Microstructure and high-temperature mechanical properties of Nb-silicide based in-situ composites

LIU Dong-ming(刘东明)^1,2, JIANG Rong-li (姜荣丽)², SHA Jiang-bo(沙江波)²

1. School of Materials Science and Engineering, South Campus, Shandong University, Ji’nan 250061, China

2. School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics,

Beijing 100083, China

Received 28 July 2006; accepted 15 September 2006

Abstract: Nb-15W-18Si-xHf (x = 0, 5, 10 and 15, mole fraction, %) alloys were prepared by arc melting and then homogenized at 1 750 ℃ for 50 h. The microstructure and mechanical behaviors, such as Vickers hardness, fracture toughness, room- and high- temperature strength of the alloys were investigated. The microstructure of the annealed Nb-15W-18Si-xHf alloys is composed of large primary Nb_SS dendrite and fine eutectic mixture of Nb_SS and M₅Si₃ silicide. Both the hardness and the fracture toughness show an increase tendency at room temperature with increase of Hf content. The 0.2% compressive yield strength, σ_0.2 of the alloys with 5 %Hf and 10%Hf are larger than 960 MPa at 1 200 ℃, and about 500 MPa even at 1 500 ℃.

Key words: Nb-15W-18Si-xHf alloys; niobium; silicide; microstructure; strength; fracture toughness

1 Introduction

Nb-silicide based in-situ composites show excellent performance in strength, creep resistance and long-term chemical and morphological stability at elevated temperatures[1-6]. Thus, there have been considerable interests in development of this system for applications as the next generation turbine airfoil materials in recent years[7-12]. As far as the high-temperature strength is concerned, extensive studies have shown that the as-cast Nb-10Si alloy has the bend strength of 400 MPa at 1 470 K[13-14]. The bend strength of Nb-16Si prepared by arc melting and hot isostatic pressing (HIP) is about 400 MPa at 1 670 K[13]. By continuously increasing Si content toward the eutectic composition of 18.7%, the arc-melted sample has the compressive yield strength of 300 MPa at 1 673 K[15].

In the previous work reported by SHA et al [16], the Nb-18Si-10Ti-10Mo-15W alloy prepared by arc melting demonstrates a compressive yield strength σ_0.2 of 350 MPa at 1 770 K. Although this value is very high, our research suggests that it can be further improved by substituting element Hf for Mo and Ti. The aim of this work is to study the microstructure and high-temperature mechanical properties of niobium silicide in-situ composites, Nb-15W-18Si-xHf (x=0, 5, 10 and 15 %). It is shown that Hf addition is beneficial to the fracture toughness at room temperature. Meanwhile, the strength of the Hf-added alloys shows considerable improvement compared with the Hf-free sample at 1 200℃, and has a slight increase at 1 500 ℃ when the Hf content is less than 10%.

2 Experimental

The nominal composition was Nb-15W-18Si-xHf (x=0, 5, 10 and 15). Elements with 99.95%(mass fraction) or higher purity were used. Ingots of about 50 g were prepared by arc melting and each alloy was melted five to six times to homogenize the compositions. The ingots were then annealed at 1 750 ℃ for 50 h in a vacuum of 1×10^-4 Pa, followed by furnace cooling to room temperature. The X-ray diffraction (XRD) analysis was performed to identify the phase constitution of alloys. The microstructure was observed using a scanning electron microscope. Vickers hardness (HV) was measured under an applied load of 1.96 N. Compressive tests were conducted at elevated temperatures at an initial strain rate of 3×10^-4 s^-1 in vacuum. The dimensions of the compressive specimens were 3 mm in diameter and 5 mm in length, and cylinder side surface of each specimen was polished using 800-mesh SiC paper prior to testing. Fracture toughness, K_Q, at room temperature was measured using a three-point bending test. The three-point-bend specimen was 25 mm in length, 6 mm in width, and 3 mm in thickness. A notch was introduced up to a/w=0.5(a is the notch length, w is the specimen width) by electro discharge machining with a wire diameter of 0.15 mm. The samples were loaded until failure using a crosshead displacement of 0.5 mm/min. The only point of deviation from ASTM E-399 standard used in this work was the omission of fatigue pre-cracking. For this reason, the toughness values for the three-point bending experiments were reported as K_Q rather than K_IC.

3 Results and discussion

3.1 Microstructure

Fig.1 shows the X-ray diffraction pattern of the Nb-15W-18Si-xHf alloy annealed at 1 750 ℃ for 50 h. It can be seen that these alloys contain Nb_SS and (Nb, W, Hf)₅Si₃ silicide (referred to as M₅Si₃ hereafter, M denotes elements Nb, W or Hf etc) phases. A two-phase mixture was observed as illustrated in Fig.2, where the bright and the dark areas correspond to the Nb_SS and the M₅Si₃ phases, respectively. Solidification begins with the formation of primary N­b_SS dendrite. As the dendrites grow and thicken, solute is rejected into the remained liquid until it reaches the eutectic composition. The subsequent eutectic solidification results in a micro- structure consisting of fine Nb_ss and M₃Si and forms the matrix of the composite. After annealing at  1 750 ℃,     the M₃Si silicide decomposes into M₅Si₃ and Nb_SS completely through eutectoid reaction. Compared with    the Hf-free sample, the Hf containing samples exhibit more volume fraction of eutectic micro- structure. In addition, the primary Nb_SS dendrites become fine with increasing Hf content. This may result from that Hf can accelerate the nucleation rate of the Nb_SS during solidification.

Fig.1 X-ray diffraction patterns of Nb-15W-18Si-xHf alloy annealed at 1 750 ℃ for 50 h

Fig.2 Scanning electron micrograph (BSE images) of Nb-15W-18Si-xHf alloy annealed at 1 750 ℃ for 50 h: (a) x=0; (b) x=15

Because the present alloys containing 18% Si approach the eutectic composition of the binary Nb-Si system, the microstructure would be mainly composed of eutectoid composition according to the Nb-Si phase diagram. For the addition of W, however, the microstructure of the Nb-15W-18Si-0Hf in Fig.2(a) shows more primary volume fraction of the Nb_SS than that of hypoeutectic Nb-14Si alloy[17]. This suggests that the addition of W may cause the eutectic line to shift  towards the Si-rich side of the phase diagram.

3.2 Mechanical behaviors at room temperature

The Vickers hardness (Hv) of the Nb-15W-18Si based alloy as a function of the Hf content at room temperature is shown in Fig.3. Hv increases with increase of Hf content. This promotion is due to solid-solution hardening of Hf on the Nb_SS. Fig.3 demonstrates the fracture toughness K_Q vs the Hf content. K_Q increases slightly from 5.1 MPa·m^1/2 for the 0 Hf sample to 7.2 MPa·m^1/2 for the 15 % Hf sample. The improvement in K_Q results from the fine primary dendrite in the Hf-containing samples.

Fig.3 Vickers hardness (Hv) and fracture toughness K_Q of Nb-15W-18Si based alloy vs Hf content at room temperature

3.3 Mechanical response at elevated temperatures

Fig.4(a) shows the compressive stress—strain curves of the Nb-15W-18Si-xHf alloys at 1 500 ℃. Compared with the Hf-free sample, the stress of the Hf-containing samples decreases slowly after reaching the peak stress, σ_max, and retains at a high level even at a plastic strain larger than 10%, which indicates the material’s capacity against creep also. They exhibit strong work-hardening behavior and substantial plastic deformation before failure at high temperatures. The 5%Hf alloy has the highest peak stress σ_max, while the 15%Hf alloy has the minimum value. In addition, the Hf-containing alloys show sufficient compressive ductility at 1 500 ℃.

The compressive stress—strain curves of the Nb-15W-18Si-xHf alloys at 1 200 ℃ show a similar tendency to that of the corresponding samples at 1 500 ℃, as seen in Fig.4(b). The compiled 0.2% yield strengths, σ_0.2, at 1 500 ℃ and 1 200 ℃, as a function of the Hf content, are summarized in Table 1. At 1 200 ℃, σ_0.2 increases with increasing Hf content, and reaches the highest value at 5%Hf, then it shows a decrease tendency with the Hf increase to 15 %. At 1 500 ℃, σ_0.2 drops from 570 MPa for the Hf-free sample to 330 MPa for the 15 % Hf sample. Our previous work[18] shows that an outstanding solid-solution hardening on the monolithic Nb_SS at high temperatures is perfectly carried out by the Hf addition. The strength decease may result

from that the Hf addition weakens Nb₅Si₃ silicide at temperature higher than 1 200 ℃. Actually, more extensive work should be done to clarify the mechanism of the strength change in these alloys at elevated temperatures. It should be noted that the four alloys demonstrate relatively high 0.2% yield strength at elevated temperatures. For instance, the 5%Hf and the 10%Hf alloys exhibit σ_0.2 values higher than 500 MPa at 1 500℃. Considering that σ_0.2 of the as-cast Nb-18Si-

10Ti-10Mo-15W at 1 500 ℃ is improved from 350 MPa to 650 MPa through directional solidification (DS)[16], one can conclude that the strength of the Nb-15W-18Si-xHf alloys can be further improved by DS.

Fig. 4 Compressive stress—strain curves of Nb-15W- 18Si-xHf alloys at (a) 1 500 ℃ and (b) 1 200 ℃

Table 1 Mechanical properties of annealed Nb-15W-18Si-xHf alloys at elevated temperatures(MPa)

4 Conclusions

1) The microstructure of the cast and annealed Nb-15W-18Si-xHf alloys is composed of the large primary Nb_SS dendrite and a fine eutectic mixture of the Nb_SS and M₅Si₃ silicide. The primary Nb_SS dendrites become fine and the volume fraction of the eutectic mixture displays a slight increase when the Hf content changes from 0 to 15 %.

2) Both the Vickers hardness and the fracture toughness show an increase tendency at room temperature with increase of Hf content.

3) The 5%Hf and 10%Hf samples show consider- able high-temperature strength. Their compressive 0.2% yield strength are higher than 500 MPa at 1 500 ℃, and 960 MPa at 1 200 ℃.

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(Edited by PENG Chao-qun)

Foundation item: Project(50671002) supported by the National Natural Science Foundation of China

Corresponding author: SHA Jiangbo; Tel: +86-10-82315989; E-mail: jbsha@buaa.edu.cn