Effect of temperature on the tensile properties and dislocation structures of FeAl alloys

Effect of temperature on the tensile properties and dislocation structures of FeAl alloys

Materials Science and Engineering A249 (1998) 206 – 216 Effect of temperature on the tensile properties and dislocation structures of FeAl alloys Din...

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Materials Science and Engineering A249 (1998) 206 – 216

Effect of temperature on the tensile properties and dislocation structures of FeAl alloys Dingqiang Li, Dongliang Lin (T.L. Lin) *, Yi Liu 1 School of Materials Science and Engineering, Open Laboratory of Education Ministry of China for High Temperature Materials and High Temperature Tests, Shanghai Jiao Tong Uni6ersity, 1954 Huashan Road, Shanghai 200030, P. R. China Received 14 January 1997; received in revised form 12 January 1998

Abstract Tensile deformation and fracture behavior of three B2 structural intermetallic FeAl alloys — one binary alloy (Fe – 36.5Al) and two ternary alloys (Fe–36.5Al–5Cr and Fe–36.5Al– 2Ti) — were investigated at different temperatures between room temperature and 1000°C. The specimens were prepared by hot-rolling of ingots and heat treatment. The elongation to failure of the alloys was found to increase when the testing temperature increased from room temperature to 300°C, decrease slightly from 300 to 500°C, and increase dramatically from 600 to 1000°C. On the other hand, the yield strength of the alloys decreased slowly until 500°C, then increased slightly, and decreased dramatically between 700 and 1000°C. The abnormal yield effect of these alloys occurred in the temperature range 500–700°C. The addition of chromium or titanium was found to improve the tensile properties of FeAl alloys, especially at elevated temperatures (800–1000°C). The ternary Fe – 36.5Al – 5Cr or Fe – 36.5Al – 2Ti alloy had a higher elongation and higher yield strength than the binary Fe – 36.5Al alloy. The fracture modes of these alloys when deformed at room temperature are a mixture of intergranular fracture and transgranular cleavage. With increase in temperature, the percentage of transgranular fracture generally increased, except for that observed around 500°C. Dislocations with a Ž111 Burgers vector are found in the FeAl alloy deformed at higher temperatures. It is suggested that the good ductility of the FeAl alloys deformed at higher temperatures may result from glide as well as climb of the dislocations with a Ž111 Burgers vector during deformation. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Dislocation structure; Fracture behavior; Temperature effects; Tensile deformation

1. Introduction Intermetallic FeAl is usually considered as a potential structural material for use at high temperatures. It has a B2 crystal structure which exists over a wide range of compositions, about 36 – 50at.%Al at room temperature, and is maintained to its melting point [1]. Together with their low cost, FeAl alloys possess a reasonably high specific modulus, high strength-toweight ratio [2], and excellent oxidation and corrosion resistance [3]. These characteristics make FeAl alloys attractive candidates for high temperature applications in harsh environments. * Corresponding author. Tel.: +86 21 62812544; fax: + 86 21 62820892. 1 Present address: State Key Laboratory for Fatigue and Fracture of Materials, Institute of Metal Research, Academia Sinica, Shengyang 110015, P.R. China.

0921-5093/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0921-5093(98)00507-3

However, polycrystalline FeAl alloys, as well as single crystals, exhibit brittleness at room temperature, which limits their application. Insufficient high temperature strength also limits the use of FeAl alloys at elevated temperatures. A number of investigations on the compressive properties of single and polycrystalline FeAl alloys have been carried out at various temperatures [4–8]. In addition, tensile properties have also been tested on iron-rich FeAl alloys of various grain sizes, produced by hot-working of cast alloys [9], extrusion of rapidly solidified [10,11] and gas-atomized powder [12,13], and extrusion of melt-spun ribbon [14]. The results have shown that the yield strength of an FeAl alloy is more or less independent of temperature, or decreases slowly as temperature increases from room temperature to a certain higher temperature, such as 700 [9], 600 [11] or

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500°C [15], above which the yield strength decreases rapidly. Sainfort et al. [9] reported that the elongation to failure increased monotonically as the temperature increased, from 8% at room temperature to more than 40% at 968°C, but Mendiratta et al. [11] and Gaydosh et al. [13] reported that elongation to failure increased monotonically only between room temperature and a transition temperature, above which elongation to failure decreased rapidly. Sainfort et al. [9] first reported that the fracture mode of an iron-rich FeAl alloy was mainly intergranular at room temperature but transgranular at higher temperatures. However, Mendiratta et al. [11] reported that the fracture mode of iron-rich FeAl alloys was transgranular from room temperature to 500°C but was intergranular when the ductility dropped at 600°C. Crimp et al. [17], Gaydosh et al. [13] and Baker et al. [16] confirmed the conclusion drawn by Mendiratta et al. [11]. In addition, other investigators [6,8,18,19] examined the effect of other factors, such as composition, grain size and strain rate, on the mechanical properties of FeAl alloys. The effect of temperature on tensile properties and fracture mode of FeAl alloys produced by hot-rolling of ingots has not been sufficiently addressed; only Sainfort et al. have examined tensile properties of FeAl alloy at and above the transition temperature. Some studies [9,11,15] have shown that the ductility of polycrystalline iron-rich FeAl alloys increases, or decreases slowly to 500–700°C, above which it decreases rapidly. This observation may be attributed to the primary role of cavitation in failure. As the alloys investigated in these studies were produced by hot extrusion of powder or solidified by melt spinning, there was more cavitation in these alloys [13]. Since these alloys were prepared by hot-rolling and heat treatment, the amount of cavitation is expected to be significantly reduced, and

Fig. 1. Graph of elongation to failure as a function of temperature for FeAl alloys.

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Fig. 2. Graph of yield stress as a function of temperature for FeAl alloys.

therefore the high temperature ductility of the FeAl alloy is expected to increase. Since the binary FeAl compound exists over a wide range of compositions and possessed considerable solubility for third element additions, it offers the potential of producing an acceptable material by alloying. It has been shown [20–22] that chromium addition produces a beneficial effect on the ductility of the DO3–Fe3Al alloy and that addition of titanium increases the ductility and strength of Fe3Al alloys at high temperature. There are few reports, however, regarding the effect of a third element such as chromium or titanium on the mechanical properties of FeAl alloys in the range from room temperature to 1000°C. Third element additions may improve room temperature ductility and/or increase elevated temperature strength, which is desirable for application of the FeAl alloy. Early studies showed that the B2 FeAl alloy undergoes a transition in slip vector, from Ž111 to Ž100, with increasing temperature [6,16,23,24]. The predominant slip vector in an FeAl compound is Ž111 at room temperature, where the slip system is sufficient for plastic flow in a polycrystal [25]. However, the predominant slip vector in an FeAl compound at high temperature is Ž100, and the slip system is not sufficiently independent for plastic flow in a polycrystal [26]. Baker et al. [16] found that in an Fe–36.9 at.% Al alloy the Ž111 slip vector was predominant up to 423°C but that only Ž100 type dislocations were present above 423°C. Therefore, the transition temperature of this FeAl alloy should be below 423°C. The temperature at which ductility dropped was above 623°C, which was 200°C higher than the temperature of the dislocation slip vector transition. In this paper, the effects of temperature on tensile deformation and fracture behavior are investigated. The specimens investigated in our tests were produced by hot-rolling of ingots and then heat treatment for

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recrystallization. Thus they are free from the cavitation found in the specimens of previous investigations [10–13]. Emphasis is placed on the effect of specimen preparation and alloy addition on high temperature ductility. Transmission electron microscopy (TEM) observations were made to examine the dislocation structure during deformation at intermediate and ele-

Fig. 3. Optical microstructures of FeAl alloys. (a) Fe–36.5Al, (b) Fe– 36.5Al – 5Cr, (c) Fe–36.5Al–2Ti.

vated temperatures and to clarify the deformation mechanisms.

2. Experimental procedure Alloys with compositions of Fe–36.5Al, Fe–36.5Al– 5Cr and Fe–36.5Al–2Ti (compositions given in atom percent throughout) were prepared by arc melting under argon using commercially pure iron (99%), aluminum (99.99%), chromium (99.8%) and titanium (99.8%). The alloy ingots were homogenized for 24 h at 1000°C, then hot-rolled at 1050–950°C clad in stainless steel sheets, with a total reduction of 50–60%. Tensile specimens with a gauge section of 12.0 × 3.2 ×1.0 mm were cut from the ingots by electro-discharge machining. All specimens were ground with grit emery papers from 100 grade to 600 grade to give a scratch-free surface, heated at 820°C for 1 h for recrystallization, and then held at 700°C for 2 h for ordering. All heat treatments were done in air. Tensile tests were performed on a Shimadzu AG-100kNA testing machine with a furnace. The specimens were strained in air to fracture under tension at a constant cross-head speed of 10 mm min − 1 (initial strain rate about 1.39 ×10 − 2 s − 1) in air at temperatures from room temperature to 1000°C. Testing temperatures were controlled using a thermocouple directly attached to the gauge section of a specimen. Stress and strain data were calculated from the load–time charts. Yield strength was determined using the 0.2% offset method when yielding was continuous. Elongation to failure calculated from the load– time chart agreed closely with the change in gauge length of the specimens. After tensile tests the specimens were etched by swabbing with the following mixture (in volume percent): 33HNO3 + 33HCl+33H2O+ 1HF. Grain size was measured using the line intercept method. Fracture surfaces were examined in an Hitachi S-520 scanning electron microscope (SEM) operated at 20 kV. Qualitative analyses of the composition of the second phase were determined by energy dispersive spectroscopy (EDS) built into the SEM. The amount of transgranular or intergranular fracture was determined using a systematic point count method. TEM foils were cut from the gauge section of a specimen of binary FeAl alloy which was additionally annealed at 1000°C for 24 h for full recrystallization, strained to 1% plastic deformation at 500 and 900°C, respectively, and then cooled quickly to maintain the dislocation configurations. The foils were ion-beam milled using a Gatan Model 600 with a gun voltage of 5 kV and a current of 1 mA. The dislocations were imaged using two-beam conditions in a JEM-200CX electron microscope operated at 200 kV. Dislocation analyses were performed using the g · b=0 invisibility criterion.

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Fig. 4. (1) Scanning electron micrograph of fracture surfaces of an Fe – 36.5Al alloy tensioned in air at different temperatures. (2) Scanning electron micrograph of fracture surfaces of an Fe–36.5Al–5Cr alloy tensioned in air at different temperatures. (3) Scanning electron micrograph of fracture surfaces of an Fe – 36.5Al–2Ti alloy tensioned in air at different temperatures.

3. Results

3.1. Tensile properties In Fig. 1 elongation to failure of the FeAl alloys is plotted against temperature. It shows that the elongation to failure of the three alloys increases slowly between room temperature to 300°C, then decreases

slowly between 300 and 500°C. Usually, elongation to failure increases monotonically and dramatically from temperatures of above 500°C for the FeAl–Cr and FeAl–Ti alloys and above 600°C for the binary FeAl alloy. Elongation to failure of the binary FeAl alloy is larger than that of the FeAl–Ti alloy but is smaller than that of the FeAl–Cr alloy at temperatures between room temperature and 500°C. Elongation to

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Fig. 4. (Continued)

failure of the FeAl – Cr alloy is the largest of these three FeAl alloys. Fig. 2 shows the plots of yield stress of the three alloys as a function of temperature. At temperatures between room temperature and 500°C, the yield stress of the three FeAl alloys tends to drop, but increases slightly between 500 and 700°C, and then decreases severely at 700°C. Fig. 2 also illustrates that the yield strength of both the FeAl – Cr and the FeAl – Ti alloys

is higher than that of the binary FeAl alloy, except in the case of the FeAl–Cr alloy deformed at room temperature.

3.2. Microstructure The optical micrographs of binary FeAl, ternary FeAl–Cr and FeAl–Ti alloys in Fig. 3 show that the average grain sizes of FeAl, FeAl–Cr and FeAl–Ti

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Fig. 4. (Continued)

alloys were 450, 390 and 350 mm, respectively. The black spots in the optical metallographs are pits etched by the acid mixture. EDS analysis confirmed that they were not second phases.

3.3. Fracture surface Fig. 4 presents the fractographs of the three alloys tested at different temperatures. All the fracture modes

of the alloys are a mixture of intergranular fracture and transgranular cleavage at room temperature. However, the percentage of transgranular cleavage or intergranular fracture surface varies with the third element additions. The percentage area of transgranular cleavage of the binary FeAl alloy is the highest, while the percentage area of intergranular fracture of the FeAl–Ti alloy is the highest. At 300°C, the fracture surfaces of all three FeAl alloys are predominantly transgranular. At

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Fig. 5. Transmission electron micrograph of an Fe – 36.5Al alloy deformed at 500°C.

500°C, however, the fracture mode of these alloys is again predominantly intergranular. At 700°C, all fractographs of these alloys show some quasi-planar cleavage characteristics, and the fracture mode appears to be transgranular fracture. At 900°C, fracture surfaces are mostly transgranular and show considerable plasticity.

3.4. Dislocation structure The dislocation structure of the binary Fe–36.5Al alloy deformed at temperatures of 500 and 900°C was examined by TEM. The micrographs in Fig. 5 show that curved and straight dislocation lines coexist in the FeAl alloy deformed at 500°C. The dislocations labeled ‘1’ and ‘4’ are in contrast for g =1( 10 (Fig. 5(a)), but are out of constrast for g =1( 10 (Fig. 5(b)) and g= 011 (Fig. 5(d)). According to the g · b = 0 invisibility criterion and the fact that the usual dislocations of Ž111 and/or Ž100 Burgers vectors exist in FeAl alloys, the Burgers vector of the dislocations labeled ‘1’ and ‘4’ is [111( ]. The dislocation labeled ‘2’ or ‘3’ is in contrast for g =11( 0 (Fig. 5(c)) and g =011 (Fig. 5(d)), but out of contrast for g=1( 1( 0 (Fig. 5(a)) and g = 1( 10 (Fig. 5(b);

thus the Burgers vector of the dislocation labeled ‘2’ or ‘3’ is [001]. Fig. 6 shows a weak-beam (1g/3g) image of dislocations with g= 011 and beam direction around [111] in an Fe–36.5Al alloy deformed at 900°C. Helical dislocation dipoles (areas labeled ‘A’) and square dislocation loops (area labeled ‘B’) have been found. The helical dislocation dipoles are coupled with an anti-phase boundary (arrowed). Fig. 7 shows the images of helical dislocation dipoles in different two-beam conditions. These helical dislocation dipoles are identified to have the Burgers vector [111] using the g · b = 0 invisibility criterion. Trace analysis shows that the axis of the helices is [111], indicating that the helices are due to the climb of [111] dislocations. Fig. 8 shows the images of the square dislocation loops at various two-beam diffraction conditions. Fig. 8(a) was taken with the beam direction near [111], while Fig. 8(b) was taken with the beam direction near [110], in which the loops are end-on. Trace analysis indicates that the loops lie on the (110) plane and the Burgers vector of the loops is [100].

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Fig. 6. The helical dislocation dipoles and square dislocation loops in an Fe – 36.5Al alloy deformed at 900°C.

Fig. 9 shows that there are a large number of glide dislocations in the binary FeAl alloy deformed at 900°C. The Burger vector of these dislocations is [111] according to the g · b = 0 invisibility criterion.

4. Discussion

4.1. Tensile deformation and fracture beha6ior In our examination, the ductility of these FeAl alloys increased dramatically when the temperature rose from 700 to 1000°C. This phenomenon is similar to that occurring in many common metals whose ductility increases with the temperature increase, but it is different from the results reported by Baker and Gaydosh [16], in which, because of the onset of grain boundary cavitation, the ductility decreased at elevated temperatures. The difference between the two phenomena could be due to the different methods of preparing the FeAl alloys. Since the alloys investigated in our study were prepared by hot-rolling, the amount of cavitation, which has an important effect on the deformation behavior of FeAl alloys, was much less than in the FeAl alloys prepared by extrusion of rapidly solidified, gas-atomized powder, or by extrusion of melt-spun ribbon, etc. [10–13]. Another important reason for the different deformation behaviors observed may be that a Ž111{110} slip system was found in the FeAl alloys deformed at high temperatures. It is known that a Ž100{110} slip system is insufficient for uniform plastic flow in a polycrystal FeAl alloy, but the larger elongation to failure in our investigation could not be explained only by

glide and climb of dislocation with the Ž100 Burger vector existing in the alloys. The discovery of dislocations with a Ž111 Burgers vector shows that glide of the dislocations with a Ž111 Burgers vector takes place during deformation of the FeAl alloy at elevated temperatures. The observed Ž111{110} slip system can provide five independent slip systems, which are sufficient for uniform plastic flow in a polycrystal alloy and bring a good ductility for the FeAl alloys deformed at the same temperatures. However, as helices are formed during the dislocation climb, a number of helices existing in the FeAl alloy shows that dislocation climb took an important role during the deformation. The dislocation climb process can decrease the tendency of transgranular fracture, and thus increase the ductility of the FeAl alloy. Thus the good ductility of FeAl alloys deformed at elevated temperatures could result from the glide of dislocations with a Ž111 Burgers vector on a {110} plane and the climb of dislocations with the same Burgers vector. Additionally, all alloys in our investigation have a considerably large grain size, about 400 mm, so the effect of grain boundary on the mechanical properties and fracture is expected to be limited. The general trend of ductility and yield strength change with increasing temperature was similar for the three FeAl alloys examined in this study. In the plots of elongation to failure as a function of temperature (Figs. 1 and 2), there is a ductility minimum at about 500°C similar to the phenomenon previously reported by other investigators [9,11,15], who found that there was a ductility drop at about 500–700°C in FeAl alloys. On comparing Figs. 1 and 2, an interesting phenomenon can be seen that both ductility and yield strength exhibit a minimum at : 500°C. Fractography analyses

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Fig. 7. The helical dislocation taken at different two-beam conditions in an Fe – 36.5Al alloy deformed at 900°C.

show that the percentage of intergranular fracture area in the fracture surface of the binary FeAl alloy at this temperature was much higher than that of the alloy tested at other temperatures. However, no impurity segregation in the grain boundaries of the FeAl alloys was observed. Thus, this phenomenon is different from the intermediate-temperature brittleness that takes place in some metals and alloys, which is caused by impurities segregating at grain boundaries. It is possible that both the ductility and yield minima may be connected to the decrease in grain boundary cohesive strength. Further investigation is also needed to clarify

the mechanism of the intermediate-temperature brittleness of the FeAl alloys.

4.2. Effect of Cr or Ti as alloying element Chromium is a well-known beneficial element for Fe3Al alloys and has a strong tendency to increase the room temperature ductility and decrease the room temperature yield strength of these alloys [20,21]. It manifests a similar effect on FeAl alloys. In our investigations, elemental chromium added to FeAl alloy increased both elevated temperature strength and duc-

Fig. 8. Square loops taken at different two-beam conditions showing that loops lie on (110) plane.

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Fig. 9. Transmission electron micrograph of an Fe – 36.5Al alloy deformed at 900°C.

tility of the binary FeAl alloy. Some investigators found that chromium produced solid solution strengthening at 600–700°C [27]. Since 5 at.% Cr is totally dissolved in the FeAl alloy, it is thought that chromium addition also produced solution strengthening at elevated temperatures. Additionally, the effect of chromium addition partly derives from the chromium atoms occupying the vacant sites and leads to a decrease in the number of point defects which play an important role in the formation of cavitation, which results in the increase of elevated temperature ductility of FeAl alloys. Titanium dissolved totally in the FeAl alloy. It can increase the high temperature ductility. Thus the higher strength of the FeAl alloy containing Ti can be considered as resulting from solid solution strengthening in the FeAl alloy. From the plots of elongation to failure and yield strength as a function of temperature for ternary FeAl– Cr, FeAl–Ti and binary FeAl alloys, it was found that no significant difference exists in the temperature dependence of tensile properties for the binary and the ternary FeAl alloys. This indicates that chromium or titanium additions do not change the mechanism of deformation behavior of FeAl alloys within the temperature range investigated.

5. Summary The addition of 5 at.% Cr or 2 at.% Ti produces a beneficial effect on tensile properties of Fe – 36.5Al alloy in the temperature range room temperature to 1000°C, especially in the higher temperature range 800 –1000°C. The elongation to failure and yield strength of the two

ternary FeAl alloys are higher than those of the binary Fe–36.5Al alloy when tested between room temperature and 1000°C. In general, for the three alloys tested, the elongation to failure increases while the yield strengths decrease, except at around 500°C, where both elongation to failure and yield stress show a minimum. The fracture mode of all of these alloys was a mixture of intergranular fracture and transgranular cleavage at room temperature. The percentage of transgranular fracture area increases with the temperature increase except at around 500°C. When the temperature increased to 900°C a ductile fracture appeared. The glide and climb of dislocations with a Ž111 Burgers vector found in the binary FeAl alloy deformed at elevated temperatures led to the high elongation to failure of the FeAl alloys at elevated temperatures.

Acknowledgements This work was supported by the National Natural Science Foundation of the People’s Republic of China.

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