Simulation of Phase Equilibrium in Alloys on the Basis of System Fe-Al
L.A. Marshuk, V.P. Yermakova, O.Yu. Sheshukov Ul. Amudsena 101, Yekaterinburg, 620016, Institute of Metallurgy of the Ural Division of the Russian Academy of Sciences, [email protected]
ABSTRACT Modelling of phase composition of Fe-Al alloys carried out with use of a programm complex the ACTPA-4 (TERRA), the composition of phases of any heterogeneous systems intended for thermodynamic calculation. The knowledge of phase composition of Fe-Al alloys is necessary from the point of view of transfer of structural attributes of charge materials through a liquid state to ingots and receptions of structure of the aluminium iron securing high parameters of its heat-resistance. The object of investigation were Fe-Al alloys with mass. % 17-70 Al. Calculations carried out for a temperature interval from 25 up to 1900 оС. In calculations took into account thermodynamic and thermochemical properties Fe, Al, Si-carbides, Fe-aluminides and Fe-silicides. Effect of a relationship of various elements in an alloy on their phase composition was investigated. The increase in Al amount in alloy results in the increase Fe-monoaluminides fraction. At increment of the aluminium contents up to 30 % FeAl amount decreases and the increase in FeAl2, and then leading growth of FeAl3 phase about suppression of FeAl and FeAl2 phases at contents before 60 % Al and higher. Results of simulation of alloy composition on the basis of system Fe-Al have allowed to predict phase composition of these alloys and to specify it by metallography, X-ray phase and X-ray microspectroanalysis methods.
Introduction In recent years Fe-Al-System alloys are very popular and are applied extensively in the metallurgy as desoxidants, extenders or alloy additives to increase the amount and the stability of aluminum applications. Our task was to determine an optimal alloy composition based on the Fe-Al- system and to define this alloy obtaining parameters (the liquid metal crystallization rate) in the context of the further addition of this alloy to cast iron to provide higher heat resistance values of aluminum cast iron. The phase composition of Fe-Al alloys in the liquid and the solid state shall be known with relation to transfer structural features to the alloy during the crystallization with different rates and to transfer structural features of burden materials to ingots through the liquid phase, and to obtain the cast iron structure providing its higher heat resistance values. In this article we used the АSTRA-4 (TERRA-4) software system, designed for phase composition thermodynamic calculations of some heterogeneous systems, and studied the equilibrium phase composition of Fe-Al-based system in the wide range of aluminum concentrations from 17% to 70%. We applied the perfect association solution model to the metal melt taking into account the short-range order in the high-temperature liquid melts.
Result and Discussions
We shall note that the ferroaluminium structures were never studied in such wide range of the aluminum concentrations (from 17% to 70%). Despite of some
1-123 international references there is no such established concept as a metallography of Fe- Al-system alloy.
Our calculations were performed for the temperature range from 25 to 1900оС. The thermodinamical and thermochemical properties of carbides of Fe, Al, and Si, as well as aluminides and silicides of Fe were taken into account. We studied the influence of the concentration ratio of different elements in the melt to their phase composition. We performed the result simulation for compositions closed to actually obtained during the experimental melts and further studied by means of various metallography methods including quantitative and qualitative optical metallography using image analysis computer system of Thixomet Pro and SIAMS-700, X-ray microanalysis, and X-ray phase analysis. Alloy compositions are listed in the Table А.
Table 1 – Compositions of Fe-Al-system alloys. Sample Al Fe C Si Cu S P 1 17.69 68.85 0.209 0.148 0.225 0.275 0.0155 2 25.52 63.50 0.205 0.352 0.096 0.052 0.0358 3 31.25 57.60 0.191 0.910 0.096 0.025 0.0100 4 37.52 60.18 0.573 0.625 0.138 0.020 0.0077 5 62.30 33.61 0.381 1.068 0.075 0.013 0.0140 The thermodinamical simulation results – phase concentration vs. temperature dependencies - are displayed in Figures 1-5.
FeAl
% 60
. s s Fe a m
, e s
a 40 h p
f o
n o i t a r
t 20 n
e Fe3C c
n FeAl2Fe3Si Al
o FeSi C
0 100 300 500 700 900 1100 1300 1500 1700 1900 Temperature, oC
Figure 1 – Phase concentration dependencies, alloy sample 1 (17.69 % Al).
1-124 80 %
FeAl . s s a
m 60
, e s a h p f 40 o n o i t Fe a r t n 20 Al e FeAl2 c Fe3C n FeAl3 o FeSi
C Fe3Si 0 100 300 500 700 900 1100 1300 1500 1700 1900 Temperature, oC Figure 2 – Phase concentration dependencies, alloy sample 2 (25,52 % Al).
80 %
.
s FeAl s a m
, 60 e s a h p f o 40 n o i Fe t a r t
n FeAl2 e 20 Al c n FeSi o FeAl3 C Fe3Si Fe3C 0 100 300 500 700 900 1100 1300 1500 1700 1900 Temperature, oC Figure 3 – Phase concentration dependencies, alloy sample 3 (31,25 % Al).
1-125 FeAl %
. s
s 60 a m
, e s a h p
40 f o
n o i t Fe Al a
r FeAl2 t
n 20 e c
n FeAl3 o
C Fe3C Al4C3 FeSi Fe3Si 0 100 300 500 700 900 1100 1300 1500 1700 1900 Temperature, oC
Figure 4 – Phase concentration dependencies, alloy sample 4 (37,53 % Al).
FeAl3 80 %
. s s a m
, 60 e s Al a h p
f o 40 n
o FeAl i t a r t
n FeAl2 e 20 c Fe n o
C SiC FeSi C 0 100 300 500 700 900 1100 1300 1500 1700 1900 Temperature, oC Figure 5 – Phase concentration dependencies, alloy sample 5 (62,30 % Al).
These figures show that the FeAl phase, assumed as a matrix, is disappeared in alloys with higher aluminum concentrations according to our calculations. The FeAl3 intermetallic phase and some amount of free aluminum are found in alloys with the Al concentration more than 53,0%. Moreover, the free aluminum ratio increases highly with its concentration in the metal increases. The results of thermodinamical simulation comparative analysis, of X-ray phase analysis, and of X-ray microanalysis are listed on the Table 2.
1-126 Table 2 – Comparing of calculation results and metallography analysis results Aluminum Phase composition and amount Concentration, Thermodinamical X-ray Phase X-ray Fe – Al Mass. % Calculations Analysis Microanalysis Diagram FeAl – 62% FeAl FeAl 17.0 α – solid solution of Fe3Al AlxOy Fe3Al Fe – 38% Fe2Al5 FeAl – 90% FeAl FeAl 25.5 α - solid solution of Fe3Al – Fe2Al3 Fe – 10% Fe2Al5 FeAl – 85% FeAl FeAl α - solid solution of FeAl 31.1 Fe3Al, FeAl2, FeAl2 Fe – 11 % FeAl2 Al2O3 AlxOy FeAl3 – 4% FeAl – 71 % FeAl Fe3Al – 16 % FeAl FeAl 37.1 Fe3Al FeAl3 – 9 % FeAl2 FeAl2 AlxFey Al4C3 – 2 % FeAl FeAl – 86 % FeAl 3 3 α-solid 64.5 α-solid solution of α-solid solution – solution of Al – 14 % of Al Al
The results of X-ray phase analysis, and of X-ray microanalysis confirmed substantially the results of the metallography analysis and of the thermodinamical calculations in terms of phase compositions appeared in alloys with the Al concentrations from 17 to 70 mass. %. Increasing the aluminum concentration in the alloy leads initially to increasing the ferrum monoalumide ratio. While the aluminum concentration increases up to 30% the amount of FeAl decreases, the FeAl2 concentrations increases at first, and further the FeAl3 phase concentration starts to increase ahead with FeAl and FeAl2 elimination at the aluminum concentrations more than 60%. The simulation results of Fe-Al-system alloy enables to predict the phase composition of these alloys and to ascertain it using methods of metallography, of X-ray phase analysis, and of X-ray microanalysis.
SUMMARY
The ASTRA-4 software system allows performing preliminary thermodinamical calculations of the phase composition, appearing in Fe-Al-system. The results obtained supported to selection of the alloy composition and its cooling rate from its liquid state – fast cooling is an attempt to fix the “structure” of the liquid alloy, and slow cooling is “transition” through all treatment steps from the liquid state to the low-temperature equilibrium state. The results of methods used in this article enable to select a ferroalloy with the optimal structure, obtained by means of concentration adjustment of leading elements and of liquid metal cooling rate. We determined that the ferroaluminum with the optimal structure required to obtain a cast iron with the higher heat resistance, must have a 30-40 % Аl
1-127 concentration and shall be cooled after melting with the maximum rate. The great amount of the most disperse particles of the oxide phase, which must be nucleus and to support the cast iron refinement due to their large contact surface with the alloy treated, are in the alloy with such composition.
ACKNOWLEDGEMENT
This work was supported by the Russian Foundation for Basic Research (grants 10-08-00361_a)
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