Blanco Rodríguez, Jesús Ángel

Gorría Korres, Pedro

Martínez Blanco, David

Smith, Ronald I.

Repositorio Institucional de la Universidad de Oviedo

This content has been downloaded from IOPscience. Please scroll down to see the full text.Download details:IP Address: 156.35.192.3This content was downloaded on 17/03/2015 at 08:24Please note that terms and conditions apply.Investigating the martensite-austenite transformation on mechanically alloyed FeNi solidsolutionsView the table of contents for this issue, or go to the journal homepage for more2011 J. Phys.: Conf. Ser. 325 012019(http://iopscience.iop.org/1742-6596/325/1/012019)Home Search Collections Journals About Contact us My IOPscience      Investigating the martensite-austenite transformation on mechanically alloyed FeNi solid solutions D. Martínez-Blanco1, P. Gorria2, J. A. Blanco2, R. I. Smith3 1Unidad de Magnetometría, SCT´s, Univ. Oviedo, Julián Clavería 8, 33006 Oviedo, Spain. 2 Departamento de Física, Univ. Oviedo, Calvo Sotelo s/n, 33007 Oviedo, Spain. 3 ISIS facility, RAL, Chilton, Didcot, Oxon OX11, 0QX, United Kingdom. E-mail: pgorria@uniovi.es Abstract. The martensite-austenite transformation on Fe70Ni30 and Fe75Ni25 nanostructured solid solutions has been investigated by neutron thermo-diffraction experiments carried out between 300 and 1000 K. We observe that the difference between the temperatures at which the martensitic transformation starts (Ai) and finishes (Af) exceeds 250 K, being five times larger than that of the as-cast coarse-grained conventional alloys. The main reason for this striking phenomenon is the drastic microstructural changes produced during the severe mechanical milling process, giving rise to a large reduction of the crystalline mean size (below 20 nm) and the generation of a considerable microstain (reaching 1%). 1.  Introduction Mechanical alloying technique has been largely used for the synthesis of metastable Fe binary metastable alloys [1], either in amorphous [2] or nanocrystalline states [3,4], even in the case of solid solutions with almost vanishing miscibility of the elemental constituents [5,6]. Moreover, these alloys have attracted great interest because very often they exhibit anomalous magnetic behaviour, such as the invar anomalies observed in Fe-Cu [7]. In particular, Fe100-xNix solid solutions obtained by means of mechanical alloying or ball milling is one of the most studied Fe-binary systems during the last two decades [8-13]. The crystal structure of these compounds depends on the relative percentage of Fe and Ni in the composition, being face centred cubic (FCC) for x ≥ 35, body centred cubic (BCC) for x ≤ 20 and a mixture of both BCC and FCC for intermediate nominal compositions [8,10]. The microstructure of these powder materials at the sub-micrometer length scale consists of agglomerated grains with mean crystalline sizes below 20 nm, favouring the formation of heterogeneous and highly disordered grain boundaries [14-16]. The reduction of the mean crystalline size makes more relevant the role played by the grain boundaries in the macroscopic physical-chemical properties, which in many cases largely differ from those of coarse-grained alloys with the same composition [17-21]. On the other hand, it is well known that BCC FeNi alloys, the martensitic phases, prepared by conventional annealing undergo the so-called MA transformation (BCCàFCC) on heating. The initial, Ai, and final, Af, temperatures have been chosen as those corresponding to the existence of 10 and 90% of the FCC (austenitic) phase in the powder sample, respectively. The difference, ΔT = Af - Ai, is composition-dependent and does not exceed 50 K in any case [22]. Moreover, on cooling from 5th Meeting of the Spanish Neutron Scattering Association IOP PublishingJournal of Physics: Conference Series 325 (2011) 012019 doi:10.1088/1742-6596/325/1/012019Published under licence by IOP Publishing Ltd 1      high temperatures, the reverse structural transformation (FCCàBCC) takes place with an important hysteresis in temperature. Previous thermomagnetic measurements have revealed that this MA transition is observed also in FeNi mechanically alloyed samples [8,23,24]. However, most of the studies concerning the MA structural transformations, have been carried out by using room temperature RT) x-ray diffraction and Mössbauer spectroscopy on samples that have been previously annealed at high temperatures and then cooled down to RT [25-27]. On the other hand, powder neutron thermo-diffraction is considered as a fundamental tool for in situ studies of the temperature dependence of nuclear and/or magnetic structure in metastable systems [28]. Hence, in the present paper we report a detailed structural analysis of MA transformation on Fe70Ni30 and Fe75Ni25 powder samples together with the temperature-induced microstructural relaxation taken place in the martensitic phases of these as-milled alloys during heating up from RT to 1000 K. 2.  Experimental details Powders of elemental Fe, with maximal particle size of 60 µm and purity of 99.95%, and Ni, with average particle size of 7 µm and purity of 99.9%, were milled in a high-energy ball mill (Retsch PM/400) during 50 h under controlled Ar atmosphere. The structural characterization was performed using “in situ" neutron thermo-diffraction experiments at the POLARIS time of flight diffractometer (ISIS Facility, UK) in the temperature range from RT to 1000 K. The samples were heated under vacuum at a heating rate of 2 K/min and each diffraction pattern was collected every 5 min. The Rietveld refinement of the diffraction patterns was performed using the FullProf Suite package [29] on patterns measured on the detector bank C (that with the higher experimental resolution) of POLARIS instrument. The interplanar distance range, from 0.3 to 3.2 Ǻ, allows us to fit at least 35 reflections for BCC and/or FCC crystal structures. Nevertheless, a reduced d-space range (0.75-2.25 Ǻ) is showed on the diffraction figures of the article in order to better appreciate the profile of the Bragg peaks. 3.  Results and discussion The figure 1 shows the neutron thermo-diffractograms of the Fe70Ni30 and Fe75Ni25 samples collected during the heating up from room temperature. In the as-milled state a mixture of both BCC and FCC phases is present in the powder samples (see fig. 2), with an approximate 78(1) and 81(1)% of BCC phase, respectively (see fig. 3). The MA structural transformation of the majority BCC phase takes place approximately between 600 and 850 K, where the Bragg reflections associated with the FCC (BCC) phase increase (decrease) their intensity. The Bragg reflections of the BCC phase disappear at high temperatures (above 850 K). This progressive transformation is better observed in Figure 1. In addition, a continuous sharpening of the BCC Bragg peaks, due to the temperature induced microstructural relaxation of the as-milled BCC nanostructure, is also observed along the similar temperature range. Moreover, the BCC and FCC phases in the as-milled samples seem to have roughly the same composition as indicated by their almost equal values for the respective lattice parameters. On the other hand, Figure 3 displays the relative phase percentages and the microstructural analysis obtained from the fits of the neutron diffraction patterns of Fe70Ni30 and Fe75Ni25 samples, respectively. The mean maximum strain and the mean crystalline size for the BCC phase have been determinate by line-profile analysis using a linear combination of spherical harmonics and Stephens´ formulation [30]. It is worth noting the following features: (i) the MA transformation spreads out in temperature on both samples for more than 250 K, being five times larger than the values observed on coarse-grained conventional alloys [22]; (ii) an interconnection between the martensitic transformation and their microstructural relaxation can been established. Hence, the microstructure of the metastable as-milled solid solutions, characterized by a large maximum strain (~1%) and a small crystalline size (< 20 nm), is responsible for a large broadening of the Bragg peaks (see figures 1 and 2). Concerning the huge enlargement of the temperature range for the MA transformation, it could be related to both the high concentration of crystalline defects and the presence of extended grain boundaries. 5th Meeting of the Spanish Neutron Scattering Association IOP PublishingJournal of Physics: Conference Series 325 (2011) 012019 doi:10.1088/1742-6596/325/1/0120192      5006007008009002.02 2.12(110)(111)d (Å)1.02.21.21.41.61.82.00.8300Temperature(K)9007005001000400600800Intensity(a.u.)(a)Intensity(a.u.) d (Å)1.02.21.21.41.61.82.00.8300Temperature(K)90070050010004006008005006007008009002.02 2.12(110)(111)Intensity(a.u.)(b)Intensity(a.u.) Figure 1. 3D-plot showing the neutron thermo-diffractograms for the Fe70Ni30 (a) and Fe75Ni25 (b) samples collected from RT to 1000 K. The right-top insets show the contour plots of the temperature dependence of the interplanar distance for the Bragg reflections (110) and (111) characteristic for the BCC and FCC crystal structures, respectively, in the temperature range marked with dashed lines in the figure.  5th Meeting of the Spanish Neutron Scattering Association IOP PublishingJournal of Physics: Conference Series 325 (2011) 012019 doi:10.1088/1742-6596/325/1/0120193      In our case, the high values of the maximum strain reached on these materials provokes a decrease of at least 100 K in the Ai respect to that observed on coarse-grained conventional alloys. Furthermore, the degree of anisotropy of the microstrain and the existence of structural defects located at the grain boundaries lead to a temperature-expanded martensite-austenite transformation due to the step-by-step individual transformation of remaining isolated particles with lower values for the microstrain. On the other hand, as the density of structural defects and the size of the grain boundaries are being progressively reduced during the temperature-induced relaxation, the mean crystalline size of the BCC domains grows following an exponential trend. The latter is produced by a coalescence of neighbor BCC nanograins. 1062 1063 1064 1065 106Rwp=2.7%Rexp=2.6%(a)Fe70Ni30 (initial)Rwp=2.4%Rexp=1.6%(b)Fe75Ni25 (initial)0.75 1.00 1.25 1.50 1.75 2.00 2.251062 1063 1064 1065 106Rwp=2.8%Rexp=1.6%(c)Fe70Ni30 (final)0.75 1.00 1.25 1.50 1.75 2.00 2.25Rwp=3.3%Rexp=2.4%(d)Fe75Ni25 (final)Intensity (arb. units)d (Å) Figure 2. Observed (red crosses) and calculated (blue solid line) room temperature neutron powder diffraction patterns for the Fe70Ni30 (a-c) and Fe75Ni25 (b-d) of the samples in the as-milled state (initial) and after heating process (final). Positions of the Bragg reflections belonging to FCC and BCC crystal structures are represented by vertical bars (first and second series, respectively). The difference (observed-calculated pattern) is depicted at the bottom of each figure.  Finally, if we compare the room temperature neutron diffraction patterns corresponding to the as-milled and that of the samples after the heating process (see figure 2) the most noticeable features are: (i) a large sharpening of the Bragg peaks have occurred due to the microstructural relaxation and (ii) while in the Fe70Ni30 sample only the FCC phase is observed (the reverse BCC-FCC transformation should occur at lower temperatures), the situation in the Fe75Ni25 sample is much more complex, because both BCC and FCC coexist at room temperature, indicating that the BCC-FCC transformation has not been fully completed yet. 5th Meeting of the Spanish Neutron Scattering Association IOP PublishingJournal of Physics: Conference Series 325 (2011) 012019 doi:10.1088/1742-6596/325/1/0120194      020406080100FCC-Fe70Ni30BCC-Fe70Ni30Relative weigth percentage90 %10 %AiAf(a)020406080100FCC-Fe75Ni25BCC-Fe75Ni25Relative weigth percentage90 %10 %AiAf(b)0.00.20.40.60.81.01.21.4050100150200300 400 500 600 700 800 900 1000Maximum Strain, <ε> (%)Mean Size, <τ> (nm)(c)BCC-Fe70Ni300.00.20.40.60.81.01.21.4050100150200300 400 500 600 700 800 900 1000Maximum Strain, <ε> (%)Mean Size, <τ> (nm)(d)BCC-Fe75Ni25Temperature (K) Figure 3. Temperature evolution of the relative phase percentages (top) and microstructure (mean maximum strain, < ε >, and mean crystalline size, < τ >) for the BCC phases of the Fe70Ni30 and Fe75Ni25 samples. Vertical bars on microstructure plots represent the degree of anisotropy, not the estimated error. 4.  Summary and conclusions High-energy ball milling has been used to synthesize Fe70Ni30 and Fe75Ni25 metastable solid solutions with a mixture of BCC and FCC crystal structures on the as-milled state. Their metastable nature largely affects the thermodynamical equilibrium phase diagram, in particular the martensitic transformation of the BCC phases. This transformation takes places at lower temperatures (≈ 550 K) and is enlarged over a wider temperature range (250 K) than in conventional cold-rolled FeNi alloys. The latter, as revealed by neutron thermo-diffraction experiments, is clearly correlated with the temperature-induced microstructural relaxation.  Acknowledgements The authors would like to thank FEDER and Spanish MICINN for financial support under project MAT2008-06542-C04-03. The SCTs at the University of Oviedo are also acknowledged. We also thank ISIS facility for neutron beam time. References [1] Zhang D L 2004 Prog. Mater. Sci. 49 537. [2] Gorria P et al. 2009 Phys. Status Solidi (RRL) 3 28. [3] Gorria P et al. 2009 Acta Mater. 57 1724. [4] Álvarez P et al. 2010 J. Phys.: Condens. Matter 22 216005. [5] Ma E 2005 Prog. Mater. Sci. 50 413. 5th Meeting of the Spanish Neutron Scattering Association IOP PublishingJournal of Physics: Conference Series 325 (2011) 012019 doi:10.1088/1742-6596/325/1/0120195      [6] Gorria P et al. 2005 Phys. Rev. B 72 014401. [7] Gorria P et al. 2004 Phys. Rev. B 69 214421. [8] Kuhrt C, Schultz L 1993 J. Appl. Phys. 73 1975. [9] Hong L, Fultz B 1996 J. Appl. Phys. 79 3946. [10] Kaloshkin S D et al. 2001 Physica B 299 570. [11] Shilling P J et al. 2003 Phys. Rev. B 68 224204. [12] Gorria P et al. 2005 J. Mag. Magn. Mater. 294 159. [13] Guittoum A et al. 2008 Philosophical Magazine 88 1085. [14] Pekala M et al. 1999 J. Non-Crystal. Solids 250-252 757. [15] Hamzaoui R et al. 2003 Mater. Sci. Eng. A 360 299. [16] Valderruten J F et al. 2006 Physica B 384 316. [17] Jartych E et al. 2000 J. Mag. Magn. Mater. 208 221. [18] Hamzaoui R et al. 2004 Mater. Sci. Eng. A 381 363. [19] Liu Y et al. 2007 Eur. Phys. J. Appl. Phys. 37 197. [20] Gorria P et al. 2009 Phys. Rev. B 80 064421. [21] Gorria P et al. 2009 Phys. Status Solidi (RRL) 3 115. [22] Magnetic Properties of Metals, d-Elements, Alloys and Compounds 1991 ed. H P J Wijn  (Springer, Berlin). [23] Pekala M et al. 1999 Nanostructured Mater. 11 789. [24] Martinez-Blanco D et al. 2007 J. Mag. Magn. Mater. 316 328. [25] Hays V et al. 1999 Nanostructured Mater. 7 411. [26] Kaloshkin S D et al. 2001 J. Non-Crystal. Solids 287 239. [27] Zhu L et al. 2001 J. Mater. Sci. 36 5571. [28] Fernández Barquín L et al. 1998 J. Phys.: Condens. Matter 10 5027. [29] Rodríguez-Carvajal J 1993 Physica B 192 55. [30] Martínez-Blanco D et al. 2008 J. Phys.: Condens. Matter 20 335213. 5th Meeting of the Spanish Neutron Scattering Association IOP PublishingJournal of Physics: Conference Series 325 (2011) 012019 doi:10.1088/1742-6596/325/1/0120196

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Investigating the martensite-austenite transformation on mechanically alloyed FeNi solid solutions

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Investigating the martensite-austenite transformation on mechanically alloyed FeNi solid solutions

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