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Chinese Journal of Materials Research  2015, Vol. 29 Issue (9): 641-648    DOI: 10.11901/1005.3093.2014.551
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Effect of Cr Addition on the Microstructure and Room-temperature Fracture Toughness of Nb-Si Based Ultra-high Temperature Alloys
Song ZHANG,Xiping GUO()
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China
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Song ZHANG,Xiping GUO. Effect of Cr Addition on the Microstructure and Room-temperature Fracture Toughness of Nb-Si Based Ultra-high Temperature Alloys. Chinese Journal of Materials Research, 2015, 29(9): 641-648.

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Abstract  

Four Nb-Si based ultra-high temperature alloys with compositions of Nb-22Ti-16Si-4Hf-3Al-xCr (x = 0, 3%, 5% and 10%, atomic fraction) were prepared by vacuum non-consumable arc melting and then heat-treated at 1450℃ for 50 h. The effects of Cr content on the microstructure and room-temperature fracture toughness of the alloys were investigated. The results showed that the Cr addition did not change the crystal structure of the silicide γ(Nb, X)5Si3. The amount of γ(Nb, X)5Si3 rose while that of the eutectic Nbss/γ(Nb, X)5Si decreased with the increasing Cr content. Furthermore, a low melting point of three-phase eutectic composed of Nbss, (Nb, X)5Si3 and Cr2(Nb, X) was observed in Cr-containing alloys and its amount gradually increased with the increasing Cr addition. After 1450℃/50 h heat-treatment, the original Nbss dendrites as well as eutectic colonies disappeared, and the microstructural uniformity was significantly ameliorated. The alloys with 0, 3% and 5% Cr were all composed of phases Nbss and γ(Nb, X)5Si3, while three-phase equilibrium of Nbss, γ(Nb, X)5Si3 and Cr2(Nb, X) was observed in the alloy with 10% Cr. However, the room-temperature fracture toughness of the as-cast alloys exhibited a decreasing trend with Cr addition.
Key words:  metallic materials      Nb-Si based ultra-high temperature alloy      arc-melting      heat-treatment      room-temperature fracture toughness      Cr addition     
Received:  30 September 2014     
Fund: *Supported by National Natural Science Foundation of China Nos. 51371145, 51071124, 51431003 and U1435201, and Special Research Fund for Doctoral Disciplines in Colleges and Universities of Ministry of Education, China, No. 20096102110012.
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Song ZHANG
Xiping GUO

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https://www.cjmr.org/EN/10.11901/1005.3093.2014.551     OR     https://www.cjmr.org/EN/Y2015/V29/I9/641

Fig.1  XRD spectra of the as-cast (a) and heat-treated (b) Nb-Si based ultrahigh temperature alloys
Fig.2  BSE images of the as-cast Nb-Si based ultrahigh temperature alloys, (a) 0Cr-AC, (b) 3Cr-AC, (c) 5Cr-AC and (d) 10Cr-AC
Fig.3  BSE images of Nbss/(Nb, X)5Si3/Cr2(Nb, X) eutectic present in the as-cast Nb-Si based ultrahigh temperature alloys, (a) 3Cr-AC, (b) 5Cr-AC and (c) 10Cr-AC
Alloy Phase Nb Ti Si Hf Al Cr
0Cr-AC Nbss γ(Nb, X)5Si3 73.6 43.6 19.1 13.0 1.8 36.3 2.4 4.7 3.1 2.4 - -
3Cr-AC Nbss γ(Nb, X)5Si3 72.2 42.2 18.1 14.8 1.6 35.6 2.2 4.8 3.5 2.3 2.4 0.3
5Cr-AC Nbss γ(Nb, X)5Si3 66.9 41.6 18.7 15.3 1.5 34.7 2.2 5.1 4.2 2.8 4.5 0.5
10Cr-AC Nbss γ(Nb, X)5Si3 65.6 40.4 18.3 14.2 1.2 36.7 2.1 5.6 3.4 2.1 9.4 1.0
Table 1  Compositions of the phases present in the as-cast Nb-Si based ultrahigh temperature alloys (%, atomic fraction)
Fig.4  BSE images of the Nb-Si based ultrahigh temperature alloys heat-treated at 1450℃ for 50 h, (a) 0Cr-HT, (b) 3Cr-HT, (c) 5Cr-HT and (d) 10Cr-HT
Alloy Phase Nb Ti Si Hf Al Cr
0Cr-HT Nbss γ(Nb, X)5Si3 70.2 43.0 23.2 12.3 1.0 37.5 2.0 4.9 3.6 2.3 - -
3Cr-HT Nbss γ(Nb, X)5Si3 63.7 41.3 24.5 14.8 0.8 36.2 1.9 4.9 3.8 2.4 5.3 0.4
5Cr-HT Nbss γ(Nb, X)5Si3 61.7 42.3 23.5 14.6 0.6 35.2 1.5 4.7 4.7 2.6 8.0 0.6
10Cr-HT Nbss γ(Nb, X)5Si3 Cr2(Nb, X) 57.6 40.2 27.5 24.9 13.7 10.0 0.4 37.4 9.5 1.3 5.1 3.3 4.1 2.6 1.7 11.7 1.0 48.0
Table 2  Compositions of the phases present in the Nb-Si based ultrahigh temperature alloys heat-treated at 1450℃ for 50 h (%, atomic fraction)
Alloy 0Cr-AC 3Cr-AC 5Cr-AC 10Cr-AC
Specimen 1 2 3 1 2 3 1 2 3 1 2 3
KQ /MPam1/2 13.3 14.6 12.3 11.6 12.4 11.7 11.3 11.9 12.2 9.9 11.0 10.0
Average KQ /MPam1/2 13.4 11.9 11.8 10.3
Table 3  Room-temperature fracture toughness of the as-cast Nb-Si based ultrahigh temperature alloys
Fig.5  SEM images of the fracture surfaces of the as-cast Nb-Si based ultrahigh temperature alloys, (a) 0Cr-AC, (b) 3Cr-AC, (c) 5Cr-AC and (d) 10Cr-AC
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