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Improving microstructure and mechanical behaviour of polycrystalline Ni50Mn29Ga21 after hot extrusion

Poster at MSE 2012, Materials Science Engineering, 25.-27. September 2012, Darmstadt
Verbesserung von Gefüge und mechanischem Verhalten von polykristallinem Ni50Mn29Ga21 nach dem Strangpressen
: Weisheit, Linda; Böhm, Andrea; Navarro Y De Sosa, Iñaki; Chulist, Robert; Skrotzki, Werner; Neugebauer, Reimund

Poster urn:nbn:de:0011-n-2159887 (192 KByte PDF)
MD5 Fingerprint: ec8711f510e5f99a675b841247145c48
Created on: 7.11.2012

2012, 1 Folie
International Conference "Materials Science Engineering" (MSE) <2012, Darmstadt>
Poster, Electronic Publication
Fraunhofer IWU ()
MSM; magnetic shape memory alloy; NiMnGa; polycrystal; heat treatment

In this article the results of improving microstructure, texture and mechanical behaviour of hot extruded polycrystalline Ni50Mn29Ga21 using post heat treatments and training methods are summarized. The aim is to produce a strongly textured coarse grained material that shows proper mechanical behaviour for magnetic-field-induced strain (MFIS). That means stress-strain curves with martensitic plateau on low stress levels with high strain values. To optimize the microstructure concerning grain size and texture, treatments were performed on cube shaped samples with a 5M modulated martensitic structure. The initial microstructure of the hot extruded material shows a grain size of 100 µm and a cyclic fibre texture along the extrusion direction measured using high-energy synchrotron radiation. The stress-strain curve demonstrates no martensitic plateau. To increase grain size and texture different heat treatments were applied. The results showed that only heat treatment at 1000°C leads to increased grain size. Using annealing without load a maximum grain size of about 300 µm was achieved. However, synchrotron measurements revealed a reduced texture. To achieve a stronger texture the samples were compressed to 5% strain at 1000°C and then further annealed without load for 60 min. This treatment leads to a maximum grain size of 4 mm. Applying a second annealing step under load at low temperatures produces a sharp texture. After these treatments the stress-strain curves exhibit a martensitic plateau at stress levels of about 10 MPa. For further mechanical improvement a thermo-mechanical training was used including compression in two directions and heating the material to 80°C for thermal resetting. This training leads to a stress-strain behaviour with more reduced martensitic plateau and a maximum residual strain of 6%. Hence, MFIS should be achieved and the results will be applicable to fabricate improved magnetically driven actuator materials by hot extrusion as efficient preparation route.
The work has been supported by the DFG within SPP 1239 (code: BO 1096/1-3) in Germany.