- Email: [email protected]
Effect of chromium and aluminum addition on anisotropic and microstructural characteristics of ball milled nanocrystalline iron
Journal of Alloys and Compounds 671 (2016) 164e169
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of chromium and aluminum addition on anisotropic and microstructural characteristics of ball milled nanocrystalline iron Rajiv Kumar a, b, c, *, Joydip Joardar e, R.K. Singh Raman c, d, V.S. Raja b, S.V. Joshi e, S. Parida b a
IITB-Monash Research Academy, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India Department of Mechanical and Aerospace Engineering, Monash University, VIC 3800 Australia d Department of Chemical Engineering, Monash University, VIC 3800 Australia e International Advanced Research Centre for Powder Metallurgy and New Materials, Hyderabad, India b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 July 2015 Received in revised form 10 January 2016 Accepted 10 February 2016 Available online 12 February 2016
Prior studies on synthesis of nanocrystalline elements have discussed the effect of ball milling on lattice parameter, crystallite size, and micro-strain. For elemental milled powders, the anisotropic peak broadening does not change with increasing milling time. However, the effect of alloying addition on the anisotropic behavior of ball milled nanocrystalline powders remains an unexplored area. Here we report the effect of chromium and aluminum addition on the anisotropic behavior of iron in nanocrystalline Fe e20Cre5Al (wt%) alloy powders synthesized by ball milling. The experimental results show that the anisotropic behavior of iron changes towards isotropic with milling. This change was also correlated to the theoretically calculated anisotropic factor from the change in elastic constant of iron due to milling. Addition of alloying elements exhibited a monotonic rise in the lattice parameter with crystallite size, which was attributed to the excess grain boundary interfacial energy and excess free volume at grain boundaries. Transmission electron microscopy image confirmed the crystallite size and nature of dislocation obtained using modified Williamson-Hall method. © 2016 Elsevier B.V. All rights reserved.
Keywords: Mechanical alloying X-ray diffraction Nanocrystalline materials Anisotropic behavior Dislocation density Lattice parameter
1. Introduction In recent years, nanocrystalline materials are being widely investigated due to unique properties like mechanical, physical, and corrosion resistance [1e6]. These materials have been developed by electro-deposition [7], severe plastic deformation [8], aluminoethermic reaction [9], chemical vapor deposition [10], and high-energy ball milling [4,11] process. Among all these techniques, high-energy ball milling is widely employed to synthesize artifactfree bulk nanocrystalline materials [1,2,12e14]. During high-energy ball milling, high impact force is imposed into the powder which induces change in crystallite size and lattice parameter, development of micro-strain, dislocation density, excess free volume and excess grain boundary interfacial energy [1,13,14]. All these structural changes play a significant role during consolidation and also
* Corresponding author. IITB-Monash Research Academy, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India. E-mail address: [email protected] (R. Kumar). http://dx.doi.org/10.1016/j.jallcom.2016.02.096 0925-8388/© 2016 Elsevier B.V. All rights reserved.
alter physical [15], mechanical [16] and corrosion [4] properties of materials after consolidation. Nanocrystalline materials, produced by severe plastic deformation such as high-energy ball milling, have shown nonmonotonous changes in lattice parameter with crystallite size, an initial lattice contraction followed by a lattice expansion [1,13,14,17]. Qin et al. [13] have proposed that the non-monotonous change in lattice parameter depends on the equilibrium between interfacial stresses developed due to excess grain boundary (GB) interfacial energy and the excess free volume at grain boundaries. The increase in lattice parameter is manifested as a shift in x-ray diffraction (XRD) peak to lower angle, while the reduction in crystallite size and the development of micro-strain during milling are reflected by peak broadening. XRD peak broadening of iron during milling is a non-monotonous function of diffraction angle which is demonstration of anisotropic behavior of iron [2]. Such anisotropic behavior is correlated to the unequal elastic modulus of iron in different crystallographic directions. Accordingly, Williamson-Hall (WH) plot for milled iron powder exhibits
R. Kumar et al. / Journal of Alloys and Compounds 671 (2016) 164e169
anisotropic broadening of iron, which does not change with increase of milling time [2]. Synthesis of nanocrystalline elemental iron, nickel, copper, and tungsten has discussed the effect of ball milling on lattice parameter, crystallite size and micro-strain [1,2,13,14,18e20]. However, the effect of alloying addition on anisotropic behavior of ball milled nanocrystalline ferrous system remains an unexplored area. The effect of chromium and aluminum addition on anisotropic behavior of iron in nanocrystalline Fee20Cre5Al (wt%) alloy powder synthesized by ball milling is reported in the present study. The WH technique was used to verify the anisotropic behavior of the alloy whereas the crystallite size was determined by modified Williamson-Hall (MWH) method. The change in lattice parameter of alloy with its crystallite size was calculated and this change was correlated with the excess GB interfacial energy and the excess free volume developed in the alloy at grain boundaries owing to nanocrystallization.
2. Experimental procedure
165
3. Results and discussion 3.1. X-ray diffraction of milled powder Fig. 1a shows XRD patterns of ball milled Fee20Cre5Al powder blends at different stages of milling. It is evident from Fig. 1a that the intensity of major aluminum peak (2q ¼ 38.47) shows a gradual drop with progress of milling and disappearance by end of 5 h of milling. Such an observation is attributed to the formation of a solid solution of chromium and aluminum with iron. Iron and aluminum have a tendency to form intermetallic phases like FeAl and Fe3Al during ball milling [23]. However, such intermetallic peaks were not detected in the XRD pattern, plausibly because the aluminum content (~5 wt %) was too low to enable their formation. Careful analysis of the XRD patterns in Fig. 1a also reveals a gradual shift of peak positions toward lower diffraction angle with increase in milling time. The observed peak shift indicates an increase in lattice parameter. The lattice expansion occurs due to dissolution of aluminum and chromium into iron to form an alloy, shown in Fig. 1b. The lattice parameter for Fee20Cre5Al alloy after 20 h milling is 0.28851 nm which is comparable with the lattice parameter reported by Capdevila et al. [24].
2.1. Materials and methods The nanocrystalline Fee20Cre5Al alloy powder was prepared using high-energy planetary ball mill (Pulverisette P-5; Fritsch GmbH, Germany). The starting materials, iron powder (99.9% purity and average particle size < 37 mm) and chromium powder (99.9% purity and average particle size < 28 mm) were obtained from Alfa Aesar while aluminum powder (99.9% purity and average particle size < 1.3 mm) was procured from Hunan Jinhao Aluminum Industrial Co. Ltd, China. As-received powders were milled for composition Fee20Cre5Al at 250 rpm for durations up to 20 h, using toluene as the inert medium. The ball-to-powder weight ratio was maintained at 10:1. Mill was periodically stopped for 15 min after every 1 h of milling to avoid excessive temperature of the tungsten carbide vessel. Powder samples were collected at 2, 5, 10, 15, and 20 h of milling to characterize the microstructural changes during milling. As received iron powder was annealed at 900 C for 10 h into a convention tubular furnace (Model No. HCS/HTF/1000/ 90-900, Supplier: Heat & Control Systems, Mumbai, India) of capacity 1000 C and the forming gas (95% Ar þ 5% H2) was purged throughout the annealing process. The XRD line broadening of the fully annealed powder was considered as an instrumental broadening. The whole XRD analysis of Fee20Cre5Al alloy powder was done after considering this as an instrumental broadening.
2.2. Characterization of milled powder The microstructural changes in iron at different milling times were determined by XRD, using RigakueSmart Lab system (Rigaku Corp, Japan) with Cu Ka radiation (l ¼ 0.154056 nm), with a step size of 0.01 in 2q range of 20e120 . HighScore Plus (version 3.0d) [21] was used for multiple peak fitting of XRD patterns of the milled powder using the pseudo-Voigt function [22] and corresponding peak broadening was calculated after considering the instrumental broadening. The extent of peak broadening and change in peak position were analyzed to estimate crystallite size, dislocation density, lattice parameter, change in excess GB interfacial energy, and excess free volume at grain boundaries. Nanocrystalline Fee20Cre5Al alloy powder was formed after 20 h of milling that was confirmed using transmission electron microscopy, TEM (JOEL JEM-2100F), and selected area electron diffraction (SAED).
3.2. Analysis of anisotropic behavior of iron The grain refinement and development of micro-strain/lattice distortion, due to defects, leads to XRD peak broadening as visible in Fig. 1a. The micro-strain/lattice distortion developed along any crystallographic direction depends on the elastic modulus along that direction [25]. The lattice distortion is favorable along the crystallographic direction with lower elastic modulus as it requires less energy to develop defects [26]. Consequently, in an anisotropic material like Fe, more broadening is expected along the crystallographic direction with lowest elastic modulus. The elastic modulus of pure iron has identical value of 221 GPa in [100], [112] and [220] crystallographic directions, whereas the elastic moduli in [200] and [310] crystallographic directions are 132 GPa and 154 GPa, respectively [25]. Many authors [2,5,27,28] have reported significant broadening along [200] and [310] directions in iron powder after ball milling. This indicates that the peak broadening in milled iron powder is not a monotonous function of diffraction angle, possibly due to its distinct elastic modulus in different crystallographic directions. The elastic modulus of iron in the crystallographic direction can be reduced after addition of chromium and aluminum, since the elastic constant of iron [25] is higher that of chromium [29] and aluminum [25], shown in Table 1. The elastic modulus (E) of BCC system depends on either stiffness (C11, C12, C44) or compliance (S11, S12, S44) constant of crystal and direction cosines (l, m, n) of reflection, and can be calculated using following formula [25].
1 1 ¼ S11 2 ðS11 S12 Þ S44 l2 m2 þ m2 n2 þ n2 l2 E 2
(1)
The change in elastic modulus of iron due to alloying addition can be ascertained using WH technique. Considering classical WH method, the total peak broadening due to crystallite size (dV) and micro-strain (ε) are related by following expression [30,31]:
DK ¼
0:9 2sinq þ 2ε dV l
(2)
where K ¼ 2sinq/l, and DK ¼ (2Dq)cosq/l is caused by peak broadening. WH plots for Fee20Cre5Al alloy powder for different milling time are shown in Fig. 2. It is evident from Fig. 2 that the
166
R. Kumar et al. / Journal of Alloys and Compounds 671 (2016) 164e169
Fig. 1. Effect of milling time on a) x-ray diffraction pattern of Fee20Cre5Al alloy powder, and b) lattice parameter of Fee20Cre5Al alloy powder.
Table 1 Stiffness and compliance constant of iron, chromium, and aluminum. Metal
C11 (GPa)
C12 (GPa)
C44 (GPa)
S11(TPa1)
S12(TPa1)
S44(TPa1)
Iron Chromium Aluminum
237.0 339.8 108.2
141.0 58.6 61.3
116.0 99.0 28.5
7.587 3.090 15.659
2.830 0.441 5.663
8.621 10.101 35.088
Fig. 2. WH plot for ball-milled Fee20Cre5Al alloy powder.
broadening along [200] and [310] directions shows an abrupt rise up to 5 h of milling, in compare to other directions. The broadening of each peak in the diffraction pattern of Fee20Cre5Al alloy powder shows a gradual increase with increase in milling time and follows a similar anisotropic broadening trend as that reported in the case of pure iron up to 5 h of milling [2]. This indicates that the elastic modulus of iron, in Fee20Cre5Al blend, has not been changed significantly up to 5 h of milling. However, on further increase in milling time, the XRD peaks exhibit faster broadening along [112] and [310] directions. The change of elastic modulus with increase in milling time indicates that the anisotropic nature of iron tends towards isotropic with increase in milling time. The effect of alloying element addition on anisotropic behavior of iron can be explained by anisotropic factor, A ¼ 2C44/(C11eC12) [32]. A model was proposed by Giri [33] for calculating the elastic constant (stiffness constant) for binary alloy system. Based on the model of Giri, the elastic constants (C11, C12, and C44) were derived for ternary alloy system as follows:
aABC a a a ¼ mA AA þ mB BB þ mC CC ; ABC C11 C11 C11 C11
(3)
aABC a a a ¼ mA AA þ mB BB þ mC CC ; ABC C12 C12 C12 C12
(4)
aABC a a a ¼ mA AA þ mB BB þ mC CC ABC C44 C44 C44 C44
(5)
where A, B, and C represent metal, and ABC represents alloy. a, and m are lattice parameter, and mole fraction respectively. The subscript/superscript (A, B, C, and ABC) on lattice parameter, mole fraction, and elastic constants indicates the corresponding properties of metal/alloy. The elastic constants of Fee20Cre5Al alloy were calculated using equations (3)e(5) whereas the elastic modulus of Fee20Cre5Al alloy was calculated using equation (1). The elastic constants and elastic moduli of Fee20Cre5Al alloy are shown in Table 2 and it was found that the anisotropic factor of Fee20Cre5Al alloy is 1.36 that is close to one. This illustrates that the anisotropic behavior of iron (A ¼ 2.41) can be changed into isotropic after addition of an adequate amount of chromium and aluminum. WH plot of Fee20Cre5Al alloy powder exhibits that the anisotropic broadening tendency of iron tends toward isotropic after 15 h milling. Continuation of milling till 20 h did not exhibit any significant change in peak broadening as compare to 15 h. This indicates the complete dissolution of chromium and aluminum in iron after 15 h of milling. 3.3. Calculation of crystallite size and nature of dislocation Estimation of average crystallite size from WH plot requires drawing a linear fit for the data points in Fig. 2. However, the WH plot shows a considerable scattered data points up to 10 h of milling, rendering it is difficult to calculate crystallite size with sufficient accuracy. Therefore, the XRD data of milled powder was analyzed using MWH method proposed by Unger et al. [34]. The method can be expressed as follows:
R. Kumar et al. / Journal of Alloys and Compounds 671 (2016) 164e169
167
Table 2 Elastic constant, elastic modulus and anisotropic factor for Fe and Fee20Cre5Al alloy. Metal/Alloy
C11 (GPa)
C12 (GPa)
C44 (GPa)
Fe
237
141
116
Anisotropic factor A ¼ 2C44/(C11eC12)
2.42
Fee20Cre5Al
215.9
97.64
80.73 1.36
1=2 0:9 pM2 b2 1=2 DΚz þ r1=2 KChkl þ OðK2 CÞ dv 2
(6)
where K ¼ 2sinq/l, and DK ¼ (2Dq)cosq/l is caused by peak broadening, dV is crystallite size, M is a constant related to the effective outer cut-off radius of dislocations, r and b are the average dislocation density and Burgers vector of dislocation, respectively. O involves higher-order term that can be neglected, C is the contrast factor for dislocation, Ch00 is the average dislocation contrast factor for (h00) reflections, Chkl ¼ C h00 ð1 qH2 Þ is the average contrast factor of dislocation for (hkl) reflections, wherein q indicates the nature of dislocation in the system [35] and H2 ¼ (h2k2þk2l2þl2h2)/(h2þk2þl2)2. The theoretical value of q can be evaluated with the help of elastic constants as explained by Unger et al. [34]. For pure iron, the theoretical value of q are 2.67, 1.28 and 1.98 correspond to pure screw, pure edge and mixed (50% edge and 50% screw) dislocations respectively [34]. For calculating the q-value experimentally, MWH equation can be rearranged in term of contract factor as follows [34]:
h
ðDΚÞ2 a K2
i
¼ brC h00 1 qH2
(7)
where a ¼ (0.9/dv)2, and b ¼ pM2b2/2. The value of a in the above equation can be calculated by standard linear regression technique [36]. The standard linear regression technique provides a best linear fit for the plot of the left hand side of equation (7) verse H2. The experimental value of q would be given by x-intercept of the best-fit linear plot of equation (7), as shown in Fig. 3a for 2 h milled Fee20Cre5Al alloy powder. The q-value was calculated for different milling time and found that dislocations present in the
(hkl)
Elastic Modulus (GPa)
110 200 112 220 310 110 200 112 220 310
220.58 131.81 220.58 220.58 154.14 188.09 155.10 188.09 188.09 165.55
powders were changed into pure edge dislocation after ball milling (Fig. 3b). The MWH plots for milled Fee20Cre5Al alloy powder after 5 and 20 h of milling are shown in Fig. 4a. The crystallite size of the milled powder, as estimated from the intercept on the ordinate axis of the best linear fit line in the MWH plot, is shown in Fig. 4b. Initially, the crystallite size shows a rapid decline till about 5 h of milling. Subsequently, it shows only marginal drop and finally reaches about 13 nm after 20 h of milling. The dislocation density was calculated using the model given by Williamson and Smallman as follows [37]:
r¼
6pE ðε=bÞ2 G lnðr=r0 Þ
(8)
where E is Young's modulus (200.58 GPa for Fee20Cre5Al alloy), G is shear modulus (78.1 GPa for Fee20Cre5Al alloy), b (0.24824 nm) is Burgers vector and, r and r0 is the outer and inner cut-off radius respectively. Young modulus and shear modulus of Fee20Cre5Al alloy were calculated using rule of mixture [38]. The calculated dislocation density of the Fee20Cre5Al milled powder is shown in Fig. 4b. The dislocation density of milled Fee20Cre5Al alloy powder increases with increase in milling time and reaches about 1.3 1016 m2 after 20 h of milling.
3.4. Transmission electron microscopy The formation of nanocrystalline Fee20Cre5Al alloy powder after 20 h of milling is also confirmed by TEM studies (Fig. 5a). TEM image shows that the average crystallite size is less than 20 nm. SAED of 20 h ball milled Fee20Cre5Al alloy powder exhibits dotted ring pattern (Fig. 5b), which also indicates that the crystallite size is
Fig. 3. a) Calculation of experimental q-value for Fee20Cre5Al alloy powder after 2 h of milling, and b) effect of milling time on nature of dislocation developed in Fee20Cre5Al alloy powder after milling.
168
R. Kumar et al. / Journal of Alloys and Compounds 671 (2016) 164e169
Fig. 4. a) MWH plots (only 5 h and 20 h of milling), and b) effect of milling time on crystallite size and dislocation density of Fee20Cre5Al alloy powder.
Fig. 5. a) TEM image, b) SAED pattern, and c) High-resolution TEM image after 20 h milling of Fee20Cre5Al alloy powder, Inset shows the inverse FFT images of high-resolution TEM.
in the nanometer regime. The strong contrast variation, noted in the TEM image, indicates the presence of defects, like dislocations. The dislocations developed after 20 h of milling in the alloy powder are illustrated using high-resolution TEM image in Fig. 5c. The inverse Fast Fourier Transform (FFT) of the image at the locations A and B of Fig. 5c shows the existence of extra half plane, which indicates the development of edge dislocations after 20 h of milling. 3.5. Variation of lattice parameter with crystallite size The formation of nanocrystalline alloy powder during ball milling is associated with excess interfacial energy due to large
grain boundary fraction and excess free volume owing to the growth of vacancies, vacancy clusters, and dislocation density at grain boundaries [13]. The excess GB interfacial energy leads to contraction whereas excess free volume leads to expansion of the lattice [1,13,14]. As a consequence of contraction and expansion, the relative change in lattice parameter with respect to crystallite size of pure metals, like Fe, Cu, Ni, W usually shows a non-monotonic behavior during milling [1,14]. However, the relative change in the lattice parameter of Fee20Cre5Al alloy powder in the present work monotonically increases with crystallite size (Fig. 6a). This variation can be explained with the help of the excess GB interfacial energy and the excess free volume at grain boundaries, which were
Fig. 6. Effect of crystallite size on a) change in lattice parameter, and b) excess interfacial energy (gExcess ) and excess free volume (DV) of Fee20Cre5Al alloy powder. gb
R. Kumar et al. / Journal of Alloys and Compounds 671 (2016) 164e169
calculated based on the models proposed by Nazarov et al. [39] and Chattopadhyaya et al. [40]. Qin et al. [13] have shown that the excess free volume at grain boundaries plays a major role in generating lattice distortion in compare to the excess GB interfacial energy and subsequently leads to lattice expansion. In case of Fee20Cre5Al alloy powder, the excess GB interfacial energy was found to decrease exponentially whereas the excess free volume at grain boundaries increased linearly with decrease in crystallite size (Fig. 6b). Thus, it is evident that the excess free volume dominates over the excess GB interfacial energy for Fee20Cre5Al alloy powder, which suggests the observed lattice expansion behavior during ball milling. 4. Conclusion Nanocrystalline Fee20Cre5Al alloy powder was synthesized using high-energy ball milling of elemental powders. The formation of solid solution of aluminum and chromium with iron was observed during ball milling. This led to change in the elastic modulus of iron and, consequently, the anisotropic behavior of iron has tended to transform into isotropic by addition of chromium and aluminum. In addition, it can say that 20 wt% Cr along with 5 wt% Al would not sufficient to change anisotropic behavior of iron into isotropic. XRD analysis of the milled powder using MWH technique illustrated a gradual drop in the average crystallite size to 13 nm and the development of pure edge dislocation in the nanocrystalline alloy powder with milling time. The crystallite size and nature of dislocation, estimated using XRD, for the alloy after 20 h of milling were also confirmed by TEM. A monotonous variation in lattice parameter was observed with crystallite size in nanocrystalline Fee20Cre5Al alloy powder.
[13]
[14]
[15]
[16]
[17] [18]
[19] [20] [21] [22] [23]
[24]
[25] [26]
[27]
References [28] [1] G.K. Rane, U. Welzel, S.R. Meka, E.J. Mittemeijer, Non-monotonic lattice parameter variation with crystallite size in nanocrystalline solids, Acta Mater 61 (2013) 4524e4533, http://dx.doi.org/10.1016/j.actamat.2013.04.021. [2] R.K. Khatirkar, B.S. Murty, Structural changes in iron powder during ball milling, Mater. Chem. Phys. 123 (2010) 247e253, http://dx.doi.org/10.1016/ j.matchemphys.2010.04.004. [3] H. Liu, W. Tang, Y. Wang, C. Liu, G. Xu, Z. Zheng, Structural evolutions of the Fee40Ale5Cr powders during mechanical alloying and subsequent heat treatment, J. Alloys Compd. 506 (2010) 963e968, http://dx.doi.org/10.1016/ j.jallcom.2010.07.130. [4] R.K. Singh Raman, R.K. Gupta, Oxidation resistance of nanocrystalline vis- a-vis microcrystalline FeeCr alloys, Corros. Sci. 51 (2009) 316e321, http:// dx.doi.org/10.1016/j.corsci.2008.10.020. [5] T.R. Malow, C.C. Koch, Grain growth in nanocrystalline iron prepared by mechanical attrition, Acta Mater 45 (1997) 2177e2186, http://dx.doi.org/ 10.1016/S1359-6454(96)00300-X. [6] T.D. Shen, C.C. Koch, T.Y. Tsui, G.M. Pharr, On the elastic moduli of nanocrystalline Fe, Cu, Ni, and Cu-Ni alloys prepared by mechanical milling/ alloying, J. Mater. Res. 10 (1995) 2892e2896. [7] A.A. Karimpoor, U. Erb, K.T. Aust, G. Palumbo, High strength nanocrystalline cobalt with high tensile ductility, Scr. Mater 49 (2003) 651e656, http:// dx.doi.org/10.1016/S1359-6462(03)00397-X. [8] S. Cheng, E. Ma, Y.M. Wang, L.J. Kecskes, K.M. Youssef, C.C. Koch, et al., Tensile properties of in situ consolidated nanocrystalline Cu, Acta Mater 53 (2005) 1521e1533, http://dx.doi.org/10.1016/j.actamat.2004.12.005. [9] H. Wang, P. La, X. Liu, Y. Wei, T. Shi, Effect of annealing on microstructure and mechanical properties of large dimensional bulk nanocrystalline FeeAleCr alloy prepared by aluminothermic reaction, Mater. Sci. Eng. A 582 (2013) 1e7, http://dx.doi.org/10.1016/j.msea.2013.05.074. [10] D.W. Lee, J.H. Yu, T.S. Jang, B.K. Kim, Nanocrystalline iron particles synthesized by chemical vapor condensation without chilling, Mater. Lett. 59 (2005) 2124e2127, http://dx.doi.org/10.1016/j.matlet.2005.02.045. [11] R.K. Gupta, R.K. Singh Raman, C.C. Koch, Grain growth behaviour and consolidation of ball-milled nanocrystalline Fee10Cr alloy, Mater. Sci. Eng. A 494 (2008) 253e256, http://dx.doi.org/10.1016/j.msea.2008.04.019. [12] J. Eckert, J.C. Holzer, C.E. Krill, W.L. Johnson, Structural and thermodynamic
[29] [30]
[31] [32] [33] [34]
[35]
[36] [37]
[38]
[39]
[40]
169
properties of nanocrystalline fcc metals prepared by mechanical attrition, J. Mater. Res. 7 (1992) 1751e1761. W. Qin, T. Nagase, Y. Umakoshi, J.A. Szpunar, Lattice distortion and its effects on physical properties of nanostructured materials, J. Phys. Condens. Matter 19 (2007) 236217, http://dx.doi.org/10.1088/0953-8984/19/23/236217. A.K. Srivastav, N. Chawake, B.S. Murty, Grain size dependent non-monotonic lattice parameter variation in nanocrystalline W : the role of nonequilibrium grain boundary structure, Scr. Mater 98 (2015) 20e23, http:// dx.doi.org/10.1016/j.scriptamat.2014.11.005. C. Menapace, I. Lonardelli, M. Tait, A. Molinari, Nanostructured/ultrafine multiphase steel with enhanced ductility obtained by mechanical alloying and spark plasma sintering of powders, Mater. Sci. Eng. A 517 (2009) 1e7, http:// dx.doi.org/10.1016/j.msea.2009.03.021. S. Libardi, M. Zadra, F. Casari, A. Molinari, Mechanical properties of nanostructured and ultrafine-grained iron alloys produced by spark plasma sintering of ball milled powders, Mater. Sci. Eng. A 478 (2008) 243e250, http:// dx.doi.org/10.1016/j.msea.2007.05.111. C. Solliard, M. Flueli, Surfaces stress and size effect on the lattice parameter in small particles of gold and platinum, Surf. Sci. 156 (1985) 487e494. Y.H. Zhao, H.W. Sheng, K. Lu, Microstructure evolution and thermal properties in nanocrystalline Fe during mechanical attrition, Acta Mater 49 (2001) 365e375. K. Lu, Y.H. Zhao, Experimental evidences of lattice distortion in nanocrystalline materials, Nanostructured Mater. 12 (1999) 559e562. H.H. Tian, M. Atzmon, Kinetics of microstructure evolution in nanocrystalline Fe powder during mechanical attrition, Acta Mater 47 (1999) 1255e1261. €nig, G. Ne nert, The HighScore suite, Powder T. Degen, M. Sadki, E. Bron, U. Ko Diffr. 29 (2014) S13eS18, http://dx.doi.org/10.1017/S0885715614000840. T. Ida, M. Ando, H. Toraya, Extended pseudo-Voigt function for approximating the Voigt profile, J. Appl. Crystallogr. 33 (2000) 1311e1316. H.G. Jiang, H.M. Hu, E.J. Lavernia, Synthesis of Fe-rich FeeAl nanocrystalline solid solutions using ball milling, J. Mater. Res. 14 (2011) 1760e1770, http:// dx.doi.org/10.1557/JMR.1999.0238. C. Capdevila, M.K. Miller, I. Toda, J. Chao, Influence of the aea0 phase separation on the tensile properties of Fe-base ODS PM 2000 alloy, Mater. Sci. Eng. A 527 (2010) 7931e7938, http://dx.doi.org/10.1016/j.msea.2010.08.083. G.E. Dieter, Mechanical Metallurgy, SI Matrix, McGraw-Hill, Landon, New York, 1988. che, Structural and microstructural study of F.Z. Bentayeb, S. Alleg, J.M. Grene Fee31Cre12Co mixture prepared by ball milling, J. Alloys Compd. 434e435 (2007) 477e480, http://dx.doi.org/10.1016/j.jallcom.2006.08.111. J.S.C. Jang, C.C. Koch, The Hall-Petch relationship in nanocrystalline iron produced by ball milling, Scr. Metall. Mateialia. 24 (1990) 1599e1604. r, A. Borbe ly, J. Lendvai, Dislocations and grain size in ball A. Revesz, T. Unga milled iron powder, Nanostructured Mater 7 (1996) 779e788. H.M. Ledbetter, R.P. Reed, Elastics properties of metals and alloys, I. iron, nickel and iron-nickel alloys, J. Phys. Chem. 2 (1973) 531e617. ly, Dislocations and grain size in electrodeposited T. Ungar, A. Revesz, A. Borbe nanocrystalline Ni determined by the modified Williamson-Hall and WarrenAverbach procedures, J. Appl. Crystallogr. 31 (1998) 554e558. G.K. Williamson, W.H. Hall, X-ray line broadening from filed aluminum and wolfrom, Acta Metall. 1 (1953) 22e31. J. Roesler, H. Harders, M. Baeker, Mechanical Behaviour of Engineering Materials, first ed., Springer-Verlag Berlin Heidelberg, 2007. A.K. Giri, Prediction of elastic constants of substitutional alloys, Mater. Lett. 17 (1993) 353e356. Re r, I. Dragomir, A. v e sz, A. Borbe ly, The contrast factors of dislocations T. Unga in cubic crystals: the dislocation model of strain anisotropy in practice, J. Appl. Crystallogr. 32 (1999) 992e1002, http://dx.doi.org/10.1107/ S0021889899009334. S.R. Aghdaee, V. Soleimanian, Dislocations, crystallite size, and planar faults in nanocrystalline ceria, Powder Diffr. 24 (2009) 228e233, http://dx.doi.org/ 10.1154/1.3187210. P.R. Bevington, D.K. Robinson, Data Reduction and Error Analysis for the Physical Sciences, third ed., McGraw-Hill, New York, USA, 1969. G.K. Williamson, R.E. Smallman III, Dislocation densities in some annealed and cold-worked metals from measurements on the X-ray Debye-Scherrer spectrum, Philos. Mag. 1 (1956) 34e46, http://dx.doi.org/10.1080/ 14786435608238074. S.Y. Chang, C.E. Li, Y.C. Huang, H.F. Hsu, J.W. Yeh, S.J. Lin, Structural and thermodynamic factors of suppressed interdiffusion kinetics in multicomponent high-entropy materials, Sci. Rep. 4 (2014) 4162, http:// dx.doi.org/10.1038/srep04162. A.A. Nazarov, A.E. Romanov, R.Z. Valiev, Random disclination ensembles in ultrafine-grained materials produced by severe plastic deformation, Scr. Mater 34 (1996) 729e734. P.P. Chattopadhyay, P.M.G. Nambissan, S.K. Pabi, I. Manna, Polymorphic BCC to FCC transformation of nanocrystalline niobium studied by positron annihilation, Phys. Rev. B 63 (2001), http://dx.doi.org/10.1103/PhysRevB.63.054107, 054107/1e7.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of chromium and aluminum addition on anisotropic and microstructural characteristics of ball milled nanocrystalline iron Rajiv Kumar a, b, c, *, Joydip Joardar e, R.K. Singh Raman c, d, V.S. Raja b, S.V. Joshi e, S. Parida b a
IITB-Monash Research Academy, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India Department of Mechanical and Aerospace Engineering, Monash University, VIC 3800 Australia d Department of Chemical Engineering, Monash University, VIC 3800 Australia e International Advanced Research Centre for Powder Metallurgy and New Materials, Hyderabad, India b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 July 2015 Received in revised form 10 January 2016 Accepted 10 February 2016 Available online 12 February 2016
Prior studies on synthesis of nanocrystalline elements have discussed the effect of ball milling on lattice parameter, crystallite size, and micro-strain. For elemental milled powders, the anisotropic peak broadening does not change with increasing milling time. However, the effect of alloying addition on the anisotropic behavior of ball milled nanocrystalline powders remains an unexplored area. Here we report the effect of chromium and aluminum addition on the anisotropic behavior of iron in nanocrystalline Fe e20Cre5Al (wt%) alloy powders synthesized by ball milling. The experimental results show that the anisotropic behavior of iron changes towards isotropic with milling. This change was also correlated to the theoretically calculated anisotropic factor from the change in elastic constant of iron due to milling. Addition of alloying elements exhibited a monotonic rise in the lattice parameter with crystallite size, which was attributed to the excess grain boundary interfacial energy and excess free volume at grain boundaries. Transmission electron microscopy image confirmed the crystallite size and nature of dislocation obtained using modified Williamson-Hall method. © 2016 Elsevier B.V. All rights reserved.
Keywords: Mechanical alloying X-ray diffraction Nanocrystalline materials Anisotropic behavior Dislocation density Lattice parameter
1. Introduction In recent years, nanocrystalline materials are being widely investigated due to unique properties like mechanical, physical, and corrosion resistance [1e6]. These materials have been developed by electro-deposition [7], severe plastic deformation [8], aluminoethermic reaction [9], chemical vapor deposition [10], and high-energy ball milling [4,11] process. Among all these techniques, high-energy ball milling is widely employed to synthesize artifactfree bulk nanocrystalline materials [1,2,12e14]. During high-energy ball milling, high impact force is imposed into the powder which induces change in crystallite size and lattice parameter, development of micro-strain, dislocation density, excess free volume and excess grain boundary interfacial energy [1,13,14]. All these structural changes play a significant role during consolidation and also
* Corresponding author. IITB-Monash Research Academy, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India. E-mail address: [email protected] (R. Kumar). http://dx.doi.org/10.1016/j.jallcom.2016.02.096 0925-8388/© 2016 Elsevier B.V. All rights reserved.
alter physical [15], mechanical [16] and corrosion [4] properties of materials after consolidation. Nanocrystalline materials, produced by severe plastic deformation such as high-energy ball milling, have shown nonmonotonous changes in lattice parameter with crystallite size, an initial lattice contraction followed by a lattice expansion [1,13,14,17]. Qin et al. [13] have proposed that the non-monotonous change in lattice parameter depends on the equilibrium between interfacial stresses developed due to excess grain boundary (GB) interfacial energy and the excess free volume at grain boundaries. The increase in lattice parameter is manifested as a shift in x-ray diffraction (XRD) peak to lower angle, while the reduction in crystallite size and the development of micro-strain during milling are reflected by peak broadening. XRD peak broadening of iron during milling is a non-monotonous function of diffraction angle which is demonstration of anisotropic behavior of iron [2]. Such anisotropic behavior is correlated to the unequal elastic modulus of iron in different crystallographic directions. Accordingly, Williamson-Hall (WH) plot for milled iron powder exhibits
R. Kumar et al. / Journal of Alloys and Compounds 671 (2016) 164e169
anisotropic broadening of iron, which does not change with increase of milling time [2]. Synthesis of nanocrystalline elemental iron, nickel, copper, and tungsten has discussed the effect of ball milling on lattice parameter, crystallite size and micro-strain [1,2,13,14,18e20]. However, the effect of alloying addition on anisotropic behavior of ball milled nanocrystalline ferrous system remains an unexplored area. The effect of chromium and aluminum addition on anisotropic behavior of iron in nanocrystalline Fee20Cre5Al (wt%) alloy powder synthesized by ball milling is reported in the present study. The WH technique was used to verify the anisotropic behavior of the alloy whereas the crystallite size was determined by modified Williamson-Hall (MWH) method. The change in lattice parameter of alloy with its crystallite size was calculated and this change was correlated with the excess GB interfacial energy and the excess free volume developed in the alloy at grain boundaries owing to nanocrystallization.
2. Experimental procedure
165
3. Results and discussion 3.1. X-ray diffraction of milled powder Fig. 1a shows XRD patterns of ball milled Fee20Cre5Al powder blends at different stages of milling. It is evident from Fig. 1a that the intensity of major aluminum peak (2q ¼ 38.47) shows a gradual drop with progress of milling and disappearance by end of 5 h of milling. Such an observation is attributed to the formation of a solid solution of chromium and aluminum with iron. Iron and aluminum have a tendency to form intermetallic phases like FeAl and Fe3Al during ball milling [23]. However, such intermetallic peaks were not detected in the XRD pattern, plausibly because the aluminum content (~5 wt %) was too low to enable their formation. Careful analysis of the XRD patterns in Fig. 1a also reveals a gradual shift of peak positions toward lower diffraction angle with increase in milling time. The observed peak shift indicates an increase in lattice parameter. The lattice expansion occurs due to dissolution of aluminum and chromium into iron to form an alloy, shown in Fig. 1b. The lattice parameter for Fee20Cre5Al alloy after 20 h milling is 0.28851 nm which is comparable with the lattice parameter reported by Capdevila et al. [24].
2.1. Materials and methods The nanocrystalline Fee20Cre5Al alloy powder was prepared using high-energy planetary ball mill (Pulverisette P-5; Fritsch GmbH, Germany). The starting materials, iron powder (99.9% purity and average particle size < 37 mm) and chromium powder (99.9% purity and average particle size < 28 mm) were obtained from Alfa Aesar while aluminum powder (99.9% purity and average particle size < 1.3 mm) was procured from Hunan Jinhao Aluminum Industrial Co. Ltd, China. As-received powders were milled for composition Fee20Cre5Al at 250 rpm for durations up to 20 h, using toluene as the inert medium. The ball-to-powder weight ratio was maintained at 10:1. Mill was periodically stopped for 15 min after every 1 h of milling to avoid excessive temperature of the tungsten carbide vessel. Powder samples were collected at 2, 5, 10, 15, and 20 h of milling to characterize the microstructural changes during milling. As received iron powder was annealed at 900 C for 10 h into a convention tubular furnace (Model No. HCS/HTF/1000/ 90-900, Supplier: Heat & Control Systems, Mumbai, India) of capacity 1000 C and the forming gas (95% Ar þ 5% H2) was purged throughout the annealing process. The XRD line broadening of the fully annealed powder was considered as an instrumental broadening. The whole XRD analysis of Fee20Cre5Al alloy powder was done after considering this as an instrumental broadening.
2.2. Characterization of milled powder The microstructural changes in iron at different milling times were determined by XRD, using RigakueSmart Lab system (Rigaku Corp, Japan) with Cu Ka radiation (l ¼ 0.154056 nm), with a step size of 0.01 in 2q range of 20e120 . HighScore Plus (version 3.0d) [21] was used for multiple peak fitting of XRD patterns of the milled powder using the pseudo-Voigt function [22] and corresponding peak broadening was calculated after considering the instrumental broadening. The extent of peak broadening and change in peak position were analyzed to estimate crystallite size, dislocation density, lattice parameter, change in excess GB interfacial energy, and excess free volume at grain boundaries. Nanocrystalline Fee20Cre5Al alloy powder was formed after 20 h of milling that was confirmed using transmission electron microscopy, TEM (JOEL JEM-2100F), and selected area electron diffraction (SAED).
3.2. Analysis of anisotropic behavior of iron The grain refinement and development of micro-strain/lattice distortion, due to defects, leads to XRD peak broadening as visible in Fig. 1a. The micro-strain/lattice distortion developed along any crystallographic direction depends on the elastic modulus along that direction [25]. The lattice distortion is favorable along the crystallographic direction with lower elastic modulus as it requires less energy to develop defects [26]. Consequently, in an anisotropic material like Fe, more broadening is expected along the crystallographic direction with lowest elastic modulus. The elastic modulus of pure iron has identical value of 221 GPa in [100], [112] and [220] crystallographic directions, whereas the elastic moduli in [200] and [310] crystallographic directions are 132 GPa and 154 GPa, respectively [25]. Many authors [2,5,27,28] have reported significant broadening along [200] and [310] directions in iron powder after ball milling. This indicates that the peak broadening in milled iron powder is not a monotonous function of diffraction angle, possibly due to its distinct elastic modulus in different crystallographic directions. The elastic modulus of iron in the crystallographic direction can be reduced after addition of chromium and aluminum, since the elastic constant of iron [25] is higher that of chromium [29] and aluminum [25], shown in Table 1. The elastic modulus (E) of BCC system depends on either stiffness (C11, C12, C44) or compliance (S11, S12, S44) constant of crystal and direction cosines (l, m, n) of reflection, and can be calculated using following formula [25].
1 1 ¼ S11 2 ðS11 S12 Þ S44 l2 m2 þ m2 n2 þ n2 l2 E 2
(1)
The change in elastic modulus of iron due to alloying addition can be ascertained using WH technique. Considering classical WH method, the total peak broadening due to crystallite size (dV) and micro-strain (ε) are related by following expression [30,31]:
DK ¼
0:9 2sinq þ 2ε dV l
(2)
where K ¼ 2sinq/l, and DK ¼ (2Dq)cosq/l is caused by peak broadening. WH plots for Fee20Cre5Al alloy powder for different milling time are shown in Fig. 2. It is evident from Fig. 2 that the
166
R. Kumar et al. / Journal of Alloys and Compounds 671 (2016) 164e169
Fig. 1. Effect of milling time on a) x-ray diffraction pattern of Fee20Cre5Al alloy powder, and b) lattice parameter of Fee20Cre5Al alloy powder.
Table 1 Stiffness and compliance constant of iron, chromium, and aluminum. Metal
C11 (GPa)
C12 (GPa)
C44 (GPa)
S11(TPa1)
S12(TPa1)
S44(TPa1)
Iron Chromium Aluminum
237.0 339.8 108.2
141.0 58.6 61.3
116.0 99.0 28.5
7.587 3.090 15.659
2.830 0.441 5.663
8.621 10.101 35.088
Fig. 2. WH plot for ball-milled Fee20Cre5Al alloy powder.
broadening along [200] and [310] directions shows an abrupt rise up to 5 h of milling, in compare to other directions. The broadening of each peak in the diffraction pattern of Fee20Cre5Al alloy powder shows a gradual increase with increase in milling time and follows a similar anisotropic broadening trend as that reported in the case of pure iron up to 5 h of milling [2]. This indicates that the elastic modulus of iron, in Fee20Cre5Al blend, has not been changed significantly up to 5 h of milling. However, on further increase in milling time, the XRD peaks exhibit faster broadening along [112] and [310] directions. The change of elastic modulus with increase in milling time indicates that the anisotropic nature of iron tends towards isotropic with increase in milling time. The effect of alloying element addition on anisotropic behavior of iron can be explained by anisotropic factor, A ¼ 2C44/(C11eC12) [32]. A model was proposed by Giri [33] for calculating the elastic constant (stiffness constant) for binary alloy system. Based on the model of Giri, the elastic constants (C11, C12, and C44) were derived for ternary alloy system as follows:
aABC a a a ¼ mA AA þ mB BB þ mC CC ; ABC C11 C11 C11 C11
(3)
aABC a a a ¼ mA AA þ mB BB þ mC CC ; ABC C12 C12 C12 C12
(4)
aABC a a a ¼ mA AA þ mB BB þ mC CC ABC C44 C44 C44 C44
(5)
where A, B, and C represent metal, and ABC represents alloy. a, and m are lattice parameter, and mole fraction respectively. The subscript/superscript (A, B, C, and ABC) on lattice parameter, mole fraction, and elastic constants indicates the corresponding properties of metal/alloy. The elastic constants of Fee20Cre5Al alloy were calculated using equations (3)e(5) whereas the elastic modulus of Fee20Cre5Al alloy was calculated using equation (1). The elastic constants and elastic moduli of Fee20Cre5Al alloy are shown in Table 2 and it was found that the anisotropic factor of Fee20Cre5Al alloy is 1.36 that is close to one. This illustrates that the anisotropic behavior of iron (A ¼ 2.41) can be changed into isotropic after addition of an adequate amount of chromium and aluminum. WH plot of Fee20Cre5Al alloy powder exhibits that the anisotropic broadening tendency of iron tends toward isotropic after 15 h milling. Continuation of milling till 20 h did not exhibit any significant change in peak broadening as compare to 15 h. This indicates the complete dissolution of chromium and aluminum in iron after 15 h of milling. 3.3. Calculation of crystallite size and nature of dislocation Estimation of average crystallite size from WH plot requires drawing a linear fit for the data points in Fig. 2. However, the WH plot shows a considerable scattered data points up to 10 h of milling, rendering it is difficult to calculate crystallite size with sufficient accuracy. Therefore, the XRD data of milled powder was analyzed using MWH method proposed by Unger et al. [34]. The method can be expressed as follows:
R. Kumar et al. / Journal of Alloys and Compounds 671 (2016) 164e169
167
Table 2 Elastic constant, elastic modulus and anisotropic factor for Fe and Fee20Cre5Al alloy. Metal/Alloy
C11 (GPa)
C12 (GPa)
C44 (GPa)
Fe
237
141
116
Anisotropic factor A ¼ 2C44/(C11eC12)
2.42
Fee20Cre5Al
215.9
97.64
80.73 1.36
1=2 0:9 pM2 b2 1=2 DΚz þ r1=2 KChkl þ OðK2 CÞ dv 2
(6)
where K ¼ 2sinq/l, and DK ¼ (2Dq)cosq/l is caused by peak broadening, dV is crystallite size, M is a constant related to the effective outer cut-off radius of dislocations, r and b are the average dislocation density and Burgers vector of dislocation, respectively. O involves higher-order term that can be neglected, C is the contrast factor for dislocation, Ch00 is the average dislocation contrast factor for (h00) reflections, Chkl ¼ C h00 ð1 qH2 Þ is the average contrast factor of dislocation for (hkl) reflections, wherein q indicates the nature of dislocation in the system [35] and H2 ¼ (h2k2þk2l2þl2h2)/(h2þk2þl2)2. The theoretical value of q can be evaluated with the help of elastic constants as explained by Unger et al. [34]. For pure iron, the theoretical value of q are 2.67, 1.28 and 1.98 correspond to pure screw, pure edge and mixed (50% edge and 50% screw) dislocations respectively [34]. For calculating the q-value experimentally, MWH equation can be rearranged in term of contract factor as follows [34]:
h
ðDΚÞ2 a K2
i
¼ brC h00 1 qH2
(7)
where a ¼ (0.9/dv)2, and b ¼ pM2b2/2. The value of a in the above equation can be calculated by standard linear regression technique [36]. The standard linear regression technique provides a best linear fit for the plot of the left hand side of equation (7) verse H2. The experimental value of q would be given by x-intercept of the best-fit linear plot of equation (7), as shown in Fig. 3a for 2 h milled Fee20Cre5Al alloy powder. The q-value was calculated for different milling time and found that dislocations present in the
(hkl)
Elastic Modulus (GPa)
110 200 112 220 310 110 200 112 220 310
220.58 131.81 220.58 220.58 154.14 188.09 155.10 188.09 188.09 165.55
powders were changed into pure edge dislocation after ball milling (Fig. 3b). The MWH plots for milled Fee20Cre5Al alloy powder after 5 and 20 h of milling are shown in Fig. 4a. The crystallite size of the milled powder, as estimated from the intercept on the ordinate axis of the best linear fit line in the MWH plot, is shown in Fig. 4b. Initially, the crystallite size shows a rapid decline till about 5 h of milling. Subsequently, it shows only marginal drop and finally reaches about 13 nm after 20 h of milling. The dislocation density was calculated using the model given by Williamson and Smallman as follows [37]:
r¼
6pE ðε=bÞ2 G lnðr=r0 Þ
(8)
where E is Young's modulus (200.58 GPa for Fee20Cre5Al alloy), G is shear modulus (78.1 GPa for Fee20Cre5Al alloy), b (0.24824 nm) is Burgers vector and, r and r0 is the outer and inner cut-off radius respectively. Young modulus and shear modulus of Fee20Cre5Al alloy were calculated using rule of mixture [38]. The calculated dislocation density of the Fee20Cre5Al milled powder is shown in Fig. 4b. The dislocation density of milled Fee20Cre5Al alloy powder increases with increase in milling time and reaches about 1.3 1016 m2 after 20 h of milling.
3.4. Transmission electron microscopy The formation of nanocrystalline Fee20Cre5Al alloy powder after 20 h of milling is also confirmed by TEM studies (Fig. 5a). TEM image shows that the average crystallite size is less than 20 nm. SAED of 20 h ball milled Fee20Cre5Al alloy powder exhibits dotted ring pattern (Fig. 5b), which also indicates that the crystallite size is
Fig. 3. a) Calculation of experimental q-value for Fee20Cre5Al alloy powder after 2 h of milling, and b) effect of milling time on nature of dislocation developed in Fee20Cre5Al alloy powder after milling.
168
R. Kumar et al. / Journal of Alloys and Compounds 671 (2016) 164e169
Fig. 4. a) MWH plots (only 5 h and 20 h of milling), and b) effect of milling time on crystallite size and dislocation density of Fee20Cre5Al alloy powder.
Fig. 5. a) TEM image, b) SAED pattern, and c) High-resolution TEM image after 20 h milling of Fee20Cre5Al alloy powder, Inset shows the inverse FFT images of high-resolution TEM.
in the nanometer regime. The strong contrast variation, noted in the TEM image, indicates the presence of defects, like dislocations. The dislocations developed after 20 h of milling in the alloy powder are illustrated using high-resolution TEM image in Fig. 5c. The inverse Fast Fourier Transform (FFT) of the image at the locations A and B of Fig. 5c shows the existence of extra half plane, which indicates the development of edge dislocations after 20 h of milling. 3.5. Variation of lattice parameter with crystallite size The formation of nanocrystalline alloy powder during ball milling is associated with excess interfacial energy due to large
grain boundary fraction and excess free volume owing to the growth of vacancies, vacancy clusters, and dislocation density at grain boundaries [13]. The excess GB interfacial energy leads to contraction whereas excess free volume leads to expansion of the lattice [1,13,14]. As a consequence of contraction and expansion, the relative change in lattice parameter with respect to crystallite size of pure metals, like Fe, Cu, Ni, W usually shows a non-monotonic behavior during milling [1,14]. However, the relative change in the lattice parameter of Fee20Cre5Al alloy powder in the present work monotonically increases with crystallite size (Fig. 6a). This variation can be explained with the help of the excess GB interfacial energy and the excess free volume at grain boundaries, which were
Fig. 6. Effect of crystallite size on a) change in lattice parameter, and b) excess interfacial energy (gExcess ) and excess free volume (DV) of Fee20Cre5Al alloy powder. gb
R. Kumar et al. / Journal of Alloys and Compounds 671 (2016) 164e169
calculated based on the models proposed by Nazarov et al. [39] and Chattopadhyaya et al. [40]. Qin et al. [13] have shown that the excess free volume at grain boundaries plays a major role in generating lattice distortion in compare to the excess GB interfacial energy and subsequently leads to lattice expansion. In case of Fee20Cre5Al alloy powder, the excess GB interfacial energy was found to decrease exponentially whereas the excess free volume at grain boundaries increased linearly with decrease in crystallite size (Fig. 6b). Thus, it is evident that the excess free volume dominates over the excess GB interfacial energy for Fee20Cre5Al alloy powder, which suggests the observed lattice expansion behavior during ball milling. 4. Conclusion Nanocrystalline Fee20Cre5Al alloy powder was synthesized using high-energy ball milling of elemental powders. The formation of solid solution of aluminum and chromium with iron was observed during ball milling. This led to change in the elastic modulus of iron and, consequently, the anisotropic behavior of iron has tended to transform into isotropic by addition of chromium and aluminum. In addition, it can say that 20 wt% Cr along with 5 wt% Al would not sufficient to change anisotropic behavior of iron into isotropic. XRD analysis of the milled powder using MWH technique illustrated a gradual drop in the average crystallite size to 13 nm and the development of pure edge dislocation in the nanocrystalline alloy powder with milling time. The crystallite size and nature of dislocation, estimated using XRD, for the alloy after 20 h of milling were also confirmed by TEM. A monotonous variation in lattice parameter was observed with crystallite size in nanocrystalline Fee20Cre5Al alloy powder.
[13]
[14]
[15]
[16]
[17] [18]
[19] [20] [21] [22] [23]
[24]
[25] [26]
[27]
References [28] [1] G.K. Rane, U. Welzel, S.R. Meka, E.J. Mittemeijer, Non-monotonic lattice parameter variation with crystallite size in nanocrystalline solids, Acta Mater 61 (2013) 4524e4533, http://dx.doi.org/10.1016/j.actamat.2013.04.021. [2] R.K. Khatirkar, B.S. Murty, Structural changes in iron powder during ball milling, Mater. Chem. Phys. 123 (2010) 247e253, http://dx.doi.org/10.1016/ j.matchemphys.2010.04.004. [3] H. Liu, W. Tang, Y. Wang, C. Liu, G. Xu, Z. Zheng, Structural evolutions of the Fee40Ale5Cr powders during mechanical alloying and subsequent heat treatment, J. Alloys Compd. 506 (2010) 963e968, http://dx.doi.org/10.1016/ j.jallcom.2010.07.130. [4] R.K. Singh Raman, R.K. Gupta, Oxidation resistance of nanocrystalline vis- a-vis microcrystalline FeeCr alloys, Corros. Sci. 51 (2009) 316e321, http:// dx.doi.org/10.1016/j.corsci.2008.10.020. [5] T.R. Malow, C.C. Koch, Grain growth in nanocrystalline iron prepared by mechanical attrition, Acta Mater 45 (1997) 2177e2186, http://dx.doi.org/ 10.1016/S1359-6454(96)00300-X. [6] T.D. Shen, C.C. Koch, T.Y. Tsui, G.M. Pharr, On the elastic moduli of nanocrystalline Fe, Cu, Ni, and Cu-Ni alloys prepared by mechanical milling/ alloying, J. Mater. Res. 10 (1995) 2892e2896. [7] A.A. Karimpoor, U. Erb, K.T. Aust, G. Palumbo, High strength nanocrystalline cobalt with high tensile ductility, Scr. Mater 49 (2003) 651e656, http:// dx.doi.org/10.1016/S1359-6462(03)00397-X. [8] S. Cheng, E. Ma, Y.M. Wang, L.J. Kecskes, K.M. Youssef, C.C. Koch, et al., Tensile properties of in situ consolidated nanocrystalline Cu, Acta Mater 53 (2005) 1521e1533, http://dx.doi.org/10.1016/j.actamat.2004.12.005. [9] H. Wang, P. La, X. Liu, Y. Wei, T. Shi, Effect of annealing on microstructure and mechanical properties of large dimensional bulk nanocrystalline FeeAleCr alloy prepared by aluminothermic reaction, Mater. Sci. Eng. A 582 (2013) 1e7, http://dx.doi.org/10.1016/j.msea.2013.05.074. [10] D.W. Lee, J.H. Yu, T.S. Jang, B.K. Kim, Nanocrystalline iron particles synthesized by chemical vapor condensation without chilling, Mater. Lett. 59 (2005) 2124e2127, http://dx.doi.org/10.1016/j.matlet.2005.02.045. [11] R.K. Gupta, R.K. Singh Raman, C.C. Koch, Grain growth behaviour and consolidation of ball-milled nanocrystalline Fee10Cr alloy, Mater. Sci. Eng. A 494 (2008) 253e256, http://dx.doi.org/10.1016/j.msea.2008.04.019. [12] J. Eckert, J.C. Holzer, C.E. Krill, W.L. Johnson, Structural and thermodynamic
[29] [30]
[31] [32] [33] [34]
[35]
[36] [37]
[38]
[39]
[40]
169
properties of nanocrystalline fcc metals prepared by mechanical attrition, J. Mater. Res. 7 (1992) 1751e1761. W. Qin, T. Nagase, Y. Umakoshi, J.A. Szpunar, Lattice distortion and its effects on physical properties of nanostructured materials, J. Phys. Condens. Matter 19 (2007) 236217, http://dx.doi.org/10.1088/0953-8984/19/23/236217. A.K. Srivastav, N. Chawake, B.S. Murty, Grain size dependent non-monotonic lattice parameter variation in nanocrystalline W : the role of nonequilibrium grain boundary structure, Scr. Mater 98 (2015) 20e23, http:// dx.doi.org/10.1016/j.scriptamat.2014.11.005. C. Menapace, I. Lonardelli, M. Tait, A. Molinari, Nanostructured/ultrafine multiphase steel with enhanced ductility obtained by mechanical alloying and spark plasma sintering of powders, Mater. Sci. Eng. A 517 (2009) 1e7, http:// dx.doi.org/10.1016/j.msea.2009.03.021. S. Libardi, M. Zadra, F. Casari, A. Molinari, Mechanical properties of nanostructured and ultrafine-grained iron alloys produced by spark plasma sintering of ball milled powders, Mater. Sci. Eng. A 478 (2008) 243e250, http:// dx.doi.org/10.1016/j.msea.2007.05.111. C. Solliard, M. Flueli, Surfaces stress and size effect on the lattice parameter in small particles of gold and platinum, Surf. Sci. 156 (1985) 487e494. Y.H. Zhao, H.W. Sheng, K. Lu, Microstructure evolution and thermal properties in nanocrystalline Fe during mechanical attrition, Acta Mater 49 (2001) 365e375. K. Lu, Y.H. Zhao, Experimental evidences of lattice distortion in nanocrystalline materials, Nanostructured Mater. 12 (1999) 559e562. H.H. Tian, M. Atzmon, Kinetics of microstructure evolution in nanocrystalline Fe powder during mechanical attrition, Acta Mater 47 (1999) 1255e1261. €nig, G. Ne nert, The HighScore suite, Powder T. Degen, M. Sadki, E. Bron, U. Ko Diffr. 29 (2014) S13eS18, http://dx.doi.org/10.1017/S0885715614000840. T. Ida, M. Ando, H. Toraya, Extended pseudo-Voigt function for approximating the Voigt profile, J. Appl. Crystallogr. 33 (2000) 1311e1316. H.G. Jiang, H.M. Hu, E.J. Lavernia, Synthesis of Fe-rich FeeAl nanocrystalline solid solutions using ball milling, J. Mater. Res. 14 (2011) 1760e1770, http:// dx.doi.org/10.1557/JMR.1999.0238. C. Capdevila, M.K. Miller, I. Toda, J. Chao, Influence of the aea0 phase separation on the tensile properties of Fe-base ODS PM 2000 alloy, Mater. Sci. Eng. A 527 (2010) 7931e7938, http://dx.doi.org/10.1016/j.msea.2010.08.083. G.E. Dieter, Mechanical Metallurgy, SI Matrix, McGraw-Hill, Landon, New York, 1988. che, Structural and microstructural study of F.Z. Bentayeb, S. Alleg, J.M. Grene Fee31Cre12Co mixture prepared by ball milling, J. Alloys Compd. 434e435 (2007) 477e480, http://dx.doi.org/10.1016/j.jallcom.2006.08.111. J.S.C. Jang, C.C. Koch, The Hall-Petch relationship in nanocrystalline iron produced by ball milling, Scr. Metall. Mateialia. 24 (1990) 1599e1604. r, A. Borbe ly, J. Lendvai, Dislocations and grain size in ball A. Revesz, T. Unga milled iron powder, Nanostructured Mater 7 (1996) 779e788. H.M. Ledbetter, R.P. Reed, Elastics properties of metals and alloys, I. iron, nickel and iron-nickel alloys, J. Phys. Chem. 2 (1973) 531e617. ly, Dislocations and grain size in electrodeposited T. Ungar, A. Revesz, A. Borbe nanocrystalline Ni determined by the modified Williamson-Hall and WarrenAverbach procedures, J. Appl. Crystallogr. 31 (1998) 554e558. G.K. Williamson, W.H. Hall, X-ray line broadening from filed aluminum and wolfrom, Acta Metall. 1 (1953) 22e31. J. Roesler, H. Harders, M. Baeker, Mechanical Behaviour of Engineering Materials, first ed., Springer-Verlag Berlin Heidelberg, 2007. A.K. Giri, Prediction of elastic constants of substitutional alloys, Mater. Lett. 17 (1993) 353e356. Re r, I. Dragomir, A. v e sz, A. Borbe ly, The contrast factors of dislocations T. Unga in cubic crystals: the dislocation model of strain anisotropy in practice, J. Appl. Crystallogr. 32 (1999) 992e1002, http://dx.doi.org/10.1107/ S0021889899009334. S.R. Aghdaee, V. Soleimanian, Dislocations, crystallite size, and planar faults in nanocrystalline ceria, Powder Diffr. 24 (2009) 228e233, http://dx.doi.org/ 10.1154/1.3187210. P.R. Bevington, D.K. Robinson, Data Reduction and Error Analysis for the Physical Sciences, third ed., McGraw-Hill, New York, USA, 1969. G.K. Williamson, R.E. Smallman III, Dislocation densities in some annealed and cold-worked metals from measurements on the X-ray Debye-Scherrer spectrum, Philos. Mag. 1 (1956) 34e46, http://dx.doi.org/10.1080/ 14786435608238074. S.Y. Chang, C.E. Li, Y.C. Huang, H.F. Hsu, J.W. Yeh, S.J. Lin, Structural and thermodynamic factors of suppressed interdiffusion kinetics in multicomponent high-entropy materials, Sci. Rep. 4 (2014) 4162, http:// dx.doi.org/10.1038/srep04162. A.A. Nazarov, A.E. Romanov, R.Z. Valiev, Random disclination ensembles in ultrafine-grained materials produced by severe plastic deformation, Scr. Mater 34 (1996) 729e734. P.P. Chattopadhyay, P.M.G. Nambissan, S.K. Pabi, I. Manna, Polymorphic BCC to FCC transformation of nanocrystalline niobium studied by positron annihilation, Phys. Rev. B 63 (2001), http://dx.doi.org/10.1103/PhysRevB.63.054107, 054107/1e7.