Aliya Amenova¹, Dauletkhan Smagulov¹

¹Kazakh national technical university after K. Satpayev, 050013 Almaty, Kazakhstan

QUANTITATIVE ANALYSIS OF THE Al – Ni – Fe – Mn – Zr – Si PHASE DIAGRAM AS A BASE OF HEAT-RESISTANT CAST ALUMINUM ALLOYS OF NEW GENERATION

Abstract. The phase composition of the Al–Ni–Fe–Mn–Zr–Si system was analyzed with respect to new-generation heat resistant casting aluminum alloys based on a Ni-containing eutectic, which are strengthened by the Al₃Zr (L12) nanoparticles.

Keywords: cast aluminum alloy, Al₃Zr (L12) nanoparticles, microstructure, phase composition, polythermal sections, high strength, eutectic, economically doped, heat resistant, thermally stable.

Introduction

Castings from aluminum alloys are extensively used in different branches of mechanical engineering, which is conditioned by a good combination of mechanical and technological properties with a small weight [1–3]. However, aluminum alloys still have not found wide use in fields where heating details to 300–350⁰ C is possible, although they are promising, especially in combination with new technologies of depositing multifunctional coatings [4, 5].

One of the most essential disadvantages of industrial aluminum alloys is their lowered thermal stability. Working temperatures for even the best AM5 alloys based on the Al–Cu system (GOST (State Standard) 1583–93) do not exceed 250–300⁰ C [3]. It is known that the heat resistance of aluminum alloys can be substantially increased due to doping with increased concentrations of transition metals (TM) [6, 7]. It was suggested that heat resistant alloys be formed based on the (Al)+Al₃Ni eutectic (nikalines) due to doping with nickel and other transition metals (Mn, Zr, Cr, Sc, V, etc) [8-10]. The field of use of these alloys is focused on conventional casting technologies and existing equipment. The production cycle of the acquisition of articles prepared from them is much shorter when compared with grade alloys based on the Al–Cu system (particularly, the stage of quenching is absent).

The most balanced complex of properties was obtained for the Al–4%Ni–2%Mn–0,5%Zr composite, which became the base of nikaline AN4Mts2. The latter passed the pilot testing in conditions of OAO IL (Moscow) and OAO VASO (Voronezh) [8, 9]. Nickel is necessary for the formation of dispersed eutectic (Al)+Al₃Ni during crystallization and manganese and zirconium is for obtaining secondary inclusions Al₆Mn and Al₃Zr during annealing. A special role is played by zirconium, which under certain technological modes allows us to realize the dispersion strengthening due to Al₃Zr nanoparticles. Secondary inclusions of the phase Al₃Zr (dispersoids), with dimensions less than 10 nm, are the most effective hardeners in aluminum alloys [1-6]. Strengthening from these nanoparticles, in contrast with secondary inclusions of Al₂Cu, is retained after prolonged annealing at temperature of 300–350⁰ C.

However, nikaline AN4Mts2 should be considered a more likely model composite, because it assumes low iron and silicon contents; i.e., high purity aluminum is required for its production.

The studies carried out in recent years in the MISA, showed the expediency of creating of new cast and wrought aluminum alloys with a high content of zirconium and 0.8% [10, 11]. It was obtained two patents of ALCOA (United States) [10, 11]. It is shown that the content of Fe and Si can be changed in a sufficiently wide range (up to 1% Fe and 1% Si).

Phase analysis of the Al–Ni–Fe–Mn–Zr–Si system’s composition

The presence of iron and silicon substantially complicates the phase composition when compared with the base composite. First of all, it concerns to crystallization phases. Seven phases, namely, Al₃Fe, Al₆(Fe,Mn), Al₉FeNi, Al₈Fe₂Si, Al₁₅(Fe,Mn)₃Si₂, Al₅FeSi, and (Si) are added to the Al₃Ni phase, which is the only one in nikaline AN4Mts2. Besides these phases, there are added the phases Al₁₅Mn₂Si₃ and (Si) to the secondary inclusions. It is shown the chemical composition and density of the alloys’ phase components in the table 1.

Table 1. Characteristics of present phases in the Al-Ni-Mn-Fe-Si-Zr alloys

Formula	Indication	Composition, [mass %]	Density, [g/cm^3]
Si	(Si)	~100Si	2,3
Al3Fe	Al3Fe	37 Fe	3,90
Al6Mn	Al6	25,3Mn	3,09–3,27
Al3Ni	e (Al3)	42Ni	3,95
Al3Zr	–	53Zr	4,11
Al5FeSi	b (Al5)	25–30Fe, 12–15Si	3,45
Al8Fe2Si	a (Al8)	30–33Fe, 6–13Si	3,58
Al9FeNi	T (Al9)	4,5–14Fe, 18–28Ni	3,4
Al15(Fe,Mn)3Si2	am (Al15)	0–31Fe, 1,5–29Mn, 8–13Si	3,55

Sections calculated using the Thermo-Calc software (the TTAL5 database) shows the influence of iron and silicon on the phase composition of the AN4Mts2 alloys more vividly. Since zirconium forms no phases excluding Al₃Zr, the Al–Ni–Mn–Fe–Si system is considered at the first stage.

Taking into account the fact that primary crystals of all intermetallic compounds are deliberately unknown, we calculated the boundaries of primary crystallization of various phases in various sections (liquidus projections).

a) b)

Figure 1. The boundaries of the primary crystallization of the cross sections of Al-Ni-Fe-Mn. a) at 1,5% Mn; b) at 2% Ni and 0,5% Fe

The joint influence of iron and nickel at 1,5% Mn (Fig. 1a) shows that, at 2% Ni and 0,5% Fe, primary crystals of the Al₆(FeMn) phase should inevitably form, but at a nickel concentration lower than ~3%, the admissible Fe content exceeds 0,5%. On the other hand, silicon can lead to the formation of primary crystals on the Al₁₅(Fe,Mn)₃Si₂ phase only with its sufficiently high concentration (>2%), which is seen from Fig. 1b.

Polythermal sections show that (see Fig. 2a) during crystallization all the phases (Al₉FeNi, Al₆(Fe,Mn), and Al₃Ni) crystallize in a comparatively narrow temperature range (<10 K). The amount of 0.15% Si is sufficient for the appearance of the Al₁₅(Fe,Mn)₃Si₂ phase and strongly complicate the structure of polythermal sections (Fig. 2b). The crystallization range of the Al₉FeNi phase considerably widens as the silicon concentration increases (at 1.5% Si, it is ~90 K).

a) b)

Figure 2. Polythermal sections of Al-Ni-Fe-Mn and Al-Ni-Fe-Mn-Si systems a) at 2% Ni and 1% Mn;

b) at 2% Ni, 1% Mn and 0,5% Fe;

c) 2%Ni, 0,5%Fe and 1,5%Mn

c)

Curves of the stages of nonequilibrium crystallization T–Qs calculated according to the Scheil–Gulliver model of Thermo-Calc software are presented in Fig. 3. It is show temperature dependences of the summary weight fractions of solid phases (Qs) [12].

Figure 3. Effect of silicon on the nature of the non-equilibrium solidification of Al-2% Ni-1% Mn-0,5% Fe: a) 0% Si; b) 0,1% Si

It can be seen from Fig. 3a that, in the absence of silicon, the nonequilibrium crystallization of the Al–2%Ni–0,5%Fe–1%Mn alloy slightly differs from equilibrium with an insignificant increase in ΔT. After the formation of primary (Al) crystals, eutectic reaction L®(Al) + Al₉FeNi proceeds in a wide temperature range and further crystallization is completed according to the reaction L®(Al) +Al₉FeNi + Al₃Ni. However, the addition of only 0.1% Si widens the crystallization range to 50 K (see Fig. 3b) due to a decrease in nonequilibrium solidus (up to ~600°C).

The character of dependences T–Qs (see Fig. 3) assumes the negative influence of silicon on the hot brittleness of nikalines, which is confirmed by the experimental data. A second negative effect of this element is associated with the fact that, as the Si concentration in the alloy increases, the solubility of zirconium in the solid solution (C_Zr–(Al)) decreases. Nickel, iron, and manganese exert an insufficient effect on the value of C_Zr–(Al); to the contrary, the presence of 1% Si leads to an almost twofold decrease in C_Zr–(Al) ,which decreases the potential of strengthening due to the formation of nanoparticles of the Al₃Zr phase during annealing (see Fig. 1b). Thus, in contrast with iron, which can be considered as the doping component as applied to economically doped nikalines, silicon should be limited rather rigorously.

Conclusions

The phase composition of the Al–Ni–Fe–Mn–Zr–Si system was analyzed with respect to new-generation heat resistant casting aluminum alloys based on a Ni-containing eutectic, which are strengthened by the Al₃Zr (L12) nanoparticles. The presence of iron and silicon significantly complicates the phase composition as compared with quaternary (Al–Ni–Mn–Zr) alloys. Silicon strongly expands the crystallization range (being ~60°C already at 0.1%), which increases the tendency of the alloy to form hot cracks during casting. It is shown that economically doped nikaline AN2ZhMts substantially exceeds the most heat resistant cast aluminum alloys of the AM5 grade in the totality of its main characteristics (heat resistance and mechanical and production properties).

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