureless melt infiltration

Abstract

The ordered intermetallic compound B2-NiAl is an attractive structural material for high-temperature application due to its high melting point (1638 ◦C), low density (5.90 g/cm3), high Young’s modulus (190 GPa), and excellent oxidation resistance at high temperature. Additionally, the thermal conductivity of NiAl is 4–8 times greater than that of Ni-based superalloys [1]. However, it is inherently brittle and has inadequate elevated temperature strength. Researchers have made efforts to incorporate a reinforcing phase in the NiAl matrix to overcome its shortcomings. Mechanical alloying, hot-pressing aided exothermic synthesis, hot-pressing and reaction synthesis, and so on for the incorporation of second phase such as TiC, Al2O3, TiB2, ZrO2, or AlN into NiAl, are the successful techniques so far [2–7]. Tuan [2] reported that the strength of NiAl/Al2O3was higher than the values predicted by the rule of mixtures. Xing [7] also reported that NiAl/(0–20 vol%)TiC showed higher compressive and tensile strength from room temperature to 1000 ◦C than polycrystalline NiAl. However, little information is available on NiAl/ceramic with low NiAl content, which can be expected to have higher wear resistance and better high temperature properties. In the present study, pressureless melting infiltration [8, 9], a simple and economic route to produce ceramic/metal composites, is used for preparing NiAl/TiC composite with about 75 vol%TiC. The microstructural characteristics, crystalline structure properties of the composites were investigated by optical microscopes (Ziess, Jenaphot 2000, Germany), transmission electron microscopy (TEM, Hitachi, H-9000NA, Japan), energy dispersive spectroscopy (EDS, Rontec) attached to the TEM and X-ray radiation (XRD) on Rigaku, Geigerflex/D. In the present study, the starting materials were TiC powder (H. C. Strack, Germany, D50 = 1–2 μm) and stoichiometric NiAl intermetallic smelted in our laboratory. TiC powder was de-agglomerated in 2-propanol medium with WC/Co balls, then dried and sieved through a 100 mesh sieve before forming. Disk-shaped TiC powder preform of 19 mm × 7 mm, and was uniaxially pressed in a steel die up to 32 MPa, followed by cold isostatic pressing at 200 MPa. The relative density of the as-pressed TiC was 60% of the theoretical density of TiC (4.93 g/cm3), which was determined by immersing in mercury. Infiltration experiments were performed in a graphite furnace in argon atmosphere. A piece of NiAl was placed on the top of the as-pressed TiC preform in an alumina crucible. Taking the sintering of TiC preform before NiAl melting and infiltrating into account, the amount of the NiAl piece for the infiltration was equal to 25 vol% of the TiC preform. Infiltration schedule, heating at 25 ◦C/min, dwelling at 1750 ◦C for 5 min or for 40 min, and cooling at 10 ◦C/min, was used. Experiments showed that NiAl infiltrated into the porous TiC preform completely in just 5 min at 1750 ◦C. The densification degree of the composite was estimated from the following equation,