Vapor-Condensed Composite Materials Ni–Al₂O₃, NiCr–Al₂O₃ with Oxide Nanophase

Abstract

The structure and mechanical properties of Ni–Al₂O₃ and NiCr–Al₂O₃ dispersion-strengthened materials condensed from the vapor phase were investigated. It was shown that nanoparticles are uniformly distributed throughout the volume of the condensates. The factor affecting the structure is the contact interaction at the particle–matrix interphase boundary. The studied mechanical properties at temperatures of 700 and 1000 ± 20 °C show that small concentrations of dispersed Al₂O₃ particles (0.25–0.4%) in Ni–Al₂O₃ condensates and up to 1% Al₂O₃ in NiCr–Al₂O₃ condensates lead to an increase in strength characteristics and non-monotonic decrease in plasticity. Based on the obtained data, the optimal concentration of the strengthening Al₂O₃ nanophase in condensed dispersion-strengthened Ni–Al₂O₃ and NiCr–Al₂O₃ materials was determined, which ensures a high level of strength and plasticity.

1 Introduction

Dispersion-reinforced materials (DRM) were obtained exclusively by powder metallurgy methods until recently. The authors of [1] conducted detailed studies of a wide range of properties of these materials. Many of them have found practical application. Obtaining DRM is a multi-stage technological process that requires strict adherence to technological discipline at all stages of their production.

High-speed electron beam evaporation–condensation as a new technological process has found wide application for the deposition of various protective coatings on products, primarily on the blades of gas turbines. The rate of vapor flow deposition on substrates of various configurations can reach 150 μm/min, which allows to deposit quite a lot of material on the substrates. Therefore, it is of scientific and practical interest to use the specified technological process, controlled at the atomic-molecular level, to obtain massive (separated from the substrate) composite materials.

Fundamental studies to establish the basic physical and mechanical laws of the formation of thick (0.01–2 mm) condensates were carried out at the Paton’s Institute of Electric Welding under the leadership of Academician Movchan [2, 3]. The main physico-chemical regularities of the formation of thick vacuum condensates of some pure metals, alloys, oxides, carbides, borides were determined, and their physico-mechanical characteristics were studied depending on the composition and condensation parameters.

Composite dispersion-reinforced porous and layered (microlayer) materials should be included in the new materials obtained by vapor deposition in a vacuum.

Currently, intensive research is being conducted on new composite materials condensed from the vapor phase with a reinforcing nanophase (oxides, carbides, borides, refractory metals). Dispersion-strengthened composite materials that condense from the vapor phase (condensates) consist of a polycrystalline metal or ceramic matrix with nanodisperse particles of the second phase uniformly distributed by volume. By varying the substrate temperature and cooling rate, the average crystallite size of the matrix can be varied from several hundred microns to several hundred nanometers, and the particle size of the master phase can be varied from several nanometers to several microns. As a result of the influence on the morphology, dispersion and nature of the distribution of the strengthening phase, it is possible to obtain in dispersion-strengthened materials a combination of properties that are unattainable in ordinary alloys [4, 5].

The use of stable refractory compounds as strengthening phases, e.g., oxides that do not actively interact with the base metal and do not dissolve in it up to its melting temperature, ensures the preservation of the microheterogeneous structure and dislocation substructure up to pre-melting temperatures. This allows you to preserve long-term operational characteristics of materials (0.9–0.95 T_mel).

2 Results and Discussions

Condensed from the vapor phase dispersion-strengthened materials Ni–Al₂O₃ (KDSM) were obtained on laboratory and industrial equipment manufactured at the IES named after Paton of the National Academy of Sciences of Ukraine. Sheet rectangular condensates (220 × 320 × 0.8–2) mm with a concentration gradient of dispersed oxide nanophase were obtained for research.

A similar technological technique makes it possible to obtain a significant number of samples of different composition. Alternating vapor flow deposition was carried out on a substrate with Art. 3, processed to a purity class of 0.63 at two condensation temperatures of 700 and 1000 ± 20 °C. For easy separation of condensates from the substrate, a separating layer of calcium fluoride (CaF₂) with a thickness of 20–40 microns was previously applied to the surface on which condensation was carried out. For industrial applying, condensates were formed in the form of cylindrical sheet blanks with a thickness of 1–4 mm and a diameter of 800 mm.

In the work, the structure, chemical, phase composition and mechanical properties were investigated according to known standard methods [4].

According to the Fig. 1 of the Ni–Al₂O₃ KDSM structure that dispersed aluminum oxide nanoparticles are evenly distributed throughout the volume of the condensate. The determining factor that affects the structure and, as a result, the mechanical properties of KDSM is the contact interaction at the particle–matrix interphase boundary. The quantitative criterion of such contact interaction is the wetting angle. It is largely influenced by the environment in which the crystallizing liquid phase interacts with the solid oxide particle, the purity of the metal and oxide phases, the condensation temperature, and other factors. Currently, a sufficient number of studies have been conducted, which are based on their contact interaction when obtaining new materials in metal (alloy)—MeO systems in a vacuum [6]. KDSM Ni–Al₂O₃ with an acceptable set of mechanical characteristics can be obtained in a narrow interval of oxide phase concentration (up to 0.6 wt.%).

Fig. 1

Structure of KDSM Ni–Al₂O₃ (×30,000)

Using the methods of electron microscopic and X-ray phase analysis, it was established that only nickel and aluminum oxide are present in the material.

Figure 2a, b shows the dependences of strength limits (σ), yield strength (σ_0,2) and relative elongation (δ) of Ni–Al₂O₃ condensates obtained at substrate temperatures (T_s) of 700 and 1000 ± 20 °C.

Fig. 2

Dependencies of strength, yield strength and relative elongation on the content of Al₂O₃ in Ni–Al₂O₃ KDSM obtained at temperatures: a—700 °C; b—1000 °C

The analysis of obtained data shows that small concentrations of dispersed Al₂O₃ particles lead to an increase in strength characteristics and a non-monotonic decrease in plasticity in a relatively narrow range of Al₂O₃ concentrations (0.25–0.4 wt.%).

The increase in plasticity is explained by the fulfillment of the structural condition: the average grain size of the metal matrix (D₃) is equal to the average distance between dispersed particles of the strengthening phase (Λ) [2].

It should be emphasized that the maximum of the curves of the dependence of plasticity on the content of Al₂O₃ shifts toward a higher content of aluminum oxide with an increase in the temperature of the substrate. The absolute values of the plasticity of two-phase Ni–Al₂O₃ materials with an optimal content of dispersed particles increase with an increase in the condensation temperature. For example, at T_s = 1000 °C, KDSM Ni–(0.35–0.4) mass % Al₂O₃ has a value of relative elongation greater than that of pure nickel (Fig. 2b).

In terms of strength characteristics, Ni–Al₂O₃ KDSM is not inferior to industrial VDU-2 DUM (98% Ni + 2% HfO₂), obtained by powder metallurgy methods.

In more complex two-phase condensed systems that form solid solutions, e.g., NiCr-Al₂O₃, qualitatively similar changes in mechanical characteristics are observed (Fig. 3a, b). The strength limit and yield strength increase in a wider range of Al₂O₃ concentrations (up to 1%). However, with this content of dispersed refractory particles, condensates have low plasticity. A similar change of the mechanical characteristics in the KDSM is caused by the almost complete absence of particle–matrix interphase interface interaction. The marginal wetting angle of Al₂O₃ with liquid nickel ranges from 150 to 1150. Due to the lack of mutual action in the condensates, pores are formed, which leads to a loss of strength and plasticity. Improvement of interphase interaction in the NiCr–Al₂O₃ system (up to 85o) leads to some increase in strength and plasticity in a wide range of Al₂O₃ concentrations compared to Ni–Al₂O₃ composites.

Fig. 3

Dependencies of strength, yield strength and relative elongation on the content of Al₂O₃ in NiCr–Al₂O₃ KDSM obtained at temperatures: a—700 °C; b—1000 °C

3 Conclusion

Thus, as a result of the research of the mechanical properties of condensed dispersion-strengthened materials (KDSM) Ni–Al₂O₃, NiCr–Al₂O₃, the optimal concentration of the reinforcing nanophase Al₂O₃ in KDSM Ni–Al₂O₃ and NiCr–Al₂O₃ was determined, which ensures a high level of strength and plasticity.