Effect of High Speed High Pressure Torsion ... - Science Direct

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ScienceDirect Materials Today: Proceedings 2S (2015) S897 – S900

International Conference on Martensitic Transformations, ICOMAT-2014

Effect of high speed high pressure torsion parameters on grain refinement of coned shape Fe based SMA active elements G. Guraua, C. Guraua, F. M. B. Fernandesb, L. G. Bujoreanuc,* a Faculty of Engineering, “Dunărea de Jos” University of Galati, Galaţi,800201, Romania CENIMAT/I3N, Faculdade de Ciências e Tecnologia (FCT), UNL, 2829-516-Monte de Caparica, Portugal c Faculty of Materials Science and Engineering, The "Gheorghe Asachi" Technical University of Iaşi, Romania b

Abstract High Speed High Pressure Torsion (HS-HPT) refers to processing that involves a combination of high pressure with torsional straining. This technique allows the efficient grain refinement on shape memory alloy brittle (CuAlNi) or with very low plasticity (FeMnSiCr). HS-HPT involve slippage between anvils and sample this means the SPD occurs at specific temperature. The recrystallization during HS-HPT on shape memory alloys determines their functionality without supplemental heat treatments. The samples manufactured by HS-HPT are discs, rings or coned shaped, 20-40 mm in diameter and 1mm - 0.2 mm thickness. Varying HS-HPT parameters coned shaped elements with increasing deformation degree were produced from Fe28Mn6Si7Cr [mass %] shape memory alloy. Scanning electron microscopy (SEM) observations, highlighted ultrafine structure. Experiments show that the mechanical properties of the alloy can be enhanced by HS-HPT. © 2014 The Authors. Published by Elsevier Ltd. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license Selection and Peer-review under responsibility of the chairs of the International Conference on Martensitic Transformations (http://creativecommons.org/licenses/by-nc-nd/4.0/). This is Peer-review an open access article under theofCC license (http://creativecommons.org/licenses/by-nc-nd/3.0/). 2014. Selection and under responsibility the BY-NC-ND chairs of the International Conference on Martensitic Transformations 2014. Keywords: Severe Plastic Deformation; High Pressure Torsion; Shape Memory Alloy; Martensitic transformation; FeMnSiCr; Grain refining

1. Introduction Materials nanostructuring by Severe Plastic Deformation (SPD) has been a breakthrough in materials processing. High Speed High Pressure Torsion (HS-HPT) is a severe plastic deformation method based on traditional HPT [1-

* Corresponding author. Tel.: +40-727-48-64-06 E-mail address: [email protected]

2214-7853 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and Peer-review under responsibility of the chairs of the International Conference on Martensitic Transformations 2014. doi:10.1016/j.matpr.2015.07.426

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G. Gurau et al. / Materials Today: Proceedings 2S (2015) S897 – S900

4]. Regarding HPT, few limits can be identified. One of them is the shape (generally disks or rings) and dimensions of the samples. Another one are referring at the plasticity of metals and alloys processed. The first limitation was overcome by newly developed HS-HPT method, by obtaining coned disk shape samples, as shown in the following.This technology is dedicated especially for shape memory alloys generally brittle or difficulty deformable and aims to reduce grain size up to nanoscale. The fact of reducing the grain to submicron or nanometer size gives many technological advantages determined by significant enhancement of properties. The essential parameters of HS-HPT are: (i) the initial pressure; (ii) the rotation speed of the superior anvil applied to the samples; (iii) the temperature developed by friction between the sample and the anvils; (iv) the number of revolutions after heating caused by friction; and (v) final pressure the rotation speed. The force and displacement of the inferior anvil should be strictly correlated, too. The best results are obtained for the operating mode that applies a large force from the beginning, and maintains it throughout the deformation. The brittle alloys require low initial strength, increasing in process. The fundamental difference between classic HPT and HS-HPT consists in implementation of an elevated rotation speed of the order of hundreds rpm and the slippage of superior anvil and sample. The temperature developed by friction between the sample and the anvils increases up to 8000C and the number of revolutions is under one revolution. Correlation between these parameters determines the final shape of the sample and its structure. HS-HPT is a reliable method which leads to reproducible results on the strength of high automation which integrates temperature, displacement and force sensors, being able to produce large disks rings or coned shaped samples, even starting from shape memory alloys in cast state. This technology allows fabrication of disks with diameters over 35 mm up to 45 mm, noticeable larger as compared to classic HPT. 2.Experimental The HS-HPT machine (Figure 1) used in present experiments has been described in previous paper extensively [5]. Rotation speed was varied between 410 and 1900 rpm and the pressure levels were selected in range 0.225, to 3.310 GPa, respectively.

Fig. 1. Active part of HS-HPT machine (a and c), as received FeMnSiCr in cast state, crowned shape and severe plastic deformed active cone elements (b and d).

The maximum force applied was 127 kN. During HS-HPT processing, torque, speed (Figure 2a and b) and force were monitored (Figure 3). The true plastic strain achieved in the processed material is estimate using the relationship: =ln(hi/hf,) where hi and hf denote initial and final thickness of the sample, respectively. The true strains achieved in case of Fe28Mn6Si5Cr were 0.15 up to 2.86.


Fig. 2. Variation of motor parameters in HS-HPT parameters (SMA FeMnSiCr, logarithmic strain 1.41) .

The structural observations aiming to emphasize processing effects were performed by Zeiss scanning electron microscopy (SEM) on as-cast as well as after subjected to HS-HPT processing with 2.33 and 2.86 deformation degrees.

Fig. 3. Force versus time plots (a) logarithmic strain 2.86, (b) logarithmic strain 1.41.

3. Results and discussions The method is used in order to lead to range ultrafine-grained materials (UFG) [6-8]. Based on severe plastic deformation considerations, also the original HS-HPT technology introduces a variety of extrinsically lattice defects, which modify martensitic transformation parameters, and enhance precipitation. The alloys subjected to HS-HPT, reduced their grain size due to microstructure fragmentation by compression and rotation. The UFG materials contain especially a very high density of grain boundaries in structure which is considered the significant key in the novel properties manifested [9,10]. At the beginning of HS-HPT process, the samples as well as the tools are at ambient temperature. The technology involves heat generation by intense friction between the anvils and the sample. The SPD takes place quickly, when the plasticity threshold is attained. The entire process last between 2 to 20 s, closely related to true strain achieved. Throughout deformation the massive heat enables dynamic recrystallization but leads to strong grain refinement [6, 7, 11]. Even if HS-HPT is applied at ambient temperature it is equivalent to the treatment at an elevated temperature, named effective temperature, T eff.

899

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20 m

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Fig. 4. SEM micrographs FeMnSiCr SMA (a) in cast state and after HS-HPT with logarithmic strain: 2.33 (c) and (d), 2.86.

At this temperature may also be induced mass transfer similar to diffusion, explained by an increased concentration of defects during high pressure torsion [12]. It is known that the annealing at elevated temperature leads to the grain growth. However, the severe plastic deformation enables the prevalence of fine grain, caused by atomic movements for the material under higher deformation [13]. SEM observations were performed in order to study the morphological changes caused by deformation, on HSHPT structure. The resulting micrographs on the samples were being shown in Figure 4. With increasing deformation degree to 2.86, generalized fibering can be observed while neither grains boundaries nor observable size-individual grains are present. The Vickers hardness values on diameter increased slightly from the center (300 HV) to peripheral zone (380 HV); the gradient on diameter was approximately 16 HV/ mm. This occurs mostly due to the temperature's asymmetric profile of dies that reduce thermal field on the edge [14]. 4. Conclusions The technological parameters of HS-HPT method were described systematically. Different amounts of deformation were imposed. A large number of determinations on shape memory alloys show that HS-HPT leads to obtain fine and ultrafine structures. It is possible to process FeMnSiCr cone shape elements having thickness below 0.5 mm and different grain refinement, recommended for miniaturization solutions in modular systems. The parts achieved are larger and more homogeneous plastic deformation than in case of HPT. Acknowledgements This work was financially supported by UEFISCDI by means of the project PN.II-PT-PCCA-2011-3.1-0174, contract 144/ 2012. References [1] R.Z. Valiev, Y. Estrin, Z. Horita, T.G. Langdon, M.J. Zehetbauer, Y.T. Zhu, JOM 4 (2006) 33–39. [2] J.M. Jani, M. Leary, A. Subic, M.A. Gibson, Mater Des. 56 (2014) 1078–1113. [3] T. Waitz, V. Kazykhanov, H.P. Karnthaler, Acta Mater. 52 (2004) 137–47. [4] A.P. Zhilyaev, T.G. Langdon, Prog. Mater. Sci. 53 (2008) 893–979. [5] G. Gurău, C. Gurău, O. Potecasu, P. Alexandru, L.-G. Bujoreanu, J. Mater. Eng. Perform. 23 (2014) 2396–2402. [6] K.K. Mahesh, F.M. Braz Fernandes, G. Gurau, J. Mat. Sci. 47 (2012) 6005–6014. [7] K.K. Mahesh, F.M. Braz Fernandes, R.J.C. Silva, G. Gurau, Phys. Procedia. 10 (2010) 22–27. [8] F.M. Braz Fernandes, K.K. Mahesh, R.J.C. Silva, C. Gurau, G. Gurau, Phys. Status Solidi C 7 (2010) 1348–1350. [9] X. Sauvage, G. Wilde, S.V. Divinski, Z. Horita, R.Z. Valiev, , Mater. Sci. Eng. A 539 (2012) 22–29. [10] Y. Estrin, A. Vinogradov, Acta Mater. 61 (2013) 782–817. [11] F.M.J. Starink, X. Cheng, S. Yang, Acta Mater. 61 (2013) 183–192. [12] B. Straumal, A.R. Kilmametov, Y.O. Kucheev, K.I. Kolesnikova, A. Korneva, P. Zieba, B. Baretzky, JETP Letters 100 (2014) 376–379. [13] B. Straumal, A. Korneva, P. Zieba, Arch. Civ. Mech. Eng. 14 (2014) 242–249. [14] D. Rao, K. Huber, J. Heerens, J.F. dos Santos, N. Huber, Mater. Sci. Eng. A 565 (2013) 44–50.