Аinagul Тоleuova, Dauletkhan Smagulov

K.I. Satpaev Кazakh National Technical University, 050013, Каzakhstan, Almaty

Studying aluminum angle of Al–Cu–Mn–Zr system phase diagram as a base for obtaining refractory aluminum alloys

Introduction

Recently there have rather actively been studied metastable condensed systems possessing a number of new physical-and-mechanical properties differing from the properties of equilibrium systems. A special place among them there is taken by nanostructured materials whose volume significant part make grains boundaries. Traditionally non-equilibrium (amorphous, nano- and microcrystal) states in aluminum alloys are obtained by methods of the melt fast quenching, mechanical alloy-forming and others.

With new technologies development there are made demands for structural materials quality, particularly, to aluminum alloys with transition metals possessing high operational and special physical properties, such as refractoriness, plasticity, fracture toughness and some others. To achieve these aims there are improved methods of materials treating in both liquid and solid states. To the first group there are referred temperature-temporal treatment and the melt high-speed crystallizing; to the second one – intense plastic deformation and thermal treatment.

Fast-quenched aluminum alloys containing 0.05 wt% by weight transition metals (Zr, Fe, Cr) showed their good advantage as a base for prospective refractory granular alloys mainly due to forming in them oversaturated solid solutions. However, obtaining oversaturated solutions by the method of high-speed crystallizing is connected with great technical difficulties. In this connection there arises the necessity to look for other methods of obtaining necessary structural state and the outside effects, for example, intense plastic deformation that would assist the forming of certain structural states ensuring the necessary level of the abovementioned alloys operational characteristics.

In the present work with the aim of determining the zone of concentrations and temperatures at which there can be achieved the maximum level of refractoriness, there has been carried out the quantitative analysis of Al–Cu–Mn and Al–Cu–Mn–Zr systems phase diagrams. With the help of Thermo-Calc program there have been calculated the corresponding isothermal and polythermal sections of phase diagrams, as well as determined the temperatures of liquidus and solidus.

Experimental studies

The analysis of the alloys chemical composition (Table 1) shows that type 1201 alloys have significantly higher copper concentration but lower manganese and zirconium concentrations than in ALTEK alloy. This difference in concentration of alloying elements defined the key difference of these alloys structure and properties.

Table 1. Some deformable alloys composition based on Al–Cu-Mn-Zr system

Grade	Cu, at%	Mn, at%	Zr, at%	Others
D20^1	6.0–7.0	0.4–0.8	0.2	Ti
1201^2	5.8–6.8	0.2–0.4	0.1–0.25	Ti, V
АА 2219^3	5.8–6.8	0.2–0.4	0.1–0.25	Ti, V
ALTEK^4	1.2–2.4	1.2–2.2	0.15–0.6	Sc, V

¹BSt, ²SSt 4784-97, ³specification of The Aluminum Association (USA) ⁴ RF pat. No 2252975 (publ. 27.05.2005, Bull. No 15)

Adding zirconium to binary alloys is known to lead to forming Al₃Zr phase [1]. Zirconium is known to increase greatly the liquidus temperature in binary alloys. The calculation shows that copper presence effects but little the degree of this increase that is demonstrated by polythermal sections shown in Figure 1, as well as the data presented in Table 2.

From Table 2 we can see that a mere addition of copper doesn’t almost effect the alloy crystallizing character. In non-equilibrium conditions of crystallizing manganese solubility in aluminum increases, and a ternary compound formation is suppressed. That’s why in such alloys alongside with (Al) there co-exist phases Al₂Cu and Al₆Mn. After forming virgin crystals (Al) there occurs separating phases Al₂Cu and Al₂₀Cu₂Mn₃ in the following reaction: L→(Al) + Al₂Cu + Al₂₀Cu₂Mn₃ at the temperature 547 °С. With the further increasing of copper concentration there are not observed significant changes.

Table 2. Parameters of Al–Cu–Mn–Zr system characteristic alloys crystallizing

Cu, at%	tL, °C	tS, °C	Phases
2	730	628	(Al) + Al20Cu2Mn3 + Al3Zr + Al6Mn
5	731	576	(Al) + Al20Cu2Mn3+ Al3Zr

Though in literature there are no data on building a diagram of 4-component Al–Cu–Mn–Zr system, phase zones distribution in the aluminum angle of this system in solid state can be predicted based on the existing information.

а b

Fig.1. Polythermal sections of Al–Cu–Mn–Zr system with varying zirconium content: а) 2 at% Cu and 1.5 at% Mn; b) 6.5 at% Cu and 0.5 at% Mn

One of the most important characteristics of any alloy is the liquidus (T_L) and the solidus (T_S) temperature. With the help of these temperatures there are determined the modes of thermal treatment, temperatures of alloys melting and casting. The results of calculating T_L and T_S for some alloys of Al –Cu–Mn– Zr system are shown in Table 2. Starting from the calculation results we can conclude that copper doesn’t effect T_L significantly but decreases T_S obviously. On the other hand, adding 0.4 at% Zr already increases the liquidus over 800 °C. The temperature effect on the phase zones location is shown on the calculated polythermal sections with varying manganese content (Figure 2). It’s obvious that with copper concentration decreasing from 2 to 1 at% there decreases the probability of phase Al₂Cu forming. The temperature effect is reflected by polythermal sections with varying manganese content shown in Figure 2. Here we can see that copper content decrease from 2 to 1 at% decreases the probability of forming Al₂Cu phase.

а b

Fig.2. Polythermal sections of Al–Cu–Mn–Zr system with varying manganese content: а) 2 at% Cu; b) 1 at% Cu: calculation for metastable phase Al₃Zr (L1₂)

Conclusions

In the work based on Thermo-Calc program there has been carried out the analysis of Al–Cu–Mn and Al–Cu–Mn-Zr phase diagrams as a base for cast and deformable refractory aluminum alloys.

To develop refractory alloys designed for operating up to 350 ⁰С there are suggested the alloys of Al–Cu–Mn–Zr system. As compared to industrial alloys of 1201 type it is suggested to decrease copper content and to increase manganese content. This will permit to obtain in the final structure the maximum number of the secondary aluminides Al₂₀Cu₂Mn₃ that (alongside with dispersoids Ll2) assist the hardening, especially at increased temperatures. Besides, new alloys don’t require homogenization (as the maximum plasticity is achieved in a cast state), that permits to decrease significantly the deformed half-finished products cost.

References

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