Effect of Si content on mechanical properties of Nb-Si based alloys with Nb_SS/ Nb₅Si₃ structure

ZHENG Peng(郑 鹏), SHA Jiang-bo(沙江波), LIU Dong-ming(刘东明),

GONG Sheng-kai(宫声凯), XU Hui-bin(徐惠彬)

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

Beijing 100083, China

Received 28 July 2006; accepted 15 September 2006

Abstract: In Nb-Si based alloys with a two-phase Nb_SS/ intermetallic Nb₅Si₃ structure, the Nb₅Si₃ provides high-temperature strength, while the Nb solid-solution phase, Nb_SS, contributes to room-temperature ductility and toughness. The results show that in Nb-15W-10Hf-xSi alloys (x = 0.5, 5 and 18, mole fraction, %), the volume fraction of the Nb₅Si₃ is 0 for the 0.5% Si sample, 15% for the 5% Si sample and the 50% for 18% Si sample. With increasing Si content, i.e., the Nb₅Si₃ fraction, the high-temperature strength is improved considerably, but room-temperature ductility and toughness are degraded. For the sample Nb-15W-10Hf-18Si with 50% Nb₅Si₃, the compressive strength at 1 500 ℃ and the room-temperature fracture toughness are 500 MPa and 6.8 MPa?m^1/2, respectively, those for the Nb₅Si₃ free sample, Nb-15W-10Hf-0.5Si, are 190 MPa and 13.6 MPa?m^1/2.

Keywords: Nb-Si based alloys; Nb-15W-10Hf-0.5Si alloy; Nb-15W-10Hf-18Si; Nb₅Si₃; mechanical properties

1 Introduction

With relatively high melting point of about 2 743 K and excellent room-temperature ductility, niobium is one of the most promising refractory metals for service at extremely elevated temperatures. The most efficient approaches to improve the high-temperature strength of Nb have undergone by solid-solution strengthening, or forming intermetallics with Nb, such as Nb₅Si₃ silicide [1-6]. The Nb-Si binaries have equilibrium phases of Nb solid-solution Nb_SS and Nb₅Si₃ silicide, in which the Nb_SS offers room-temperature ductility, and the silicide supplies strength and creep resistance at elevated temperatures. Currently, most researchers have focused on eutectic composition Nb-18Si based alloys. In these alloys, the Nb₅Si₃ silicide is the matrix[1-3, 7]. In early work[7], breakthrough of the high-temperature strength has been made in Nb-10Mo-15W-10Ti-18Si alloy. The compressive yield strength of the alloy is as high as 550-650 MPa even at 1 500 ℃. Its ductility and fracture toughness at room temperature, however, are insufficient, and less than 1% and 6 MPa×m^1/2, respectively, which would be a barrier for aerospace applications. In fact, the brittle Nb₅Si₃ phase showing fracture toughness as low as 3 MPa×m^1/2 dominates the balance between toughness and strength of the bulk two-phase Nb_SS/Nb₅Si₃ silicide structure[5], but these properties as a function of the brittle Nb₅Si₃ fraction, or the Si content, are still unknown. To investigate the relationship between toughness and strength and probe the possibility of aerospace applications of the two-phase Nb_SS/Nb₅Si₃ structure, a series of Nb-Si based alloys with various volume fractions of the Nb₅Si₃ phase were designed in this work, and their mechanical behaviors at room and high temperatures were measured and discussed.

2 Experimental

The composition of Nb-15W-10Hf was used as base and Nb was further replaced by 0.5%, 5% and 18%(mole fraction) Si. Elements with 99.95%(mass fraction) or higher purity were used.  Button ingots were prepared by arc-melting in 100 kPa argon for 5 or 6 times to homogenize compositions throughout ingots. The ingots were finally annealed at 1 700 ℃ in vacuum for 50 h to ensure the equilibrium microstructures. Microstructures were observed using a scanning electron microscope (SEM). Fracture toughness K_Q at room temperature was measured using three-point-bending test. The three-point-bending specimen is 30 mm in length, 6 mm and 3 mm in width and thickness, respectively. A notch was introduced up to a/w=0.5(a is the notch length, w is the specimen width) by electrical discharge machining with a wire diameter of 0.1 mm. Compression tests were conducted at room temperature, 1 200 ℃ and 1 500 ℃ in vacuum at an initial strain rate of 3?10^-4 s^-1. The dimensions of the compression cylinders are 3 mm in diameter and 5 mm in length, and every surface of samples were polished using 800 mesh SiC paper prior to testing.

3 Results and discussion

3.1 Microstructures

Typical SEM microstructures (backscatter scanning electron images) of the three annealed samples are shown in Fig.1. The bright phase is Nb-base solid solution (referred to as Nb_SS hereafter) and the dark phase is (Nb, W, Hf)₅Si₃ silicide (referred to as 5-3 silicide hereafter). The Nb-15W-10Hf-0.5Si alloy shows the monolithic Nb_SS, in which all the alloying elements solidify, as seen in Fig.1(a). The Nb-15W-10Hf-5Si alloy has a dentritical Nb_SS-dominant microstructure, and the 5-3 silicide with a volume fraction of about 15% distributes on the interstitials of the Nb_SS dendrites, as seen in Fig.1(b). As for the Nb-15W-15Hf-18Si alloy, the microstructure consists of the Nb_SS dendrite and the 5-3 silicide matrix (Fig.1(c)). The volume fraction of the 5-3 silicide is about 50%.

3.2 Mechanical response at room temperature

Fig.2 shows the compressive stress—strain curves at room temperature. In the monolithic Nb_SS (Nb-15W-

10Hf-0.5Si) or the Nb_SS-dominant (Nb-15W-10Hf-5Si) microstructure, strong plastic deformation hardening exists. The maximum compressive strain, ε_max, of both alloys, range in 5%-11% (ε_max is defined as the plastic strain corresponding to the peak stress). With a 50% 5-3 silicide, the stress—strain curve of the Nb-15W-10Hf-18Si alloy only shows a low linear portion. The sample then collapses in a brittle manner with no plastic deformation, which is similar to that of the monolithic 5-3 silicide phase[1]. It is indicated that this brittle response is dominated by the 5-3 silicide phase, although this sample contains about 50% ductile Nb_SS phase.

Fracture toughness, K_Q, of notched three-point-bending specimens as a function of the Si contents, as well as 0.2% yield strength, σ_0.2 and ε_max，are given in Table 1. It can be seen that the Nb-15W-10Hf-0.5Si alloy  has the highest K_Q value of 13.6 MPa×m^1/2, while the Nb-15W-10Hf-5Si alloy containing 15% 5-3 silicide has a K_Q value of 11.8 MPa×m^1/2. With the 5-3 silicide matrix, the Nb-15W-10Hf-18Si alloy shows the lowest K_Q value of 6.8 MPa×m^1/2. Referring to the microstructures in Fig.1 suggests that the Nb_SS benefits significantly to ε_max and K_Q only when the Nb_SS dominates the microstructure. Apparently, ε_max strongly depends on the volume fraction of the 5-3 silicide.

Fig.1 SEM microstructures of annealed Nb-W-Hf-Si alloys: (a)   Nb-15W-10Hf-0.5Si; (b) Nb-15W-10Hf-5Si; (c) Nb-15W-10Hf-18Si

Fig.2 Compressive stress—strain curves of Nb-W-Hf-Si alloys at room temperature

Table 1 Mechanical properties of Nb-15W-10Hf-xSi alloys at room-temperature

3.3 High-temperature compressive behavior

Fig.3 presents the compressive stress—strain curves of the annealed Nb-15W-10Hf-(0.5-18)Si alloys at elevated temperatures. At 1 200 ℃, as seen in Fig.3(a), both the Nb-15W-10Hf-0.5Si and Nb-15W-10Hf-5Si alloys always keep strong work hardening after yield though the plastic strain is larger than 10%, then their stress reach constant values of about 750 MPa and 1 100 MPa, respectively. The Nb-15W-10Hf-18Si alloy has the highest peak stress s_max, but its work-hardening region in strain scale is shorter than that of the Nb_SS-dominant microstructure. The stress decreases quickly after reaching the peak value, which also indicates insufficiency of the ductility of the 5-3 silicide matrix. At 1 500 ℃, as shown in Fig.3(b), the three samples almost hold the same compressive stress—strain characteristic as at 1 200 ℃.

Fig.3 Compressive stress—strain curves of Nb-W-Hf-Si alloys at  1 200 ℃(a) and 1 500 ℃(b)

Data for the s_0.2 as a function of the nominal Si contents are summarized in Table 2. The s_0.2 basically tends to increase monotonously with the Si contents. At  1 200℃, the 5-3 silicide free sample (Nb-15W-10Hf-

0.5Si) shows the s_0.2 of 410 MPa, lower than 960 MPa of the 5-3 silicide matrix sample (Nb-15W-10Hf-18Si). Based on the strength at 1 200 ℃ and 1 500 ℃, the key role of Si on the high-temperature strength is recognized. On the other hand, the strength depends mainly on the volume fraction of 5-3 silicide at the elevated temperatures.

Table 2 0.2% yield strength of Nb-15W-10Hf-xSi alloys at high temperatures (MPa)

As mentioned above, the room-temperature toughness and the high-temperature strength are conflictive with each other in the Nb-Si based alloys. We have tried to obtain a balance between the low-temperature ductility or toughness and the high-temperature strength in this work, especially by designing the role of Si in a Nb_SS/5-3 silicide two-phase structure. From Tables 1 and 2, it is known that the Nb-15W-10Hf-0.5Si alloy has proper high-temperature strength, accompanied with outstanding room- temperature ductility and toughness. Solid-solution hardening Nb by W and Hf, as well as small amount of Si, is considered the main factor. W and Si have a positive effect on the solid-solution hardening of the Nb_SS[7-9]. The solid-solution hardening on Nb supplied by Hf attributes to the large size misfit of about 9% and the structural difference that is b.c.c (Nb) to h.c.p (Hf). With such a larger size misfit, the lattice parameter of the Nb_SS that is increased from 0.332 7 nm for the Nb-15W-0.5Si alloy to 0.340 1 nm when 10%(mole fraction) Hf is further added, the Vickers hardness, therefore, is increased from 320 to 380 [10]. Even the Nb_SS phase was significantly hardened by the Hf addition at room temperature and higher temperature, and it still remained enough room-temperature ductility and toughness. With an hcp crystal structure, Hf may bring more slip systems into the b.c.c Nb_SS phase. In general, these additive slip systems make the dislocation active and the plastic deformation easy, which is the mechanism that contributes to the room-temperature ductility and fracture toughness of the Nb_SS phase. CHAN [11] has also pointed out that Hf and Ti are the only alloying elements that do not increase the BDTT of the Nb_SS; on the other hand, Hf and Ti are the only toughening and ductilizating elements for Nb. The lowering of the Peierls-Nabarro barrier energy by Hf or Ti addition promotes dislocation mobility, and thereby increases both the fracture toughness and ductility of the Nb_SS.

In view of the high-temperature strength, the 5-3 silicide carries out its hardening function significantly, but the ductility of the Nb_SS/5-3 silicide structure is sensitive to the volume fraction of the brittle silicide. In this work, a considerable degradation in the ductility takes place at 15% fraction of the 5-3 silicide, as seen in Table 1. When the fraction of the 5-3 silicide is as high as 50%, i.e., the 5-3 silicide phase becomes the matrix, and, at this moment, the Nb_SS/5-3 silicide structure shows no plasticity, and its fracture toughness, however, is dropped to half of the 5-3 silicide free sample, even the microstructure contains 50% metallic and toughening Nb_SS phase. Combining Table 1 and results in Refs.[7, 8] suggests that the volume fraction of the 5-3 silicide absolutely dominates the bulk ductility and toughness of the Nb_SS/5-3 silicide two-phase structure. On the other hand, for a constant fraction of the 5-3 silicide, toughening the Nb_SS phase may bring little improvement in the toughness of the bulk two-phase structure. It is pointed out that decrease of the 5-3 silicide fraction would be a dominant approach to improve the ductility or toughness of the Nb_SS/5-3 silicide structure. Degradation of strength caused by decreasing of the 5-3 silicide, however, can be supplied partly by solid-solution hardening the metallic Nb_SS phase[7-10]. Subsequently, a balance between the high-temperature strength and the room-temperature ductility or toughness can be obtained in a Nb_SS-dominant two-phase Nb_SS/5-3 silicide structure.

4 Conclusions

1)  With increase of the Si contents from 0.5%, through 5% to 18%(mole fraction), the volume fraction of the 5-3 silicide in the Nb_SS/5-3 silicide structure increases from 0, through 15% to 50%. The 5-3 silicide is matrix for the alloy containing 18%(mole fraction) Si.

2)  The 5-3 silicide strongly affects the room-

temperature toughness and the high-temperature strength. The strength of the sample containing 50% 5-3 silicide at 1 200 ℃ and 1 500 ℃ are approximately 2.5 times of the 5-3 silicide free sample, while the room- temperature toughness of the former is just half of the latter. 

3)  The ductility of the 5-3 silicide free sample at room temperature is about 11%, while the 50% 5-3 silicide sample is entirely brittle, and has no plastic deformation.

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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 Jiang-bo; Tel: +86-10-82315989(100); E-mail: jbsha@buaa.edu.cn