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Nanocrystalline Al–Fe intermetallics – light weight alloys with high hardness
Intermetallics 18 (2010) 47–50
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Nanocrystalline Al–Fe intermetallics – light weight alloys with high hardness M. Krasnowski*, T. Kulik Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, 02-507 Warsaw, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 May 2009 Received in revised form 4 June 2009 Accepted 5 June 2009 Available online 16 July 2009
Nanocrystalline Al–Fe alloys containing 60–85 at.% Al were produced by consolidation of mechanically alloyed nanocrystalline or amorphous (Al85Fe15 composition) powders at 1000 C under a pressure of 7.7 GPa. The hardness of the alloys varied between 5.8 and 9.5 GPa, depending on the Al content. The specific strength, calculated using an approximation of the yield strength according to the Tabor relation, was between 544 and 714 kNm/kg. Based on the results obtained, we infer that application of high pressure affected crystallisation of amorphous Al85Fe15 alloy, influencing the phase composition of the crystallisation product, and phase changes in nanocrystalline Al80Fe20 alloy, inhibiting them. Ó 2009 Published by Elsevier Ltd.
Keywords: A. Aluminides A. Nanostructured intermetallics C. Powder metallurgy, including consolidation
1. Introduction Iron aluminides can be of considerable technological interest due to their advantageous properties, in particular a high specific strength, high specific stiffness, good strength at intermediate temperatures and excellent corrosion resistance at elevated temperatures [1]. Al-rich iron aluminides are characterised by low density, but also by lower strength and hardness than Fe3Al or FeAl ones. However, strength and hardness can be improved by grain size refinement, especially to nanometric scale. Nanocrystalline materials exhibit enhanced properties, such as high strength and hardness, compared to the materials with conventional grain size [2–4]. Having their strength increased, Al-based alloys will possess high specific strength. Nanocrystalline materials are extensively produced by the mechanical alloying (MA) process, but in the form of a powder [5], hence consolidation is a necessary step for milled powders to have possible practical applications. Compaction of nanocrystalline powders into bulk, full-density material providing nanocrystalline structure’s maintenance is difficult, since application of high temperature, which is required for good consolidation of powders, i.e. to remove all porosity and to obtain good interparticle bonding, can cause grain growth. To overcome the problem of nanoscale microstructure coarsening, the employment of a high pressure during consolidation and limiting of the high temperature
* Corresponding author. E-mail address: [email protected] (M. Krasnowski). 0966-9795/$ – see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.intermet.2009.06.006
exposure time can be utilised. High pressure hot-pressing method has been successfully used for producing bulk nanocrystalline samples [4,6–9]. Recently, we have demonstrated that application of a high pressure influences grain growth at elevated temperature by hindering it [4,6,8]. Considering the grain growth as a diffusional process, this hindering can be explained by the fact that the diffusion coefficient decreases with pressure [10]. Hence, high pressure can reduce grain boundary mobility. We have also shown that the nanocrystalline FeAl intermetallic as well as FeAl–TiC nanocomposites produced by MA followed by high pressure hotpressing consolidation possess relatively high hardness (namely, 1235HV0.2 (12.12 GPa) and 1608HV0.2 (15.77 GPa) for FeAl and FeAl–TiC respectively) in comparison with their microcrystalline counterparts [6,4]. Aluminium-rich Al–Fe powder alloys have been prepared using the MA process [11–13]. However, works devoted to consolidation of these powders are very scarce [14,15]. In the current work, we obtained bulk nanocrystalline Al–Fe alloys containing 60–85 at.% Al by consolidation of mechanically alloyed nanocrystalline or amorphous powders. The structural and phase transformations taking place during consolidation were studied and the produced compacts were characterised.
2. Experimental Alx–Fe100x (x ¼ 60, 65, 70, 75, 80, and 85) powder alloys (all compositions are given in at.% throughout this paper) were synthesized by mechanical alloying in a SPEX 8000 D ball mill. The details of this experimental step can be found in Ref. [13].
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M. Krasnowski, T. Kulik / Intermetallics 18 (2010) 47–50
A press equipped with a toroid-type high pressure cell was used for consolidation of the milled powders. The shape of the cell and the material of the gasket ensured that the compacting conditions were close to isostatic ones. The compaction process was performed under a pressure of 7.7 GPa at a temperature of 1000 C for 3 min. The loading, at a rate 0.5 GPa/min, was done prior to the heating. The heating and cooling rate was 1000 C/min. The structural investigations of consolidated samples were carried out by X-ray diffraction (XRD) method using a Philips 1830 diffractometer using CuKa radiation. The lattice parameter and the mean crystallite size, the latter determined by the Williamson-Hall method, were calculated from the XRD data taking into account CuKa1 radiation, after Ka2 stripping using the Rachinger method. The instrumental broadening was determined using an Si standard and subtracted from the experimental breadth to obtain a ‘‘physical’’ broadening of each diffraction line, which was then used for the Williamson-Hall calculations. In order to check the quality of consolidation, a Nikon Epiphot 200 light microscope was used for observations of the produced materials’ surface. Samples for light metallography were prepared using standard polishing technique. The Vickers microhardness and hardness of the compacts were measured under a load of 200 g and 1 kg respectively, imposed for 15 s. Vickers microhardness value was the average of at least 25 indentations. The density of the bulk samples was determined using a Gibertini E154 balance equipped with a device for measuring the density of solids (Archimedes method). Basing on mass measurements performed during density determination, open porosity of the consolidated samples was calculated. 3. Results and discussion The phase and structural evolution occurring in the Al–Fe powders during mechanical alloying and the characterisation,
including study of thermal behaviour, of the milling products prepared for consolidation have already been described and analysed in detail [13]. It has been shown that three kinds of structure were produced: (i) nanocrystalline supersaturated Fe(Al) solid solution for the Al60Fe40 Al65Fe35 alloys, (ii) nanocrystalline Al5Fe2 intermetallic, at least partially ordered, for the Al75Fe25 and Al80Fe20 alloys, (iii) amorphous for the Al85Fe15 alloy. In the case of Al70Fe30 composition, a two-phase Al5Fe2 þ Fe(Al) alloy was obtained [13]. Fig. 1 shows the XRD patterns of the milled powders before and after consolidation. Comparing the spectra of bulk samples with those of the powders before consolidation, one can see that in all the alloys, except the Al75Fe25 one, hot-pressing caused phase changes. For the alloys containing 60, 65 and 70% of Al, the (100) and (111) superlattice reflections of the ordered B2 structure appear, which evidences the ordering of the Fe(Al) solid solution and its transformation into an FeAl intermetallic compound. In the case of the Al65Fe35 alloy, besides the ordering, precipitation of an Al5Fe2 intermetallic from the supersaturated Fe(Al) solid solution is evident from the XRD pattern. The lattice parameter of the FeAl phase decreased in comparison with the one of the solid solution before consolidation and is equal to 2.911 Å and 2.908 Å for Al60Fe40 and Al65Fe35 respectively. The same phase changes were noticed during heating of the as-milled alloys containing 60, 65 and 70% of Al in the calorimeter [13]. For the Al80Fe20 alloy, appearance of diffraction lines, which are attributed to an Al13Fe4 phase, besides those of Al5Fe2 intermetallic is evident. In the case of the Al85Fe15 alloy instead of the diffraction halo, peaks are present, which indicates that the amorphous phase crystallised during consolidation. However, these diffraction peaks cannot be assigned to any phase in Al–Fe system [16] nor to any Al–Fe phase in the ICDD PDF4 database, hence products of the crystallisation under high pressure of the amorphous alloy are metastable phases. Some of the peaks can be indexed in cubic system and attributed to a phase with bcc structure and unit cell parameter of 2.964 Å. It is worthwhile to
Fig. 1. XRD patterns of the Al–Fe alloys: (a) before consolidation, (b) after consolidation.
M. Krasnowski, T. Kulik / Intermetallics 18 (2010) 47–50
49
Table 1 Phase composition, mean grain size (d), microhardness HV0.2 together with standard deviation given in percentage (Sd%), hardness HV1, estimated yield strength, density (r), open porosity (p) and specific yield strength of the produced bulk samples. Alloy
Phase composition
d [nm]
HV0.2 (Sd%) [GPa]
HV1 [GPa]
Estimated yield strength [GPa]
r [g/cm3]
p [%]
Specific yield strength [kNm/kg]
Al60Fe40 Al65Fe35 Al70Fe30 Al75Fe25 Al80Fe20 Al85Fe15
FeAl FeAl þ Al5Fe2 FeAl þ Al5Fe2 Al5Fe2 Al5Fe2 þ Al13Fe4 bcc þ ?
37 40 (FeAl) 33 & 26 32
9.57 9.36 9.67 8.46 7.44 6.02
9.45 9.12 9.3 7.7 7.33 5.79
3.15 3.04 3.1 2.57 2.44 1.93
4.898 4.465 4.34 3.988 3.668 3.546
0.2 w0 0.3 w0 0.4 w0
643 681 714 644 666 544
34 (bcc)
(4%) (3%) (3.5%) (5%) (4%) (3.5%)
comment on the pressure’s effect on crystallisation of the amorphous Al85Fe15 alloy and on phase transformation in the Al80Fe20 alloy upon thermal stimulus. During heating in the calorimeter, when material was under atmospheric pressure, the crystallisation products were an Al13Fe4 phase and an fcc Al [13], while during heating throughout the consolidation process, when high pressure was applied, crystallisation yielded metastable phases different from the ones mentioned above. Hence, in our case, we can infer that application of high pressure affects crystallisation of the amorphous alloy, influencing the phase composition of the products of this process. The pressure effect on crystallisation of mechanically alloyed Al85Fe15 alloy was also reported by Gu et al. [14], however, the effect was different than in our work. They found that the applied pressure enhances the fcc Al precipitation from amorphous phase. For the Al80Fe20 alloy, heating in the calorimeter induced complete transformation of the Al5Fe2 phase into an Al13Fe4 phase [13], whereas heating during hot-pressing caused only partial transformation and a two-phase alloy was produced. Thus, employment of high pressure in this case affects phase changes, inhibiting them. Hindering of phase transformation by high pressure was reported in the case of an Fe50Al50 alloy, where ordering of Fe(Al) solid solution, which occurred in the calorimeter below 450 C [17], did not take place during consolidation at 750 C in the same device as used in this work [18]. Besides the signs of phase changes, another feature in the bulk materials’ XRD patterns is that all the peaks became a little sharper than those in the patterns of the milled powders before consolidation. This reduction in peaks width is due to the increase of the mean crystallite size and the decrease of the mean lattice strain. The estimated mean crystallite sizes for the phases in the compacted samples (where it was reasonable) are given in Table 1. These data show that a limited growth of grains took place during hot-pressing and the nanoscale grain size has been retained after applied consolidation process. The product of crystallisation of the amorphous alloy is also nanocrystalline.
The produced samples were investigated using light microscopy. Images of the surface of all the polished samples, with different concentrations of Al, are very similar. As an example, Fig. 2 shows micrographs of the Al60Fe40 alloy. One can see that the surface is smooth and free of pores or voids between bonded powder particles, which is an evidence of a good quality of consolidation. The compacted materials were also characterised by microhardness, hardness, density and open porosity measurements. Microhardness measurements give information about resistance of material to plastic deformation by indentation in a more local scale than hardness. Hence, microhardness test can verify the samples homogeneity in terms of hardness. The average Vickers microhardness and hardness of the produced materials are listed in Table 1. The standard deviation of the measured microhardness values is quite small, which indicates uniformity of this property in each sample. No information about hardness/microhardness of bulk nanocrystalline Al–Fe alloys with similar content of Al to that in this study has been found in literature. However, data for some Al–Fe alloys or Al-based alloys are available and can be quoted for comparison. For example, the microhardness of a nanocrystalline FeAl intermetallic prepared by mechanical alloying and hot forging has been found to be 680HV0.3 (6.67 GPa) [19], while of a nanocrystalline FeAl alloy obtained by mechanical milling of intermetallic compound followed by explosive shock wave consolidation was 683HV0.5 (6.7 GPa) [20]. The microhardness of a nanocrystalline Fe50Al50 alloy produced in the same way as the Al–Fe alloys in this work was 1235HV0.2 (12.12 GPa) [6]. For a hot-pressed Al85Ni5Y8Co2 alloy a hardness value around 500 HV (4.91 GPa) was obtained [21], while for an Al85Ni10La5 alloy consolidated by spark plasma sintering and subsequently annealed, a hardness of 450 HV (4.41 GPa) was achieved [22]. On the basis of the quoted results, we assume that the microhardness and hardness of the bulk nanocrystalline Al–Fe alloys obtained in this work are relatively high. The density and the open porosity values of the compacted materials are shown in Table 1. The open porosity does not exceed
Fig. 2. Micrographs of the polished surface of the Al60Fe40 alloy: (a) lower, (b) higher magnification.
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M. Krasnowski, T. Kulik / Intermetallics 18 (2010) 47–50
Fig. 3. Vickers hardness and specific yield strength of the produced alloys as a function of Al content.
0.4% (it is therefore negligible for all the alloys), which indicates that the samples are well consolidated. Most methods for preparing truly nanocrystalline materials do not allow for significant quantities or sizes of artefact-free bulk materials [23]. However, some processing routes are capable of producing limited sizes of artefact-free nanostructured materials. Due to small samples’ size, many studies of the nanocrystalline materials’ strength were based on hardness measurements [24]. According to the Tabor relation, the Vickers hardness divided by 3 should approximate the yield strength [24,25] and so estimated values are also given in Table 1. The specific yield strength, defined by the ratio of yield strength to density, is listed in Table 1 too. No data concerning specific yield strength of Al–Fe alloys with similar content of Al has been found in literature. Amorphous Mg-based alloys are referred as low density materials with high specific strength [26]. The specific strength of an Mg85Cu5Ca10 melt spun ribbon calculated using the Tabor relation was 300 kNm/kg [27]. In the case of an Mg-based bulk metallic glass/TiB2 composite, for which the Tabor relation was satisfied, the specific strength of 360 kNm/kg was reported [26]. Taking into account the cited results, we conclude that the specific yield strength of the materials produced in this work is relatively high. For better presentation, the hardness and the specific yield strength of the obtained nanocrystalline Al–Fe alloys as a function of Al content are displayed in Fig. 3. The alloys containing 60, 65 and 70% of Al have similar hardness, higher than others, while five alloys, except the Al85Fe15 one, posses specific yield strength at the same level. The Al70Fe30 bulk nanocrystalline alloy has the highest value of specific yield strength. 4. Conclusions In summary, bulk nanocrystalline Al–Fe alloys containing 60, 65, 70, 75, 80 and 85 at.% of Al were produced by hot-pressing consolidation of mechanically alloyed powders. Consolidation was performed at 1000 C under a pressure of 7.7 GPa for 3 min.
A limited growth of grains took place during hot-pressing and the nanoscale grain size was retained after the applied consolidation of nanocrystalline powders. Consolidation of the amorphous Al85Fe15 powder caused crystallisation and appearance of metastable nanocrystalline phases, different to those which crystallised during heating under atmospheric pressure. The standard deviation of the HV0.2 microhardness of the bulk samples is quite small, which indicates uniformity of this property in each material. The hardness of the alloys varied between 5.79 and 9.45 GPa (590 and 963 HV1), while the specific yield strength, approximated using the Tabor relation, varied between 544 and 714 kNm/kg, depending on the Al content. The open porosity of bulk samples is negligible. These results show that the quality of consolidation with preserving nanometric grain size is satisfactory and the hardness as well as specific yield strength of the produced materials is relatively high. Based on the results obtained, we also infer that application of high pressure affected crystallisation of the amorphous Al85Fe15 alloy, influencing the phase composition of the crystallisation’s product, as well as thermally induced phase changes in the Al80Fe20 alloy, inhibiting them. Acknowledgements This work was supported by the Ministry of Science and Higher Education (grant no. N507 057 31/1290). The authors would like to thank Dr. S. Gierlotka (Institute of High Pressure Physics of the Polish Academy of Sciences, Warsaw, Poland) for assistance in performing the hot-pressing consolidation. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
Deevi SC, Sikka VK, Liu CT. Prog Mater Sci 1997;42:177–92. Gleiter H. Nanostruct Mater 1992;1:1–19. Nieman GW, Weertman JR, Siegel RW. Nanostruct Mater 1992;1:185–90. Krasnowski M, Kulik T. Intermetallics 2007;15:1377–83. Suryanarayana C. Prog Mater Sci 2001;46:1–184. Krasnowski M, Kulik T. Intermetallics 2007;15:201–5. Krasnowski M, Antolak A, Kulik T. J Alloy Compd 2007;434–435C:344–7. Krasnowski M, Kulik T. Scripta Mater 2007;57:553–6. M. Krasnowski, T. Kulik. J AlloyCompd 10.1016/j.jallcom.2008.07.166. Shewmon PG. Diffusion in solids. New York: McGraw-Hill; 1963. Huang B, Ishihara KN, Shingu PH. Mater Sci Eng 1997;A231:72–9. Enzo S, Mulas G, Frattini R. Mater Sci Forum 1998;269–272:385–90. Krasnowski M, Kulik T. Mater Chem Phys 2009;116:631–7. Gu XJ, Ye F, Zhou F, Lu K. Mater Sci Eng 2000;A278:61–5. M. Krasnowski, T. Kulik. Mater J Alloy Compd, in press. Massalski TB. Binary phase diagrams. Metals Park, OH: ASM International; 1986. Krasnowski M, Grabias A, Kulik T. J Alloys Compd 2006;424:119–27. Krasnowski M, Kulik T. Scripta Mater 2003;48:1489–94. ˜ oz MA, Dodge A, Morris DG. Nanostruct Mater 1999;11:873–85. Morris-Mun Varin RA, Czujko T, Bystrzycki J, Calka A. Mater Sci Eng 2002;A329–331:213–21. Kawamura Y, Mano H, Inoue A. Scripta Mater 2001;44:1599–604. Sasaki TT, Hono K, Vierke J, Wollgarten M, Banhart J. Mater Sci Eng 2008;A490:343–50. C.C. Koch, I.A. Ovid’ko, S. Seal, S. Veprek. Structural nanocrystalline materiale, Cambridge; 2007. Youngdahl CJ, Sanders PG, Eastman JA, Weertman JR. Scripta Mater 1997;37:809–13. Tabor D. Br J Appl Phys 1956;7:159–66. Xu Y-K, Ma H, Xu J, Ma E. Acta Mater 2005;53:1857–66. Guo F, Poon SJ, Gu X, Shiflet GJ. Scripta Mater 2007;56:689–92.
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Nanocrystalline Al–Fe intermetallics – light weight alloys with high hardness M. Krasnowski*, T. Kulik Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, 02-507 Warsaw, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 May 2009 Received in revised form 4 June 2009 Accepted 5 June 2009 Available online 16 July 2009
Nanocrystalline Al–Fe alloys containing 60–85 at.% Al were produced by consolidation of mechanically alloyed nanocrystalline or amorphous (Al85Fe15 composition) powders at 1000 C under a pressure of 7.7 GPa. The hardness of the alloys varied between 5.8 and 9.5 GPa, depending on the Al content. The specific strength, calculated using an approximation of the yield strength according to the Tabor relation, was between 544 and 714 kNm/kg. Based on the results obtained, we infer that application of high pressure affected crystallisation of amorphous Al85Fe15 alloy, influencing the phase composition of the crystallisation product, and phase changes in nanocrystalline Al80Fe20 alloy, inhibiting them. Ó 2009 Published by Elsevier Ltd.
Keywords: A. Aluminides A. Nanostructured intermetallics C. Powder metallurgy, including consolidation
1. Introduction Iron aluminides can be of considerable technological interest due to their advantageous properties, in particular a high specific strength, high specific stiffness, good strength at intermediate temperatures and excellent corrosion resistance at elevated temperatures [1]. Al-rich iron aluminides are characterised by low density, but also by lower strength and hardness than Fe3Al or FeAl ones. However, strength and hardness can be improved by grain size refinement, especially to nanometric scale. Nanocrystalline materials exhibit enhanced properties, such as high strength and hardness, compared to the materials with conventional grain size [2–4]. Having their strength increased, Al-based alloys will possess high specific strength. Nanocrystalline materials are extensively produced by the mechanical alloying (MA) process, but in the form of a powder [5], hence consolidation is a necessary step for milled powders to have possible practical applications. Compaction of nanocrystalline powders into bulk, full-density material providing nanocrystalline structure’s maintenance is difficult, since application of high temperature, which is required for good consolidation of powders, i.e. to remove all porosity and to obtain good interparticle bonding, can cause grain growth. To overcome the problem of nanoscale microstructure coarsening, the employment of a high pressure during consolidation and limiting of the high temperature
* Corresponding author. E-mail address: [email protected] (M. Krasnowski). 0966-9795/$ – see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.intermet.2009.06.006
exposure time can be utilised. High pressure hot-pressing method has been successfully used for producing bulk nanocrystalline samples [4,6–9]. Recently, we have demonstrated that application of a high pressure influences grain growth at elevated temperature by hindering it [4,6,8]. Considering the grain growth as a diffusional process, this hindering can be explained by the fact that the diffusion coefficient decreases with pressure [10]. Hence, high pressure can reduce grain boundary mobility. We have also shown that the nanocrystalline FeAl intermetallic as well as FeAl–TiC nanocomposites produced by MA followed by high pressure hotpressing consolidation possess relatively high hardness (namely, 1235HV0.2 (12.12 GPa) and 1608HV0.2 (15.77 GPa) for FeAl and FeAl–TiC respectively) in comparison with their microcrystalline counterparts [6,4]. Aluminium-rich Al–Fe powder alloys have been prepared using the MA process [11–13]. However, works devoted to consolidation of these powders are very scarce [14,15]. In the current work, we obtained bulk nanocrystalline Al–Fe alloys containing 60–85 at.% Al by consolidation of mechanically alloyed nanocrystalline or amorphous powders. The structural and phase transformations taking place during consolidation were studied and the produced compacts were characterised.
2. Experimental Alx–Fe100x (x ¼ 60, 65, 70, 75, 80, and 85) powder alloys (all compositions are given in at.% throughout this paper) were synthesized by mechanical alloying in a SPEX 8000 D ball mill. The details of this experimental step can be found in Ref. [13].
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M. Krasnowski, T. Kulik / Intermetallics 18 (2010) 47–50
A press equipped with a toroid-type high pressure cell was used for consolidation of the milled powders. The shape of the cell and the material of the gasket ensured that the compacting conditions were close to isostatic ones. The compaction process was performed under a pressure of 7.7 GPa at a temperature of 1000 C for 3 min. The loading, at a rate 0.5 GPa/min, was done prior to the heating. The heating and cooling rate was 1000 C/min. The structural investigations of consolidated samples were carried out by X-ray diffraction (XRD) method using a Philips 1830 diffractometer using CuKa radiation. The lattice parameter and the mean crystallite size, the latter determined by the Williamson-Hall method, were calculated from the XRD data taking into account CuKa1 radiation, after Ka2 stripping using the Rachinger method. The instrumental broadening was determined using an Si standard and subtracted from the experimental breadth to obtain a ‘‘physical’’ broadening of each diffraction line, which was then used for the Williamson-Hall calculations. In order to check the quality of consolidation, a Nikon Epiphot 200 light microscope was used for observations of the produced materials’ surface. Samples for light metallography were prepared using standard polishing technique. The Vickers microhardness and hardness of the compacts were measured under a load of 200 g and 1 kg respectively, imposed for 15 s. Vickers microhardness value was the average of at least 25 indentations. The density of the bulk samples was determined using a Gibertini E154 balance equipped with a device for measuring the density of solids (Archimedes method). Basing on mass measurements performed during density determination, open porosity of the consolidated samples was calculated. 3. Results and discussion The phase and structural evolution occurring in the Al–Fe powders during mechanical alloying and the characterisation,
including study of thermal behaviour, of the milling products prepared for consolidation have already been described and analysed in detail [13]. It has been shown that three kinds of structure were produced: (i) nanocrystalline supersaturated Fe(Al) solid solution for the Al60Fe40 Al65Fe35 alloys, (ii) nanocrystalline Al5Fe2 intermetallic, at least partially ordered, for the Al75Fe25 and Al80Fe20 alloys, (iii) amorphous for the Al85Fe15 alloy. In the case of Al70Fe30 composition, a two-phase Al5Fe2 þ Fe(Al) alloy was obtained [13]. Fig. 1 shows the XRD patterns of the milled powders before and after consolidation. Comparing the spectra of bulk samples with those of the powders before consolidation, one can see that in all the alloys, except the Al75Fe25 one, hot-pressing caused phase changes. For the alloys containing 60, 65 and 70% of Al, the (100) and (111) superlattice reflections of the ordered B2 structure appear, which evidences the ordering of the Fe(Al) solid solution and its transformation into an FeAl intermetallic compound. In the case of the Al65Fe35 alloy, besides the ordering, precipitation of an Al5Fe2 intermetallic from the supersaturated Fe(Al) solid solution is evident from the XRD pattern. The lattice parameter of the FeAl phase decreased in comparison with the one of the solid solution before consolidation and is equal to 2.911 Å and 2.908 Å for Al60Fe40 and Al65Fe35 respectively. The same phase changes were noticed during heating of the as-milled alloys containing 60, 65 and 70% of Al in the calorimeter [13]. For the Al80Fe20 alloy, appearance of diffraction lines, which are attributed to an Al13Fe4 phase, besides those of Al5Fe2 intermetallic is evident. In the case of the Al85Fe15 alloy instead of the diffraction halo, peaks are present, which indicates that the amorphous phase crystallised during consolidation. However, these diffraction peaks cannot be assigned to any phase in Al–Fe system [16] nor to any Al–Fe phase in the ICDD PDF4 database, hence products of the crystallisation under high pressure of the amorphous alloy are metastable phases. Some of the peaks can be indexed in cubic system and attributed to a phase with bcc structure and unit cell parameter of 2.964 Å. It is worthwhile to
Fig. 1. XRD patterns of the Al–Fe alloys: (a) before consolidation, (b) after consolidation.
M. Krasnowski, T. Kulik / Intermetallics 18 (2010) 47–50
49
Table 1 Phase composition, mean grain size (d), microhardness HV0.2 together with standard deviation given in percentage (Sd%), hardness HV1, estimated yield strength, density (r), open porosity (p) and specific yield strength of the produced bulk samples. Alloy
Phase composition
d [nm]
HV0.2 (Sd%) [GPa]
HV1 [GPa]
Estimated yield strength [GPa]
r [g/cm3]
p [%]
Specific yield strength [kNm/kg]
Al60Fe40 Al65Fe35 Al70Fe30 Al75Fe25 Al80Fe20 Al85Fe15
FeAl FeAl þ Al5Fe2 FeAl þ Al5Fe2 Al5Fe2 Al5Fe2 þ Al13Fe4 bcc þ ?
37 40 (FeAl) 33 & 26 32
9.57 9.36 9.67 8.46 7.44 6.02
9.45 9.12 9.3 7.7 7.33 5.79
3.15 3.04 3.1 2.57 2.44 1.93
4.898 4.465 4.34 3.988 3.668 3.546
0.2 w0 0.3 w0 0.4 w0
643 681 714 644 666 544
34 (bcc)
(4%) (3%) (3.5%) (5%) (4%) (3.5%)
comment on the pressure’s effect on crystallisation of the amorphous Al85Fe15 alloy and on phase transformation in the Al80Fe20 alloy upon thermal stimulus. During heating in the calorimeter, when material was under atmospheric pressure, the crystallisation products were an Al13Fe4 phase and an fcc Al [13], while during heating throughout the consolidation process, when high pressure was applied, crystallisation yielded metastable phases different from the ones mentioned above. Hence, in our case, we can infer that application of high pressure affects crystallisation of the amorphous alloy, influencing the phase composition of the products of this process. The pressure effect on crystallisation of mechanically alloyed Al85Fe15 alloy was also reported by Gu et al. [14], however, the effect was different than in our work. They found that the applied pressure enhances the fcc Al precipitation from amorphous phase. For the Al80Fe20 alloy, heating in the calorimeter induced complete transformation of the Al5Fe2 phase into an Al13Fe4 phase [13], whereas heating during hot-pressing caused only partial transformation and a two-phase alloy was produced. Thus, employment of high pressure in this case affects phase changes, inhibiting them. Hindering of phase transformation by high pressure was reported in the case of an Fe50Al50 alloy, where ordering of Fe(Al) solid solution, which occurred in the calorimeter below 450 C [17], did not take place during consolidation at 750 C in the same device as used in this work [18]. Besides the signs of phase changes, another feature in the bulk materials’ XRD patterns is that all the peaks became a little sharper than those in the patterns of the milled powders before consolidation. This reduction in peaks width is due to the increase of the mean crystallite size and the decrease of the mean lattice strain. The estimated mean crystallite sizes for the phases in the compacted samples (where it was reasonable) are given in Table 1. These data show that a limited growth of grains took place during hot-pressing and the nanoscale grain size has been retained after applied consolidation process. The product of crystallisation of the amorphous alloy is also nanocrystalline.
The produced samples were investigated using light microscopy. Images of the surface of all the polished samples, with different concentrations of Al, are very similar. As an example, Fig. 2 shows micrographs of the Al60Fe40 alloy. One can see that the surface is smooth and free of pores or voids between bonded powder particles, which is an evidence of a good quality of consolidation. The compacted materials were also characterised by microhardness, hardness, density and open porosity measurements. Microhardness measurements give information about resistance of material to plastic deformation by indentation in a more local scale than hardness. Hence, microhardness test can verify the samples homogeneity in terms of hardness. The average Vickers microhardness and hardness of the produced materials are listed in Table 1. The standard deviation of the measured microhardness values is quite small, which indicates uniformity of this property in each sample. No information about hardness/microhardness of bulk nanocrystalline Al–Fe alloys with similar content of Al to that in this study has been found in literature. However, data for some Al–Fe alloys or Al-based alloys are available and can be quoted for comparison. For example, the microhardness of a nanocrystalline FeAl intermetallic prepared by mechanical alloying and hot forging has been found to be 680HV0.3 (6.67 GPa) [19], while of a nanocrystalline FeAl alloy obtained by mechanical milling of intermetallic compound followed by explosive shock wave consolidation was 683HV0.5 (6.7 GPa) [20]. The microhardness of a nanocrystalline Fe50Al50 alloy produced in the same way as the Al–Fe alloys in this work was 1235HV0.2 (12.12 GPa) [6]. For a hot-pressed Al85Ni5Y8Co2 alloy a hardness value around 500 HV (4.91 GPa) was obtained [21], while for an Al85Ni10La5 alloy consolidated by spark plasma sintering and subsequently annealed, a hardness of 450 HV (4.41 GPa) was achieved [22]. On the basis of the quoted results, we assume that the microhardness and hardness of the bulk nanocrystalline Al–Fe alloys obtained in this work are relatively high. The density and the open porosity values of the compacted materials are shown in Table 1. The open porosity does not exceed
Fig. 2. Micrographs of the polished surface of the Al60Fe40 alloy: (a) lower, (b) higher magnification.
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Fig. 3. Vickers hardness and specific yield strength of the produced alloys as a function of Al content.
0.4% (it is therefore negligible for all the alloys), which indicates that the samples are well consolidated. Most methods for preparing truly nanocrystalline materials do not allow for significant quantities or sizes of artefact-free bulk materials [23]. However, some processing routes are capable of producing limited sizes of artefact-free nanostructured materials. Due to small samples’ size, many studies of the nanocrystalline materials’ strength were based on hardness measurements [24]. According to the Tabor relation, the Vickers hardness divided by 3 should approximate the yield strength [24,25] and so estimated values are also given in Table 1. The specific yield strength, defined by the ratio of yield strength to density, is listed in Table 1 too. No data concerning specific yield strength of Al–Fe alloys with similar content of Al has been found in literature. Amorphous Mg-based alloys are referred as low density materials with high specific strength [26]. The specific strength of an Mg85Cu5Ca10 melt spun ribbon calculated using the Tabor relation was 300 kNm/kg [27]. In the case of an Mg-based bulk metallic glass/TiB2 composite, for which the Tabor relation was satisfied, the specific strength of 360 kNm/kg was reported [26]. Taking into account the cited results, we conclude that the specific yield strength of the materials produced in this work is relatively high. For better presentation, the hardness and the specific yield strength of the obtained nanocrystalline Al–Fe alloys as a function of Al content are displayed in Fig. 3. The alloys containing 60, 65 and 70% of Al have similar hardness, higher than others, while five alloys, except the Al85Fe15 one, posses specific yield strength at the same level. The Al70Fe30 bulk nanocrystalline alloy has the highest value of specific yield strength. 4. Conclusions In summary, bulk nanocrystalline Al–Fe alloys containing 60, 65, 70, 75, 80 and 85 at.% of Al were produced by hot-pressing consolidation of mechanically alloyed powders. Consolidation was performed at 1000 C under a pressure of 7.7 GPa for 3 min.
A limited growth of grains took place during hot-pressing and the nanoscale grain size was retained after the applied consolidation of nanocrystalline powders. Consolidation of the amorphous Al85Fe15 powder caused crystallisation and appearance of metastable nanocrystalline phases, different to those which crystallised during heating under atmospheric pressure. The standard deviation of the HV0.2 microhardness of the bulk samples is quite small, which indicates uniformity of this property in each material. The hardness of the alloys varied between 5.79 and 9.45 GPa (590 and 963 HV1), while the specific yield strength, approximated using the Tabor relation, varied between 544 and 714 kNm/kg, depending on the Al content. The open porosity of bulk samples is negligible. These results show that the quality of consolidation with preserving nanometric grain size is satisfactory and the hardness as well as specific yield strength of the produced materials is relatively high. Based on the results obtained, we also infer that application of high pressure affected crystallisation of the amorphous Al85Fe15 alloy, influencing the phase composition of the crystallisation’s product, as well as thermally induced phase changes in the Al80Fe20 alloy, inhibiting them. Acknowledgements This work was supported by the Ministry of Science and Higher Education (grant no. N507 057 31/1290). The authors would like to thank Dr. S. Gierlotka (Institute of High Pressure Physics of the Polish Academy of Sciences, Warsaw, Poland) for assistance in performing the hot-pressing consolidation. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
Deevi SC, Sikka VK, Liu CT. Prog Mater Sci 1997;42:177–92. Gleiter H. Nanostruct Mater 1992;1:1–19. Nieman GW, Weertman JR, Siegel RW. Nanostruct Mater 1992;1:185–90. Krasnowski M, Kulik T. Intermetallics 2007;15:1377–83. Suryanarayana C. Prog Mater Sci 2001;46:1–184. Krasnowski M, Kulik T. Intermetallics 2007;15:201–5. Krasnowski M, Antolak A, Kulik T. J Alloy Compd 2007;434–435C:344–7. Krasnowski M, Kulik T. Scripta Mater 2007;57:553–6. M. Krasnowski, T. Kulik. J AlloyCompd 10.1016/j.jallcom.2008.07.166. Shewmon PG. Diffusion in solids. New York: McGraw-Hill; 1963. Huang B, Ishihara KN, Shingu PH. Mater Sci Eng 1997;A231:72–9. Enzo S, Mulas G, Frattini R. Mater Sci Forum 1998;269–272:385–90. Krasnowski M, Kulik T. Mater Chem Phys 2009;116:631–7. Gu XJ, Ye F, Zhou F, Lu K. Mater Sci Eng 2000;A278:61–5. M. Krasnowski, T. Kulik. Mater J Alloy Compd, in press. Massalski TB. Binary phase diagrams. Metals Park, OH: ASM International; 1986. Krasnowski M, Grabias A, Kulik T. J Alloys Compd 2006;424:119–27. Krasnowski M, Kulik T. Scripta Mater 2003;48:1489–94. ˜ oz MA, Dodge A, Morris DG. Nanostruct Mater 1999;11:873–85. Morris-Mun Varin RA, Czujko T, Bystrzycki J, Calka A. Mater Sci Eng 2002;A329–331:213–21. Kawamura Y, Mano H, Inoue A. Scripta Mater 2001;44:1599–604. Sasaki TT, Hono K, Vierke J, Wollgarten M, Banhart J. Mater Sci Eng 2008;A490:343–50. C.C. Koch, I.A. Ovid’ko, S. Seal, S. Veprek. Structural nanocrystalline materiale, Cambridge; 2007. Youngdahl CJ, Sanders PG, Eastman JA, Weertman JR. Scripta Mater 1997;37:809–13. Tabor D. Br J Appl Phys 1956;7:159–66. Xu Y-K, Ma H, Xu J, Ma E. Acta Mater 2005;53:1857–66. Guo F, Poon SJ, Gu X, Shiflet GJ. Scripta Mater 2007;56:689–92.