Thermomechanical treatment of a Fe-Mn-Al alloy

Analysis of the hardnees and the metalographic struture, show in a FeMnAl alloy the existence of a γ fase and a β precipitate, activitate by Si during the casting processes. During the recovery and recrystalisation temperatures there are the formation of fringes, coarsed grains of γ fase and β precipitate . The thermomechanical treatments give to the material excellent properties of mechanical resistance and good ductility, without the β collaboration.


INTRODUCTION
Studies of ordered alloys Fe3Al and Fe3Si [6,12] has enabled the development of the Fe-(5w%Al)-(1.5w%Si)magnetic alloy itself, with excellent resistance to corrosion and high resistance to oxidation at high temperatures, by forming a thin surface layer of alumina, Al2O3.
The composition limit for the alloy is 1.5w%Si and 5w%Al [1,17], which allows their use as well as resistors (80 μΩcm), combined with moderate mechanical strength properties and resistance to oxidation.
Their mechanical fragility is attributes to the structure BCC and to the interstitial defects of the alloy.
Because their FCC structure the (55w%Fe-35w%Mn-10w%Al) alloy [1,16], presents at high temperatures a better mechanical and a good plasticity properties.
A combination of (35w%-40w%Mn) and 15w%Al of that alloy produces an improper structure β-Mn, and a lamellar ferritic structure B2 that do not maintain their austenitic structure at high temperature [2,13,17].
The addition of Carbon to the alloy [8] stabilizes the austenite, gives inoxidizable property up to 650•C, similar to the inox steel Fe-Ni-Cr, and superior mechanical conditions up to 815C [7,10] to the carbon steels 304 AISI and 347 AISI.
During cold rolling operation that alloy hardens quickly wanting intermediate thermal anneals.
Addition of 3w% of elements as B, Cr, Mo, Ni, Nb, Ti, do not improve the best properties of this alloy [3,14,1518].*corresponding The table 1 shows for example some mechanical properties of different Fe cast-alloys, Some properties of the austenitic alloy (56w%Fe.30w%Mn, 10w%Al, 3w%Si, 1.0w%C) [4] are presented here: remarkable corrosion resistance to boiling sea water during 118 hours [9], what is adequate to use as helix for vessels long journey; imperceptible superficial corrosion after to be into sea water during 32 days at room temperature, as show similar works [1,5,9,10]; their properties σR (MPa), σE(MPa), between 24° C and 815° C, and of cold reduction increment, are higher than that austenitic Fe-Ni-Cr steels [10]; their ductility is similar to traditional inox steels, which permits various percentages of reduction; the σE of the alloy can reach to high values of 1660(MPa) for 80% of cold reduction [10]; studies of that Fe-Mn-Al-Si-C austenitic steel, at three dynamic strain aging rates in the range of 172-345•C, show the occurrence of peaks in strength, in ductility and work hardening parameters, the three well-defined peaks for the work hardening were displaced to higher temperatures with increase in strain rate, and the activation energy involved was determined and the possible mechanism of strain ageing was proposed [4]; the oxidization resistance of the alloy at 700° C for 70 hours is almost the same for the inox AISI 304 at 800° C for 40 hours with an stable oxidized superficial layer [11]; their electrical resistivity varies from 168 μΩcm to 180 μΩcm between 80•C and 800° C, values that are superior on the Ni-Cr and Kanthal alloys [11].
The existence of two metallographic structures β and γ, was revealed by the optical metallography and the Scanning Electronic Microscope (SEM-Instituto de Quimica-UNESP), having the β phase a little concentration of Cr and a smallest concentration of Fe and Mn than in the γ phase.
The existence of β phase it is attributed to the Si influence on the casting procedure.
These two phases stay inside the (γ+β) band of the Horizontal Section at 7600C of the Fe-Mn-Al Ternary Equilibrium Diagram [2].

ANALYTICAL PROGRAMS
Two programs were settled, trough micro hardness and micrographic analysis, to determine the temperatures of stress relief, and of re-crystallization, as well as to evaluate the mechanical properties for technological applications.
1º) Program -Sequence of thermomechanical treatments; a)Cold rolling to 40% of reduction (0.2 cm thickness), b) Anneal at 1473K, 1 hour and quench to normal temperature, c) Anneal from 300K to 1200K, in successive steps of 100K for 1 hour to measure the respectives micro-hardness, figure 1, and metallographic structure observation, Table 2.

CONCLUSION Under the metallographic point of view
The component Si during the cooling speed of the molten state alloy produces a blend of γ phase and β precipitates [2].The calculated grain proportion of γ and β was γ ∕ β = 522.
By collaboration of the fringes the recrystallization of γ phase grains starts at temperature of 800K (1•) program) and at 1000K (2•) program) with dimensions less than 12 μm.
The β phase starts to be multi-fractured at 1000K and no collaborates on the hardness of the material.

Under the technological point of view
1•) Program -In the range of 600K-800K the heat treatment ensures to the alloy an excellent strength, hardness of 385Vickers, with a adequate grain sizes, and at 1180K, an hardness of 250 Vickers, a favourable mechanical conditions to produce by cold rolling process, plates blades, wires, nuts or bolts.2•) Program -Internal stress relieves is proven in the heat treatment (range 600K -700K), because the low hardness, 337Vickers.
For technical applications the higher hardness recommended is 422Vickers, despite of more energy consumption and less resistance to oxidation, figure 4 General hardness values, here marked, agree with the data contained in the works cited on the bibliography.

Table 3 -
Micrographic observation of annealed samples