Preparation of Niobium Aluminium Alloy Based on Shock Compression Method

Abstract

A new method based on a light-gas gun has been proposed to synthesize Nb-Al alloys, and a recovery capsule has been investigated. The copper-coated sample is accessible after shock wave loading. In this paper, we successfully synthesize Nb-Al alloys, which are high-temperature structural materials. X-ray diffraction is employed to clarify the structural characteristics of compounds after impact, and the simulation of X-ray diffractions is employed to clarify the structure. In detail, tetragonal NbAl₃ alloys certainly appeared in the recovery capsule; this alloy is considered to be best candidate.

1. Introduction

As one of the dynamic high-pressure loading devices, a light-gas gun can realize high-temperature and high-pressure conditions by the collision of a projectile with a target [1]. In 1946, the United States successfully developed the first light gas gun; it has become an important tool for studying the equation of state of materials under high pressure, the electromagnetic properties, the impact of new materials, the simulation of high-speed collision planetary, etc. [2,3,4]. Compared with conventional synthesis methods, shock wave synthesis technology caused by light-gas guns has the advantages of fast loading time, uniform pressure distribution, fine particle size and good sintering performance [3,4].

To the best of our knowledge, there are a lot of newly formed materials and new compression-induced phases under shock compression. For instance, diamond, wurtzite-type BN and spinel-type nitrides and a new phase of carbon nitrides were synthesized via shock wave [5,6,7,8]. Moreover, the Hugoniot relations of spinel-type ceramics of Fe₃O₄ and AlON suggested that the post-spinel phases could be formed above 100 GPa [9,10].

Nb-Al alloys recently attracted wide attention as high-temperature structural materials due to their low density, high melting point, high toughness and excellent high temperature strength [11,12]. It is well known that the Nb-Al binary system consists of Nb₃Al, Nb₂Al and NbAl₃ intermetallics [13,14]. Among them, there is no literature to show the preparation of Nb-Al alloys under shock waves produced by light-gas guns. Nevertheless, the structural analogue A15 Nb₃Si was synthesized using a two-stage light-gas gun [15]. In the reaction-assisted shock consolidation of Nb-Al alloys, the NbAl₃ and Nb₂Al compounds were identified with X-ray diffraction. Unfortunately, as the plane wave caused by the chemical-induced shock was not uniform, the products were mixed with other impurities [16]. To obtain high-quality alloys, model experiments were planned and carried out using a two-stage light-gas gun.

2. Experimental Methods

The experimental device is schematically shown in Figure 1 [17]. The diameter and thickness of the reaction chamber’s geometrical dimensions (first warehouse) were 12.1 mm and 4 mm, respectively. The copper flyer with 3 mm thickness and recovery capsule in synthesis section collided with each other with a flyer velocity that ranged from 1200 to 3300 m/s, caused by a two-stage light-gas gun. As far as we know, copper is generally chosen to synthesize and recover high-pressure phases because copper is known for having high shock-impedance and thermal conductivity. The original samples were prepared by mixing 99.95% pure niobium powder and 99.95% pure aluminum powder, and the mass ratio 60:40 of Nb:Al mixture ratio was mixed under nitrogen atmosphere. Additionally, then the original samples were made into a 12 mm diameter sheet sample (cylindrical, of 3.81 × 10³ kg/m³ density) via preforming. The purpose of this step was to prevent the aluminium powder from deflagration and to avoid the oxidation of the samples. To ensure that the shock wave generated by high-speed collision passed through the sample to achieve the maximum balance pressure, a high-quality steel frame was used to fix the recovery capsule. As shown in Figure 2 [18], all samples were placed in the first warehouse of the recovery capsule, which included a pedestal and a screw cap. In order to avoid the effects of sparse waves, the pedestal and screw cap were made of the same material. Six samples came from same batch; the considered parameters are listed in

Table 1. Parameters of Nb and Al powder compact.

Number	Diameter (mm)	Thickness (mm)	Mass (g)	Density	Porosity
No. 1	12	3.16	1.361	3.81	38.8%
No. 2	12	3.21	1.382	3.81	38.8%
No. 3	12	3.56	1.537	3.82	38.6%
No. 4	12	3.36	1.455	3.82	38.6%
No. 5	12	3.54	1.526	3.81	38.8%
No. 6	12	3.26	1.408	3.81	38.8%

. Finally, the recovered samples were characterized using X-ray diffraction. The X’Pert Pro MPD X was used; the manufacturer of the X-ray was HollandPanalytical (Almelo, The Netherlands).

The X-ray diffraction calculations of Nb-Al alloys were performed using the first principles implemented in the CASTEP (Cambridge Serial Total Energy Package) code [19,20]. The diffraction peaks were applied based on the Pseudo-Voight function. The models for diffraction simulation in this paper were tetragonal NbAl₃, whose parameters are a = 3.8458 Å, c = 8.6094 Å [21], and cubic Nb₇Al, whose parameters are a = 3.275 Å [22]. The lattice parameter of pure cubic metal model for Nb is a = 3.3 Å [23], for Al it is a = 4.056 Å [24], and for Cu it is a = 3.613 Å [25], respectively.

3. Results and Discussion

As shown in

Table 2. The shock compression loading parameters at different impact velocities.

Number.	Mass of Sheet (g)	Impact Velocity (km/s)	Impact Pressure (GPa)
No. 1	1.361	2.14	44.31
No. 2	1.382	1.63	31.79
No. 3	1.537	2.52	54.44
No. 4	1.45	1.28	23.91
No. 5	1.526	2.60	56.65
No. 6	1.408	3.28	86.91

, the alloys were synthesized with different impact velocities. The impact pressures were calculated using the Hugoniot equation $P=\rho (C_0+\lambda \mu )\mu$ [26], where the Hugoniot parameter of copper is C₀ = 3.933 km/s and λ = 1.5 [27]. The density $\rho$ of copper is 8.93 g/cm³. The impact velocity $\mu$ is expressed by the symmetric collision equation $\mu =\frac{W}{2}$ [28], where the $W$ is the velocity of flyer measured with the magnetic speed measuring system. One copper recovery capsule and copper-coated samples are shown as examples in Figure 3; it can be seen that the recovery capsule is geometrically maintained. The powder compact after shock wave loading was very thin and had a metal lustre.

Figure 4 and Figure 5 show the X-ray data of sample No. 3 and sample No. 5, respectively. Each substance is represented by a colour. Comparison of the shape and size of these diffraction peaks implies that the NbAl₃ and Nb₇Al were successfully synthesized. Interestingly, the NbAl₃ compound was produced in both two different impact velocities. A strong peak appeared in the 2ϴ range of 38 to 40°, which corresponded to the (112) surface. It is suggested that the NbAl₃ (112) is the most easily synthetic plane in Nb-Al alloys. The characteristics of NbAl₃ alloy synthesized at 2.52 km/s are comparable to that of the alloy synthesized at 2.6 km/s, where the (204) surface and (004) surface appear at different impact velocities, respectively. The impact of a shock wave travelling with sufficient velocity would impart enough energy to promote the shock synthesis of alloys.

To support our experimental observation of alloys, the structure and reflex powder diffraction were studied based on the Pseudo-Voight function. The crystal models of I4/mmm NbAl₃, Im-3m Nb₇Al, Im-3m Nb, Fm-3m Al and Fm-3m Cu were built for X-ray diffraction simulation. Simulated diffraction images compared with X-ray data are shown in Figure 4 and Figure 5. In all, the X-ray diffraction peaks of Nb-Al alloys included NbAl₃ and Nb₇Al. In detail, the pure metal Nb (110) had a diffraction peak in the 2ϴ at 38°, and the pure metal Al (200), (311) and (220) had diffraction peaks in the 2ϴ at 44, 65 and 78°, respectively. The experimental peaks of pure metals indicate that the synthesis of Nb-Al alloys contained the residual metal power. Copper (111) and (200) peaks in the 2ϴ at 42 and 45° revealed that copper permeated to Nb-Al alloys under impact. At an impact velocity of 2.52 km/s, it can be seen that the plane (112) of NbAl₃ and the plane (110) of Nb₇Al overlapped with pure cubic Nb, because they have the same atomic distributions in plane, as shown in Figure 6. It is predicted that of the formation of surfaces NbAl₃ (112) and Nb₇Al (110) is easier. As the impact velocity increased to 2.60 km/s, the peaks of Nb₇Al disappeared, while those of NbAl₃ were still retained. This suggests that the cubic crystal $(a=c)$ transitioned to a tetragonal ($a\ne c$) structure under impact, because the shock wave was actually axial.

The crystal structures reveal that the crystal transition from cubic to tetragonal is axial under impact. When the impact pressure increased, the Nb and Al atoms moved axially, where the atoms were in plane (110) along direction [001]. The pure cubic metals transferred to Nb₇Al retained the cubic structure, and those transferred to NbAl₃ became tetragonal in structure. In all, those planes had strong diffraction peaks and they did not change under impact, revealing that the NbAl₇ and Nb₃Al alloys are easy to be synthesized from pure Nb and Al under shock waves. This is evidence to support our experiment results. Although the diffraction angles of certain crystal planes were almost equal between Al and Nb and Nb-Al alloys, the alloying process in general powder metallurgy is based on atomic diffusion, and the same is true for the shock compression of powders.

4. Conclusions

In summary, the results demonstrate that we synthesized two Nb-Al alloys via shock waves. The products after impact were measured using X-ray diffraction. In order to illustrate the micro-transition of synthesis and the characteristics of products, we used X-ray diffraction simulation to display the structural characteristics of Nb-Al systems. These experiments and calculations provide a new synthetic pathway for Nb-Al alloys. In addition, the design of a recovery capsule was proven to be feasible.