Abstract
Selective Laser Melting was successfully used as a fabrication method to produce Ni-Mn-Ga and Ni-Mn-Ga-Fe ferromagnetic shape memory alloys. The starting material in a powder form with an average particle size of about 17.6 µm was produced by milling of as melt-spun ribbons. The microstructure, phase composition, and martensitic transformation behavior of both powder precursors and laser melted alloys were investigated by several methods, including high energy X-ray diffraction, electron microscopy, and vibrating sample magnetometry. The as laser melted materials are chemically homogenous and show a typical layered microstructure. Both alloy compositions have a duplex structure consisting either of austenite and 10M martensite (Ni-Mn-Ga) or a mixture of 14M and NM martensitic phases (Ni-Mn-Ga-Fe), contrary to the as milled powder precursors showing fcc structure in both cases. The forward martensitic transformation takes place at 336 and 325 K for Ni-Mn-Ga and Ni-Mn-Ga-Fe, respectively, while the magnetic response is much stronger for Ni-Mn-Ga than for the quaternary alloy. The results show that Selective Laser Melting allows for producing of good quality, homogenous materials. However, their microstructural features and consequently shape memory behavior should be tailored by additional heat treatment.
Highlights
Ferromagnetic Ni-Mn-Ga shape memory alloys capable of 12% magnetic field induced strain, high-frequency response, and extended fatigue lifetime (~2 × 109 cycles) were widely studied in the past two decades as promising candidates for smart materials devices [1,2]
ResPualrttsicalnedshDaipsecuasnsdiosnize distribution of the initial powder source is illustrated through the ePxaermticplelasrhyaNpei-aMnnd-sGizaepdoiwstrdiebrut(iFoinguorfeth1ea)i;ntihtiealrpesouwltdsefrosrotuhreceNisi-Milluns-Gtraat-eFde tphorwoudgehrs thweereexesmimpillaarryaNndi-Mthnu-sGaarepnowotdsehro(wFing.uProew1ad)e;rthceharerascutletrsisfotircsthweeNrei-eMvnal-uGaat-eFde fproomwdSeErMs wimeraegseimainlaalryasnisd(tFhiugsuraere1nao).t Isthohwasn.bPeeonwdfoeurncdhatrhaacttepriasrtitcicslewsereexheivbailtuaatetydpfircoaml cSoEaMrse immaogrpehaonlaolgyys,is a(Fnidgutrhee1iar ).mItehanas bsiezeen, foesutnimdattheadt pbaarstiecdlesoenxhniboirtmaatlypaiscyalmcmoaertsreicmsoirz-e pdhiosltorigbyu, tainodn,thweairs m17e.a3nμsmize(,Feigstuimrea1tebd).bTahseedhoignhneostrmfraelqauseynmcymoeftrpicasritziceledissitzreibsuwtiaosn,inwtahse 1r7a.3ngμemo(fF1ig–u25reμ1mb),. wThheilehiaghsemstalflreaqmuoeunncyt ooff ppaarrttiicclleessiwzeasswaabsovine t4h0e μramng(eFiogfu1r–e251bμ)m
Phase compositions of as milled, as milled and annealed and selected laser melted NiMn-Ga and Ni-Mn-Ga-Fe powders were further examined with high-energy synchrotron radiation (Figure 4)
Summary
Ferromagnetic Ni-Mn-Ga shape memory alloys capable of 12% magnetic field induced strain, high-frequency response (kHz), and extended fatigue lifetime (~2 × 109 cycles) were widely studied in the past two decades as promising candidates for smart materials devices [1,2]. More recently, semi-constrained polycrystalline Ni-Mn-Ga alloys, produced as foams, have been shown to yield up to 8.7% recoverable strain [3], which has subsequently triggered vivid research interest in the development of alternative synthesis routes for Ni-Mn-Ga alloys This interest is increased by prospects of a less time-consuming and more predefined geometry-specific manufacturing approach as opposed to single crystal growth. One has to bear in mind though, that heavy pulverization, improving powder size and particle shape uniformity, can simultaneously severely degrade the structure, stabilize the martensite phase, or even suppress martensitic transformation [27,28] Such radical structure interference would entail careful optimization of 3D printing regime and application of additional heating stage for homogenization and structure recovery [26]. It is demonstrated that through 3D printing, some functional behavior of Ni-Mn-Ga and Ni-Mn-Ga-Fe is restored, but additional heat treatment is unavoidable for maximizing the ultimate response
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