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Synthesis and characterization of bulk amorphous steels
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Journal of Non-Crystalline Solids 354 (2008) 3284–3290 www.elsevier.com/locate/jnoncrysol
Synthesis and characterization of bulk amorphous steels M. Iqbal a,b,*, J.I. Akhter b,*, H.F. Zhang a, Z.Q. Hu a,1 a
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China b Physics Division, Pakistan Institute of Nuclear Science and Technology, P.O. Nilore, Islamabad, Pakistan Received 11 October 2007; received in revised form 28 January 2008 Available online 18 April 2008
Abstract Bulk amorphous steels (BASs) are a novel class of advanced materials having very attractive physical, thermal and mechanical properties and have applications as structural materials. Two BASs Fe50Cr14Mo14C14B6M2 (M = Y and Dy) were designed following the Greer’s confusion principle and cylinders of thickness 3–5 mm were synthesized by Cu mold casting technique. Characterization was carried out by techniques of X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) with attachment of energy dispersive spectroscopy (EDS). The alloys show high glass-forming ability (GFA) as well as high thermal stability. Hardness and elastic moduli of the present steels were found to be about 3–4 times higher as compared to the conventional steels. Steel containing Dy has superior mechanical and thermal properties as compared to the steel containing Y. Ó 2008 Elsevier B.V. All rights reserved. PACS: 61.05.cp; 61.10.Nz; 61.43.Dq; 62.20.x Keywords: Amorphous metals; Metallic glasses; Alloys; X-ray diffraction; Hardness; Scanning electron microscopy
1. Introduction Metallic glasses have been extensively studied during the last two decades owing to their various potential technological applications. Fe-based bulk metallic glasses (BMGs) are of special importance due to their relatively low cost and unique mechanical properties such as ultrahigh strength and high corrosion resistance as compared to conventional materials [1]. Among these the amorphous steels have shown great potential for some structural applications [2]. In order to compete commercially with conventional materials, amorphous steels need to be made of
*
Corresponding authors. Address: Physics Division, Pakistan Institute of Nuclear Science and Technology, P.O. Nilore, Islamabad, Pakistan. Tel.: +92 51 2207224; fax: +92 51 9290275 (J.I. Akhter). E-mail addresses: [email protected] (M. Iqbal), jiakhter@ yahoo.com, [email protected] (J.I. Akhter), [email protected] (Z.Q. Hu). 1 Tel.: +86 24 23971827, +86 24 23992092; fax: +86 24 23992092. 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.02.009
relatively inexpensive components, while maintaining a large critical size for a given application. Bulk amorphous steels (BASs) offer important advantages over their crystalline counterparts such as much lower material cost, higher strength, better magnetic properties, better corrosion resistance and higher thermal stability as well as better glassforming ability (GFA) [3,4]. Structural amorphous steels (SASs) having thickness up to 12–16 mm have been produced with glass transition temperature Tg above 900 K by Cu mold casting [5–8]. However, most of these steels have negligible ductility and they are brittle [9]. The limited GFA of amorphous steels is another major concerning point. Efforts have been devoted to improve the GFA of Fe-based alloys in order to enhance their ability to form bulk glassy steels under conventional industrial conditions, using, for example, commercial-grade raw materials, low vacuum, conventional casting techniques, etc. Recently, a great progress has been made in fabricating bulk amorphous steels [5–10] and nonmagnetic bulk amorphous steels have been developed which are three to four times
M. Iqbal et al. / Journal of Non-Crystalline Solids 354 (2008) 3284–3290
stronger as well as have superior corrosion resistance compared to the ordinary steels. A number of compositions of BASs have been reported, in the recent past [5,9,11], with varied amounts of Fe contents. However, the supercooled liquid region of these steels has been less than 50 K in most cases indicating low GFA of these materials. Therefore, improvement of GFA and brittleness are still major tasks in this regard. In the present study two FeCrMoCB steels have been synthesized by adding 2 at.% Y and Dy to investigate their effect on thermal and mechanical properties. 2. Experimental The steels of compositions Fe50Cr14Mo14C14B6M2, (where M = Y and Dy), designated as S1 and S2 in Table 1, were designed according to the Greer’s confusion principle [12]. The alloy buttons were produced from 1-3N pure constituent materials by arc melting at least four times to get chemical homogeneity. The Fe–B master alloy was used along with other alloy constituents having precise weights. The final casting of bulk samples having thickness (3– 5 mm) and length 60 mm was done in an induction furnace by Cu mold casting. In order to determine the thermal parameters, low temperature DSC was performed at heating rate ‘r’ of 10, 20 and 40 K/min. High temperature DSC was also performed at 20 and 40 K/min using NETZSCH DSC 404C to determine the melting and liquid temperatures. Samples of both steels were annealed at various temperatures ranging from 873 to 1123 K for 20 minutes under inert atmosphere. For structural characterization, XRD Table 1 Steel composition (at.%) with their designations and density (g/cm3) Steel
Composition
q1 Arc melted (±0.002)
q2 Induction cast (±0.002)
Difference q1 q2
S1 S2
Fe50Cr14Mo14C14B6Y2 Fe50Cr14Mo14C14B6Dy2
8.431 8.600
8.277 8.355
0.154 0.245
S1
Intensity (a.u.)
11 1
2
2 873 K/20 min
20
30
40
Fig. 1(a) and (b) shows the XRD patterns of as-cast samples of the steels S1 and S2 of varying thickness. It is clear that both the steels with thickness upto 4 mm show broad bands in the XRD patterns, which indicate amorphous nature of the steels. XRD patterns of both the steels having thickness 5 mm revealed that they are partially crystalline. Physical appearance of induction cast ingots shows excellent metallic luster, indicating the amorphous nature of the steels. XRD analysis of the annealed samples revealed that both the steels contain crystalline phases, namely, c-Fe and Cr23C6 as shown in Fig. 1(a) and (b). SEM examination of as-cast polished samples of both steels reveal featureless surface with out any second phase particles or segregation which reconfirms the amorphous nature of the steel samples. Fig. 2(a)–(c) shows the microstructure of the samples annealed at 1123 K for 20 min. EDS analysis confirmed the presence of Fe and Cr rich precipitates. The magnified view of crystalline matrix is shown in Fig. 2(b) indicating presence of precipitates in steel S1. In addition to the above mentioned phases, the steel S1 contains dendritic structure as shown in Fig. 2(a).
γ -Fe 2
50
2 S2 1 1 1 11 1 1 1 1
Cr23C6 1 11 2
1123 K/20 min
11
11
11
1
953 K/20 min
As-cast dia. 5 mm
As-cast dia. 5 mm
As-cast dia. 4 mm
As-cast dia. 4 mm
As-cast dia. 3 mm
60
2 theta (deg)
2
1
11 1123 K/20 min
2
3. Results
Cr23C6 1
1 1 1
was conducted using D/Max-2500 Rigaku diffractometer ˚ ) radiation. Uniaxial compreswith Cu Ka1 (k = 1.54056 A sion tests were applied on as-cast samples with aspect ratio 2 at a constant strain rate of 4.2 10–4/s. As-cast and annealed samples were examined in scanning electron microscope (SEM) to study the microstructure. Analysis of the samples was carried out by electron probe microanalyzer (EPMA) attached with SEM. Vicker’s hardness ‘HV’ was measured by MVK-H3 Mitutoya hardness testing machine taking average of at least eight to ten readings. Nanohardness ‘H’ and elastic modulus ‘E’ was measured employing Nanoindenter XP with Berkovich indenter using Oliver and Pharr method [13] under a load of 10 mN and taking average of at least six measurements. Density ‘q’ of arc-melted buttons and induction melted ingots was measured by Archimedes principle.
Intensity (a.u.)
γ -Fe 2
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70
80
20
30
40
50
2 theta (deg)
Fig. 1. XRD patterns of two as-cast and annealed steels S1 (a) and S2 (b).
60
70
80
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Heat flow (a.u.) Exo.
r = 10 K/min
Tg Tx
S2
Tg
Tx
S1 Tp1 450
600
750
Tp2
900
1050
1200
Temperature (K)
Heat flow (a.u.) Exo.
r = 20K/min
Tm Tl
Tg Tx
S2
S1 Tp1 450
600
750
Tp2
900
1050
1200
1350
1500
Temperature (K)
Tm Tl
Heat flow (a.u.) Exo.
r = 40K/min Tg
S2
Tx
Tp1 T p2
S1 Tp1
Fig. 2. Microstructure of annealed samples at 1123 K/20 min. Steel S1 at low magnification (a) magnified view of crystalline matrix (b) and steel S2 (c).
450
600
750
Tp2
900
1050
1200
1350
1500
Temperature (K)
Fig. 3. Low (a) and high temperature DSC scans (b,c) of two BASs S1 and S2 at heating rates of 10, 20 and 40 K/min.
Low and high temperature DSC were conducted at the heating rates of 10, 20 and 40 K/min and DSC scans are shown in Fig. 3(a)–(c). The results reveal that multistage crystallization occurs in both the steels. The thermal parameters like glass transition temperature Tg, crystallization temperature Tx, peak temperature Tp, melting and liquid temperatures Tm and Tl are taken from the DSC scans. The maximum variation in these temperature measurements is less than ±0.5 K. Using these values the supercooled liquid region DTx (=Tx Tg), reduced glass transition temperature Trg1 (=Tg/Tm) and Trg2 (=Tg/Tl), gamma ‘c’ parameter (=Tx/(Tl + Tg)) and delta ‘d’ parameter (=Tx/(Tl Tg)) were calculated. In addition to these parameters, Weinberg parameter KW [=(Tx Tg)/Tm], Hruby parameter KH (=(Tx Tg)/(Tm Tx)), thermal parameter KLL (=Tx/(Tg + Tm)), K1 (=Tm Tg),
Table 2a Thermal parameters by low temperature DSC of two steels S1 and S2 Steel
r
Tg
Tx1
Tp1
DTx
Tx2
Tp2
S1 S2
10 10
823 835
88 899
893 908
65 64
913 923
938 943
All temperatures are in K.
K2 = DTx, K3 (=Tx/Tm), K4 (=(Tp Tx)(Tx Tg)/Tm) and stability parameter KSP (=(Tp Tx)(Tx Tg)/Tg) [14] were also evaluated. All the parameters are summarized in Tables 2a–2c. These parameters are generally used to indicate the thermal stability and GFA [15,16]. The maximum supercooled liquid region was found to be 70 K for S2. The values of DTx are better than those reported by
M. Iqbal et al. / Journal of Non-Crystalline Solids 354 (2008) 3284–3290
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Table 2b Thermal parameters of two BASs at heating rates of 20 and 40 K/min from high temperature DSC Steel
r
Tg
Tx1
DTx
Tm
Tl
Tg/Tm
Tg/Tl
c
d
b
S1 S1 S2 S2
20 40 20 40
825 840 840 859
892 908 905 929
67 68 65 70
1373 1385 1384 1375
1413 1435 1418 1415
0.601 0.606 0.607 0.625
0.584 0.585 0.592 0.607
0.399 0.399 0.401 0.409
1.516 1.526 1.566 1.671
2.71 2.75 2.89 3.38
All temperatures are in K.
Table 2c Few more thermal parameters of two BASs at heating rates ‘r’ Steel
r
Tp1
Tx2
Tp2
K1
K2
K3
K4
KH
KW
KLL
S1 S1 S2 S2
20 40 20 40
909 919 919 958
929 938 934 1018
946 958 949 1028
548 545 544 516
67 68 65 70
0.6497 0.6556 0.6539 0.6756
0.8590 0.5401 0.6763 1.5069
0.1393 0.1426 0.1357 0.1570
0.049 0.041 0.047 0.051
0.0406 0.0408 0.0407 0.0416
All temperatures are in K.
Lu et al. [17] for Fe-based alloys containing 2 at.% Y. The results on Trg, c parameter (=Tx/(Tl + Tg)) and d parameter (=Tx/(Tl Tg)) [18] indicate that the present values are better than many Fe-based alloys. Steel S2 containing Dy has the highest values of all the parameters. From the data reported by Gu et al. [9] for more than 20 Fe-based BMGs, DTx, Trg (Tg/Tl), c and d parameters were calculated and the maximum values are 67 K, 0.584, 0.391 and 1.488, respectively. The values of thermal parameters determined in the present study were found to be 70 K, 0607, 0.409 and 1.671 respectively as given in Table 2b, which indicate good GFA of the present steels. Thermal parameters for present steels were also found to be better than Luo’s steels [19]. Thermal parameters like KH, KW, KLL, KSP, K1, K2, K3 and K4, etc. [15] are summarized in Table 2c. In order to calculate the activation energy for crystallization, Kissinger and Ozawa plots are drawn as shown in Fig. 4(a) and (b) for both steels. The data was fitted to Kissinger equation lnðr=T 2p Þ ¼ Eac =RT p + constant and Ozawa equation ln(r) = Eac/Tp + constant, where ‘r’ is the heating rate, Tp is the peak temperature in DSC curves, Eac is the activation energy, R is real gas constant 8.3145 J/mol K, with slope E/R = B, where B is a constant. The activation energy for first and second stage crystallization was deter-
mined from Kissinger and Ozawa plots using the values of R and B. The results are summarized in Table 3, which again indicates high thermal stability of both the steels. The density of BASs was found to be in the range of 8.3– 8.4 g/cm3 as given in Table 1, which is higher than the density found by Lu and Liu for BASs (7.8–7.9 g/cm3) [5]. Density of arc-melted ingots was found to be higher than the as-cast induction cast samples (3 mm thick) because the arc-melted alloy buttons contain crystalline phases while as-cast samples are fully amorphous. Results on HV of as-cast and the annealed samples are given in Table 4. It shows that the hardness of as-cast steels S1 and S2 is 1219 ± 15 and 1272 ± 15 respectively. The HV exceeds from 1500 at annealing temperature 1123 K for both the steels. The maximum value of hardness was found to be 1550 for S2. Nanohardness H (GPa) of as-cast steels was measured and given in Table 4, which is found to be higher than many BMGs, e.g. Zr-based BMGs. The as-cast steels S1 and S2 have H in the range of 16.7–17.2 GPa. With annealing temperature, H also increases and maximum value was found to be 22.3 GPa for S2 at 1123 K. The elastic modulus E (GPa) is one of the fundamental properties of materials as it shows the bonding between the atoms 4.0
-9.5
-10.0
ln(r)
2
ln(r/Tp )
3.5
S1
S2 -10.5
3.0
2.5
-11.0
-11.5 1.04
S1
S2
1.06
1.08
1.10 -1
1000/Tp(K )
1.12
2.0 1.04
1.06
1.08
1.10 -1
1000/Tp(K )
Fig. 4. Kissinger (a) and Ozawa (b) plots for both steels.
1.12
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Table 3 Activation energies (kJ/mol) of two steels S1and S2 Steel
Eac1 Kissinger
Ozawa
S1 S2
348.4 ± 2.6 358.3 ± 2.8
363.6 ± 2.6 374.2 ± 2.8
Difference (%)
Eac2
Difference (%)
Kissinger
Ozawa
4.34 4.44
505.6 ± 9.5 436.8 ± 14.4
521.4 ± 9.5 472.7 ± 15.2
3.12 8.22
Table 4 Mechanical properties of the steels S1 and S2 Sample
HV (±15)
H (±0.2) GPa
E (±5) GPa
H/E
h (nm)
hc (nm)
hf (nm)
hmax (nm)
hf/hmax
%R
S1 S1 S1 S2 S2 S2
1219 1280 1515 1272 1420 1548
17.2 17.9 21.5 16.7 21.3 22.3
263 267 321 257 326 337
0.0652 0.0670 0.0668 0.0651 0.0655 0.0660
179.2 180.7 165.8 185.8 165.7 162.5
142.1 143.0 130.1 148.2 130.5 127.4
133.7 128.6 112.9 134.6 113 111.6
187.0 180.8 165.8 187.5 165.7 162.5
0.715 0.711 0.681 0.718 0.682 0.687
28.5 28.9 31.9 28.2 31.8 31.3
as-cast 873 K 1123 K as-cast 953 K 1123 K
shown in Fig. 5(a) and (b). The maximum load (Pmax) used is 10 mN. The development of pop-in marks (displacement discontinuities) in first loading of the indent and pop-out marks in the unloading curves were observed. The pop-in marks indicate a sudden penetration of the tip of the indent into the sample. The non-uniform penetration is also due to sudden plastic deformation or formation of cracks. Iqbal et al. and Wang et al. [20–24] have also observed pop-in marks in loading curves of BMGs. The
and is dependent on interatomic distances. The elastic moduli of the present steels (given in Table 4) were found to be much higher than many BMGs [20–24]. The elastic moduli E of as-cast steels S1and S2 were found to be 263 and 257 GPa respectively, which are higher than many BMGs. Maximum elastic modulus was found to be 337 for S2 at 1123 K. Loading and unloading curves (P–h curves) obtained by nanoindentation of as-cast and heat-treated S1 and S2 are S1
As-cast 873 K/20 min 1123 K/20 min
10
Loading curves
Pop-out
Pop-out Pop-in marks
6
Loading curves
25
50
75
100
125
150
Penetration depth h (nm)
175
200
Pmax
4
Pop-out Unloading curves
hf
0
0
Pop-in marks
2
Unloading curves
hf hmax 0
Pmax
Load (mN)
Load(mN)
6
2
As-cast 953 K 1123 K
8
8
4
S2
10
225
Pop-in marks
0
25
50
hmax 75
100
125
150
175
200
225
Penetration depth h (nm)
Fig. 5. Loading and unloading curves (P-h curves) of two as-cast and annealed steels S1 (a) and S2 (b) showing pop-in and pop-out marks.
Fig. 6. Shear band formation in compression tested as-cast steels S1 (a) and S2 (b).
M. Iqbal et al. / Journal of Non-Crystalline Solids 354 (2008) 3284–3290
higher slope of the unloading curves indicates a higher stiffness or elastic modulus while higher penetration depth of indent shows lower hardness. Fig. 6(a) and (b) shows SEM micrographs of as-cast fractured samples of both the steels, which reveal the formation of parallel and curved shear bands without liquid droplets and veins patterns. 4. Discussion Two bulk amorphous steels were synthesized from low purity materials to investigate the effect of Y and Dy, having large atomic sizes (0.18015 nm and 0.17740 nm respectively), on the thermal stability, GFA and mechanical properties. Over all analysis of the results suggests that both the steels have good GFA as well as thermal stability. The GFA of BMGs has been described in terms of a number of thermal parameters like DTx, Trg, c, d and b [Tg Tx/(Tl Tx)2] [25–27]. Chen et al. [18] reported these thermal parameters of a number of Fe-based alloys. The thermal parameters DTx, Trg, c, d and b for the Fe48Cr15Mo14C15B6Y2 alloy, having composition similar to the present steels, were 47 K, 0.587, 0.385, 1.418 and 2.225, respectively. It is clear from Table 2b that most of the parameters of the present steels are higher than those reported by Chen et al. [18]. Shen and Schwarz [28] have reported DTx to be an important parameter regarding the estimation of GFA of Fe-based alloys. The striking feature of the present study was that supercooled liquid regions of 67 and 70 K were achieved for steels containing Y and Dy respectively. The high values of Tg and DTx suggest high thermal stability of the present steels. The enhanced thermal stability of the alloys is also evident from the high values of the activation energy for crystallization. The value is much higher for the steel S2, for which even sample with thickness 5 mm was fairly amorphous. The other important feature of the present study was that mechanical properties of the synthesized steels were much higher compared to the conventional steels. The microhardness values of 1219 and 1272 are comparable to Lu’s [5] steels, while these values are much higher than 960–1150 reported by Hess et al. [3] for Fe48Cr15C15Mo14B6Er2 bulk amorphous steel. Like microhardness, the nanohardness and elastic moduli of the present steels were found to be higher than many BMGs [20–24]. It was observed that the microhardness and elastic moduli increase by more than 25% as the samples are annealed. The enhancement in hardness and elastic moduli is due to nucleation of crystalline phases, which act as obstacles to the dislocation movement [29]. Nanohardness to elastic modulus ratio, i.e. H/E of as-cast and annealed steel samples was calculated and found to be in the range of 0.0651–0.0670, which is comparable with H/E ratios reported by Wang et al. [24]. H/E ratio 0.1 indicates that bonding in these BMGs is most probably of covalent nature. The elastic recovery hf /hmax and percentage elastic recovery of displacement on unloading % R = [(hmax
3289
hf)/hmax) 100%] are two important parameters that were calculated from the P–h curves, where hf is the final indentation depth and hmax is the maximum penetration depth of the indenter. This parameter is independent of indentation depth due to self-similar geometry of the indenter. The limits of this parameter are generally 0 6 hf/hmax 6 1, where the lower limit corresponds to fully elastic deformation and the upper limit reflects characteristic of rigid plastic materials, for which there is no elastic recovery [30]. The value of elastic recovery (hf/hmax) for glass and Al are 0.687 and 0.951, respectively. The elastic recovery limits in present case are found to be in the range of 0.681–0.718, while percentage elastic recovery % R ranges in between 28.2% and 31.9% for the steels under study. 5. Conclusions Two amorphous steels having diameter 4 mm and very good thermal and mechanical properties were synthesized by Cu mold casting technique. The maximum supercooled liquid region of 70 K was achieved for steel S2. High values of thermal parameters for the present steels indicate better glass-forming ability. High temperature DSC of present steels reveals multistage crystallization. Higher values of activation energy for crystallization show higher thermal stability of the present steels. Hardness and elastic moduli of the present steels were found to be about 3–4 times higher as compared to the conventional steels. Steel S2 containing Dy has superior mechanical/thermal properties than steels containing Y. Acknowledgements M. Iqbal is grateful to IMR and Chinese Academy of Sciences for offering PhD Scholarship. National Natural Science Foundation of China (Grant No. 50731005) and the Ministry of Science and Technology of China (2006CB605201, 2005DFA50860) supported this work. Thanks are also due to Profs. W.S. Sun, W. Wei, X.P. Song, J.Z. Zhao, A.M. Wang and H. Li for good cooperation and extending help during the experimental part of the work. Thanks to PAEC officials for providing the chance to do this work at IMR, Shenyang, China. References [1] K.F. Yao, C.Q. Zhang, Appl. Phys. Lett. 90 (2007) 61901-1. [2] J. Cheney, K. Vecchio, Bulk Metallic Glasses, in: P.K. Liaw, R.A. Buchanan, (Eds.), TMS (The Minerals, Metals and Materials Society), 2006, p. 135. [3] P.A. Hess, S.J. Poon, G.J. Shiflet, R.H. Dauskardt, J. Mater. Res. 20 (2005) 783. [4] H. Chiriac, N. Lupu, Mater. Sci. Eng. A 375–377 (2004) 255. [5] Z.P. Lu, C.T. Liu, J.R. Thompson, W.D. Porter, Phys. Rev. Lett. 92 (2004) 245503. [6] V. Ponnambalam, S.J. Poon, G.J. Shiflet, J. Mater. Res. 19 (2004) 1320.
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Journal of Non-Crystalline Solids 354 (2008) 3284–3290 www.elsevier.com/locate/jnoncrysol
Synthesis and characterization of bulk amorphous steels M. Iqbal a,b,*, J.I. Akhter b,*, H.F. Zhang a, Z.Q. Hu a,1 a
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China b Physics Division, Pakistan Institute of Nuclear Science and Technology, P.O. Nilore, Islamabad, Pakistan Received 11 October 2007; received in revised form 28 January 2008 Available online 18 April 2008
Abstract Bulk amorphous steels (BASs) are a novel class of advanced materials having very attractive physical, thermal and mechanical properties and have applications as structural materials. Two BASs Fe50Cr14Mo14C14B6M2 (M = Y and Dy) were designed following the Greer’s confusion principle and cylinders of thickness 3–5 mm were synthesized by Cu mold casting technique. Characterization was carried out by techniques of X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) with attachment of energy dispersive spectroscopy (EDS). The alloys show high glass-forming ability (GFA) as well as high thermal stability. Hardness and elastic moduli of the present steels were found to be about 3–4 times higher as compared to the conventional steels. Steel containing Dy has superior mechanical and thermal properties as compared to the steel containing Y. Ó 2008 Elsevier B.V. All rights reserved. PACS: 61.05.cp; 61.10.Nz; 61.43.Dq; 62.20.x Keywords: Amorphous metals; Metallic glasses; Alloys; X-ray diffraction; Hardness; Scanning electron microscopy
1. Introduction Metallic glasses have been extensively studied during the last two decades owing to their various potential technological applications. Fe-based bulk metallic glasses (BMGs) are of special importance due to their relatively low cost and unique mechanical properties such as ultrahigh strength and high corrosion resistance as compared to conventional materials [1]. Among these the amorphous steels have shown great potential for some structural applications [2]. In order to compete commercially with conventional materials, amorphous steels need to be made of
*
Corresponding authors. Address: Physics Division, Pakistan Institute of Nuclear Science and Technology, P.O. Nilore, Islamabad, Pakistan. Tel.: +92 51 2207224; fax: +92 51 9290275 (J.I. Akhter). E-mail addresses: [email protected] (M. Iqbal), jiakhter@ yahoo.com, [email protected] (J.I. Akhter), [email protected] (Z.Q. Hu). 1 Tel.: +86 24 23971827, +86 24 23992092; fax: +86 24 23992092. 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.02.009
relatively inexpensive components, while maintaining a large critical size for a given application. Bulk amorphous steels (BASs) offer important advantages over their crystalline counterparts such as much lower material cost, higher strength, better magnetic properties, better corrosion resistance and higher thermal stability as well as better glassforming ability (GFA) [3,4]. Structural amorphous steels (SASs) having thickness up to 12–16 mm have been produced with glass transition temperature Tg above 900 K by Cu mold casting [5–8]. However, most of these steels have negligible ductility and they are brittle [9]. The limited GFA of amorphous steels is another major concerning point. Efforts have been devoted to improve the GFA of Fe-based alloys in order to enhance their ability to form bulk glassy steels under conventional industrial conditions, using, for example, commercial-grade raw materials, low vacuum, conventional casting techniques, etc. Recently, a great progress has been made in fabricating bulk amorphous steels [5–10] and nonmagnetic bulk amorphous steels have been developed which are three to four times
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stronger as well as have superior corrosion resistance compared to the ordinary steels. A number of compositions of BASs have been reported, in the recent past [5,9,11], with varied amounts of Fe contents. However, the supercooled liquid region of these steels has been less than 50 K in most cases indicating low GFA of these materials. Therefore, improvement of GFA and brittleness are still major tasks in this regard. In the present study two FeCrMoCB steels have been synthesized by adding 2 at.% Y and Dy to investigate their effect on thermal and mechanical properties. 2. Experimental The steels of compositions Fe50Cr14Mo14C14B6M2, (where M = Y and Dy), designated as S1 and S2 in Table 1, were designed according to the Greer’s confusion principle [12]. The alloy buttons were produced from 1-3N pure constituent materials by arc melting at least four times to get chemical homogeneity. The Fe–B master alloy was used along with other alloy constituents having precise weights. The final casting of bulk samples having thickness (3– 5 mm) and length 60 mm was done in an induction furnace by Cu mold casting. In order to determine the thermal parameters, low temperature DSC was performed at heating rate ‘r’ of 10, 20 and 40 K/min. High temperature DSC was also performed at 20 and 40 K/min using NETZSCH DSC 404C to determine the melting and liquid temperatures. Samples of both steels were annealed at various temperatures ranging from 873 to 1123 K for 20 minutes under inert atmosphere. For structural characterization, XRD Table 1 Steel composition (at.%) with their designations and density (g/cm3) Steel
Composition
q1 Arc melted (±0.002)
q2 Induction cast (±0.002)
Difference q1 q2
S1 S2
Fe50Cr14Mo14C14B6Y2 Fe50Cr14Mo14C14B6Dy2
8.431 8.600
8.277 8.355
0.154 0.245
S1
Intensity (a.u.)
11 1
2
2 873 K/20 min
20
30
40
Fig. 1(a) and (b) shows the XRD patterns of as-cast samples of the steels S1 and S2 of varying thickness. It is clear that both the steels with thickness upto 4 mm show broad bands in the XRD patterns, which indicate amorphous nature of the steels. XRD patterns of both the steels having thickness 5 mm revealed that they are partially crystalline. Physical appearance of induction cast ingots shows excellent metallic luster, indicating the amorphous nature of the steels. XRD analysis of the annealed samples revealed that both the steels contain crystalline phases, namely, c-Fe and Cr23C6 as shown in Fig. 1(a) and (b). SEM examination of as-cast polished samples of both steels reveal featureless surface with out any second phase particles or segregation which reconfirms the amorphous nature of the steel samples. Fig. 2(a)–(c) shows the microstructure of the samples annealed at 1123 K for 20 min. EDS analysis confirmed the presence of Fe and Cr rich precipitates. The magnified view of crystalline matrix is shown in Fig. 2(b) indicating presence of precipitates in steel S1. In addition to the above mentioned phases, the steel S1 contains dendritic structure as shown in Fig. 2(a).
γ -Fe 2
50
2 S2 1 1 1 11 1 1 1 1
Cr23C6 1 11 2
1123 K/20 min
11
11
11
1
953 K/20 min
As-cast dia. 5 mm
As-cast dia. 5 mm
As-cast dia. 4 mm
As-cast dia. 4 mm
As-cast dia. 3 mm
60
2 theta (deg)
2
1
11 1123 K/20 min
2
3. Results
Cr23C6 1
1 1 1
was conducted using D/Max-2500 Rigaku diffractometer ˚ ) radiation. Uniaxial compreswith Cu Ka1 (k = 1.54056 A sion tests were applied on as-cast samples with aspect ratio 2 at a constant strain rate of 4.2 10–4/s. As-cast and annealed samples were examined in scanning electron microscope (SEM) to study the microstructure. Analysis of the samples was carried out by electron probe microanalyzer (EPMA) attached with SEM. Vicker’s hardness ‘HV’ was measured by MVK-H3 Mitutoya hardness testing machine taking average of at least eight to ten readings. Nanohardness ‘H’ and elastic modulus ‘E’ was measured employing Nanoindenter XP with Berkovich indenter using Oliver and Pharr method [13] under a load of 10 mN and taking average of at least six measurements. Density ‘q’ of arc-melted buttons and induction melted ingots was measured by Archimedes principle.
Intensity (a.u.)
γ -Fe 2
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70
80
20
30
40
50
2 theta (deg)
Fig. 1. XRD patterns of two as-cast and annealed steels S1 (a) and S2 (b).
60
70
80
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Heat flow (a.u.) Exo.
r = 10 K/min
Tg Tx
S2
Tg
Tx
S1 Tp1 450
600
750
Tp2
900
1050
1200
Temperature (K)
Heat flow (a.u.) Exo.
r = 20K/min
Tm Tl
Tg Tx
S2
S1 Tp1 450
600
750
Tp2
900
1050
1200
1350
1500
Temperature (K)
Tm Tl
Heat flow (a.u.) Exo.
r = 40K/min Tg
S2
Tx
Tp1 T p2
S1 Tp1
Fig. 2. Microstructure of annealed samples at 1123 K/20 min. Steel S1 at low magnification (a) magnified view of crystalline matrix (b) and steel S2 (c).
450
600
750
Tp2
900
1050
1200
1350
1500
Temperature (K)
Fig. 3. Low (a) and high temperature DSC scans (b,c) of two BASs S1 and S2 at heating rates of 10, 20 and 40 K/min.
Low and high temperature DSC were conducted at the heating rates of 10, 20 and 40 K/min and DSC scans are shown in Fig. 3(a)–(c). The results reveal that multistage crystallization occurs in both the steels. The thermal parameters like glass transition temperature Tg, crystallization temperature Tx, peak temperature Tp, melting and liquid temperatures Tm and Tl are taken from the DSC scans. The maximum variation in these temperature measurements is less than ±0.5 K. Using these values the supercooled liquid region DTx (=Tx Tg), reduced glass transition temperature Trg1 (=Tg/Tm) and Trg2 (=Tg/Tl), gamma ‘c’ parameter (=Tx/(Tl + Tg)) and delta ‘d’ parameter (=Tx/(Tl Tg)) were calculated. In addition to these parameters, Weinberg parameter KW [=(Tx Tg)/Tm], Hruby parameter KH (=(Tx Tg)/(Tm Tx)), thermal parameter KLL (=Tx/(Tg + Tm)), K1 (=Tm Tg),
Table 2a Thermal parameters by low temperature DSC of two steels S1 and S2 Steel
r
Tg
Tx1
Tp1
DTx
Tx2
Tp2
S1 S2
10 10
823 835
88 899
893 908
65 64
913 923
938 943
All temperatures are in K.
K2 = DTx, K3 (=Tx/Tm), K4 (=(Tp Tx)(Tx Tg)/Tm) and stability parameter KSP (=(Tp Tx)(Tx Tg)/Tg) [14] were also evaluated. All the parameters are summarized in Tables 2a–2c. These parameters are generally used to indicate the thermal stability and GFA [15,16]. The maximum supercooled liquid region was found to be 70 K for S2. The values of DTx are better than those reported by
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Table 2b Thermal parameters of two BASs at heating rates of 20 and 40 K/min from high temperature DSC Steel
r
Tg
Tx1
DTx
Tm
Tl
Tg/Tm
Tg/Tl
c
d
b
S1 S1 S2 S2
20 40 20 40
825 840 840 859
892 908 905 929
67 68 65 70
1373 1385 1384 1375
1413 1435 1418 1415
0.601 0.606 0.607 0.625
0.584 0.585 0.592 0.607
0.399 0.399 0.401 0.409
1.516 1.526 1.566 1.671
2.71 2.75 2.89 3.38
All temperatures are in K.
Table 2c Few more thermal parameters of two BASs at heating rates ‘r’ Steel
r
Tp1
Tx2
Tp2
K1
K2
K3
K4
KH
KW
KLL
S1 S1 S2 S2
20 40 20 40
909 919 919 958
929 938 934 1018
946 958 949 1028
548 545 544 516
67 68 65 70
0.6497 0.6556 0.6539 0.6756
0.8590 0.5401 0.6763 1.5069
0.1393 0.1426 0.1357 0.1570
0.049 0.041 0.047 0.051
0.0406 0.0408 0.0407 0.0416
All temperatures are in K.
Lu et al. [17] for Fe-based alloys containing 2 at.% Y. The results on Trg, c parameter (=Tx/(Tl + Tg)) and d parameter (=Tx/(Tl Tg)) [18] indicate that the present values are better than many Fe-based alloys. Steel S2 containing Dy has the highest values of all the parameters. From the data reported by Gu et al. [9] for more than 20 Fe-based BMGs, DTx, Trg (Tg/Tl), c and d parameters were calculated and the maximum values are 67 K, 0.584, 0.391 and 1.488, respectively. The values of thermal parameters determined in the present study were found to be 70 K, 0607, 0.409 and 1.671 respectively as given in Table 2b, which indicate good GFA of the present steels. Thermal parameters for present steels were also found to be better than Luo’s steels [19]. Thermal parameters like KH, KW, KLL, KSP, K1, K2, K3 and K4, etc. [15] are summarized in Table 2c. In order to calculate the activation energy for crystallization, Kissinger and Ozawa plots are drawn as shown in Fig. 4(a) and (b) for both steels. The data was fitted to Kissinger equation lnðr=T 2p Þ ¼ Eac =RT p + constant and Ozawa equation ln(r) = Eac/Tp + constant, where ‘r’ is the heating rate, Tp is the peak temperature in DSC curves, Eac is the activation energy, R is real gas constant 8.3145 J/mol K, with slope E/R = B, where B is a constant. The activation energy for first and second stage crystallization was deter-
mined from Kissinger and Ozawa plots using the values of R and B. The results are summarized in Table 3, which again indicates high thermal stability of both the steels. The density of BASs was found to be in the range of 8.3– 8.4 g/cm3 as given in Table 1, which is higher than the density found by Lu and Liu for BASs (7.8–7.9 g/cm3) [5]. Density of arc-melted ingots was found to be higher than the as-cast induction cast samples (3 mm thick) because the arc-melted alloy buttons contain crystalline phases while as-cast samples are fully amorphous. Results on HV of as-cast and the annealed samples are given in Table 4. It shows that the hardness of as-cast steels S1 and S2 is 1219 ± 15 and 1272 ± 15 respectively. The HV exceeds from 1500 at annealing temperature 1123 K for both the steels. The maximum value of hardness was found to be 1550 for S2. Nanohardness H (GPa) of as-cast steels was measured and given in Table 4, which is found to be higher than many BMGs, e.g. Zr-based BMGs. The as-cast steels S1 and S2 have H in the range of 16.7–17.2 GPa. With annealing temperature, H also increases and maximum value was found to be 22.3 GPa for S2 at 1123 K. The elastic modulus E (GPa) is one of the fundamental properties of materials as it shows the bonding between the atoms 4.0
-9.5
-10.0
ln(r)
2
ln(r/Tp )
3.5
S1
S2 -10.5
3.0
2.5
-11.0
-11.5 1.04
S1
S2
1.06
1.08
1.10 -1
1000/Tp(K )
1.12
2.0 1.04
1.06
1.08
1.10 -1
1000/Tp(K )
Fig. 4. Kissinger (a) and Ozawa (b) plots for both steels.
1.12
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Table 3 Activation energies (kJ/mol) of two steels S1and S2 Steel
Eac1 Kissinger
Ozawa
S1 S2
348.4 ± 2.6 358.3 ± 2.8
363.6 ± 2.6 374.2 ± 2.8
Difference (%)
Eac2
Difference (%)
Kissinger
Ozawa
4.34 4.44
505.6 ± 9.5 436.8 ± 14.4
521.4 ± 9.5 472.7 ± 15.2
3.12 8.22
Table 4 Mechanical properties of the steels S1 and S2 Sample
HV (±15)
H (±0.2) GPa
E (±5) GPa
H/E
h (nm)
hc (nm)
hf (nm)
hmax (nm)
hf/hmax
%R
S1 S1 S1 S2 S2 S2
1219 1280 1515 1272 1420 1548
17.2 17.9 21.5 16.7 21.3 22.3
263 267 321 257 326 337
0.0652 0.0670 0.0668 0.0651 0.0655 0.0660
179.2 180.7 165.8 185.8 165.7 162.5
142.1 143.0 130.1 148.2 130.5 127.4
133.7 128.6 112.9 134.6 113 111.6
187.0 180.8 165.8 187.5 165.7 162.5
0.715 0.711 0.681 0.718 0.682 0.687
28.5 28.9 31.9 28.2 31.8 31.3
as-cast 873 K 1123 K as-cast 953 K 1123 K
shown in Fig. 5(a) and (b). The maximum load (Pmax) used is 10 mN. The development of pop-in marks (displacement discontinuities) in first loading of the indent and pop-out marks in the unloading curves were observed. The pop-in marks indicate a sudden penetration of the tip of the indent into the sample. The non-uniform penetration is also due to sudden plastic deformation or formation of cracks. Iqbal et al. and Wang et al. [20–24] have also observed pop-in marks in loading curves of BMGs. The
and is dependent on interatomic distances. The elastic moduli of the present steels (given in Table 4) were found to be much higher than many BMGs [20–24]. The elastic moduli E of as-cast steels S1and S2 were found to be 263 and 257 GPa respectively, which are higher than many BMGs. Maximum elastic modulus was found to be 337 for S2 at 1123 K. Loading and unloading curves (P–h curves) obtained by nanoindentation of as-cast and heat-treated S1 and S2 are S1
As-cast 873 K/20 min 1123 K/20 min
10
Loading curves
Pop-out
Pop-out Pop-in marks
6
Loading curves
25
50
75
100
125
150
Penetration depth h (nm)
175
200
Pmax
4
Pop-out Unloading curves
hf
0
0
Pop-in marks
2
Unloading curves
hf hmax 0
Pmax
Load (mN)
Load(mN)
6
2
As-cast 953 K 1123 K
8
8
4
S2
10
225
Pop-in marks
0
25
50
hmax 75
100
125
150
175
200
225
Penetration depth h (nm)
Fig. 5. Loading and unloading curves (P-h curves) of two as-cast and annealed steels S1 (a) and S2 (b) showing pop-in and pop-out marks.
Fig. 6. Shear band formation in compression tested as-cast steels S1 (a) and S2 (b).
M. Iqbal et al. / Journal of Non-Crystalline Solids 354 (2008) 3284–3290
higher slope of the unloading curves indicates a higher stiffness or elastic modulus while higher penetration depth of indent shows lower hardness. Fig. 6(a) and (b) shows SEM micrographs of as-cast fractured samples of both the steels, which reveal the formation of parallel and curved shear bands without liquid droplets and veins patterns. 4. Discussion Two bulk amorphous steels were synthesized from low purity materials to investigate the effect of Y and Dy, having large atomic sizes (0.18015 nm and 0.17740 nm respectively), on the thermal stability, GFA and mechanical properties. Over all analysis of the results suggests that both the steels have good GFA as well as thermal stability. The GFA of BMGs has been described in terms of a number of thermal parameters like DTx, Trg, c, d and b [Tg Tx/(Tl Tx)2] [25–27]. Chen et al. [18] reported these thermal parameters of a number of Fe-based alloys. The thermal parameters DTx, Trg, c, d and b for the Fe48Cr15Mo14C15B6Y2 alloy, having composition similar to the present steels, were 47 K, 0.587, 0.385, 1.418 and 2.225, respectively. It is clear from Table 2b that most of the parameters of the present steels are higher than those reported by Chen et al. [18]. Shen and Schwarz [28] have reported DTx to be an important parameter regarding the estimation of GFA of Fe-based alloys. The striking feature of the present study was that supercooled liquid regions of 67 and 70 K were achieved for steels containing Y and Dy respectively. The high values of Tg and DTx suggest high thermal stability of the present steels. The enhanced thermal stability of the alloys is also evident from the high values of the activation energy for crystallization. The value is much higher for the steel S2, for which even sample with thickness 5 mm was fairly amorphous. The other important feature of the present study was that mechanical properties of the synthesized steels were much higher compared to the conventional steels. The microhardness values of 1219 and 1272 are comparable to Lu’s [5] steels, while these values are much higher than 960–1150 reported by Hess et al. [3] for Fe48Cr15C15Mo14B6Er2 bulk amorphous steel. Like microhardness, the nanohardness and elastic moduli of the present steels were found to be higher than many BMGs [20–24]. It was observed that the microhardness and elastic moduli increase by more than 25% as the samples are annealed. The enhancement in hardness and elastic moduli is due to nucleation of crystalline phases, which act as obstacles to the dislocation movement [29]. Nanohardness to elastic modulus ratio, i.e. H/E of as-cast and annealed steel samples was calculated and found to be in the range of 0.0651–0.0670, which is comparable with H/E ratios reported by Wang et al. [24]. H/E ratio 0.1 indicates that bonding in these BMGs is most probably of covalent nature. The elastic recovery hf /hmax and percentage elastic recovery of displacement on unloading % R = [(hmax
3289
hf)/hmax) 100%] are two important parameters that were calculated from the P–h curves, where hf is the final indentation depth and hmax is the maximum penetration depth of the indenter. This parameter is independent of indentation depth due to self-similar geometry of the indenter. The limits of this parameter are generally 0 6 hf/hmax 6 1, where the lower limit corresponds to fully elastic deformation and the upper limit reflects characteristic of rigid plastic materials, for which there is no elastic recovery [30]. The value of elastic recovery (hf/hmax) for glass and Al are 0.687 and 0.951, respectively. The elastic recovery limits in present case are found to be in the range of 0.681–0.718, while percentage elastic recovery % R ranges in between 28.2% and 31.9% for the steels under study. 5. Conclusions Two amorphous steels having diameter 4 mm and very good thermal and mechanical properties were synthesized by Cu mold casting technique. The maximum supercooled liquid region of 70 K was achieved for steel S2. High values of thermal parameters for the present steels indicate better glass-forming ability. High temperature DSC of present steels reveals multistage crystallization. Higher values of activation energy for crystallization show higher thermal stability of the present steels. Hardness and elastic moduli of the present steels were found to be about 3–4 times higher as compared to the conventional steels. Steel S2 containing Dy has superior mechanical/thermal properties than steels containing Y. Acknowledgements M. Iqbal is grateful to IMR and Chinese Academy of Sciences for offering PhD Scholarship. National Natural Science Foundation of China (Grant No. 50731005) and the Ministry of Science and Technology of China (2006CB605201, 2005DFA50860) supported this work. Thanks are also due to Profs. W.S. Sun, W. Wei, X.P. Song, J.Z. Zhao, A.M. Wang and H. Li for good cooperation and extending help during the experimental part of the work. Thanks to PAEC officials for providing the chance to do this work at IMR, Shenyang, China. References [1] K.F. Yao, C.Q. Zhang, Appl. Phys. Lett. 90 (2007) 61901-1. [2] J. Cheney, K. Vecchio, Bulk Metallic Glasses, in: P.K. Liaw, R.A. Buchanan, (Eds.), TMS (The Minerals, Metals and Materials Society), 2006, p. 135. [3] P.A. Hess, S.J. Poon, G.J. Shiflet, R.H. Dauskardt, J. Mater. Res. 20 (2005) 783. [4] H. Chiriac, N. Lupu, Mater. Sci. Eng. A 375–377 (2004) 255. [5] Z.P. Lu, C.T. Liu, J.R. Thompson, W.D. Porter, Phys. Rev. Lett. 92 (2004) 245503. [6] V. Ponnambalam, S.J. Poon, G.J. Shiflet, J. Mater. Res. 19 (2004) 1320.
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