Microstructure evolution of as-cast Nb-Ti-C alloys

WEI Wen-qing(魏文庆)¹, WANG Hong-wei(王宏伟)¹, GAO Zeng-xin(高增新)², WEI Zun-jie(魏尊杰)¹

1. School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China;

2. Beijng Xinghang Mechanical Electrical Equipment Factory, Beijing 100074, China

Received 10 June 2009; accepted 15 August 2009

Abstract: Nb-Ti-C alloys with different Ti contents (4%-16%, mole fraction) were fabricated by vacuum non-consumable arc-melting method. The results indicate that the alloys contain niobium solid solution (Nb_ss), carbides (Nb₂C, (Nb, Ti)C) and eutectic of Nb_ss/MC. Carbides in the Nb-Ti-C alloys change from Nb₂C to (Nb, Ti)C with the increase of titanium content. Microstructures in the as-cast state include primary plate-like Nb₂C, eutectic of Nb_ss/Nb₂C, Nb_ss and secondary needle-like Nb₂C in Nb-5Ti-4C alloy, and other microstructures are primary Nb_ss, eutectic of Nb_ss/(Nb, Ti)C. Morphology of carbides are changed apparently through adding different Ti contents, and morphology of Nb_ss changes from the cellular structure to dendrite.

Key words: Nb-Ti-C alloy; carbide transformation; niobium solid solution

1 Introduction

Niobium-based alloys have been investigated as next generation high-temperature structure materials in place of current Ni-based or Co-based superalloys, which have a maximum operating temperature of about 1 400 K[1-3]. As one of refractory metal, pure niobium has a relatively low density, a very high melting point (2 741 K) and good room temperature ductility. However the strength of pure Nb decreases substantially as temperature is above 1 200 K[4]. Pure Nb has higher solid solubility of C, N, O compared with other superalloys, which suggests that Nb based alloys have great effect of solid solution strengthening[5-6]. Solid solution strengthened Nb alloys with V, Ta, Mo, Hf and W have been reported[7-9], and also some two-phase in-situ composites containing a BCC Nb solid solution (Nb_ss) and an intermetallic compound such as Nb₃Al[10-12] or Nb₅Si₃[13]. The strength of Nb-10Mo- (10-15)W-10Ti-18Si alloy which was developed by Japan can reach 800 MPa even at 1 773 K[2].

Traditional Nb alloys suffer from some problems[2-3], such as high density and poor oxidation resistance. The superalloy can keep low temperature plastic property of niobium, and hold low density, high strength, and high oxidation resistance. For Nb-Ti-C alloys the Ti reacts with oxygen to form selective oxides[10, 14], which can decrease the oxygen diffusion rate. Moreover, Ti and C can decrease the density of Nb based alloys[1, 6]. High temperature strength of the alloys can be improved by the generating of uniformly distributed carbides[15]. No systematic studies of microstructure evolution of Nb-Ti-C alloys have been reported. So, the purpose of this work is to investigate the phase composition, the distribution of carbides and Nb_ss, and microstructure evolution of as-cast Nb-Ti-C alloys.

2 Experimental

The composition design of Nb-Ti-C alloys in this work was based on binary phase diagram of Nb-C. High purity Nb, sponge Ti and TiC powder were used as raw materials. The alloys were prepared by vacuum non-consumable arc-melting method and cast into buttons. The starting materials were treated by acid washing and water scrubbing, and then uniformly mixed. The buttons were melted three times with electromagnetic mixing to ensure chemical homogeneity. The mass of buttons is about 30 g. The nominal compositions of the alloys in this work are Nb-5Ti-4C, Nb-20Ti-4C, Nb-47Ti-4C and Nb-58Ti-4C (mole fraction, %), and  the precision contents are listed in Table 1. Their microstructures and compositions of the alloys were characterized using a combination of X-ray diffraction analyzer (XRD), scanning electron microscope (SEM) and energy dispersive spectroscope (EDS).

Table 1 Composition of Nb-Ti-C alloy (mole fraction, %)

3 Results and discussion

3.1 Phase identification

The XRD patterns of samples are shown in Fig.1. It is found that the alloys contain three different phases: Nb_ss, Nb₂C and (Nb, Ti)C. Nb-5Ti-4C alloy contains Nb_ss and Nb₂C; other alloys (Nb-20Ti-4C, Nb-47Ti-4C and Nb-58Ti-4C) contain Nb_ss and (Nb, Ti)C.

Fig.1 XRD patterns of as-cast samples

The lattice parameters calculated from the XRD data are 0.330 52, 0.330 05, 0.329 63 and 0.329 42 nm for the Nb_ss in Nb-5Ti-4C, Nb-20Ti-4C, Nb-47Ti-4C and Nb-58Ti-4C alloys. More Ti elements dissolve in Nb_ss as Ti content increases, and the solid solubility of Ti in Nb_ss is improved. This phenomenon is caused by Ti element, which has little atomic radius, in place of Nb element with a BCC structure.

3.2 Microstructure evolution of Nb-Ti-C alloys with Ti content

SEM micrographs of each alloy in the as-cast state are shown in Fig.2. The compositions measured by EDS of Nb_ss and carbides of each alloy are shown in Table 2. The molar ratio of C/M (Nb, Ti), calculated from EDS data, in deep color zone of Fig.3(a), is higher than that in bright part. So it can be known that the deep color phase is carbides and the bright color phase is Nb_ss.

Fig.2(a) shows the microstructure of the Nb-5Ti-4C alloy, and a two-phase microstructure consisting of Nb_ss and Nb₂C can be observed. One kind of plate-like primary Nb₂C phase distributes uniformly in grains; and another eutectic of Nb₂C/ Nb_ss phase distributes in grain boundary (Fig.2(a)). It is excited that the third kind of needle-like carbides distributes in grains from the high magnification images of Fig.3(b).

Therefore, it can be concluded that plate-like carbides nucleate and grow up from liquid alloy firstly under the condition of non-equilibrium solidification according to binary alloy phase diagram of Nb-C; and then Nb_ss grains grow up around primary carbides, which are used as effective nucleating particles; the eutectic of Nb_ss and Nb₂C precipitates from remnant liquid in grain boundary lastly. The secondary needle-like Nb₂C with the length of 8-10 μm and width of about 1 μm, precipitates during cooling from the solidification temperature due to supersaturation of C in the Nb_ss phase because of the solubility limit of carbon reducing.

Table 2 EDS analysis of each alloy

As indicated from the XRD analysis, the alloys of Nb-20Ti-4C, Nb-47Ti-4C and Nb-58Ti-4C consist of Nb_ss with BCC structure and (Nb, Ti)C with FCC NaCl structure. It is noted that more carbides are got compared with Nb-5Ti-4C alloy, carbide network becomes densely and complex, and the primary dendrite becomes more developed with the increase of Ti content in alloys.

In Nb-20Ti-4C alloy (shown in Fig.3(a)), the eutectic structure of intermediate phase is observed, in which Nb_ss and (Nb, Ti)C are alternatively distributed. Carbides are in the majority in this eutectic zone. There is no significant secondary needle-like carbides precipitation. Secondary carbides also cannot be observed in Nb-47Ti-4C and Nb-58Ti-4C (shown in Figs.3(c) and (d)), and also plate-like precipitate carbides becomes less. The phenomenon indicates that eutectic temperature of the alloys decreases and rich carbon zone in Nb_ss disappears.

The Nb_ss becomes a coarse dendritic morphology with the increase of Ti content observed from Figs.2(b), (c) and (d), which indicates that this phase solidifies first, and then an eutectic mixture of carbide and Nb_ss phases

Fig.2 SEM micrographs of each alloy in as-cast state: (a) Nb-5Ti-4C alloy; (b) Nb-20Ti-4C alloy; (c) Nb-47Ti-4C alloy; (d) Nb-58Ti-4C alloy

Fig.3 SEM images with high magnification: (a) Nb-20Ti-4C alloy; (b) Nb-5Ti-4C alloy; (c) Nb-47Ti-4C alloy; (d) Nb-58Ti-4C alloy

forms from the interdendritic liquid finally. The atomic ratio of Nb/Ti in Nb_ss, as shown in Table 2, decreases obviously with changing Ti element content. There are also more Ti elements dissolving in Nb_ss. The increase of solid solubility of Nb_ss is beneficial to oxidation resistance of the alloys. It is noted, however, that carbides are developed observing from Fig.3(d). The atomic ratio of M(Nb, Ti)C in carbides is approximately 1 based on the calculation from Table 2 in Nb_ss of Nb-20Ti-4C, Nb-47Ti-4C, and Nb-58Ti-4C alloy.

Above all, the volume fraction, structure and style of carbides change with changing Ti content. The reaction of carbon with Ti is easier than with Nb. Carbides include Nb₂C, growing up mainly in grains as primary phase, and (Nb, Ti)C, precipitating mainly in grain boundary as secondary phase. These carbides are contributive to improve high temperature mechanical properties through dispersion strengthening, solution strengthening and boundary strengthening. We know, however, that the temperature of eutectic transformation decreases as Ti with low melting point is added. The range of crystallizing increases. So, Nb_ss has more cooling time for crystal growth. Cellular structure is obtained in Nb-5Ti-4C alloy and coarse dendritic morphology is obtained in other alloys. Moreover, this also make secondary carbide disappear, and the most carbides solidify in boundary finally.

4 Conclusions

1) Ternary Nb-Ti-C alloys consist of niobium solid solution (Nb_ss), carbides including Nb₂C and (Nb, Ti)C and eutectic of Nb_ss/MC. The styles of carbides change from Nb₂C to (Nb, Ti)C with the increase of Ti content. The atomic ratio of Ti element in Nb_ss and MC increases with the increase of Ti content.

2) Primary Nb₂C, Nb_ss, eutectic of Nb_ss and Nb₂C and secondary Nb₂C are included in Nb-5Ti-4C alloy, and the other Nb-Ti-C alloys consist of primary Nb_ss and eutectic of Nb_ss and (Nb, Ti)C.

3) Nb is effective on the solubility of Ti in Nb_ss, but the temperature of eutectic transformation decreases with the increase of Ti content. Morphology of carbides is changed apparently by adding different Ti contents. The crystallization morphology of Nb_ss changes from the cellular to dendrite.

References

[1] JIAO H S, JONES I P, AINDOW M. Microstructures and mechanical properties of Nb-Ti-C alloys[J]. Materials Science and Engineering A, 2008, 485(6): 359-366.

[2] ZHANG Xiao-ming. New research progress of Nb based supperalloys and composites in Japan[J]. Rare Metals Letters, 2005, 24(2): 3-7. (in Chinese)

[3] QU Nai-qin, CHEN Yong-lu. Application and prospect of Nb based alloys[J]. World Nonferrous Metals, 1998, 5(11): 16-19. (in Chinese)

[4] ALLAMEH S M, HAYES R W, LI M, LORIA E A, SROLOVITZ D J. Microstructure and mechanical properties of a β Nb-Ti based alloy[J]. Materials Science and Engineering A, 2002, 328: 122-132

[5] ZHENG Xin, L? Hong-jun, LI Zhong-kui. Carbides in Nb based alloys[J]. Rare Metal, 2006, 12(30): 117-120. (in Chinese)

[6] DING R G, JONES I P, JIAO H S. Effect of carbon on the microstructures and mechanical properties of as cast Nb-base alloy[J]. Materials Science and Engineering A, 2007, 485(6): 87-89.

[7] ALLAMEH S M, HAYES R W, LORIA E A, SOBOYEJO W O. Creep behavior in an extruded β solid solution Nb-Ti base alloy[J]. Materials Science and Engineering A, 2002, 329/331: 856-862.

[8] LEONARD K J, MISHURDA J C, VASUDEVAN V K. Phase equilibria at 1 100 ℃ in the Nb-Ti-Al system[J]. Materials Science and Engineering A, 2002, 329/331: 282-288.

[9] DING Ren-gen, JONES I P, JIAO Hui-sheng. Effect of Mo and Hf on the mechanical properties and microstructure of Nb-Ti-C alloys[J]. Materials Science and Engineering A, 2007, 485: 126-135.

[10] GUO Hai-sheng, GUO Xi-ping. Research progress in the directionally solidified microstructure and eutectic solidification theory of niobium-based ultrahigh-temperature alloy[J]. Metals Review, 2007, 21(4): 56-59. (in Chinese)

[11] SUN Zhi-ping, GUO Xi-ping. Alloying effects of fracture toughness of both Nb-based solid solution alloys and Nb Silicide-based alloys[J]. Metals Review, 2008, 22(4): 79-83. (in Chinese)

[12] GUO Jin-ming, GUO Xi-ping. Effects of alloying on the microstructure and high temperature oxidation resistance of Nb-Ti-Si-based alloys[J]. Rare Metals and Cemented Carbides, 2008, 36(2): 39-43.

[13] JIAO Hui-sheng, JONES I P, AINDOW M. Effect of Al, Cr and Ta additions on the oxidation behaviour of Nb-Ti-Si in situ composites at 800 ℃[J]. Materials Science and Engineering A, 2006, 416(1): 269-280.

[14] ZHOU Rui-fa, HAN Ya-fang, LI Su-suo. High temperature structure materials[M]. Beijing: National Defence Industry Press, 2006: 150-168. (in Chinese)

[15] DING Ren-gen, JIAO Hui-sheng, JONES I P. Effect of alloy composition on microstructure and high temperature properties of Nb-Zr-C ternary alloys[J]. Materials Science and Engineering A, 2003, 341: 282-288.

(Edited by YANG You-ping)

Corresponding author: WEI Wen-qing; Tel: +86-451-86403150; E-mail: hitweiwenqing@126.com