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Improvement of mechanical properties of chromium–nickel sintered compacts by multiple rolling
January 2003
Materials Letters 57 (2003) 1142 – 1150 www.elsevier.com/locate/matlet
Improvement of mechanical properties of chromium–nickel sintered compacts by multiple rolling Y. Harada a,*, M. Ohmori b, F. Yoshida c, R. Nowak c,1 a b
Department of Production Systems Engineering, Toyohashi University of Technology, 1-1, Tempaku-cho, Toyohashi 441-8580, Japan Department of Mechanical Engineering, Hiroshima Kokusai-Gakuin University, 6-20-1, Nakano, Aki-ku, Hiroshima 739-8321, Japan c Faculty of Engineering, Hiroshima University, Kagamiyama, 1-4-1, Higashihiroshima 739-8527, Japan Received 27 December 2001; accepted 4 June 2002
Abstract The present paper provides details on the thermomechanical treatment of chromium – nickel alloys with high content of chromium, which leads to considerable improvement of their mechanical properties. The investigated alloys were prepared from Cr – Ni powder mixtures containing 50% and 80% of chromium in mass. They were formed by a particular sequence of cold and hot isostatic pressing. The as-sintered compacts were essentially brittle at room temperature, while they have exhibited considerable ductility after the special thermomechanical treatments. The treatment consisted of a sequence of repeated rolling and annealing operations, which is relatively simple and applicable in the industrial production. It is found that the treatments led to a more homogeneous, better alloyed structure and enhanced interdiffusion of the component metals. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Chromium – nickel alloys; Sintering; Mechanical properties; Ductility; Thermomechanical treatment; Interdiffusion
1. Introduction The advantages of application of chromium – nickel alloys as structural and device materials have been already recognized for decades (an early review given in Ref. [1]). Chromium alloyed with iron and cobalt is appreciated particularly due to their high oxidation, corrosion and heat resistance [2], as well as the excellent creep strength at high temperatures [3]. Moreover, their low density was considered to be an *
Corresponding author. Tel.: +81-532-44-6715; fax: +81-53244-6690. E-mail address: [email protected] (Y. Harada). 1 Currently at Department of Materials Science, Helsinki University of Technology, FIN-20215, Finland.
important factor in the production of machine parts and for aerospace applications, such as air-breathing gas turbines and airframes [1,4]. In spite of the above-mentioned attractive mechanical characteristics of chromium – nickel alloys, the poor ductility prevents their wider industrial application. Consequently, the study of high-temperature strength, ductility and workability of these alloys (including pure chromium) continues for decades (review given in Ref. [5]). In particular, the emphasis has been placed on chromium-rich alloys since early 1950s, when Wain et al. [6] found a 15% elongation in chromium. Bucklin and Grant [7] reported a possible forging of the vacuum-melted chromium –nickel alloys with 35 –70% chromium. Further, Parry et al. [8] improved
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 9 4 6 - 1
Y. Harada et al. / Materials Letters 57 (2003) 1142–1150
the workability of the alloys with the above-mentioned content by a special heat treatment. The ductile-to-brittle transition of chromium alloys was extensively investigated and the optimum conditions for extrusion, forging, rolling, swaging and tubing have been established [9,10]. Moreover, Ennis and Bridges [11] provided a thorough study of the effect of heat treatment and alloying additions on structure, ductility, creep strength and corrosion-resistance of the cast Ni –Cr alloy with 50% chromium in mass. In spite of the achieved progress, the limited ductility and the difficulties associated with production of the cast chromium – nickel alloys (due to unavoidable contamination by impurities from a crucible heated to very high temperatures) with more than 50% of chromium in mass remain unsolved [12,13]. Recently, the melting technology and the sintering methods have been remarkably developed. It is, therefore, possible to produce the sintered or cast chromium –nickel alloys with desirable composition [14]. This improvement triggered our research on the sintered chromium –nickel alloys with a high content of chromium. We also noticed a new insight into physical properties of chromium alloys reported recently by Matsumoto et al. [15,16]. They clarified alloying effects on electronic structure of chromium, the enhanced yield stress of the chromium-rich metals due to surface etching [17], the increase in chromium ductility caused by environmental and alloying phenomena [18 – 20].
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Based on our previous investigation of the tensile, rolling and extrusion deformation of the sintered chromium [21 – 25] and the strain-aging of cast and sintered chromium [26,27], the present study addresses the mechanical properties of the high-purity sintered chromium – nickel alloys containing more than 50% of chromium in mass. This work aims at improving the workability of the Cr – Ni sintered alloys by the particular repeated rolling combined with a proper sequence of thermal treatment. To the best of our knowledge, the Cr –Ni alloys with such a high content of chromium have never been investigated so far, since it was impossible to obtain the high chromium alloys by traditional methods without sintering technique.
2. Experimental procedure The nickel – chromium alloys were prepared using powders of chromium and nickel with the mass purity of 99.95% each. The powder mixtures with nominal content 50%Cr – 50%Ni and 80%Cr – 20%Ni were pressed using the CIP (cold isostatic press 490 MPa at 300 K) and subsequently sintered by HIP (hot isostatic press 176 MPa at 1473 K). The obtained 15-mm thick sintered compacts were deformed to 2-mm thick sheets by a repeated rolling (the details given in Table 1). The rolling was performed with a reduction of 15% in every pass
Table 1 The special thermomechanical treatments applied in the present study to improve mechanical properties of Cr – Ni alloys with high content of chromium Content of sinter compacts
Treatment symbol
Sequence of operationsa
Temperature of the final annealing (K)
50%Cr – 50%Ni 80%Cr – 20%Ni
X1 X2 X3 Y1 Y2 Y3 Y4 Y5 Z1 Z2 Z3
R ! R ! A1 ! R ! R ! A1 ! R ! R ! A1 ! R ! R R ! R ! A1 ! R ! R ! A1 ! R ! R ! A1 ! R ! R R ! R ! A1 ! R ! R ! A1 ! R ! R ! A1 ! R ! R R ! R ! A2 ! R ! R ! A2 ! R ! R ! A2 ! R ! R R ! R ! A2 ! R ! R ! A2 ! R ! R ! A2 ! R ! R R ! R ! A2 ! R ! R ! A2 ! R ! R ! A2 ! R ! R R ! R ! A2 ! R ! R ! A2 ! R ! R ! A2 ! R ! R R ! R ! A2 ! R ! R ! A2 ! R ! R ! A2 ! R ! R R ! A2 ! R ! A2 ! R ! A2 ! R ! A2 ! R ! A2 ! R R ! A2 ! R ! A2 ! R ! A2 ! R ! A2 ! R ! A2 ! R R ! A2 ! R ! A2 ! R ! A2 ! R ! A2 ! R ! A2 ! R
1173 1273 1373 1173 1273 1373 1473 1573 1173 1273 1373
50%Cr – 50%Ni 80%Cr – 20%Ni
80%Cr – 20%Ni
a The symbols R, A1 and A2 denote rolling pass with a reduction of 15%, annealing at 1173 K (annealing time = 3.6 103 s) and 1573 K (annealing time = 18 103 s), respectively. The final annealing time was kept constant as 3.6 103 s.
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and followed by annealing process applied in a certain stage of the deformation sequence. Several annealing temperatures were selected for each finally deformed metal sheet, which resulted in as many as 19 materials. These alloys differed in nickel content, the applied rolling sequence and the final annealing temperature. The considerable number of the applied ‘‘production conditions’’ allowed us to perform extensive investigations on the mechanical behavior of the various Ni – Cr alloys, and to select the optimum operation sequence which leads to the most desired properties. The specimens for tensile tests (14 mm long, 3 mm wide and 2 mm thick in the straight portion) were cut from both of the as-sintered and the repeatedly rolled materials (refer to Table 1). The tension test was performed in vacuum using a testing machine SHIMADZU AUTOGRAPH (DSS-5000) at a strain rate of 0.006 s 1. The test temperature ranges from 195 K, 273 K, room temperature (300 K) to high temperatures between 573 and 1373 K at an increment of 100 K. At least three specimens of the same material were examined for each test condition. The microstructures of both the sintered samples and the specimens modified by thermomechanical treatment were observed by means of the powerful optical microscope Nikon ASX-II with Nomarski phase contrast. The cross-sections parallel to the top surface of the rolled sheet were observed. The areas of particular interest were investigated using electron probe X-ray microanalysis (EPMA-X-ray microanalyzer: Hitachi X-650, electron probe microanalyzer: Horiba EMAX-2200) and scanning electron microscope (SEM-JEOL JSM-6300) equipped with the energy dispersion X-ray detector (EDX-PHILIPS DX-4). This inspection was performed to determine the local composition of the alloys.
microstructures for as-sintered 80%Cr – 20%Ni and 50%Cr – 50%Ni compacts contain bright and dark areas which correspond to chromium- and nickel-rich phases, respectively (see Fig. 1). The ‘‘bright grains’’ contain almost exclusively chromium, while the most frequently registered composition of the ‘‘dark area’’ (refer to Fig. 2) was close to 20% Cr – 80% Ni. The result suggests a quite intense diffusion of chromium into the nickel grains during HIP process, similarly to the effect reported by Maykuth and Gilbert [1], which in turn, agrees with the common expectation [28]. 3.2. Mechanical properties of sintered Cr – Ni compacts For mechanical behavior of the Cr –Ni sintered compacts, we concentrated on two mechanical char-
3. Properties of sintered Cr – Ni compacts 3.1. Microscopic observations of sintered Cr – Ni compacts The microscopic observation of the sintered materials revealed a structure far from homogeneous, an aggregate of at least two phases (Cr- and Ni-rich phases). EPMA and EDX analysis show that typical
Fig. 1. Typical optical micrographs of the structure of sintered compacts obtained from powder mixture containing 80%Cr – 20%Ni (a) and 50%Cr – 50%Ni (b). The bright areas are constituted of almost pure Cr, while dark regions are Ni-rich according to EPMA and EDX analyses.
Y. Harada et al. / Materials Letters 57 (2003) 1142–1150
acteristics, namely, tensile strength and total elongation up to fracture. The temperature dependencies of tensile strength and the elongation were examined at temperatures from 293 to 1373 K (see Fig. 2a and b) for the prepared compacts. It is worth noting that tensile strength of both samples starts to decrease at 750 K (Fig. 2a), where the total elongation achieves its maximum value (Fig. 2b). The sintered compacts are essentially brittle at room temperature (RT = 300 K), exhibiting no elongation prior to fracture. Their ductility is also very limited at temperatures exceeding 973 K (Fig. 2b). Keeping in mind the results of the microscopic observations, it is easy to conclude that the brittleness of the compacts observed at room temperature is due to the presence of the brittle chromium phase
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Fig. 3. SEM micrograph obtained for the sintered 50%Cr – 50%Ni compact after its annealing at the temperature as high as 1473 K, which indicates the intergranular separation due to the difference in the coefficients of thermal expansions for chromium and nickel-rich phase.
(refer to Fig. 1), of which the ductile – brittle transition temperature is higher than RT [6,29]. The reason for the limited ductility registered above 973 K is due to the presence of thermal stress-induced interfacial cracks (see Fig. 3 for 50%Cr – 50%Ni alloy annealed at 1473 K), which leads to intergranular fracture in uniaxial tension. There is a considerable difference in the thermal expansion coefficient between chromium (6.2 10 6 deg 1—pure Cr [30]) and nickel (17.6 10 6 deg 1—20%Cr – 80%Ni [31]) phases. Since in our compact the Crrich grains (‘‘bright phase’’ in Fig. 1a and b) are surrounded by the Ni-rich area (‘‘dark phase’’ in Fig. 1a and b) when being heated up, the tensile component of thermal stress arises and acts on the interfaces of the two phases, which induces the interfacial cracking. Indeed, such interfacial cracks were clearly observed in every compacts annealed at temperatures above 1273 K.
4. Cr –Ni alloys modified by thermomechanical treatment
Fig. 2. Variation of the tensile strength (a) and total elongation up to rupture (b) versus test temperature, registered for sintered Cr – Ni compacts with different content of chromium.
As mentioned above, the as-sintered compacts of Cr– Ni alloys are essentially brittle at high temperatures above 973 K. In order to overcome the difficulty in hot working due to their brittle nature, we
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performed a sequence of repeated rolling and annealing operations to promote the alloying and create fine structures. The following three types of treatment were applied:
Treatment X: annealing at 1173 K after every two rolling passes; Treatment Y: annealing at a higher temperature, 1573 K, after every two rolling passes; Treatment Z: annealing at 1573 K after every single rolling pass. It was found from the experiments that the mechanical properties of the alloys were significantly improved by treatment Y, and the results were almost same as those obtained from treatment Z. However, the attempt by treatment X was not so successful. Therefore, in this paper, the change of mechanical properties by treatment Y will be mainly discussed. 4.1. The properties of materials after treatments X and Y The tensile strength and elongation of the Cr –Ni alloys after treatments X and Y are summarized in Figs. 4 and 5, respectively. The modification of the sintered compacts with Y (see Table 1) led to considerable enhancement of high-temperature ductility in both samples of 50%Cr – 50%Ni and 80%Cr – 20%Ni. In contrast, the 50%Cr – 50%Ni sintered compact modified by X1 (hereafter sample X1) had slightly improved ductile properties (compare Figs. 4b and 2b). The materials treated with X2 and X3 (Table 1) behaved similarly to the unmodified metals. The minimum total elongation recorded around 1000 K for both samples X and Y (Figs. 4b and 5b) is attributed to the dynamic strain aging of the chromium phase [27]. This minimum is also possibly caused by the intermediate temperature embrittlement of the nickel phase [32]. It is worth noting that the similar temperature-dependence of ductility has been reported for the cast and rolled 50%Cr – 50%Ni alloys [33]. The EDX analysis indicated that the content of chromium in both Cr- and Ni-rich phases did not change significantly after treatment X. After treatment Y, on the contrary, considerable changes in the content were found. The microstructure in Fig. 6a and b
Fig. 4. Variation of the tensile strength (a) and total elongation up to rupture (b) versus test temperature, registered for the sintered compacts after its modification by multiple rolling and annealing according to sequence X (refer to Table 1).
indicate the existence of two phases which correspond to bright and dark areas. The EDX examination proved that the ‘‘bright phase’’ (matrix) in Fig. 6a contains 72%Cr and 28%Ni (the numbers refer to mass percent), while the ‘‘dark phase’’ has composition of 47%Cr – 53%Ni (particles). Similarly, the ‘‘bright phase’’ in Fig. 6b contains 47%Cr and 53%Ni and the ‘‘dark phase’’ 72%Cr – 28%Ni. These results clearly show that treatment Y promoted the alloying successfully. As mentioned in Section 3.2, the brittleness of the original compacts at room temperature is due to the presence of brittle Cr phase, and further, their low ductility at high temperature is caused by thermal stress induced cracks. After treatment Y, the brittle Cr phase no more exists, and the thermal expansion
Y. Harada et al. / Materials Letters 57 (2003) 1142–1150
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Fig. 5. Variation of the tensile strength (a) and total elongation up to rupture (b) versus test temperature, registered for the sintered compacts after its modification with the sequence Y (for the definition refer to Table 1).
mismatch between the new two phase, 72%Cr – 28%Ni and 47%Cr– 52%Ni, is much less than that between the original phases. Consequently, by treatment Y, considerable improvement of ductility is guaranteed. The tensile strength of the sintered 50%Cr – 50%Ni compact modified by Y decreased moderately from approximately 1000 to 800 MPa with increasing temperature from RT to 873 K, while the strength of the material modified with X (Fig. 4a) remained almost constant (700 – 800 MPa). Furthermore, rapid decrease to almost zero-strength was observed at temperatures ranging from 840 to 1400 K, independent of the final annealing temperature (Figs. 4a and 5a).
Fig. 6. Typical optical micrographs of the structure of sintered compacts obtained from powder mixture containing 80%Cr – 20%Ni (a) and 50%Cr – 50%Ni (b) after their modification by multiple rolling and annealing according to sequence Y (refer to Table 1), and containing 80%Cr – 20%Ni (c) after its modification by multiple rolling and annealing according to a sequence X1 (see Table 1). Note the elongation of Cr-rich phase parallel to the rolling direction.
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small—refer to Fig. 6a), which is essentially different from that of the sintered materials (Fig. 1) and of the alloys treated with sequence X. An example of microscopic changes by treatment X, for the 80%Cr – 20%Ni sintered compact modified with X1, is given in Fig. 6c. Although the grains of chromium-rich phase, which had been originally equiaxed (refer to Fig. 1a), were elongated parallel to the rolling direction, a fine microstructure was not obtained from treatment X. The selected 50%Cr – 50%Ni sintered compacts were treated by the modified sequence X. The original procedures X were changed in such a way that annealing at 1173 K prior to the final rolling (Table 1) was replaced by annealing at 1573 K for 18 103 s.
Fig. 7. Variation of the tensile strength (a) and total elongation up to rupture (b) versus test temperature, registered for the sintered 50%Cr – 50%Ni compact after its modification by multiple rolling and annealing according to sequence X, while the annealing at 1173 K prior to the final rolling (Table 1) was replaced by the annealing at 1573 K for 18 103 s. The data for the same material modified according to sequence Y are shown for comparison.
The mechanical behavior of the sintered 80%Cr – 20%Ni compact after treatment Y is similar to that exhibited by the 50%Cr – 50%Ni sample. Consequently, we emphasize that the modification with the sequence Y is effective in improving the properties of Cr-rich sintered compacts. The modification with the sequence Y led to high ductility at high temperature— the great advantage for metal forming processes, while high strength at room temperature is appreciated for most of structural materials. The sintered Cr – Ni compacts modified by treatment Y possess a fine grain microstructure (in particular, the grain size of the 80%Cr –20%Ni alloy is very
Fig. 8. Variation of the tensile strength (a) and total elongation up to rupture (b) versus test temperature, registered for the sintered 80%Cr – 20%Ni compact after its modification by multiple rolling and annealing according to sequence Z (refer to Table 1).
Y. Harada et al. / Materials Letters 57 (2003) 1142–1150
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The tensile properties of the sintered compact prepared by modified sequence X match those of 50%Cr – 50%Ni alloy treated with the sequence Y (see Fig. 7). It should be emphasized that using the modified sequence X, we obtained a more ductile alloy at a temperature as low as 195 K.
treatments consisting of multiple rolling and annealing operations are relatively simple and can be easily applied to the industrial production.
4.2. The properties of materials after treatment Z
[1] D.J. Maykuth, A. Gilbert, Chromium and chromium alloys, Defense Metals Information Center, Report 234, Columbus, OH, 1966. [2] F.R. Morall, J. Met. 18 (1963) 1115. [3] C.T. Sims, J. Met. 15 (1965) 127. [4] R.F. Decker, J. Met. 17 (1965) 139. [5] B.C. Allen, D.J. Maykuth, R.I. Jaffee, Influence of impurity element, structure and prestrain on tensile transition temperature of chromium, NASA TN D-837, 1961. [6] H. Wain, F. Henderson, S. Johonstone, J. Inst. Met. 83 (1954) 133. [7] A.J. Bucklin, N.J. Grant, ASTM Spec. Tech. Publ. 174 (1955) 47. [8] P.J. Parry, P.J. Bridges, B. Taylor, J. Inst. Met. 97 (1969) 373. [9] S.A. Spachner, W. Rostoker, Trans. ASM 50 (1958) 838. [10] W.D. Klopp, NASA TM X-1867, 1969. [11] P.J. Ennis, P.J. Bridges, J. Inst. Met. 100 (1972) 346. [12] S. Yosida, Y. Ohba, J. Inst. Jpn. 22 (1958) 443. [13] M. Isshiki, J. Less-Common Met. 36 (1984) 157. [14] K. Simotori, Thesis, Tokyo University of Technology, 1972. [15] Y. Matsumoto, M. Morinaga, T. Nambu, T. Sakaki, J. Phys., Condens. Matter 8 (1996) 3619. [16] Y. Matsumoto, J. Fukumori, M. Morinaga, M. Furui, T. Nambu, T. Sakaki, Scr. Mater. 34 (1996) 1685. [17] M. Morinaga, Y. Murata, M. Furui, T. Wada, Scr. Mater. 37 (1997) 699. [18] M. Morinaga, T. Nambu, T. Sakai, J. Mater. Sci. 30 (1995) 1105. [19] T. Nambu, J. Fukumori, M. Morinaga, Y. Matsumoto, T. Sakaki, Scr. Mater. 32 (1995) 407. [20] Y. Matsumoto, M. Morinaga, M. Furui, Scr. Mater. 38 (1998) 321. [21] M. Ohmori, A. Kaya, Y. Harada, F. Yoshida, M. Itoh, J. Jpn. Inst. Met. 52 (1988) 223. [22] Y. Harada, M. Ohmori, F. Yoshida, M. Itoh, J. Jpn. Inst. Met. 53 (1989) 201; Y. Harada, M. Ohmori, F. Yoshida, M. Itoh, J. Jpn. Inst. Met. 53 (1989) 921. [23] Y. Harada, M. Ohmori, S. Ohnishi, J. Jpn. Inst. Met. 54 (1990) 473. [24] Y. Harada, M. Ohmori, F. Yoshida, M. Itoh, Proc. 3rd Int. Conf. Technology of Plasticity ICTP, 1990, p. 719. [25] M. Ohmori, Y. Harada, M. Yoshida, Proc. 5th Int. Conf. Technology of Plasticity ICTP, 1996, p. 251. [26] M. Ohmori, Y. Harada, F. Yoshida, M. Itoh, J. Jpn. Inst. Met. 54 (1990) 262. [27] Y. Harada, M. Ohmori, Proc. 1st Int. Conf. Ultra High Purity Base Metals SUHP94, 1994, p. 482.
Although the sequence of multiple rolling and annealing is effective in obtaining sintered Cr – Ni compacts with improved ductility, an additional operational scheme (Z) was also studied exclusively for the 80%Cr– 20%Ni compact (see Table 1). The material treated in such a way appeared to behave similarly to that modified by sequence Y (see Figs. 5 and 8). Thus, the modification of the Cr– Ni sintered compacts by sequence Z provides an alternative example of successful treatment which leads to the increase of ductility.
5. Summarizing remarks The present work provides details of treatments which allowed us to obtain sintered Cr – Ni compacts containing large amount of chromium with significantly improved ductility. The homogeneity of structure was improved by the applied multiple rolling and annealing. The as-sintered metals possessed high ductility at temperature close to 700 K, while they were entirely brittle at RT and above 1000 K, which prompted our research for the method for improving their properties. The mechanical properties of the sintered compacts have been slightly improved after multiple rolling and annealing with the sequence X. The lack of marked changes in tensile strength and elongation after treatment X was attributed to the limited changes in the structure (merely the grain elongation). In contrast, treatments Y and Z resulted from the formation of fine grain, well alloyed, homogeneous structure, leading to a significant improvement of ductility of Cr – Ni compacts. The better alloying, and consequently, the improvement of mechanical properties, has been also achieved for modified sequence X, when the thermal treatment before final rolling was performed at 1573 K. The proposed
References
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[28] M. Hansen, Constitution of Binary Alloys, McGraw-Hill, New York, 1958. [29] A. Sully, E. Brandes, K. Michell, J. Inst. Met. 81 (1953) 585. [30] Data Book, vol. 2, Japan Institute of Metals, 1984, p. 12.
[31] S.L. Hoyt, Metals properties, ASME Handbook, McGrawHill, 1954, p. 381. [32] M. Ohmori, M. Itoh, F. Yoshida, J. Jpn. Inst. Met. 47 – 49 (1983) 775. [33] Y. Harada, M. Ohmori, unpublished work.
Materials Letters 57 (2003) 1142 – 1150 www.elsevier.com/locate/matlet
Improvement of mechanical properties of chromium–nickel sintered compacts by multiple rolling Y. Harada a,*, M. Ohmori b, F. Yoshida c, R. Nowak c,1 a b
Department of Production Systems Engineering, Toyohashi University of Technology, 1-1, Tempaku-cho, Toyohashi 441-8580, Japan Department of Mechanical Engineering, Hiroshima Kokusai-Gakuin University, 6-20-1, Nakano, Aki-ku, Hiroshima 739-8321, Japan c Faculty of Engineering, Hiroshima University, Kagamiyama, 1-4-1, Higashihiroshima 739-8527, Japan Received 27 December 2001; accepted 4 June 2002
Abstract The present paper provides details on the thermomechanical treatment of chromium – nickel alloys with high content of chromium, which leads to considerable improvement of their mechanical properties. The investigated alloys were prepared from Cr – Ni powder mixtures containing 50% and 80% of chromium in mass. They were formed by a particular sequence of cold and hot isostatic pressing. The as-sintered compacts were essentially brittle at room temperature, while they have exhibited considerable ductility after the special thermomechanical treatments. The treatment consisted of a sequence of repeated rolling and annealing operations, which is relatively simple and applicable in the industrial production. It is found that the treatments led to a more homogeneous, better alloyed structure and enhanced interdiffusion of the component metals. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Chromium – nickel alloys; Sintering; Mechanical properties; Ductility; Thermomechanical treatment; Interdiffusion
1. Introduction The advantages of application of chromium – nickel alloys as structural and device materials have been already recognized for decades (an early review given in Ref. [1]). Chromium alloyed with iron and cobalt is appreciated particularly due to their high oxidation, corrosion and heat resistance [2], as well as the excellent creep strength at high temperatures [3]. Moreover, their low density was considered to be an *
Corresponding author. Tel.: +81-532-44-6715; fax: +81-53244-6690. E-mail address: [email protected] (Y. Harada). 1 Currently at Department of Materials Science, Helsinki University of Technology, FIN-20215, Finland.
important factor in the production of machine parts and for aerospace applications, such as air-breathing gas turbines and airframes [1,4]. In spite of the above-mentioned attractive mechanical characteristics of chromium – nickel alloys, the poor ductility prevents their wider industrial application. Consequently, the study of high-temperature strength, ductility and workability of these alloys (including pure chromium) continues for decades (review given in Ref. [5]). In particular, the emphasis has been placed on chromium-rich alloys since early 1950s, when Wain et al. [6] found a 15% elongation in chromium. Bucklin and Grant [7] reported a possible forging of the vacuum-melted chromium –nickel alloys with 35 –70% chromium. Further, Parry et al. [8] improved
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 9 4 6 - 1
Y. Harada et al. / Materials Letters 57 (2003) 1142–1150
the workability of the alloys with the above-mentioned content by a special heat treatment. The ductile-to-brittle transition of chromium alloys was extensively investigated and the optimum conditions for extrusion, forging, rolling, swaging and tubing have been established [9,10]. Moreover, Ennis and Bridges [11] provided a thorough study of the effect of heat treatment and alloying additions on structure, ductility, creep strength and corrosion-resistance of the cast Ni –Cr alloy with 50% chromium in mass. In spite of the achieved progress, the limited ductility and the difficulties associated with production of the cast chromium – nickel alloys (due to unavoidable contamination by impurities from a crucible heated to very high temperatures) with more than 50% of chromium in mass remain unsolved [12,13]. Recently, the melting technology and the sintering methods have been remarkably developed. It is, therefore, possible to produce the sintered or cast chromium –nickel alloys with desirable composition [14]. This improvement triggered our research on the sintered chromium –nickel alloys with a high content of chromium. We also noticed a new insight into physical properties of chromium alloys reported recently by Matsumoto et al. [15,16]. They clarified alloying effects on electronic structure of chromium, the enhanced yield stress of the chromium-rich metals due to surface etching [17], the increase in chromium ductility caused by environmental and alloying phenomena [18 – 20].
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Based on our previous investigation of the tensile, rolling and extrusion deformation of the sintered chromium [21 – 25] and the strain-aging of cast and sintered chromium [26,27], the present study addresses the mechanical properties of the high-purity sintered chromium – nickel alloys containing more than 50% of chromium in mass. This work aims at improving the workability of the Cr – Ni sintered alloys by the particular repeated rolling combined with a proper sequence of thermal treatment. To the best of our knowledge, the Cr –Ni alloys with such a high content of chromium have never been investigated so far, since it was impossible to obtain the high chromium alloys by traditional methods without sintering technique.
2. Experimental procedure The nickel – chromium alloys were prepared using powders of chromium and nickel with the mass purity of 99.95% each. The powder mixtures with nominal content 50%Cr – 50%Ni and 80%Cr – 20%Ni were pressed using the CIP (cold isostatic press 490 MPa at 300 K) and subsequently sintered by HIP (hot isostatic press 176 MPa at 1473 K). The obtained 15-mm thick sintered compacts were deformed to 2-mm thick sheets by a repeated rolling (the details given in Table 1). The rolling was performed with a reduction of 15% in every pass
Table 1 The special thermomechanical treatments applied in the present study to improve mechanical properties of Cr – Ni alloys with high content of chromium Content of sinter compacts
Treatment symbol
Sequence of operationsa
Temperature of the final annealing (K)
50%Cr – 50%Ni 80%Cr – 20%Ni
X1 X2 X3 Y1 Y2 Y3 Y4 Y5 Z1 Z2 Z3
R ! R ! A1 ! R ! R ! A1 ! R ! R ! A1 ! R ! R R ! R ! A1 ! R ! R ! A1 ! R ! R ! A1 ! R ! R R ! R ! A1 ! R ! R ! A1 ! R ! R ! A1 ! R ! R R ! R ! A2 ! R ! R ! A2 ! R ! R ! A2 ! R ! R R ! R ! A2 ! R ! R ! A2 ! R ! R ! A2 ! R ! R R ! R ! A2 ! R ! R ! A2 ! R ! R ! A2 ! R ! R R ! R ! A2 ! R ! R ! A2 ! R ! R ! A2 ! R ! R R ! R ! A2 ! R ! R ! A2 ! R ! R ! A2 ! R ! R R ! A2 ! R ! A2 ! R ! A2 ! R ! A2 ! R ! A2 ! R R ! A2 ! R ! A2 ! R ! A2 ! R ! A2 ! R ! A2 ! R R ! A2 ! R ! A2 ! R ! A2 ! R ! A2 ! R ! A2 ! R
1173 1273 1373 1173 1273 1373 1473 1573 1173 1273 1373
50%Cr – 50%Ni 80%Cr – 20%Ni
80%Cr – 20%Ni
a The symbols R, A1 and A2 denote rolling pass with a reduction of 15%, annealing at 1173 K (annealing time = 3.6 103 s) and 1573 K (annealing time = 18 103 s), respectively. The final annealing time was kept constant as 3.6 103 s.
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and followed by annealing process applied in a certain stage of the deformation sequence. Several annealing temperatures were selected for each finally deformed metal sheet, which resulted in as many as 19 materials. These alloys differed in nickel content, the applied rolling sequence and the final annealing temperature. The considerable number of the applied ‘‘production conditions’’ allowed us to perform extensive investigations on the mechanical behavior of the various Ni – Cr alloys, and to select the optimum operation sequence which leads to the most desired properties. The specimens for tensile tests (14 mm long, 3 mm wide and 2 mm thick in the straight portion) were cut from both of the as-sintered and the repeatedly rolled materials (refer to Table 1). The tension test was performed in vacuum using a testing machine SHIMADZU AUTOGRAPH (DSS-5000) at a strain rate of 0.006 s 1. The test temperature ranges from 195 K, 273 K, room temperature (300 K) to high temperatures between 573 and 1373 K at an increment of 100 K. At least three specimens of the same material were examined for each test condition. The microstructures of both the sintered samples and the specimens modified by thermomechanical treatment were observed by means of the powerful optical microscope Nikon ASX-II with Nomarski phase contrast. The cross-sections parallel to the top surface of the rolled sheet were observed. The areas of particular interest were investigated using electron probe X-ray microanalysis (EPMA-X-ray microanalyzer: Hitachi X-650, electron probe microanalyzer: Horiba EMAX-2200) and scanning electron microscope (SEM-JEOL JSM-6300) equipped with the energy dispersion X-ray detector (EDX-PHILIPS DX-4). This inspection was performed to determine the local composition of the alloys.
microstructures for as-sintered 80%Cr – 20%Ni and 50%Cr – 50%Ni compacts contain bright and dark areas which correspond to chromium- and nickel-rich phases, respectively (see Fig. 1). The ‘‘bright grains’’ contain almost exclusively chromium, while the most frequently registered composition of the ‘‘dark area’’ (refer to Fig. 2) was close to 20% Cr – 80% Ni. The result suggests a quite intense diffusion of chromium into the nickel grains during HIP process, similarly to the effect reported by Maykuth and Gilbert [1], which in turn, agrees with the common expectation [28]. 3.2. Mechanical properties of sintered Cr – Ni compacts For mechanical behavior of the Cr –Ni sintered compacts, we concentrated on two mechanical char-
3. Properties of sintered Cr – Ni compacts 3.1. Microscopic observations of sintered Cr – Ni compacts The microscopic observation of the sintered materials revealed a structure far from homogeneous, an aggregate of at least two phases (Cr- and Ni-rich phases). EPMA and EDX analysis show that typical
Fig. 1. Typical optical micrographs of the structure of sintered compacts obtained from powder mixture containing 80%Cr – 20%Ni (a) and 50%Cr – 50%Ni (b). The bright areas are constituted of almost pure Cr, while dark regions are Ni-rich according to EPMA and EDX analyses.
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acteristics, namely, tensile strength and total elongation up to fracture. The temperature dependencies of tensile strength and the elongation were examined at temperatures from 293 to 1373 K (see Fig. 2a and b) for the prepared compacts. It is worth noting that tensile strength of both samples starts to decrease at 750 K (Fig. 2a), where the total elongation achieves its maximum value (Fig. 2b). The sintered compacts are essentially brittle at room temperature (RT = 300 K), exhibiting no elongation prior to fracture. Their ductility is also very limited at temperatures exceeding 973 K (Fig. 2b). Keeping in mind the results of the microscopic observations, it is easy to conclude that the brittleness of the compacts observed at room temperature is due to the presence of the brittle chromium phase
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Fig. 3. SEM micrograph obtained for the sintered 50%Cr – 50%Ni compact after its annealing at the temperature as high as 1473 K, which indicates the intergranular separation due to the difference in the coefficients of thermal expansions for chromium and nickel-rich phase.
(refer to Fig. 1), of which the ductile – brittle transition temperature is higher than RT [6,29]. The reason for the limited ductility registered above 973 K is due to the presence of thermal stress-induced interfacial cracks (see Fig. 3 for 50%Cr – 50%Ni alloy annealed at 1473 K), which leads to intergranular fracture in uniaxial tension. There is a considerable difference in the thermal expansion coefficient between chromium (6.2 10 6 deg 1—pure Cr [30]) and nickel (17.6 10 6 deg 1—20%Cr – 80%Ni [31]) phases. Since in our compact the Crrich grains (‘‘bright phase’’ in Fig. 1a and b) are surrounded by the Ni-rich area (‘‘dark phase’’ in Fig. 1a and b) when being heated up, the tensile component of thermal stress arises and acts on the interfaces of the two phases, which induces the interfacial cracking. Indeed, such interfacial cracks were clearly observed in every compacts annealed at temperatures above 1273 K.
4. Cr –Ni alloys modified by thermomechanical treatment
Fig. 2. Variation of the tensile strength (a) and total elongation up to rupture (b) versus test temperature, registered for sintered Cr – Ni compacts with different content of chromium.
As mentioned above, the as-sintered compacts of Cr– Ni alloys are essentially brittle at high temperatures above 973 K. In order to overcome the difficulty in hot working due to their brittle nature, we
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performed a sequence of repeated rolling and annealing operations to promote the alloying and create fine structures. The following three types of treatment were applied:
Treatment X: annealing at 1173 K after every two rolling passes; Treatment Y: annealing at a higher temperature, 1573 K, after every two rolling passes; Treatment Z: annealing at 1573 K after every single rolling pass. It was found from the experiments that the mechanical properties of the alloys were significantly improved by treatment Y, and the results were almost same as those obtained from treatment Z. However, the attempt by treatment X was not so successful. Therefore, in this paper, the change of mechanical properties by treatment Y will be mainly discussed. 4.1. The properties of materials after treatments X and Y The tensile strength and elongation of the Cr –Ni alloys after treatments X and Y are summarized in Figs. 4 and 5, respectively. The modification of the sintered compacts with Y (see Table 1) led to considerable enhancement of high-temperature ductility in both samples of 50%Cr – 50%Ni and 80%Cr – 20%Ni. In contrast, the 50%Cr – 50%Ni sintered compact modified by X1 (hereafter sample X1) had slightly improved ductile properties (compare Figs. 4b and 2b). The materials treated with X2 and X3 (Table 1) behaved similarly to the unmodified metals. The minimum total elongation recorded around 1000 K for both samples X and Y (Figs. 4b and 5b) is attributed to the dynamic strain aging of the chromium phase [27]. This minimum is also possibly caused by the intermediate temperature embrittlement of the nickel phase [32]. It is worth noting that the similar temperature-dependence of ductility has been reported for the cast and rolled 50%Cr – 50%Ni alloys [33]. The EDX analysis indicated that the content of chromium in both Cr- and Ni-rich phases did not change significantly after treatment X. After treatment Y, on the contrary, considerable changes in the content were found. The microstructure in Fig. 6a and b
Fig. 4. Variation of the tensile strength (a) and total elongation up to rupture (b) versus test temperature, registered for the sintered compacts after its modification by multiple rolling and annealing according to sequence X (refer to Table 1).
indicate the existence of two phases which correspond to bright and dark areas. The EDX examination proved that the ‘‘bright phase’’ (matrix) in Fig. 6a contains 72%Cr and 28%Ni (the numbers refer to mass percent), while the ‘‘dark phase’’ has composition of 47%Cr – 53%Ni (particles). Similarly, the ‘‘bright phase’’ in Fig. 6b contains 47%Cr and 53%Ni and the ‘‘dark phase’’ 72%Cr – 28%Ni. These results clearly show that treatment Y promoted the alloying successfully. As mentioned in Section 3.2, the brittleness of the original compacts at room temperature is due to the presence of brittle Cr phase, and further, their low ductility at high temperature is caused by thermal stress induced cracks. After treatment Y, the brittle Cr phase no more exists, and the thermal expansion
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Fig. 5. Variation of the tensile strength (a) and total elongation up to rupture (b) versus test temperature, registered for the sintered compacts after its modification with the sequence Y (for the definition refer to Table 1).
mismatch between the new two phase, 72%Cr – 28%Ni and 47%Cr– 52%Ni, is much less than that between the original phases. Consequently, by treatment Y, considerable improvement of ductility is guaranteed. The tensile strength of the sintered 50%Cr – 50%Ni compact modified by Y decreased moderately from approximately 1000 to 800 MPa with increasing temperature from RT to 873 K, while the strength of the material modified with X (Fig. 4a) remained almost constant (700 – 800 MPa). Furthermore, rapid decrease to almost zero-strength was observed at temperatures ranging from 840 to 1400 K, independent of the final annealing temperature (Figs. 4a and 5a).
Fig. 6. Typical optical micrographs of the structure of sintered compacts obtained from powder mixture containing 80%Cr – 20%Ni (a) and 50%Cr – 50%Ni (b) after their modification by multiple rolling and annealing according to sequence Y (refer to Table 1), and containing 80%Cr – 20%Ni (c) after its modification by multiple rolling and annealing according to a sequence X1 (see Table 1). Note the elongation of Cr-rich phase parallel to the rolling direction.
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small—refer to Fig. 6a), which is essentially different from that of the sintered materials (Fig. 1) and of the alloys treated with sequence X. An example of microscopic changes by treatment X, for the 80%Cr – 20%Ni sintered compact modified with X1, is given in Fig. 6c. Although the grains of chromium-rich phase, which had been originally equiaxed (refer to Fig. 1a), were elongated parallel to the rolling direction, a fine microstructure was not obtained from treatment X. The selected 50%Cr – 50%Ni sintered compacts were treated by the modified sequence X. The original procedures X were changed in such a way that annealing at 1173 K prior to the final rolling (Table 1) was replaced by annealing at 1573 K for 18 103 s.
Fig. 7. Variation of the tensile strength (a) and total elongation up to rupture (b) versus test temperature, registered for the sintered 50%Cr – 50%Ni compact after its modification by multiple rolling and annealing according to sequence X, while the annealing at 1173 K prior to the final rolling (Table 1) was replaced by the annealing at 1573 K for 18 103 s. The data for the same material modified according to sequence Y are shown for comparison.
The mechanical behavior of the sintered 80%Cr – 20%Ni compact after treatment Y is similar to that exhibited by the 50%Cr – 50%Ni sample. Consequently, we emphasize that the modification with the sequence Y is effective in improving the properties of Cr-rich sintered compacts. The modification with the sequence Y led to high ductility at high temperature— the great advantage for metal forming processes, while high strength at room temperature is appreciated for most of structural materials. The sintered Cr – Ni compacts modified by treatment Y possess a fine grain microstructure (in particular, the grain size of the 80%Cr –20%Ni alloy is very
Fig. 8. Variation of the tensile strength (a) and total elongation up to rupture (b) versus test temperature, registered for the sintered 80%Cr – 20%Ni compact after its modification by multiple rolling and annealing according to sequence Z (refer to Table 1).
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The tensile properties of the sintered compact prepared by modified sequence X match those of 50%Cr – 50%Ni alloy treated with the sequence Y (see Fig. 7). It should be emphasized that using the modified sequence X, we obtained a more ductile alloy at a temperature as low as 195 K.
treatments consisting of multiple rolling and annealing operations are relatively simple and can be easily applied to the industrial production.
4.2. The properties of materials after treatment Z
[1] D.J. Maykuth, A. Gilbert, Chromium and chromium alloys, Defense Metals Information Center, Report 234, Columbus, OH, 1966. [2] F.R. Morall, J. Met. 18 (1963) 1115. [3] C.T. Sims, J. Met. 15 (1965) 127. [4] R.F. Decker, J. Met. 17 (1965) 139. [5] B.C. Allen, D.J. Maykuth, R.I. Jaffee, Influence of impurity element, structure and prestrain on tensile transition temperature of chromium, NASA TN D-837, 1961. [6] H. Wain, F. Henderson, S. Johonstone, J. Inst. Met. 83 (1954) 133. [7] A.J. Bucklin, N.J. Grant, ASTM Spec. Tech. Publ. 174 (1955) 47. [8] P.J. Parry, P.J. Bridges, B. Taylor, J. Inst. Met. 97 (1969) 373. [9] S.A. Spachner, W. Rostoker, Trans. ASM 50 (1958) 838. [10] W.D. Klopp, NASA TM X-1867, 1969. [11] P.J. Ennis, P.J. Bridges, J. Inst. Met. 100 (1972) 346. [12] S. Yosida, Y. Ohba, J. Inst. Jpn. 22 (1958) 443. [13] M. Isshiki, J. Less-Common Met. 36 (1984) 157. [14] K. Simotori, Thesis, Tokyo University of Technology, 1972. [15] Y. Matsumoto, M. Morinaga, T. Nambu, T. Sakaki, J. Phys., Condens. Matter 8 (1996) 3619. [16] Y. Matsumoto, J. Fukumori, M. Morinaga, M. Furui, T. Nambu, T. Sakaki, Scr. Mater. 34 (1996) 1685. [17] M. Morinaga, Y. Murata, M. Furui, T. Wada, Scr. Mater. 37 (1997) 699. [18] M. Morinaga, T. Nambu, T. Sakai, J. Mater. Sci. 30 (1995) 1105. [19] T. Nambu, J. Fukumori, M. Morinaga, Y. Matsumoto, T. Sakaki, Scr. Mater. 32 (1995) 407. [20] Y. Matsumoto, M. Morinaga, M. Furui, Scr. Mater. 38 (1998) 321. [21] M. Ohmori, A. Kaya, Y. Harada, F. Yoshida, M. Itoh, J. Jpn. Inst. Met. 52 (1988) 223. [22] Y. Harada, M. Ohmori, F. Yoshida, M. Itoh, J. Jpn. Inst. Met. 53 (1989) 201; Y. Harada, M. Ohmori, F. Yoshida, M. Itoh, J. Jpn. Inst. Met. 53 (1989) 921. [23] Y. Harada, M. Ohmori, S. Ohnishi, J. Jpn. Inst. Met. 54 (1990) 473. [24] Y. Harada, M. Ohmori, F. Yoshida, M. Itoh, Proc. 3rd Int. Conf. Technology of Plasticity ICTP, 1990, p. 719. [25] M. Ohmori, Y. Harada, M. Yoshida, Proc. 5th Int. Conf. Technology of Plasticity ICTP, 1996, p. 251. [26] M. Ohmori, Y. Harada, F. Yoshida, M. Itoh, J. Jpn. Inst. Met. 54 (1990) 262. [27] Y. Harada, M. Ohmori, Proc. 1st Int. Conf. Ultra High Purity Base Metals SUHP94, 1994, p. 482.
Although the sequence of multiple rolling and annealing is effective in obtaining sintered Cr – Ni compacts with improved ductility, an additional operational scheme (Z) was also studied exclusively for the 80%Cr– 20%Ni compact (see Table 1). The material treated in such a way appeared to behave similarly to that modified by sequence Y (see Figs. 5 and 8). Thus, the modification of the Cr– Ni sintered compacts by sequence Z provides an alternative example of successful treatment which leads to the increase of ductility.
5. Summarizing remarks The present work provides details of treatments which allowed us to obtain sintered Cr – Ni compacts containing large amount of chromium with significantly improved ductility. The homogeneity of structure was improved by the applied multiple rolling and annealing. The as-sintered metals possessed high ductility at temperature close to 700 K, while they were entirely brittle at RT and above 1000 K, which prompted our research for the method for improving their properties. The mechanical properties of the sintered compacts have been slightly improved after multiple rolling and annealing with the sequence X. The lack of marked changes in tensile strength and elongation after treatment X was attributed to the limited changes in the structure (merely the grain elongation). In contrast, treatments Y and Z resulted from the formation of fine grain, well alloyed, homogeneous structure, leading to a significant improvement of ductility of Cr – Ni compacts. The better alloying, and consequently, the improvement of mechanical properties, has been also achieved for modified sequence X, when the thermal treatment before final rolling was performed at 1573 K. The proposed
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