ISSN:
1662-8985
Source:
Scientific.Net: Materials Science & Technology / Trans Tech Publications Archiv 1984-2008
Topics:
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes:
Off-stoichiometric Ni2MnGa ferromagnetic shape memory alloys undergo a martensitictransformation (MT) from the L21 cubic phase to a martensitic crystal lattice consisting, in themajority of cases, of a periodic stacking sequence of nearly closed-packed planes with periodicityof 5, 7 or 2 planes, denoted as 10M (five layered tetragonal), 14M (seven layered orthorhombic) and2M (non-modulated tetragonal).In addition to the parent to martensite transformation, Ni-Mn-Ga alloys tend to show stress ortemperature induced intermartensitic transformations (IMTs) towards the most stable 2M phase,through the sequences 10M→14M→2M or 14M→2M depending on the first formed martensite.The IMTs reported in the literature show a variety of characteristics such as reversibility,completeness, hysteresis and temperature of occurrence, but, as a general trend, the role of internalstresses in favouring the occurrence of IMTs is recognised.Recently it has been shown that the L21 order degree favours the occurrence of theintermartensitic transformation from 14M to 2M martensite, stabilising the non modulatedmartensite through a decrease of its free energy with respect to the layered martensite. From thispoint of view, the occurrence of intermartensitic transformations in Ni-Mn-Ga alloys appears as a“chemical“ free energy effect. Aiming to go deeply into this aspect, in this work the occurrence ofIMTs and their properties have been examined for an extensive set of off-stoichiometric Ni2MnGa.The results show the existence of a relationship between the IMTs temperatures and the alloyscomposition, as well as the dependence of all observed IMTs (i.e., 10M→14M, 14M→2M and theircorresponding reverse transformations) on the L21 order degree
Type of Medium:
Electronic Resource
URL:
http://www.tib-hannover.de/fulltexts/2011/0528/01/41/transtech_doi~10.4028%252Fwww.scientific.net%252FAMR.52.47.pdf
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