The effect of (ZrB2-SiC) addition on microstructure and mechanical properties of NbMo-matrix composites fabricated by hot-pressing
Introduction
Niobium-based alloys are promising candidate for high temperature structural materials because they have low density, high melting points, good ductility and high resistance to corrosion [1]. However, the strength declines rapidly when the temperature is above 1200 K [2]. The addition of refractory metal Molybdenum (Mo) to Nb could make up for this shortcoming of Nb at high temperature by solution strengthening, and the degree of strengthening reaches its maximum extent when Nb/Mo volume ratio is approximately 1 [3]. Zirconium diboride (ZrB2) has excellent performance at ultra-high temperature because of its ultra-high melting point (3246 °C), high hardness and other desirable mechanical properties [4,5]. In our early research, (0–60) vol% ZrB2-NbMo composites were fabricated by hot-pressing at the temperature of 2400 °C with a pressure of 50 MPa for 10 min in Argon, and the volume fraction of additive Nb and Mo is same (Nb/Mo = 1). The properties of (0–60) vol% ZrB2-NbMo composites at room temperature and the temperature range of 800 °C to 1300 °C were tested and analyzed. The results show ZrB2 addtion to NbMo can greatly improve the compressive strength of the composites at all tested temperature compared to NbMo solid solution, and the compressive strength of 60 vol% ZrB2-NbMo is 700.46 MPa even at 1300 °C [6,7]. Previous study reported the addition of silicon carbide (SiC) to ZrB2-based composites could improve properties and ZrB2-SiC composites had high room temperature strengths, high fracture toughness and high hardness values, in which SiC additive could limit the grain growth [[8], [9], [10]]. Jayaseelan et al. reported the oxide impurities in the ZrB2–10 vol% SiC [11], and Liang et al. observed a lot of edge dislocations in ZrB2–20 vol% SiC [12]. Fahrenholtz et al. found that ZrB2 with 30 vol% SiC has excellent strength and fracture toughness [13]. Neuman et al. reported the strength of ZrB2–30 vol% SiC up to 1600 MPa in air condition [14,15].
In this study, mixture powder of 70 vol% ZrB2 and 30 vol% SiC is added to NbMo, and the volume ratio of Nb/Mo is 1. The aim of this present work is to investigate the room temperature properties of the 15, 30, 45, 60 vol% (70 vol% ZrB2 + 30 vol% SiC)-NbMo composites fabricated by the method reported in our early literature [6]. The effects of composite additive on the microstructure and mechanical properties of the x (70% ZrB2 + 30% SiC) -NbMo were discussed.
Section snippets
Processing
Five compositions were selected (amounts in vol%):
The starting materials were high purity Nb (N, for short) and Mo (M, for short) powders (purity >99.95%, mean particle size 18–25 μm) and ZrB2 (Z, for short) powders (purity >99.95%, mean particle size 23 μm) and SiC (S, for short) powders (purity >99.9%, mean particle size
Phase composition
Fig. 1 shows the XRD patterns of five samples. NbMo solid solution and ZrC exist in all the four samples with additives, and SiC are decomposed in these four. There are no residual ZrB or ZrB2 in NM-15ZS, which indicating the ZrB2 are totally decomposed to form ZrC and NbB2 in it. There are two stable niobium borides phase formed in the x (70% ZrB2 + 30% SiC)-NbMo composites: NbB2 and Nb3B4, which is easily explained by NbB phase diagram [16]. Nb5Si3 are found in all the four composites. Mo3Si
Conclusions
x (70 vol% ZrB2 + 30 vol% SiC)-NbMo composites (x = 15 vol%, 30 vol%, 45 vol%, 60 vol%) were hot-pressed at 2400 °C with the pressure of 50 MPa for 10 min in Ar gas condition. The object is to investigate the effects of effect of (ZrB2-SiC) addition on phase composition, relative density, hardness and compressive strength at room temperature. The main conclusions are:
- (1)
The types of formed phase were decided by the amount of ZrB2 and SiC addition. The eutectic phase in 15% (70% ZrB2 + 30%
Acknowledgement
The authors would like to thank the financial support from the National Natural Science Foundation of China (No. 11372110) and the Fundamental Research Funds for the Central Universities (No. 2017XS053).
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