Residual and working stresses of a TiC/Ni3Al FGM and its structure optimization

Abstract

Metal/ceramic type functionally gradient materials [1 ] (FGMs) are new-generation composite materials for future use in aerospaee, nuclear engineering, etc. The key problem for their practical use is how to ensure no damage is caused by the thermal stresses in the preparation and working processes. In analysing the residual and working stresses of ceramic/metal FGMs, Kawasaki [2], Kato [3], and Yuan et al. [4] have done some interesting work. However, work which optimizes the composition and structure by comprehensively surveying the residual and working stresses has not been done until now. In fact, these studies are very important to the thermal stress relaxation design of FGM. I n this paper, a TiC/Ni3A1 FGM is studie& As a ceramic phase, TiC has a high electronic expansion coefficient and high-temperamre strength; as a metal phase, the intermetallic Ni3A1 has a fine creepresistance at elevated temperature, and an excellent interface bondability with TiC [5, 6]. The thermomechanical properties of TiC/Ni3A1 composites with different TiC-Ni3A1 ratios are first tested. The tested properties are then used as a basis of axisyrnmetric finite element calculation for the two different types of stresses. For a disc-like TiC/Ni3A1 FGM, by surveying the stresses, the stress sites, and the stress state at the pure ceramic side, the second optimization for the FGM compositional distribution and structure is obtained. TiC/Ni3A1 composites with Ni3A1 volume fraction of 0, 20%, 40%, 60%, 80% and 100% were sintered by the HP method for 2 h at 1300 °C and.300 MPa under Ar gas protection. These sintering conditions are the same as the conditions for sintering TiC/ Ni3A1 FGM. The sintered composites were cut and ground into 36 x 4 x 3 m m strip samples; the properties were then tested with a four-point bending test method in a materials test system. In the test, the top span of the samples was 3 0 m m and the underpart span 20 mm. Strain ganges were fixed at the centre of the samples. Young's modulus E was calculated through the tested load-defiection curve; t h e Poisson ratio /z was determined by the ratio of the tested cross-strain to the longitudinal strain; and the fracture strength of the samples was indicated as the four-point bending strength o-. The average values of E and /z were obtained from 30 samples and the average ofrom seven samples. In addition, the thermal expansion coefficient was tested by a nonloaded thermal dilatometer and the thermal diffusion coefficient ff and specific heat Cp were tested by a laser perturbation method (SHINKO RIKO TC-700). The samples were 10 mm long and 2 mm wide; they were tested at 25 °C, 300 °C, 600 °C, 800 °C and 1200 °C, respectively. The thermal conductivity A was calculated from A = ff x Cp x d, where d is the sample density. The tested properties are indicated in Table I. F ig . 1 is a finite element model for the thermal stress calculation of a disc-like TiC/Ni3A1 FGM. The material is 30 mm in diameter, 6 mm in thickness, and has 1 l gradient layers. The top side is the metal phase, the bottom is the ceramic phase, and between the two sides are the FGM interlayers. Due to the