THE EFFECT OF HEAT TREATMENT ON THE STRUCTURE OF NB AND CR DOPED FE 3. Martin ŠVEC, Věra VODIČKOVÁ, Pavel HANUS

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1 THE EFFECT OF HEAT TREATMENT ON THE STRUCTURE OF NB AND CR DOPED FE 3 AL ALLOY Martin ŠVEC, Věra VODIČKOVÁ, Pavel HANUS Department of Material Science, Technical University of Liberec, Czech Republic Abstrakt The alloyed iron aluminides with Fe 3 Al matrix are used as structural materials. Nb, Cr, Zr, Ti, Ta, Mo additives appear like promising for high-temperature applications [1]. The Nb addition leads to the formation of niobium carbides in the structure of the alloy [2]. Presence of this phase in an appropriate shape could enhance high-temperature mechanical properties of aluminides. The effect of the Nb addition on the phase composition of this alloy was studied [3,4]. The effect of the heat treatment on the structure was also observed. The alloy was annealed at 1150 C/2h and 1400 C/100h in the air for the confirmation of the phase stability. The phase composition was studied by light optical microscopy (LOM) and scanning electron microscopy (SEM) with energy dispersive analysis (EDX). Also Vickers hardness was measured for all states. High temperature mechanical properties are tested as R p0,2 in tension. Keywords Fe 3 Al type aluminides, Nb, Cr and C addition, phase structure, heat treatment, high-temperature mechanical properties 1. INTRODUCTION The iron aluminides are very promising materials for structural applications with many advantages, as f. e. excellent oxidation and sulfidation resistance, relatively high tensile strength at room temperature and lower density than stainless steels [1]. But some problematic properties (f. e. sharp drop in strength above 600 C or limited ductility at room temperature) prevent their expansion. These properties are tried to be eliminated by the alloying. Suitable additives for improving of high-temperature mechanical properties seem to be niobium or chromium [3,4]. 2. MATERIALS AND EXPERIMENTAL METHODS The alloy was prepared by vacuum induction melting and casting. Ingots (29x40x400) were annealed at 1200 C for 2 hours and then rolled at 1200 C from 29 mm thickness to 4 mm in several steps with 15 % thickness reduction in one pass. After each pass between the rolls, the material was reheated to 1200 C. The nominal chemical composition and type of heat treatment of the samples is given in Tab. 1. Table 1 The nominal chemical composition of the samples and the states Sample Nominal chemical composition (at. %) Fe Al Nb C Cr FA-NbCr AR 66,2 28,1 0,5 0,2 5 FA-NbCr HT1 FA-NbCr HT2 State as received; hot rolled to the thickness 4 mm at 1473 K annealed at 1150 C for 2 hours in air, cooled in oil annealed at 1400 C for 100 hours in air, cooled in oil

2 The microstructure of samples was investigated in a state after the oxide-polishing by suspension OP-S by the light optical microscope Nikon Epiphot 200 and by the scanning electron microscope Tescan Vega XMU equipped by Bruker detector. Also the energy dispersive X-ray analysis (EDX) were performed on the scanning electron microscope. The high-temperature tensile tests (between C) were made on the Department of Physics of Materials, Charles University in Prague. The hardness of samples was measured by the hardness tester Zwick 3212 by Vickers method with the 0,5 kg load (HV 0,5). 3. RESULTS AND DISCUSION 3.1 Characterization of the structure The alloy FA-NbCr AR Fig. 1 and 2 show the structure of alloy FA-NbCr AR The matrix of alloy is very coarse-grained and the precipitates are regularly distributed in it. The size of precipitates is about μm. The chemical composition of matrix and precipitates was verified by EDX. The results are shown in Fig. 3 and they are summarized in Tab. 2. The value of the carbon content is for reference only, because C as a light element is by EDX hardly detectable, so that the measured results are burdened to considerable error. The measured content of Fe, Al, and Cr is influenced by the surrounding matrix due to small size of precipitates. EDX analysis confirmed that these precipitates are NbC. Fig. 1 The structure of the alloy FA-NbCr AR with the precipitates of NbC Fig. 2 The detail of precipitates NbC in alloy FA-NbCr AR Fig. 3 Map of elements of precipitates in alloy FA-NbCr AR

3 Table 2 Chemical composition of matrix and precipitates of alloy FA-NbCr AR from EDX at. % C at. % Al at. % Fe at. % Nb at. % Cr Precipitate ~ 55,95 1,95 19,91 20,54 1,66 Matrix ~ 2,91 23,84 67,16 1,55 4,55 The alloy FA-NbCr HT1 The alloy FA-NbCr HT1 was annealed at 1150 C for 2 hours. The precipitates were significantly refined during this heat treatment. The shape of these precipitates was changed from irregularly rounded to acicular with dimensions approximately 10 x 2 μm. The precipitates were partially dissolved in the matrix. The structure of the alloy is shown in Fig. 4a and 4b. No significant differences in chemical composition of precipitates and matrix in compare with alloy FA-NbCr AR were found by EDX. Fig. 4a The structure of the alloy FA-NbCr HT1 with the finer precipitates of NbC Fig. 4b The detail of finer precipitates NbC in alloy FA-NbCr HT1 The alloy FA-NbCr HT2 The alloy FA-NbCr HT2 was annealed in air at 1400 C for 100 hours. The long-time annealing at the high temperatures showed significantly positive influence of chromium on the high-temperature stability of iron aluminides. The structure of alloy FA-NbC HT2 is shown in Fig. 5a and 5b. The material recrystallized during the long-time high temperature s annealing. The newly formed structure is very coarse-grained (the grain size is in the order of millimeters). The most of precipitates was dissolved in the matrix by the long-time annealing. The remaining precipitates like needle shape with dimensions 30 x 4 μm are distributed along grain boundaries.

4 Fig. 5a The structure of the alloy FA -NbCr HT2 with the needle precipitates of NbC Fig. 5b The detail of needle precipitates NbC in alloy FA-NbCr HT2 3.2 High-temperature mechanical properties The tensile curves of alloy FA-NbCr AR at different temperatures are shown In Fig. 6. Deformation rate was 10 4 s 1. The temperature dependence of yield stress is shown in Fig. 7., the values of mechanical properties obtained at high temperature tensile tests are summarized in Tab. 3. The alloy exhibits an anomalous increase in yield stress at 600 C approximately for both deformation rates. In comparison with alloy Fe-26Al-2Nb-1C in [5] (see Fig. 7) yield stress values at 650 and 800 C are obviously better. It could be due to matrix hardening by Cr addition. Fig. 6 The shape of tensile curves of alloy FA-NbCr AR at different temperatures and deformation rate 10 4 s 1

5 Fig. 7 The temperature dependence of yield stress FA-NbCr AR in comparison with [5] Table 3 Values of mechanical properties of alloy FA-NbCr AR obtained at high temperature tensile tests (deformation rate 10 4 s 1 ) temperature [ C] yield stress [MPa] ultimate strength [MPa] ductility [%] ,3 383,1 4, ,5 599,4 14, ,3 372,7 41, ,1 226,4 38, ,8 93,1 13,14 The hardness measurements of FA-NbCr HT1 show (see Table 4), that the hardness of matrix was increased after the heat treatment. This behavior can be explained by the fact, that the part of precipitates was dissolved in the matrix and solidified it by solid solution. The hardness measurement showed no significant increase of hardness in compare with short-annealed sample (see Table 4). So all the chromium was dissolved in the matrix during annealing 1150 C/2 hours and another hardening of alloy by solid solution was not possible at constant chromium content. The long time high temperature annealing influences only the homogeneity of structure. The alloy FA-NbCr HT2 was the most homogeneous of all alloys. Table 4 The results of hardness measurements of alloy FA-NbCr AR., FA-NbCr HT1 and FA-NbCr HT2 Material averange hardness Ф HV0.5 FA-NbCr AR 264 ± 3 FA-NbCr HT1 281 ± 5 FA-NbCr HT2 286 ± 1

6 4 CONCLUSIONS - The high temperature annealing (both HT1 and HT2) has significant effect on the structure of the alloy FA-NbCr in as received state. NbC precipitates are subsequently dissolved and remaining particles are refined. In case of the sufficiently fine dispersion of precipitates mechanical properties could be improved. - Cr addition affected mechanical properties of alloy FA-NbCr with the most probability due to strengthening of the matrix. Yield stress values at 650 and 800 C are obviously better than for alloy in [5]. ACKNOWLEDGMENT The research was supported by the SGS project Innovations in Material Engineering. REFERENCES [1] MC KAMEY, C. G. Iron Aluminides. In Physical Metalurgy and processing of Intermetallic Compounds, eds. STOLOFF N. S. SIKKA V. K., 1994, [2] SCHNEIDER, A. a kol. Constitution and microstructures of Fe Al M C (M = Ti, V, Nb, Ta) alloys with carbides and Laves phase. In Intermetallics 11 (2003), [3] MORRIS, D. G. Possibilities for high temperature strengthening in iron aluminides. In Intermetallics 6 (1998), [4] PALM, M. Concepts derived from phase diagram studies for the strengthening of Fe Al-based alloys. In Intermetallics 13 (2005), [5] FALAT, L., et. al. Mechanical properties of Fe Al M C (M = Ti, V, Nb, Ta) alloys with strengthening carbides and Laves phase. In Intermetallics 13 (2005),