XRD is a very good characterisation technique used to determine the crystallite size and lattice strain of different materials.
Figure 4.1: XRD pattern of pure Fe at different temperature
The above figure shows XRD pattern of pure Fe annealed at different temperatures. The XRD peaks of the samples are (110), (200) and (211). In the above figure the lattice strain is more for wider peaks which will be discussed later.
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Figure 4.2: XRD pattern of Fe-0.25 wt.% Y at different temperature
The above figure shows XRD pattern of Fe mechanically alloyed with 0.25% Y for 1500 minutes, annealed at different temperatures. The XRD peaks of the samples are (110), (200) and (211) . A high amount of broadening is observed in the as milled condition which shows the presence of large lattice strain.
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Figure 4.2: XRD pattern of Fe-0.5 wt.% Y at different temperature
The above figure shows XRD pattern of Fe mechanically alloyed with 0.50% Y for 1500 minutes, annealed at different temperatures. The XRD peaks of the samples are (110), (200) and (211). Peak broadening is observed in the as milled condition and annealing at 1200oC.
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Figure 4.4: Variation of crystallite size at different temperatures with addition of yttrium in Fe
The crystallite size is calculated by Scherrer’s formula:
The above figure displays the variation of crystallite size with Y content and annealing temperature as analysed from the XRD peaks plot obtained.In the as milled state, the crystallite size remains nearly constant till 0.25 % Y but then decreses. The crystallite size is observed to increase gradually up to 0.25 atom % Y and then decreases in samples annealed at 500oC, 1200oC . In case of samples annealed at 900oC, the crystallite size decreases progressively with increase in %Y. On annealing at 700oC, the crystallite size first decreases till 0.25 %Y and then increases. The crystallite sizes of samples annealed at 900oC are found to be larger in comparison to corresponding values at other temperatures. The maximum crystallite size is obtained at pure Fe sample annealed at 900oC as 61.85 nm and a minimum of 6.62 nm at 1200 oC.
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Figure 4.5: Variation of Lattice Strain at different temperatures with addition of yttrium. The lattice strain is calculated as :
The above plot shows the variation of lattice strain with atom % Y and temperature of annealing. The more the broadening of peaks, higher is the lattice strain. The data based on the broadening of peaks observed in the XRD plot is analysed. As we see, the lattice strain increases progressively with increase in the %Y in as milled conditionand in samples annealed at 500 oC. Hence the yttrium addition makes a positive contribution to lattice strain causing solid solution stranghtening. In case of samples annealed at 700oC and 900oC, the lattice strain first increases up to 0.25 %Y and then decreases. The lattice strain is found to decrease continuously with % Y when annealed at 1200 oC which shows that Y addition does not have any significant effect when annealing is done at high temperature . A maximum lattice strain is observed in as milled condition of Fe-0.5 atom% Y i.e. 0.0056.
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Figure 4.6: Variation of Micro hardness at different temperatures with addition of yttrium. The above figure displays the variation of micro hardness value obtained, with the %Y and annealing temperature. It is extremely clear that, when we consider samples annealed at identical temperature, the microhardness increases with increase in %Y. With increase in annealing temperature, the hardness decreases as expected. The maximum micro hardness of 1012.8 VHN is obtained in the 0.5 atom % Y sample in as milled condition while minimum of 270.6 VHN is found at 1200oC in pure Fe sample. Hence , we found that the addition of Yttrium in minute amounts enhances the hardness of Fe nanostructures.
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Figure 4.7: variation of crystallite size with different temperature of various compositions In the above plot ,we observe that the crystallite size initially increases with increase with anneal temperature in all 3 compositions. The crystallite size then decreases with temperature rise beyond 900oC. While in case of 0.25%Y , crystallite size increases progressively with temperature, a gradual increase upto 500oC followed by a near constant value upto 700oC and then a very sharp rise till 900oC is obtained in case of pure Fe. However in case of 0.5% Y, a drop in crystallite size occurs from 500oC to 700oC followed by a steep rise. The maximum crystallite sizes are observed at 900oC with the highest peak observed in case of pure Fe sample i.e 61.85 nm.
38 300 600 900
M
ic
ro
-h
ar
dn
es
s
(H
V
N
)
Temperature (in
OC )
No Y 0.25 atm% Y 0.5 atm% Y As milled 500 700 900 1200 Figure 4.8: variation of Micro hardness with different temperature of various compositionsOn an increase in anneal temperature a decrease in the micro hardness is observed. An exception is found at pure Fe where the hardness is found to decrease from 500oC to 700oC. The hardness is found to be higher for a higher %Y. Maximum micro hardness of 1012.8 VHN was obtained in the 0.5% Y sample in as milled condition while a minimum of 270.6 VHN was found in as milled condition annealed at 1200oC .
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5. Conclusions :
The hardness increases with increase in the amount of Y may be due to the mechanism of grain refinement strengthening and solid solution hardening The hardness decreases with increase in annealing temperature. However, the rate of decrease in hardness with increase in annealing temperature is less with increase in amount of yttrium added.
When considering different annealing temperatures, the crystallite size is found to decrease once the atom % Y exceeds 0.25 % , due to segregation of yttrium at the grain boundaries which pins the grain growth ,except for annealing at 700ºC. On increasing the temperature of annealing, the crystallite size increases till 900 oC and then decreases. While considering different compositions, it is observed that the crystallite size obtained in samples having 0.5 % Y is significantly less than that of pure Fe and 0.25 atom% Y. The crystallite size of Fe-0.25 atom% Y is marginally below pure Fe till 500 oC but is considerably less when annealed at higher temperatures. This shows that the pinning effect of Y is more effective when atom % Y exceeds 0.25%.
The lattice strain increases sharply with increase in yttrium content, when annealed at 500 oC and in as milled state. On further increasing of annealing temperature, it is found that the lattice strain increases upto yttrium content of 0.25 atom % and on further yttrium addition , the lattice strain decreases ; as in cases of annealing at 700 oC and 900oC. When annealed at 1200 oC , there is a decrease in lattice strain due to high temperature annealing which dominates the increase due to addition of yttrium. .40
6.Scope for further work
1. Further tests can be done by changing the parameters of mechanical milling like milling time, ball to powder weight ratio or addition of a process control agent and then optimizing the results.
2. There is further scope for improvement in mechanical properties if compaction can be done using advanced techniques like hot isostatic pressing.
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Referrences :
1. K.A. Darling, B.K. VanLeeuwen, J.E. Semones, C.C. Koch, R.O. Scattergood, L.J. Kecskes, S.N. Mathaudhu, Materials Science and Engineering A 528 (2011) 4365– 2. Prashanth KG, Scudino S, Murty BS, Eckert J. Crystallization kinetics and
consolidation of mechanically alloyed Al70Y16Ni10Co4 glassy powders, Journal of Alloys and Compounds, 477 (2009): pp. 171-177
3. Hasan Kotan, Kris A. Darling, Mostafa Saber, Carl C. Koch, Ronald O. Scattergood, Journal of Alloys and Compounds 551 (2013) 621–629
4. LIU Dong-hua, LIU Yong, ZHAO Da-peng, WANG Yan, FANG Jing-hua, WEN Yu- ren, LIU Zu-ming, Transactions Nonferrous Met. Soc. China 20(2010) 831-838 5. H. Kotan, M. Saber, C.C. Koch, R.O. Scattergood, Materials Science and Engineering
A 552 (2012) 310– 315
6. D. Cui, J. Jiang, G. Cao, E. Xiao, X. Qu, J. Univ. Sci. Technol. Beijing 15 (2008) 150–154.
7. C. C. Koch, Nanostructured Materials. Vol. 9. pp. 13-22.1997
8. M.A. Meyers, A. Mishra, D.J. Benson, Prog. Mater. Sci. 51 (2006) 427–556.
9. Suryanarayana C. Mechanical alloying and Milling, Progress in Materials Science 46 (2001): pp. 1-184
10. C. C. Koch. (2002). Nanostructured Materials - Processing, Properties, and Applications. William Andrew Publishing.
11. K. Wolski, G. Le Caer, P. Delcroix, R. Fillit, F. Th6venot, J. Le Coze, Materials ScienceandEngineeringA 207 (1996) 97-104
12. F. Tehrani, M.H. Abbasi, M.A. Golozar, M. Panjepour, Materials Science and Engineering A 528 (2011) 3961–3966
13. F. Hadefa, A. Otmania, A. Djekounb, J.M. Grenèche, Materials characterization 62 (2011)751 – 759
14. J.Z. Jiang, C. Gente, R. Bormann, Materials Science and Engineering A242 (1998) 268–277
15. T. R. Malow and C. C. Koch, Acta mater. Vol. 45, No. 5, pp. 2177-2186, 1997 16. Atkinson, H. V., Acta metall., 1988, 36, 469-491
17. Bolling, G. F. and Winegard, W. C., Acta metall. 1958,6, 283-287. 18. Grey, E. A. and Higgins, G. T., Actametall, 1973, 21,309-321.
42
20. H. Natter, M.-S. Loffler, C.E. Krill and R. Hempelmann, Scripta mater. 44 (2001) 2321–2325
21. J. M. Lifshitz and V. V. Slyozov, J. Phys. Chem. Solids 19, 35 (1961) 22. C. Wagner, Z. Electochem. 65, 581 (1961)
23. J. C. Ion, K. E, Easterling and M. F. Ashby, Acta metall. 32, 1949 (1984). 24. K. Lu, Z.F. Dong, I. Bakonyi, A. Cziraki, Acta Metall. Mater. 45 (1997) 2177
25. K.A. Darling, B.K. VanLeeuwen, J.E. Semones, C.C. Koch, R.O. Scattergood, L.J. Kecskes, S.N. Mathaudhu, Materials Science and Engineering A 528 (2011) 4365– 4371
26. Phase transformations in metals and alloys, D.A. Porter, & K.E. Easterling, Chapman & Hall, p140-142.
27. Kunok Chang, Long-Qing Chen, Modelling and simulation in material science and engineering 20 (2012) 055004 (11pp)
28. N. Moelans, B. Blanpain, P. Wollants, ActaMaterialia 55 (2007) 2173–2182
29. AzmirHarun, Elizabeth A. Holm, Mike P. Clode, Mark A. Miodownik, ActaMaterialia 54 (2006) 3261–3273
30. Zheng Chen, Feng Liu, Xiaoqin Yang, ChengjinShen, Yu Fan, Journal of Alloys and Compounds 509 (2011) 7109–7115
31. Kris A. Darling, Ryan N. Chan, Patrick Z. Wong, Jonathan E. Semones, Ronald O. Scattergood and Carl C. Koch, ScriptaMaterialia 59 (2008) 530–533
32. A. Molinari, S. Libardi, M. Leoni, P. Scardi, ActaMaterialia 58 (2010) 963–966 33. J. Svejcar, E. Dorazil, P. Pacl, P. Horky, Journal of Magnetism and Magnetic
Materials 19 (1980) 45-48
34. Heather A. Murdoch, Christopher A. Schuh, ActaMaterialia 61 (2013) 2121–2132 35. G.M. Pharr, W.C. Oliver, MRS Bulletin, 7 (1992), p. 28
36. H.R. Wilde, A. Wehrstedt, Materialprufung, 42 (2001), p. 468
37. C W Nieman, J R Weertman & R W Siegel, scripta Met. et Muter. 23(12),2013 (1989)
38. S K Ganapathi. and D A Rtgney, Scripta Met. Er Mater. 24,1675 (1990) 39. X Y Qin, X J Wu & L D Zhang, Nanostructered Materials, 5 (I), 101 (1995) 40. Yuchui Xiao & R James Kirkpatrick, J. Muter. Res. 10, 10 (1995)