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Influence of the V content on microstructure and hardness of high-energy ball milled nanocrystalline Al-V alloys
Journal of Alloys and Compounds 760 (2018) 63e70
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Influence of the V content on microstructure and hardness of high-energy ball milled nanocrystalline Al-V alloys J. Esquivel, R.K. Gupta* Department of Chemical and Biomolecular Engineering, Corrosion Engineering Program, The University of Akron, Akron, OH, 44325, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 January 2018 Received in revised form 9 May 2018 Accepted 11 May 2018 Available online 14 May 2018
Nanocrystalline Al-xV (x ¼ 0, 0.5, 2, 5, 10 and 20 at. %) alloys were produced by high-energy ball milling (HEBM) followed by consolidation at room temperature under uniaxial pressure of 3 GPa. Grain size, dispersion of the alloying element (V) and formation of solid solution were studied using X-ray diffraction (XRD) analysis and scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscopy (EDXS). High-energy ball milling imparted nanocrystalline structure and high solid solubility of V in Al - several orders of magnitude higher than the thermodynamically predicted value. Hardness of Al-xV alloys, which increased with increasing the V content, was higher than the commercial Al alloys. High hardness of the Al-xV alloys was attributed to the solid solution strengthening caused by high solid solubility of V and grain refinement <100 nm. © 2018 Elsevier B.V. All rights reserved.
Keywords: Aluminum alloys Mechanical alloying Nanocrystalline alloys Solid solution strengthening
1. Introduction Properties, including mechanical and corrosion properties, of commercial alloys are limited by the capabilities of the conventional alloy production/processing methods in designing a desired microstructure [1e3]. New manufacturing technologies along with new alloy compositions are being sought to meet the current demand of advanced materials with improved properties [1e3]. Highenergy ball milling (HEBM) or mechanical alloying (MA), originally developed for the synthesis of oxide-dispersion strengthened (ODS) nickel and iron-base superalloys for applications in aerospace industry [4e6], has been vindicated to be capable of synthesizing a variety of equilibrium and non-equilibrium materials including nanocrystalline alloys, supersaturated solid solutions, intermetallics, high entropy alloys and composites [7e12]. The properties of the high-energy ball milled alloys have been reported to be distinct and often superior from those produced by conventional casting route [4e9]. The yield strength of high-energy ball milled Mg [13,14], Al [15,16], Cu [17,18], Ti [19] and Fe [20,21] based alloys has been reported to be higher than that of commercial alloys of similar chemical composition. Compressive yield strength of Al20 wt%Cr [22] and Al-5wt.%Fe [15] alloys produced by HEBM and subsequent spark plasma sintering was reported to be 1104 and
* Corresponding author. E-mail address: [email protected] (R.K. Gupta). https://doi.org/10.1016/j.jallcom.2018.05.132 0925-8388/© 2018 Elsevier B.V. All rights reserved.
1059 MPa respectively, which is higher than the yield strength of any commercial Al alloy. High temperature oxidation resistance [23,24] and aqueous corrosion resistance [25,26] of Fe-Cr alloys produced by HEBM were superior to their microcrystalline counterpart. Similarly, nanocrystalline structure caused by HEBM has been shown effective in improving corrosion resistance of Al [27,28] and Mg based alloys [29]. High-energy ball milling has been a promising technique in the endeavor of developing new alloys with superior properties. For instance, the tradeoff between yield strength and corrosion resistance in commercial Al alloys poses numerous challenges and limits their engineering applications. Our previous work has demonstrated that HEBM along with suitable alloy composition circumvented this tradeoff and imparted simultaneous improvement in corrosion resistance and mechanical properties in Al alloys [22,27,28]. In pursuit of developing lightweight alloys with revolutionary properties, several binary Al alloys (Al-5at.%M; M: Mo, V, Nb, Cr, Ni, Si, Ti, Mn), exhibiting excellent corrosion resistance and yield strength were produced by HEBM [28]. X-ray diffraction analysis revealed the grain refinement <50 nm and high solid solubility of the alloying elements. Scanning electron microscopy was used to study the microstructure of the alloys. Uniform distribution of the alloying elements in the matrix and absence of secondary phases were observed in Al-5at.%V, Al-5at.%Ni, Al-5at.%Nb, Al-5at.%Ti, and Al-5at.%Si alloys. Fine particles of unalloyed alloying elements (<2 mm) were observed in Al-5at.%Mo, Al-5at.%Cr, and Al-5at.%Mn
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alloys [28]. The corrosion resistance and hardness of the highenergy ball milled Al-5at.% M alloys were superior to any commercial Al alloy, which was attributed to the combined influence of the high solid solubility of the alloying elements and grain refinement [28]. Among the tested alloys, Al-V, Al-Mo and Al-Nb showed the best combination of corrosion resistance and hardness [28]. The properties of the alloys can be further increased by optimizing the content of the alloying elements. Investigating high-energy ball milled Al-V alloys becomes of particular interest due to the high efficiency of V in enhancing corrosion and mechanical properties [28]. Therefore, there is merit in optimizing alloy composition and processing parameters for Al-V alloys to further improve properties and study underlying mechanisms. This paper presents an investigation on the influence of V content on the microstructure and hardness of high-energy ball milled Al-xV alloys. The grain size and solid solubility of the alloys, determined using X-ray diffraction analysis, were correlated with the hardness. Strengthening mechanisms were investigated using empirical relationships.
2. Experimental 2.1. Alloy synthesis and consolidation AlexV alloys with a nominal V content of 0, 0.5, 2, 5 10 and 20 at.% were prepared via HEBM. Starting materials, appropriate amounts of Al powder (99.70% purity and particle size ~300 mm) and V powder (99.95% and particle size <50 mm), were loaded in hardened stainless steel jars with hardened stainless steel balls (10 mm in diameter). Stearic acid (1.5 wt%) was used as process controlling agent (PCA). Steel jars were loaded and sealed inside a glove box (high purity Ar atmosphere, O2 < 25 ppm). HEBM was performed in a planetary ball mill at a speed of 280 RPM for 100 h. The ball-to-powder weight ratio was 16:1. The milling was interrupted for 30 min after every one hour of milling. Upon completion of milling, the jars were taken inside the glove box to recover the alloy powder in the Ar atmosphere. Alloy powders were subsequently cold compacted using an auto-pellet press in a tungsten carbide die under a uniaxial pressure of 3 GPa.
2.3. Hardness Vickers hardness was measured using a Wilson Tukon 1202 Vickers hardness tester. Samples were ground until 1200 grit SiC sandpaper before testing. The applied load was 25 g and dwelling time of the indenter was 10 s. For each alloy, a total of 10 indentations were applied. Distance between the adjacent indentations was kept at least 6 times the length of a single indentation. The hardness reported in this paper is an average of at least 10 measurements. 3. Results 3.1. Scanning electron microscopy (SEM) and energy dispersive Xray spectroscopy (EDXS) Back scatter electron images of the high-energy ball milled and subsequently cold compacted Al-xV alloys are presented in Fig. 1. Neither coarse intermetallics nor unalloyed V particles were observed by SEM, which divulged the ability of HEBM in causing uniform dispersion of V in the matrix. Externally fine (<1 mm) bright particles were noticed in the high-magnification images (insets in Fig. 1). EDXS analysis performed on the bright particles detected Fe and Cr which are constituents of the milling media. Abrasion of the milling media was conjectured to be the source of the fine bright particles. EDXS analysis performed on the matrix of each alloy revealed the V content to be close to the V added in the starting of the milling. Presence of the added V in the matrix and absence of V rich phases after HEBM are indicators of the formation of Al-V solid solution. Significant porosity was observed in Al-10 at.%V and Al-20 at.%V alloys, which was attributed to the high hardness of these alloys, and therefore applied consolidation pressure (3 GPa) was inadequate for removing the pores. Porosity was estimated by analyzing the SEM images using an image analyzing software. The porosity increased with increasing V content (Fig. 2). The theoretical density of the Al-xV alloys was approximated using a rule-of-mixtures. The estimated density and porosity are presented in Fig. 2. The density of the alloys increased with increasing V content in the alloy and reached to 3.61 g/cm3 for Al-20 at.%V alloy. 3.2. X-ray diffraction (XRD) analysis
2.2. Characterization X-ray diffraction (XRD) analysis was used to investigate the phases formed, solid solubility of V, and grain size. XRD was performed on as milled powders using a Cu K-alpha radiation (l ¼ 0.1541). Scan rate was 1 /minute and step size was 0.02 . For Al-20 at.%V, a high resolution XRD scan was also performed using a scan rate of 0.02 /min in the selected narrow 2q range. The grain size was calculated using Scherrer's equation after subtracting the instrumentals broadening [27,28,30]. Peak shift was used to calculate the lattice parameter which was correlated to the amount of V present in the solid solution. Procedure for calculating grain size and solid solubility is discussed in Refs. [28,31,32]. Sample preparation for the SEM characterization involved polishing up to 0.05 mm diamond suspension followed by ultrasonic cleaning in ethanol for 5 min. SEM was performed in a Tescan Lyra 3 FIB-FESEM in back scatter electron (BSE) and secondary electron (SE) modes at an accelerating voltage of 20 kV. Energy dispersive Xray spectroscopy (EDXS) analysis was also performed to analyze the composition of the different phases present in the structure. EDXS point analysis was performed at minimum of three point to characterize the chemical composition of any phase reported herein.
X-ray diffraction scans for the high-energy ball milled Al-xV are presented in Fig. 3. The diffraction peaks corresponding to various crystallographic orientations of Al matrix and Al3V intermetallics are indexed. With the exception of Al-20 at.%V, alloys did not show any additional peak of V or Al-V intermetallics, which further substantiated the observation from SEM that the added V has been incorporated into the solid solution. Increasing V content caused an increase in peak broadening and decrease in the peak intensity (Fig. 3). This indicated that grain refinement increased with increasing V content. Grain size, calculated using Scherrer equation after eliminating instrumental broadening, is presented in Fig. 4. The grain size decreased with increasing V content in the alloy. The increased grain refinement with increasing alloying content is reported in the literature for several other alloys produced by HEBM [12,33]. Suppression of recovery by increasing alloying content has been attributed to the faster grain refinement and smaller achievable grain size [12,34,35]. A zoomed-in region of the XRD scans for Al-xV alloys showing only peaks corresponding to (111) plane is presented in Fig. 3b. The peak shifts towards higher 2q values (Fig. 3b) with increasing V content. The effective atomic radius of V atom in Al-V solid solutions, estimated from first-principles calculations [36], has been
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Fig. 1. Back-scatter electron (BSE) images for high-energy ball milled a) Al, b) Al-0.5 at.%V, c) Al-2at.%V, d) Al-5at.%V, e) Al-10 at.%V and f) Al-20 at.%V alloys. Corresponding high magnification images are shown in the inset.
Fig. 2. Porosity and density of the Al-xV alloys as a function of V added to the alloy in the begining of HEBM. Porosity of the alloys was determined by analyzing SEM images. The density of the alloys was estimated by rule-of-mixtures.
reported to be 134.1 pm which is smaller than that of Al atom (142 pm). Therefore, increasing V content in the solid solution causes reduction in lattice parameters and therefore a peak shift towards higher 2q values. The peak shift was used to calculate the lattice parameters, which was correlated to the V content in solid solution [28]. Fig. 5 presents the solid solubility of the V in Al-xV alloys as a function of V added in the starting of the milling. XRD analysis showed that V content of the solid solution increased with increasing V to the alloy (Fig. 5). The solid solubility of V in Al-xV
alloys produced in this study was several orders of magnitude higher than the equilibrium solid solubility. The maximum equilibrium solubility of V in Al is reported to be 0.33 at.% at 662.10 C, which decreases to 5.22 107 at.% at 25 C [37,38]. The Al-xV alloys therefore exist as supersaturated solid solution. High energy imparted by the ball milling processes caused formation of such supersaturated solid solution. The solid solubility of V determined using XRD was lower than the V added during ball milling (Fig. 5). However, SEM analysis did not show significant unalloyed V particles or Al-V intermetallics. The V content determined using EDXS was very close to the total amount of V added in the starting of HEBM, evincing that the V added during ball milling was dispersed in the alloys and contained within the interaction volume for EDXS without forming coarse phases. X-ray diffraction analysis for Al-20 at.% V indicated presence of two phases - Al3V and super-saturated FCC Al phase in Al-20 at.% V (Fig. 3a). The lattice parameter of FCC Al phase was 3.9071 Å which was contracted by the presence of 18.01 at.% of vanadium substitutional atoms. The equilibrium phases, according to the phase diagram, in an Al 20 at.% V alloy should be Al23V4 and Al3V [39]. XRD peaks corresponding to the position and relative intensity of Al3V are present, although no evidence of Al23V4 was found. A high resolution XRD scan (0.02 /min) in the 2q range of 80e85 is presented in Fig. 6. Despite Al3V and super-saturated FCC Al overlapping at the highest intensity peaks, a clear super-saturated FCC (113) peak can be observed at 2q ¼ 81.67 - normally present at 78.23 for pure Al. No other Al-V intermetallic exhibits a peak in this position, which confirms the existence of a super-saturated FCC Al phase with 18.01 at.% V. Further analysis using advanced characterization techniques such as TEM will be required to detect
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Fig. 4. Grain size (determined from XRD analysis) vs V added to the alloys. The asterisks symbol (*) marks the estimated solid solubility.
Fig. 3. XRD scans for high-energy ball milled Al-xV alloys (x: 0, 0.5, 2, 5, 10 and 20 at.%). a) full XRD scan, b) zoomed-in region showing peak shift for Al (111).
presence of any amorphous and other crystalline phases and their distribution.
3.3. Hardness The Vickers hardness as a function of V content is presented in Fig. 7. Vickers hardness of high-energy ball milled Al-xCr [27], AlxMn [31] and Al-xFe [40] alloys are included for comparison. The yield strength as calculated by Tabor's rule (HV ¼ 3sy in MPa) is also presented on secondary y-axis [41]. Hardness of the ball milled Al in this study was 127 HV which is 102 HV higher than that of unmilled pure Al. The hardness increased with increasing V content and reached to 441.5 HV for Al-20 at.% V. The yield strength of AA7075-T651, a high strength Al alloy used widely in aircrafts, is reported to be ~575 MPa [42], which is lower than that estimated for Al-5at.% V, Al-10 at.% V and Al-20 at.% V alloys produced herein. The high hardness of the Al-xV alloys has been attributed to the grain boundary and solid solution strengthening, and is discussed in the next section.
Fig. 5. Solid solubility of V as determined from XRD analysis versus vanadium added during high-energy ball milling. The dashed line shows the linear behavior as if all the V were in the solid solution.
The specific yield strength of the Al-xV alongside some commercial alloys for comparison is presented in Fig. 8. The specific yield strength was calculated by dividing the estimate yield strength of the alloys by the density. The data for the commercial alloys was collected from the literature. Clearly, the specific yield strength of the alloys produced herein was dependent on the V content. Specific yield strength of the ball milled alloys containing 2 at. % V alloy was superior to that of many commercial alloys (Fig. 8). The yield strength of the alloys, presented in Fig. 7, was estimated form the Vickers hardness using Tabor's rule. This method of estimating yield strength from hardness has extensively been used in the literature [28,43,44]. However, it should be noted that hardness does not provide any measure of ductility and the predicted yield strength may not be precise. Therefore, tensile tests
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Fig. 8. Specific yield-strength of Al-xV alloys as produced by high-energy ball milling. Commercial alloys are included for comparison.
Fig. 6. High-resolution XRD scan, collected using a scan speed of 0.02 /minute, of Al20 at.%V alloy showing the appearance of a (113) diffraction peak correspond to FCC Al phase which contains 18.01 at.% V.
Fig. 9. Influence of V content on the equilibrium mole fraction of FCC Al and intermetallics phase at 25 C. The volume fractions were calculated using software PANDAT. Fig. 7. Vickers hardness and yield strength (Assuming Tabor's rule) of high energy ball milled Al-xV alloys in the presented as a function of total alloying content added in the begining of the HEBM. Data for alloying with Cr, Mn amd Fe from the literature, a) Gupta et al. [27], b) Darling et al. [31], and c) Nayak et al. [40], is also presented for the comparision.
should be performed to understand the mechanical properties these alloys. 4. Discussion Under equilibrium conditions, increasing V content in Al is expected to cause formation of intermetallic phases. The equilibrium volume fraction of various phases as a function of the V content at room temperature was calculated using software PANDAT [37] and is presented in Fig. 9. The content of FCC Al decreased and intermetallics increased with increasing V content in the alloy. Volume fraction FCC Al for Al-10 at.%V and Al-20 at.%V was predicted to be negligible (Fig. 9). Contrary to the thermodynamic predictions,
alloys presented herein displayed absence of intermetallics and formation of supersaturated solid solution. The solid solubility of V in Al-20 at.%V was determined to be 18.01 at.% which is 54.5 times higher than the highest solubility of V in Al predicted using software PANDAT [37]. The produced Al-xV alloys are supersaturated solid solution and should decompose upon exposure to elevated temperatures which can be exploited for improving strength by age hardening. Moreover, diffusion coefficient of V in Al is very low (4.24 1028 at 300 C [45]) which would cause sluggish coarsening of precipitates during high temperature exposure - making these alloys attractive for high temperature applications. Future research on studying the properties of the alloys at high temperature will provide further insight. The hardness of the high-energy ball milled Al-xV alloys increased with increasing V content. The high hardness of the alloys was attributed to the combined influences of the grain refinement [46,47] and extended solid solubility of V [48]. Orowan strengthening has a contribution in the alloys (such as precipitation
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hardened alloys and ODS alloys synthesized through mechanical alloying) which are strengthened by the distributing particles or dispersoids. However, literature suggests this contribution in highenergy ball milled alloys is minor [22,49,50]. Contribution from the grain size strengthening (sgb) can be calculated following the Hall-Petch equation [31,51]:
sgb ¼
k d1=2
(1)
where k is the Hall-Petch coefficient, and d is the grain size. Contribution from solid solution strengthening (sss) can be calculated using the following equation [52]:
sss ¼ HC a
(2)
where a and H are constants and C is the concentration of solute atoms in at.%. a was reported to be the 1 for many alloys, which can be used for the Al-xV alloys. Constant H is not known for V. The total yield strength of the alloys can be determined using equation [50,53]:
s ¼ so þ sgb þ sss
(3)
where so is the yield strength of pure Al, sgb is the yield strength contribution of grain boundaries, and sss is the yield strength contribution of solute atoms. Parameters used for various strengthening mechanisms (used in equations (1) and (2)) are not readily available and shows strong dependency on the grain size, and composition. Therefore, the determination of the contribution from individual strengthening mechanisms is challenging. In the present study, the high hardness of high-energy ball milled Al (without V addition) can be attributed to the grain refinement. The Hall-Petch coefficient for the highenergy ball milled Al was calculated to be 0.122 MPa m1/2 using equation (1). The calculated Hall-Petch coefficient for the ball Al milled is in good agreement with the literature where it is reported to vary from 0.012 to 0.32 MPa m1/2 [54e56]. For Al-0.5 at.%V, using equation (3) and Hall-Petch coefficient of 0.122 MPa m1/2, the value of H was calculated to be 68 MPa/at%, which is close to the H value for Cr (44.7 MPa/at%) [57] and Mn (54.8 MPa/at%) [52]. Using the estimated Hall- Petch coefficient (0.122 MPa m1/2) and H (68 MP/at%), grain size and solid solubility determined using XRD, the total yield strength was estimated using equation (3). The experimental yield strength (Fig. 7) was plotted against the estimated yield strength (from equation (3)) in Fig. 10. The estimated yield strength and experimental yield strength were in good agreement for up to 10 at.% V. The yield strength of Al-20 at.% V was much lower than the estimated value (using equation (3)), which was attributed to the inverse Hall-Petch relationship - softening caused the grain boundary sliding [58,59]. The inverse Hall-Petch relationship is a known phenomenon and has been subject of significant research [52e54]. It is suggested that inverse Hall-Petch phenomenon occurs when grain size is < 10 nm [60]. The grain size for Al- 20 at.% V alloy was estimated to be 9.3 nm (Fig. 5). The solid solution strengthening for Al-20 at.% V was estimated to be 1240 MPa which is 86% of the experimental yield strength. The strengthening due to the grain refinement (equation (3)) was 179 MPa and the Hall-Petch coefficient (k) decreased to 0.017 MPa m1/2. Such a softening and decrease in Hall-Petch coefficient has been reported for several materials in the literature [52e54]. The alloys produced herein contained a large fraction of V in the solid solution, which is expected to impart high corrosion resistance. Solid solution strengthening has been identified to cause
Fig. 10. Experimentally measured yield strength, extrapolated from the Vickers hardness vs estimated yield strength as calculated using equation 3. Composition of each of the alloy is marked.
simultaneous improvement in corrosion and mechanical properties [28]. Furthermore, a noticeable tendency to age harden has been observed in Al-5at.% V alloy in the temperature range of 200e300 C [38] while retaining the majority of supersaturation and grain stability, owing to the low diffusivity of V in Al. This opens up the possibility of further improving mechanical properties by the appropriate thermal treatment/consolidation technique. Further research on these alloys is expected to result in new Al alloys with excellent corrosion resistance and mechanical properties. The alloys produced by ball milling are in the powder form and development of suitable consolidation techniques for the production porosity free bulk components would open up new avenues for engineering application. One of the immediate applications of the alloys would be substitution of the conventional sintered components. The alloy powder can be used for additive manufacturing, coating or repairing using supersonic particle deposition.
5. Conclusions Nanocrystalline Al-xV alloys with V content varying from 0 to 20 at.% were produced successfully using high-energy ball milling followed by compaction. The characterization was performed using XRD, SEM/EDXS and hardness measurements. The main conclusions can be summarized as following: 1. X-ray diffraction analysis and SEM/EDXS revealed that HEBM caused grain refinement <100 nm, uniform dispersion of V in Al, and formation of supersaturated solid solution. No coarse intermetallic or unalloyed V particles were detected in the highenergy ball milled Al-xV alloys. 2. Grain refinement and solid solubility of V were influenced by the V content. The solid solubility of V increased with increasing V content and reached to 18.01 at.% for the Al-20 at.% alloy. Grain size decreased with increasing V content and reached to 9.3 nm for Al-20 at.% V. 3. The hardness of the Al-xV alloys varied from 127 to 441.5 HV and increased with the V content. Contribution of grain refinement and solid solution in strengthening the alloys was determined using empirical relationships reported in the literature. Solid
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solution strengthening increased with increasing V content in the alloy. 4. The yield strength (estimated from the hardness) of the alloys produced in this study was superior to that of commercial Al alloys. The high-energy ball milled Al-xV alloys are promising for further research.
Declarations of interest None. Acknowledgements The authors would like to thank Dr. Zhorro Nikolov, National Polymer Innovation Center facilities, The University of Akron, for his help in XRD experiments. References [1] K. Lu, The future of metals, Science, 328 (2010) 319e320. [2] C. Suryanarayana, C.C. Koch, Nanocrystalline materialseCurrent research and future directions, Hyperfine Interact. 130 (2000) 5e44. [3] T.G. Langdon, Twenty-five years of ultrafine-grained materials: achieving exceptional properties through grain refinement, Acta Mater. 61 (2013) 7035e7059. [4] J.S. Benjamin, Dispersion strengthened superalloys by mechanical alloying, Metall. Trans. 1 (1970) 2943e2951. [5] J.S. Benjamin, M.J. Bomford, Dispersion strengthened aluminum made by mechanical alloying, Metall. Mater. Trans. A 8 (1977) 1301e1305. [6] R.F. Singer, W.C. Oliver, W.D. Nix, Identification of dispersoid phases created in aluminum during mechanical alloying, Metall. Mater. Trans. A 11 (1980) 1895e1901. [7] C. Suryanarayana, Mechanical alloying and milling, Prog. Mater. Sci. 46 (2001) 1e184. [8] D.L. Zhang, Processing of advanced materials using high-energy mechanical milling, Prog. Mater. Sci. 49 (2004) 537e560. [9] C. Suryanarayana, N. Al-Aqeeli, Mechanically alloyed nanocomposites, Prog. Mater. Sci. 58 (2013) 383e502. [10] E. Ma, Alloys created between immiscible elements, Prog. Mater. Sci. 50 (2005) 413e509. [11] R.K. Gupta, F. Mirza, M.U.F. Khan, J. Esquivel, Aluminum containing Na2CrO4: inhibitor release on demand, Mater. Lett. 205 (2017) 194e197. [12] R.K. Gupta, B.S. Murty, N. Birbilis, An Overview of High-energy Ball Milled Nanocrystalline Aluminium Alloys, Springer International Publishing, 2017. s, Y.T. Zhu, [13] K.M. Youssef, Y.B. Wang, X.Z. Liao, S.N. Mathaudhu, L.J. Kecske C.C. Koch, High hardness in a nanocrystalline Mg 97 Y 2 Zn 1 alloy, Mater. Sci. Eng. A 528 (2011) 7494e7499. [14] M.Y. Song, Effects of mechanical alloying on the hydrogen storage characteristics of Mg-xwt% Ni (x¼ 0, 5, 10, 25 and 55) mixtures, Int. J. Hydrogen Energy 20 (1995) 221e227. [15] T.T. Sasaki, T. Ohkubo, K. Hono, Microstructure and mechanical properties of bulk nanocrystalline AleFe alloy processed by mechanical alloying and spark plasma sintering, Acta Mater. 57 (2009) 3529e3538. [16] R.W. Hayes, D. Witkin, F. Zhou, E.J. Lavernia, Deformation and activation volumes of cryomilled ultrafine-grained aluminum, Acta Mater. 52 (2004) 4259e4271. [17] D.G. Morris, M.A. Morris, Microstructure and strength of nanocrystalline copper alloy prepared by mechanical alloying, Acta Metall. Mater. 39 (1991) 1763e1770. [18] K.A. Darling, M. Rajagopalan, M. Komarasamy, M.A. Bhatia, B.C. Hornbuckle, R.S. Mishra, K.N. Solanki, Extreme creep resistance in a microstructurally stable nanocrystalline alloy, Nature 537 (2016) 378e381. [19] C. Suryanarayana, F.H. Froes, Mechanical alloying of titanium-base alloys, Adv. Mater. 5 (1993) 96e106. [20] J.C. Rawers, D. Govier, R. Doan, Nitrogen addition to iron powder by mechanical alloying, Mater. Sci. Eng. A 220 (1996) 162e167. [21] E. Bonetti, G. Scipione, G. Valdre, S. Enzo, R. Frattini, P.P. Macri, A study of nanocrystalline iron and aluminium metals and Fe 3 Al intermetallic by mechanical alloying, J. Mater. Sci. 30 (1995) 2220e2226. [22] R.K. Gupta, D. Fabijanic, T. Dorin, Y. Qiu, J.T. Wang, N. Birbilis, Simultaneous improvement in the strength and corrosion resistance of Al via high-energy ball milling and Cr alloying, Mater. Des. 84 (2015) 270e276. [23] R.K.S. Raman, R.K. Gupta, C.C. Koch, Resistance of nanocrystalline vis- a-vis microcrystalline FeeCr alloys to environmental degradation and challenges to their synthesis, Philos. Mag. 90 (2010) 3233e3260.
69
[24] R.K. Gupta, N. Birbilis, J. Zhang, Oxidation resistance of nanocrystalline alloys, in: Corros. Resist., IntechOpen, 2012, https://doi.org/10.5772/34928. [25] R.K. Gupta, R.K.S. Raman, C.C. Koch, B.S. Murty, Effect of nanocrystalline structure on the corrosion of a Fe-20Cr alloy, Int. J. Electrochem. Sci. 8 (2013) 6791e6806. [26] R.K. Gupta, N. Birbilis, The influence of nanocrystalline structure and processing route on corrosion of stainless steel: a review, Corros. Sci. 92 (2015) 1e15. [27] R.K. Gupta, D. Fabijanic, R. Zhang, N. Birbilis, Corrosion behaviour and hardness of in situ consolidated nanostructured Al and Al-Cr alloys produced via high-energy ball milling, Corrosion Sci. 98 (2015) 643e650. [28] J. Esquivel, H. Murdoch, K. Darling, R. Gupta, Excellent corrosion resistance and hardness in Al alloys by extended solid solubility and nanocrystalline structure, Mater. Res. Lett. 6 (2018) 79e83. [29] M.-H. Grosjean, M. Zidoune, L. Roue, J. Huot, R. Schulz, Effect of ball milling on the corrosion resistance of magnesium in aqueous media, Electrochim. Acta 49 (2004) 2461e2470. [30] B.D. Cullity, S.R. Stock, Element of X-ray Diffraction, AdditioneWesley, Addison-Wesley, Reading, MA, 1978. [31] K.A. Darling, A.J. Roberts, L. Armstrong, D. Kapoor, M.A. Tschopp, L.J. Kecskes, S.N. Mathaudhu, Influence of Mn solute content on grain size reduction and improved strength in mechanically alloyed Al e Mn alloys, Mater. Sci. Eng. A 589 (2014) 57e65. [32] A. Roth, An investigation of the aluminum-vanadium system, Zeitschrift Für Met. 32 (1940) 356e359. [33] D.K. Mukhopadhyay, C. Suryanarayana, F.H.S. Froes, Structural evolution in mechanically alloyed Al-Fe powders, Metall. Mater. Trans. A 26 (1995) 1939e1946. [34] J. Eckert, J.C. Holzer, C.E. Krill, W.L. Johnson, Structural and thermodynamic properties of nanocrystalline fcc metals prepared by mechanical attrition, J. Mater. Res. 7 (1992) 1751e1761. [35] C.C. Koch, Synthesis of nanostructured materials by mechanical milling: problems and opportunities, Nanostruct. Mater. 9 (1997) 13e22. [36] T. Uesugi, K. Higashi, First-principles studies on lattice constants and local lattice distortions in solid solution aluminum alloys, Comput. Mater. Sci. 67 (2013) 1e10. [37] W. Cao, S.-L. Chen, F. Zhang, K. Wu, Y. Yang, Y.A. Chang, R. Schmid-Fetzer, W.A. Oates, PANDAT software with PanEngine, PanOptimizer and PanPrecipitation for multi-component phase diagram calculation and materials property simulation, Calphad 33 (2009) 328e342. [38] J. Esquivel, M.G. Wachowiak, S. O'Brien, R.K. Gupta, Thermal stability of nanocrystalline Al-5at.% Ni and Al-5at.% V alloys produced by high-energy ball milling, J. Alloys Compd. 744 (2018) 651e657. [39] J.L. Murray, Al-V (aluminum-vanadium), Bull. Alloy Phase Diagrams 10 (1989) 351e357. [40] S.S. Nayak, M. Wollgarten, J. Banhart, S.K. Pabi, B.S. Murty, Nanocomposites and an extremely hard nanocrystalline intermetallic of AleFe alloys prepared by mechanical alloying, Mater. Sci. Eng. A 527 (2010) 2370e2378. [41] D. Tabor, The Hardness of Metals, Oxford university press, 2000. [42] I. Polmear, D.S. John, Light Alloys: from Traditional Alloys to Nanocrystals, Butterworth-Heinemann, Oxford, 2005. [43] G.W. Nieman, J.R. Weertman, R.W. Siegel, Mechanical behavior of nanocrystalline Cu and Pd, J. Mater. Res. 6 (1991) 1012e1027. [44] C.R. Bradbury, J.-K. Gomon, L. Kollo, H. Kwon, M. Leparoux, Hardness of multi wall carbon nanotubes reinforced aluminium matrix composites, J. Alloys Compd. 585 (2014) 362e367. [45] K.E. Knipling, D.C. Dunand, D.N. Seidman, Criteria for developing castable, creep-resistant aluminum-based alloyseA review, Zeitschrift Für Met. 97 (2006) 246e265. [46] G.E. Dieter, D.J. Bacon, Mechanical Metallurgy, McGraw-hill, New York, 1986. [47] R.W. Armstrong, Hallepetch Relationship: Use in Characterizing Properties of Aluminum and Aluminum Alloys, 2016. [48] T.J. Rupert, J.C. Trenkle, C.A. Schuh, Enhanced solid solution effects on the strength of nanocrystalline alloys, Acta Mater. 59 (2011) 1619e1631. [49] D.J. Bacon, U.F. Kocks, R.O. Scattergood, The effect of dislocation selfinteraction on the Orowan stress, Philos. Mag. 28 (1973) 1241e1263. [50] T.D. Shen, C.C. Koch, Formation, solid solution hardening and softening of nanocrystalline solid solutions prepared by mechanical attrition, Acta Mater. 44 (1996) 753e761. [51] N.N. Krishna, R. Tejas, K. Sivaprasad, K. Venkateswarlu, Study on cryorolled AleCu alloy using X-ray diffraction line profile analysis and evaluation of strengthening mechanisms, Mater. Des. 52 (2013) 785e790. €lander, H.-E. Ekstro €m, Strengthening [52] Ø. Ryen, B. Holmedal, O. Nijs, E. Nes, E. Sjo mechanisms in solid solution aluminum alloys, Metall. Mater. Trans. A 37 (2006) 1999e2006. [53] E. Hornbogen, E.A. Starke, Overview no. 102 Theory assisted design of high strength low alloy aluminum, Acta Metall. Mater. 41 (1993) 1e16. [54] D.B. Witkin, E.J. Lavernia, Synthesis and mechanical behavior of nanostructured materials via cryomilling, Prog. Mater. Sci. 51 (2006) 1e60. [55] E. Bonetti, L. Pasquini, E. Sampaolesi, The influence of grain size on the mechanical properties of nanocrystalline aluminium, Nanostruct. Mater. 9 (1997) 611e614.
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[56] H. Hasegawa, S. Komura, A. Utsunomiya, Z. Horita, M. Furukawa, M. Nemoto, T.G. Langdon, Thermal stability of ultrafine-grained aluminum in the presence of Mg and Zr additions, Mater. Sci. Eng. A 265 (1999) 188e196. [57] P.G. Partridge, M.C. Mcconnell, The effect of microstructure and composition on the properties of vapour quenched Al-Cr alloys-II. Tensile Properties, Acta Metall. 35 (1993) 1981e1993.
[58] J. Schiøtz, F.D. Di Tolla, K.W. Jacobsen, Softening of nanocrystalline metals at very small grain sizes, Nature 391 (1998) 561e563. [59] M.A. Meyers, A. Mishra, D.J. Benson, Mechanical properties of nanocrystalline materials, Prog. Mater. Sci. 51 (2006) 427e556. [60] T.G. Nieh, J. Wadsworth, Hall-Petch relation in nanocrystalline solids, Scr. Metall. Mater. 25 (1991) 955e958.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Influence of the V content on microstructure and hardness of high-energy ball milled nanocrystalline Al-V alloys J. Esquivel, R.K. Gupta* Department of Chemical and Biomolecular Engineering, Corrosion Engineering Program, The University of Akron, Akron, OH, 44325, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 January 2018 Received in revised form 9 May 2018 Accepted 11 May 2018 Available online 14 May 2018
Nanocrystalline Al-xV (x ¼ 0, 0.5, 2, 5, 10 and 20 at. %) alloys were produced by high-energy ball milling (HEBM) followed by consolidation at room temperature under uniaxial pressure of 3 GPa. Grain size, dispersion of the alloying element (V) and formation of solid solution were studied using X-ray diffraction (XRD) analysis and scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscopy (EDXS). High-energy ball milling imparted nanocrystalline structure and high solid solubility of V in Al - several orders of magnitude higher than the thermodynamically predicted value. Hardness of Al-xV alloys, which increased with increasing the V content, was higher than the commercial Al alloys. High hardness of the Al-xV alloys was attributed to the solid solution strengthening caused by high solid solubility of V and grain refinement <100 nm. © 2018 Elsevier B.V. All rights reserved.
Keywords: Aluminum alloys Mechanical alloying Nanocrystalline alloys Solid solution strengthening
1. Introduction Properties, including mechanical and corrosion properties, of commercial alloys are limited by the capabilities of the conventional alloy production/processing methods in designing a desired microstructure [1e3]. New manufacturing technologies along with new alloy compositions are being sought to meet the current demand of advanced materials with improved properties [1e3]. Highenergy ball milling (HEBM) or mechanical alloying (MA), originally developed for the synthesis of oxide-dispersion strengthened (ODS) nickel and iron-base superalloys for applications in aerospace industry [4e6], has been vindicated to be capable of synthesizing a variety of equilibrium and non-equilibrium materials including nanocrystalline alloys, supersaturated solid solutions, intermetallics, high entropy alloys and composites [7e12]. The properties of the high-energy ball milled alloys have been reported to be distinct and often superior from those produced by conventional casting route [4e9]. The yield strength of high-energy ball milled Mg [13,14], Al [15,16], Cu [17,18], Ti [19] and Fe [20,21] based alloys has been reported to be higher than that of commercial alloys of similar chemical composition. Compressive yield strength of Al20 wt%Cr [22] and Al-5wt.%Fe [15] alloys produced by HEBM and subsequent spark plasma sintering was reported to be 1104 and
* Corresponding author. E-mail address: [email protected] (R.K. Gupta). https://doi.org/10.1016/j.jallcom.2018.05.132 0925-8388/© 2018 Elsevier B.V. All rights reserved.
1059 MPa respectively, which is higher than the yield strength of any commercial Al alloy. High temperature oxidation resistance [23,24] and aqueous corrosion resistance [25,26] of Fe-Cr alloys produced by HEBM were superior to their microcrystalline counterpart. Similarly, nanocrystalline structure caused by HEBM has been shown effective in improving corrosion resistance of Al [27,28] and Mg based alloys [29]. High-energy ball milling has been a promising technique in the endeavor of developing new alloys with superior properties. For instance, the tradeoff between yield strength and corrosion resistance in commercial Al alloys poses numerous challenges and limits their engineering applications. Our previous work has demonstrated that HEBM along with suitable alloy composition circumvented this tradeoff and imparted simultaneous improvement in corrosion resistance and mechanical properties in Al alloys [22,27,28]. In pursuit of developing lightweight alloys with revolutionary properties, several binary Al alloys (Al-5at.%M; M: Mo, V, Nb, Cr, Ni, Si, Ti, Mn), exhibiting excellent corrosion resistance and yield strength were produced by HEBM [28]. X-ray diffraction analysis revealed the grain refinement <50 nm and high solid solubility of the alloying elements. Scanning electron microscopy was used to study the microstructure of the alloys. Uniform distribution of the alloying elements in the matrix and absence of secondary phases were observed in Al-5at.%V, Al-5at.%Ni, Al-5at.%Nb, Al-5at.%Ti, and Al-5at.%Si alloys. Fine particles of unalloyed alloying elements (<2 mm) were observed in Al-5at.%Mo, Al-5at.%Cr, and Al-5at.%Mn
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alloys [28]. The corrosion resistance and hardness of the highenergy ball milled Al-5at.% M alloys were superior to any commercial Al alloy, which was attributed to the combined influence of the high solid solubility of the alloying elements and grain refinement [28]. Among the tested alloys, Al-V, Al-Mo and Al-Nb showed the best combination of corrosion resistance and hardness [28]. The properties of the alloys can be further increased by optimizing the content of the alloying elements. Investigating high-energy ball milled Al-V alloys becomes of particular interest due to the high efficiency of V in enhancing corrosion and mechanical properties [28]. Therefore, there is merit in optimizing alloy composition and processing parameters for Al-V alloys to further improve properties and study underlying mechanisms. This paper presents an investigation on the influence of V content on the microstructure and hardness of high-energy ball milled Al-xV alloys. The grain size and solid solubility of the alloys, determined using X-ray diffraction analysis, were correlated with the hardness. Strengthening mechanisms were investigated using empirical relationships.
2. Experimental 2.1. Alloy synthesis and consolidation AlexV alloys with a nominal V content of 0, 0.5, 2, 5 10 and 20 at.% were prepared via HEBM. Starting materials, appropriate amounts of Al powder (99.70% purity and particle size ~300 mm) and V powder (99.95% and particle size <50 mm), were loaded in hardened stainless steel jars with hardened stainless steel balls (10 mm in diameter). Stearic acid (1.5 wt%) was used as process controlling agent (PCA). Steel jars were loaded and sealed inside a glove box (high purity Ar atmosphere, O2 < 25 ppm). HEBM was performed in a planetary ball mill at a speed of 280 RPM for 100 h. The ball-to-powder weight ratio was 16:1. The milling was interrupted for 30 min after every one hour of milling. Upon completion of milling, the jars were taken inside the glove box to recover the alloy powder in the Ar atmosphere. Alloy powders were subsequently cold compacted using an auto-pellet press in a tungsten carbide die under a uniaxial pressure of 3 GPa.
2.3. Hardness Vickers hardness was measured using a Wilson Tukon 1202 Vickers hardness tester. Samples were ground until 1200 grit SiC sandpaper before testing. The applied load was 25 g and dwelling time of the indenter was 10 s. For each alloy, a total of 10 indentations were applied. Distance between the adjacent indentations was kept at least 6 times the length of a single indentation. The hardness reported in this paper is an average of at least 10 measurements. 3. Results 3.1. Scanning electron microscopy (SEM) and energy dispersive Xray spectroscopy (EDXS) Back scatter electron images of the high-energy ball milled and subsequently cold compacted Al-xV alloys are presented in Fig. 1. Neither coarse intermetallics nor unalloyed V particles were observed by SEM, which divulged the ability of HEBM in causing uniform dispersion of V in the matrix. Externally fine (<1 mm) bright particles were noticed in the high-magnification images (insets in Fig. 1). EDXS analysis performed on the bright particles detected Fe and Cr which are constituents of the milling media. Abrasion of the milling media was conjectured to be the source of the fine bright particles. EDXS analysis performed on the matrix of each alloy revealed the V content to be close to the V added in the starting of the milling. Presence of the added V in the matrix and absence of V rich phases after HEBM are indicators of the formation of Al-V solid solution. Significant porosity was observed in Al-10 at.%V and Al-20 at.%V alloys, which was attributed to the high hardness of these alloys, and therefore applied consolidation pressure (3 GPa) was inadequate for removing the pores. Porosity was estimated by analyzing the SEM images using an image analyzing software. The porosity increased with increasing V content (Fig. 2). The theoretical density of the Al-xV alloys was approximated using a rule-of-mixtures. The estimated density and porosity are presented in Fig. 2. The density of the alloys increased with increasing V content in the alloy and reached to 3.61 g/cm3 for Al-20 at.%V alloy. 3.2. X-ray diffraction (XRD) analysis
2.2. Characterization X-ray diffraction (XRD) analysis was used to investigate the phases formed, solid solubility of V, and grain size. XRD was performed on as milled powders using a Cu K-alpha radiation (l ¼ 0.1541). Scan rate was 1 /minute and step size was 0.02 . For Al-20 at.%V, a high resolution XRD scan was also performed using a scan rate of 0.02 /min in the selected narrow 2q range. The grain size was calculated using Scherrer's equation after subtracting the instrumentals broadening [27,28,30]. Peak shift was used to calculate the lattice parameter which was correlated to the amount of V present in the solid solution. Procedure for calculating grain size and solid solubility is discussed in Refs. [28,31,32]. Sample preparation for the SEM characterization involved polishing up to 0.05 mm diamond suspension followed by ultrasonic cleaning in ethanol for 5 min. SEM was performed in a Tescan Lyra 3 FIB-FESEM in back scatter electron (BSE) and secondary electron (SE) modes at an accelerating voltage of 20 kV. Energy dispersive Xray spectroscopy (EDXS) analysis was also performed to analyze the composition of the different phases present in the structure. EDXS point analysis was performed at minimum of three point to characterize the chemical composition of any phase reported herein.
X-ray diffraction scans for the high-energy ball milled Al-xV are presented in Fig. 3. The diffraction peaks corresponding to various crystallographic orientations of Al matrix and Al3V intermetallics are indexed. With the exception of Al-20 at.%V, alloys did not show any additional peak of V or Al-V intermetallics, which further substantiated the observation from SEM that the added V has been incorporated into the solid solution. Increasing V content caused an increase in peak broadening and decrease in the peak intensity (Fig. 3). This indicated that grain refinement increased with increasing V content. Grain size, calculated using Scherrer equation after eliminating instrumental broadening, is presented in Fig. 4. The grain size decreased with increasing V content in the alloy. The increased grain refinement with increasing alloying content is reported in the literature for several other alloys produced by HEBM [12,33]. Suppression of recovery by increasing alloying content has been attributed to the faster grain refinement and smaller achievable grain size [12,34,35]. A zoomed-in region of the XRD scans for Al-xV alloys showing only peaks corresponding to (111) plane is presented in Fig. 3b. The peak shifts towards higher 2q values (Fig. 3b) with increasing V content. The effective atomic radius of V atom in Al-V solid solutions, estimated from first-principles calculations [36], has been
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65
Fig. 1. Back-scatter electron (BSE) images for high-energy ball milled a) Al, b) Al-0.5 at.%V, c) Al-2at.%V, d) Al-5at.%V, e) Al-10 at.%V and f) Al-20 at.%V alloys. Corresponding high magnification images are shown in the inset.
Fig. 2. Porosity and density of the Al-xV alloys as a function of V added to the alloy in the begining of HEBM. Porosity of the alloys was determined by analyzing SEM images. The density of the alloys was estimated by rule-of-mixtures.
reported to be 134.1 pm which is smaller than that of Al atom (142 pm). Therefore, increasing V content in the solid solution causes reduction in lattice parameters and therefore a peak shift towards higher 2q values. The peak shift was used to calculate the lattice parameters, which was correlated to the V content in solid solution [28]. Fig. 5 presents the solid solubility of the V in Al-xV alloys as a function of V added in the starting of the milling. XRD analysis showed that V content of the solid solution increased with increasing V to the alloy (Fig. 5). The solid solubility of V in Al-xV
alloys produced in this study was several orders of magnitude higher than the equilibrium solid solubility. The maximum equilibrium solubility of V in Al is reported to be 0.33 at.% at 662.10 C, which decreases to 5.22 107 at.% at 25 C [37,38]. The Al-xV alloys therefore exist as supersaturated solid solution. High energy imparted by the ball milling processes caused formation of such supersaturated solid solution. The solid solubility of V determined using XRD was lower than the V added during ball milling (Fig. 5). However, SEM analysis did not show significant unalloyed V particles or Al-V intermetallics. The V content determined using EDXS was very close to the total amount of V added in the starting of HEBM, evincing that the V added during ball milling was dispersed in the alloys and contained within the interaction volume for EDXS without forming coarse phases. X-ray diffraction analysis for Al-20 at.% V indicated presence of two phases - Al3V and super-saturated FCC Al phase in Al-20 at.% V (Fig. 3a). The lattice parameter of FCC Al phase was 3.9071 Å which was contracted by the presence of 18.01 at.% of vanadium substitutional atoms. The equilibrium phases, according to the phase diagram, in an Al 20 at.% V alloy should be Al23V4 and Al3V [39]. XRD peaks corresponding to the position and relative intensity of Al3V are present, although no evidence of Al23V4 was found. A high resolution XRD scan (0.02 /min) in the 2q range of 80e85 is presented in Fig. 6. Despite Al3V and super-saturated FCC Al overlapping at the highest intensity peaks, a clear super-saturated FCC (113) peak can be observed at 2q ¼ 81.67 - normally present at 78.23 for pure Al. No other Al-V intermetallic exhibits a peak in this position, which confirms the existence of a super-saturated FCC Al phase with 18.01 at.% V. Further analysis using advanced characterization techniques such as TEM will be required to detect
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Fig. 4. Grain size (determined from XRD analysis) vs V added to the alloys. The asterisks symbol (*) marks the estimated solid solubility.
Fig. 3. XRD scans for high-energy ball milled Al-xV alloys (x: 0, 0.5, 2, 5, 10 and 20 at.%). a) full XRD scan, b) zoomed-in region showing peak shift for Al (111).
presence of any amorphous and other crystalline phases and their distribution.
3.3. Hardness The Vickers hardness as a function of V content is presented in Fig. 7. Vickers hardness of high-energy ball milled Al-xCr [27], AlxMn [31] and Al-xFe [40] alloys are included for comparison. The yield strength as calculated by Tabor's rule (HV ¼ 3sy in MPa) is also presented on secondary y-axis [41]. Hardness of the ball milled Al in this study was 127 HV which is 102 HV higher than that of unmilled pure Al. The hardness increased with increasing V content and reached to 441.5 HV for Al-20 at.% V. The yield strength of AA7075-T651, a high strength Al alloy used widely in aircrafts, is reported to be ~575 MPa [42], which is lower than that estimated for Al-5at.% V, Al-10 at.% V and Al-20 at.% V alloys produced herein. The high hardness of the Al-xV alloys has been attributed to the grain boundary and solid solution strengthening, and is discussed in the next section.
Fig. 5. Solid solubility of V as determined from XRD analysis versus vanadium added during high-energy ball milling. The dashed line shows the linear behavior as if all the V were in the solid solution.
The specific yield strength of the Al-xV alongside some commercial alloys for comparison is presented in Fig. 8. The specific yield strength was calculated by dividing the estimate yield strength of the alloys by the density. The data for the commercial alloys was collected from the literature. Clearly, the specific yield strength of the alloys produced herein was dependent on the V content. Specific yield strength of the ball milled alloys containing 2 at. % V alloy was superior to that of many commercial alloys (Fig. 8). The yield strength of the alloys, presented in Fig. 7, was estimated form the Vickers hardness using Tabor's rule. This method of estimating yield strength from hardness has extensively been used in the literature [28,43,44]. However, it should be noted that hardness does not provide any measure of ductility and the predicted yield strength may not be precise. Therefore, tensile tests
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67
Fig. 8. Specific yield-strength of Al-xV alloys as produced by high-energy ball milling. Commercial alloys are included for comparison.
Fig. 6. High-resolution XRD scan, collected using a scan speed of 0.02 /minute, of Al20 at.%V alloy showing the appearance of a (113) diffraction peak correspond to FCC Al phase which contains 18.01 at.% V.
Fig. 9. Influence of V content on the equilibrium mole fraction of FCC Al and intermetallics phase at 25 C. The volume fractions were calculated using software PANDAT. Fig. 7. Vickers hardness and yield strength (Assuming Tabor's rule) of high energy ball milled Al-xV alloys in the presented as a function of total alloying content added in the begining of the HEBM. Data for alloying with Cr, Mn amd Fe from the literature, a) Gupta et al. [27], b) Darling et al. [31], and c) Nayak et al. [40], is also presented for the comparision.
should be performed to understand the mechanical properties these alloys. 4. Discussion Under equilibrium conditions, increasing V content in Al is expected to cause formation of intermetallic phases. The equilibrium volume fraction of various phases as a function of the V content at room temperature was calculated using software PANDAT [37] and is presented in Fig. 9. The content of FCC Al decreased and intermetallics increased with increasing V content in the alloy. Volume fraction FCC Al for Al-10 at.%V and Al-20 at.%V was predicted to be negligible (Fig. 9). Contrary to the thermodynamic predictions,
alloys presented herein displayed absence of intermetallics and formation of supersaturated solid solution. The solid solubility of V in Al-20 at.%V was determined to be 18.01 at.% which is 54.5 times higher than the highest solubility of V in Al predicted using software PANDAT [37]. The produced Al-xV alloys are supersaturated solid solution and should decompose upon exposure to elevated temperatures which can be exploited for improving strength by age hardening. Moreover, diffusion coefficient of V in Al is very low (4.24 1028 at 300 C [45]) which would cause sluggish coarsening of precipitates during high temperature exposure - making these alloys attractive for high temperature applications. Future research on studying the properties of the alloys at high temperature will provide further insight. The hardness of the high-energy ball milled Al-xV alloys increased with increasing V content. The high hardness of the alloys was attributed to the combined influences of the grain refinement [46,47] and extended solid solubility of V [48]. Orowan strengthening has a contribution in the alloys (such as precipitation
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hardened alloys and ODS alloys synthesized through mechanical alloying) which are strengthened by the distributing particles or dispersoids. However, literature suggests this contribution in highenergy ball milled alloys is minor [22,49,50]. Contribution from the grain size strengthening (sgb) can be calculated following the Hall-Petch equation [31,51]:
sgb ¼
k d1=2
(1)
where k is the Hall-Petch coefficient, and d is the grain size. Contribution from solid solution strengthening (sss) can be calculated using the following equation [52]:
sss ¼ HC a
(2)
where a and H are constants and C is the concentration of solute atoms in at.%. a was reported to be the 1 for many alloys, which can be used for the Al-xV alloys. Constant H is not known for V. The total yield strength of the alloys can be determined using equation [50,53]:
s ¼ so þ sgb þ sss
(3)
where so is the yield strength of pure Al, sgb is the yield strength contribution of grain boundaries, and sss is the yield strength contribution of solute atoms. Parameters used for various strengthening mechanisms (used in equations (1) and (2)) are not readily available and shows strong dependency on the grain size, and composition. Therefore, the determination of the contribution from individual strengthening mechanisms is challenging. In the present study, the high hardness of high-energy ball milled Al (without V addition) can be attributed to the grain refinement. The Hall-Petch coefficient for the highenergy ball milled Al was calculated to be 0.122 MPa m1/2 using equation (1). The calculated Hall-Petch coefficient for the ball Al milled is in good agreement with the literature where it is reported to vary from 0.012 to 0.32 MPa m1/2 [54e56]. For Al-0.5 at.%V, using equation (3) and Hall-Petch coefficient of 0.122 MPa m1/2, the value of H was calculated to be 68 MPa/at%, which is close to the H value for Cr (44.7 MPa/at%) [57] and Mn (54.8 MPa/at%) [52]. Using the estimated Hall- Petch coefficient (0.122 MPa m1/2) and H (68 MP/at%), grain size and solid solubility determined using XRD, the total yield strength was estimated using equation (3). The experimental yield strength (Fig. 7) was plotted against the estimated yield strength (from equation (3)) in Fig. 10. The estimated yield strength and experimental yield strength were in good agreement for up to 10 at.% V. The yield strength of Al-20 at.% V was much lower than the estimated value (using equation (3)), which was attributed to the inverse Hall-Petch relationship - softening caused the grain boundary sliding [58,59]. The inverse Hall-Petch relationship is a known phenomenon and has been subject of significant research [52e54]. It is suggested that inverse Hall-Petch phenomenon occurs when grain size is < 10 nm [60]. The grain size for Al- 20 at.% V alloy was estimated to be 9.3 nm (Fig. 5). The solid solution strengthening for Al-20 at.% V was estimated to be 1240 MPa which is 86% of the experimental yield strength. The strengthening due to the grain refinement (equation (3)) was 179 MPa and the Hall-Petch coefficient (k) decreased to 0.017 MPa m1/2. Such a softening and decrease in Hall-Petch coefficient has been reported for several materials in the literature [52e54]. The alloys produced herein contained a large fraction of V in the solid solution, which is expected to impart high corrosion resistance. Solid solution strengthening has been identified to cause
Fig. 10. Experimentally measured yield strength, extrapolated from the Vickers hardness vs estimated yield strength as calculated using equation 3. Composition of each of the alloy is marked.
simultaneous improvement in corrosion and mechanical properties [28]. Furthermore, a noticeable tendency to age harden has been observed in Al-5at.% V alloy in the temperature range of 200e300 C [38] while retaining the majority of supersaturation and grain stability, owing to the low diffusivity of V in Al. This opens up the possibility of further improving mechanical properties by the appropriate thermal treatment/consolidation technique. Further research on these alloys is expected to result in new Al alloys with excellent corrosion resistance and mechanical properties. The alloys produced by ball milling are in the powder form and development of suitable consolidation techniques for the production porosity free bulk components would open up new avenues for engineering application. One of the immediate applications of the alloys would be substitution of the conventional sintered components. The alloy powder can be used for additive manufacturing, coating or repairing using supersonic particle deposition.
5. Conclusions Nanocrystalline Al-xV alloys with V content varying from 0 to 20 at.% were produced successfully using high-energy ball milling followed by compaction. The characterization was performed using XRD, SEM/EDXS and hardness measurements. The main conclusions can be summarized as following: 1. X-ray diffraction analysis and SEM/EDXS revealed that HEBM caused grain refinement <100 nm, uniform dispersion of V in Al, and formation of supersaturated solid solution. No coarse intermetallic or unalloyed V particles were detected in the highenergy ball milled Al-xV alloys. 2. Grain refinement and solid solubility of V were influenced by the V content. The solid solubility of V increased with increasing V content and reached to 18.01 at.% for the Al-20 at.% alloy. Grain size decreased with increasing V content and reached to 9.3 nm for Al-20 at.% V. 3. The hardness of the Al-xV alloys varied from 127 to 441.5 HV and increased with the V content. Contribution of grain refinement and solid solution in strengthening the alloys was determined using empirical relationships reported in the literature. Solid
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solution strengthening increased with increasing V content in the alloy. 4. The yield strength (estimated from the hardness) of the alloys produced in this study was superior to that of commercial Al alloys. The high-energy ball milled Al-xV alloys are promising for further research.
Declarations of interest None. Acknowledgements The authors would like to thank Dr. Zhorro Nikolov, National Polymer Innovation Center facilities, The University of Akron, for his help in XRD experiments. References [1] K. Lu, The future of metals, Science, 328 (2010) 319e320. [2] C. Suryanarayana, C.C. Koch, Nanocrystalline materialseCurrent research and future directions, Hyperfine Interact. 130 (2000) 5e44. [3] T.G. Langdon, Twenty-five years of ultrafine-grained materials: achieving exceptional properties through grain refinement, Acta Mater. 61 (2013) 7035e7059. [4] J.S. Benjamin, Dispersion strengthened superalloys by mechanical alloying, Metall. Trans. 1 (1970) 2943e2951. [5] J.S. Benjamin, M.J. Bomford, Dispersion strengthened aluminum made by mechanical alloying, Metall. Mater. Trans. A 8 (1977) 1301e1305. [6] R.F. Singer, W.C. Oliver, W.D. Nix, Identification of dispersoid phases created in aluminum during mechanical alloying, Metall. Mater. Trans. A 11 (1980) 1895e1901. [7] C. Suryanarayana, Mechanical alloying and milling, Prog. Mater. Sci. 46 (2001) 1e184. [8] D.L. Zhang, Processing of advanced materials using high-energy mechanical milling, Prog. Mater. Sci. 49 (2004) 537e560. [9] C. Suryanarayana, N. Al-Aqeeli, Mechanically alloyed nanocomposites, Prog. Mater. Sci. 58 (2013) 383e502. [10] E. Ma, Alloys created between immiscible elements, Prog. Mater. Sci. 50 (2005) 413e509. [11] R.K. Gupta, F. Mirza, M.U.F. Khan, J. Esquivel, Aluminum containing Na2CrO4: inhibitor release on demand, Mater. Lett. 205 (2017) 194e197. [12] R.K. Gupta, B.S. Murty, N. Birbilis, An Overview of High-energy Ball Milled Nanocrystalline Aluminium Alloys, Springer International Publishing, 2017. s, Y.T. Zhu, [13] K.M. Youssef, Y.B. Wang, X.Z. Liao, S.N. Mathaudhu, L.J. Kecske C.C. Koch, High hardness in a nanocrystalline Mg 97 Y 2 Zn 1 alloy, Mater. Sci. Eng. A 528 (2011) 7494e7499. [14] M.Y. Song, Effects of mechanical alloying on the hydrogen storage characteristics of Mg-xwt% Ni (x¼ 0, 5, 10, 25 and 55) mixtures, Int. J. Hydrogen Energy 20 (1995) 221e227. [15] T.T. Sasaki, T. Ohkubo, K. Hono, Microstructure and mechanical properties of bulk nanocrystalline AleFe alloy processed by mechanical alloying and spark plasma sintering, Acta Mater. 57 (2009) 3529e3538. [16] R.W. Hayes, D. Witkin, F. Zhou, E.J. Lavernia, Deformation and activation volumes of cryomilled ultrafine-grained aluminum, Acta Mater. 52 (2004) 4259e4271. [17] D.G. Morris, M.A. Morris, Microstructure and strength of nanocrystalline copper alloy prepared by mechanical alloying, Acta Metall. Mater. 39 (1991) 1763e1770. [18] K.A. Darling, M. Rajagopalan, M. Komarasamy, M.A. Bhatia, B.C. Hornbuckle, R.S. Mishra, K.N. Solanki, Extreme creep resistance in a microstructurally stable nanocrystalline alloy, Nature 537 (2016) 378e381. [19] C. Suryanarayana, F.H. Froes, Mechanical alloying of titanium-base alloys, Adv. Mater. 5 (1993) 96e106. [20] J.C. Rawers, D. Govier, R. Doan, Nitrogen addition to iron powder by mechanical alloying, Mater. Sci. Eng. A 220 (1996) 162e167. [21] E. Bonetti, G. Scipione, G. Valdre, S. Enzo, R. Frattini, P.P. Macri, A study of nanocrystalline iron and aluminium metals and Fe 3 Al intermetallic by mechanical alloying, J. Mater. Sci. 30 (1995) 2220e2226. [22] R.K. Gupta, D. Fabijanic, T. Dorin, Y. Qiu, J.T. Wang, N. Birbilis, Simultaneous improvement in the strength and corrosion resistance of Al via high-energy ball milling and Cr alloying, Mater. Des. 84 (2015) 270e276. [23] R.K.S. Raman, R.K. Gupta, C.C. Koch, Resistance of nanocrystalline vis- a-vis microcrystalline FeeCr alloys to environmental degradation and challenges to their synthesis, Philos. Mag. 90 (2010) 3233e3260.
69
[24] R.K. Gupta, N. Birbilis, J. Zhang, Oxidation resistance of nanocrystalline alloys, in: Corros. Resist., IntechOpen, 2012, https://doi.org/10.5772/34928. [25] R.K. Gupta, R.K.S. Raman, C.C. Koch, B.S. Murty, Effect of nanocrystalline structure on the corrosion of a Fe-20Cr alloy, Int. J. Electrochem. Sci. 8 (2013) 6791e6806. [26] R.K. Gupta, N. Birbilis, The influence of nanocrystalline structure and processing route on corrosion of stainless steel: a review, Corros. Sci. 92 (2015) 1e15. [27] R.K. Gupta, D. Fabijanic, R. Zhang, N. Birbilis, Corrosion behaviour and hardness of in situ consolidated nanostructured Al and Al-Cr alloys produced via high-energy ball milling, Corrosion Sci. 98 (2015) 643e650. [28] J. Esquivel, H. Murdoch, K. Darling, R. Gupta, Excellent corrosion resistance and hardness in Al alloys by extended solid solubility and nanocrystalline structure, Mater. Res. Lett. 6 (2018) 79e83. [29] M.-H. Grosjean, M. Zidoune, L. Roue, J. Huot, R. Schulz, Effect of ball milling on the corrosion resistance of magnesium in aqueous media, Electrochim. Acta 49 (2004) 2461e2470. [30] B.D. Cullity, S.R. Stock, Element of X-ray Diffraction, AdditioneWesley, Addison-Wesley, Reading, MA, 1978. [31] K.A. Darling, A.J. Roberts, L. Armstrong, D. Kapoor, M.A. Tschopp, L.J. Kecskes, S.N. Mathaudhu, Influence of Mn solute content on grain size reduction and improved strength in mechanically alloyed Al e Mn alloys, Mater. Sci. Eng. A 589 (2014) 57e65. [32] A. Roth, An investigation of the aluminum-vanadium system, Zeitschrift Für Met. 32 (1940) 356e359. [33] D.K. Mukhopadhyay, C. Suryanarayana, F.H.S. Froes, Structural evolution in mechanically alloyed Al-Fe powders, Metall. Mater. Trans. A 26 (1995) 1939e1946. [34] J. Eckert, J.C. Holzer, C.E. Krill, W.L. Johnson, Structural and thermodynamic properties of nanocrystalline fcc metals prepared by mechanical attrition, J. Mater. Res. 7 (1992) 1751e1761. [35] C.C. Koch, Synthesis of nanostructured materials by mechanical milling: problems and opportunities, Nanostruct. Mater. 9 (1997) 13e22. [36] T. Uesugi, K. Higashi, First-principles studies on lattice constants and local lattice distortions in solid solution aluminum alloys, Comput. Mater. Sci. 67 (2013) 1e10. [37] W. Cao, S.-L. Chen, F. Zhang, K. Wu, Y. Yang, Y.A. Chang, R. Schmid-Fetzer, W.A. Oates, PANDAT software with PanEngine, PanOptimizer and PanPrecipitation for multi-component phase diagram calculation and materials property simulation, Calphad 33 (2009) 328e342. [38] J. Esquivel, M.G. Wachowiak, S. O'Brien, R.K. Gupta, Thermal stability of nanocrystalline Al-5at.% Ni and Al-5at.% V alloys produced by high-energy ball milling, J. Alloys Compd. 744 (2018) 651e657. [39] J.L. Murray, Al-V (aluminum-vanadium), Bull. Alloy Phase Diagrams 10 (1989) 351e357. [40] S.S. Nayak, M. Wollgarten, J. Banhart, S.K. Pabi, B.S. Murty, Nanocomposites and an extremely hard nanocrystalline intermetallic of AleFe alloys prepared by mechanical alloying, Mater. Sci. Eng. A 527 (2010) 2370e2378. [41] D. Tabor, The Hardness of Metals, Oxford university press, 2000. [42] I. Polmear, D.S. John, Light Alloys: from Traditional Alloys to Nanocrystals, Butterworth-Heinemann, Oxford, 2005. [43] G.W. Nieman, J.R. Weertman, R.W. Siegel, Mechanical behavior of nanocrystalline Cu and Pd, J. Mater. Res. 6 (1991) 1012e1027. [44] C.R. Bradbury, J.-K. Gomon, L. Kollo, H. Kwon, M. Leparoux, Hardness of multi wall carbon nanotubes reinforced aluminium matrix composites, J. Alloys Compd. 585 (2014) 362e367. [45] K.E. Knipling, D.C. Dunand, D.N. Seidman, Criteria for developing castable, creep-resistant aluminum-based alloyseA review, Zeitschrift Für Met. 97 (2006) 246e265. [46] G.E. Dieter, D.J. Bacon, Mechanical Metallurgy, McGraw-hill, New York, 1986. [47] R.W. Armstrong, Hallepetch Relationship: Use in Characterizing Properties of Aluminum and Aluminum Alloys, 2016. [48] T.J. Rupert, J.C. Trenkle, C.A. Schuh, Enhanced solid solution effects on the strength of nanocrystalline alloys, Acta Mater. 59 (2011) 1619e1631. [49] D.J. Bacon, U.F. Kocks, R.O. Scattergood, The effect of dislocation selfinteraction on the Orowan stress, Philos. Mag. 28 (1973) 1241e1263. [50] T.D. Shen, C.C. Koch, Formation, solid solution hardening and softening of nanocrystalline solid solutions prepared by mechanical attrition, Acta Mater. 44 (1996) 753e761. [51] N.N. Krishna, R. Tejas, K. Sivaprasad, K. Venkateswarlu, Study on cryorolled AleCu alloy using X-ray diffraction line profile analysis and evaluation of strengthening mechanisms, Mater. Des. 52 (2013) 785e790. €lander, H.-E. Ekstro €m, Strengthening [52] Ø. Ryen, B. Holmedal, O. Nijs, E. Nes, E. Sjo mechanisms in solid solution aluminum alloys, Metall. Mater. Trans. A 37 (2006) 1999e2006. [53] E. Hornbogen, E.A. Starke, Overview no. 102 Theory assisted design of high strength low alloy aluminum, Acta Metall. Mater. 41 (1993) 1e16. [54] D.B. Witkin, E.J. Lavernia, Synthesis and mechanical behavior of nanostructured materials via cryomilling, Prog. Mater. Sci. 51 (2006) 1e60. [55] E. Bonetti, L. Pasquini, E. Sampaolesi, The influence of grain size on the mechanical properties of nanocrystalline aluminium, Nanostruct. Mater. 9 (1997) 611e614.
70
J. Esquivel, R.K. Gupta / Journal of Alloys and Compounds 760 (2018) 63e70
[56] H. Hasegawa, S. Komura, A. Utsunomiya, Z. Horita, M. Furukawa, M. Nemoto, T.G. Langdon, Thermal stability of ultrafine-grained aluminum in the presence of Mg and Zr additions, Mater. Sci. Eng. A 265 (1999) 188e196. [57] P.G. Partridge, M.C. Mcconnell, The effect of microstructure and composition on the properties of vapour quenched Al-Cr alloys-II. Tensile Properties, Acta Metall. 35 (1993) 1981e1993.
[58] J. Schiøtz, F.D. Di Tolla, K.W. Jacobsen, Softening of nanocrystalline metals at very small grain sizes, Nature 391 (1998) 561e563. [59] M.A. Meyers, A. Mishra, D.J. Benson, Mechanical properties of nanocrystalline materials, Prog. Mater. Sci. 51 (2006) 427e556. [60] T.G. Nieh, J. Wadsworth, Hall-Petch relation in nanocrystalline solids, Scr. Metall. Mater. 25 (1991) 955e958.