A New Diffraction Approach To Crystal Structure Determination of Nano-twined Martensites

Olikhovska, L.; Ustinov, А.I.; Glavatska, N.; Glavatskyy, I.

doi:10.1051/esomat/200902025

“…Recently Ge et al [49] demonstrated gradual change of lattice parameters resulting from the coarsening of nanotwins during the 14M→NM transformation observed by TEM. All the mentioned effects can significantly influence the diffraction pattern and can result in an additional or missing diffraction peaks and consequent difficulties in lattice symmetry determination [50,51]. In this respect it is also interesting to note that according to Righi et al [52], the transformation of the 10M structure from commensurate to incommensurate did not result in sudden changes in a, b lattice constants.…”

Section: Limits Of the Used Lattice Approximationmentioning

confidence: 94%

The relation between lattice parameters and very low twinning stress in Ni₅₀Mn_25+xGa_25−xmagnetic shape memory alloys

Straka

¹

,

Drahokoupil

²

,

Pacherová

³

et al. 2015

Smart Mater. Struct.

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In search for the origins of the extraordinary low twinning stress of Ni-Mn-Ga magnetic shape memory alloys we studied the thermally induced changes of structure in Ni 50 Mn 25+x Ga 25−x (x=2.7-3.9) single crystal samples and compared them with twinning stress dependences. The alloys exhibited transformation to five-layered (10M) martensite structure between 297 to 328 K. All samples exhibited magnetic shape memory effect. Just below the transformation temperature the samples had very low twinning stress of about 0.1-0.3 MPa, which increased with decreasing temperature. The structural changes were monitored using X-ray diffraction in the temperature range 173-343 K. The 10M structure was approximated by monoclinic lattice with the unit cell derived from the cubic unit cell of the parent L2 1 phase. With decreasing temperature, the lattice parameters a and γ increased, c decreased, while b was nearly constant. For x ≤ 3.5, sudden sharp changes in a and b parameters additionally occurred, resulting in a = b in some regions of the phase diagram, which might be related to the refinement of twin structure of 10M martensite on nanoscale. The temperature dependences of lattice parameter γ (and c or c/a) correlate well with the temperature dependences of twinning stress in agreement with the prediction by a microstructural model of twin boundary motion. On the contrary, there is no correlation between (a − b) and twinning stress. This indicates no significant role of a/b twins or laminate in twin boundary motion mechanism and low twinning stress.

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“…Recently Ge et al [49] demonstrated gradual change of lattice parameters resulting from the coarsening of nanotwins during the 14M→NM transformation observed by TEM. All the mentioned effects can significantly influence the diffraction pattern and can result in an additional or missing diffraction peaks and consequent difficulties in lattice symmetry determination [50,51].…”

Section: Limits Of the Used Lattice Approximationmentioning

confidence: 99%

Thermally induced changes of structure in Ni$_{50}$Mn$_{25+x}$Ga$_{25-x}$ magnetic shape memory single crystals with very low twinning stress

Straka,

Drahokoupil,

Pacherová

et al. 2014

Preprint

0

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In search for the origins of the extraordinary low twinning stress of Ni-Mn-Ga magnetic shape memory alloys we studied the thermally induced changes of structure in Ni 50 Mn 25+x Ga 25−x (x=2.7-3.9) single crystal samples and compared them with twinning stress dependences. The alloys exhibited transformation to five-layered (10M) martensite structure between 297 to 328 K. All samples exhibited magnetic shape memory effect. Just below the transformation temperature the samples had very low twinning stress of about 0.1-0.3 MPa, which increased with decreasing temperature. The structural changes were monitored using X-ray diffraction in the temperature range 173-343 K. The 10M structure was approximated by monoclinic lattice with the unit cell derived from the cubic unit cell of the parent L2 1 phase. With decreasing temperature, the lattice parameters a and γ increased, c decreased, while b was nearly constant. For x ≤ 3.5, sudden sharp changes in a and b parameters additionally occurred, resulting in a = b in some regions of the phase diagram, which might be related to the refinement of twin structure of 10M martensite on nanoscale. The temperature dependences of lattice parameter γ (and c or c/a) correlate well with the temperature dependences of twinning stress in agreement with the prediction by a microstructural model of twin boundary motion. On the contrary, there is no correlation between (a − b) and twinning stress. This indicates no significant role of a/b twins or laminate in twin boundary motion mechanism and low twinning stress.

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“…While using the diffraction for characterization of the modulated structures, some notice should be given to the results of Monte Carlo modeling work for the nano-twinned orthorhombic Ni-Mn-Ga crystal [26,27]. The different appearance of the satellites and position shift of the main peaks together their splitting, pointed to that the five-layered modulated structure would compose of 3 and 2 nano-twin layers, while the seven-layered one consists of 5 and 2 layers.…”

Section: About Characterizationmentioning

confidence: 99%

Recent Development of the Magnetic Shape Memory Materials Research in Finland

Söderberg¹,

Aaltio²,

Ge³

et al. 2009

MRS Proc.

0

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Ni-Mn-Ga based magnetic shape memory (MSM) materials have been studied since 1998 in Finland at the Helsinki University of Technology (TKK, previously HUT). The large HUT-MSM-project resulted in MSM-alloys with high service temperature, 10 % field-induced-strain, as well as circumstances when and how a Ni-Mn-Ga alloy exhibits this phenomenon. The understanding of the structure and behavior of twin boundaries, and their role, for example, in the vibration damping and long-term actuation has been enhanced in the recent projects. Twin boundaries have been studied by XRD, by high-resolution transmission electron microscopy (HRTEM), and by in-situ straining in TEM, the last one in co-operation with the Institute of Physics in Prague (ASCR-IP), Czech Republic. The results obtained by neutron diffraction in cooperation with Hahn-Meitner-Institut Berlin, Institute for Metal Physics (IMP), Kiev, and Institut Laue-Langevin (ILL), Grenoble, have given new crystallographic information. Damping of Ni-Mn-Ga polymer composites has been proved to be excellent at high stiffness levels with the loss factor δ = 0.6 at E ≈ 1 GPa. This research was carried out in co-operation with the University of California Los Angeles (UCLA), USA. In the long-term actuation, a fatigue life of 2x10 9 has been recorded for a five-layered modulated Ni-Mn-Ga structure in mechanical cycling. The evolution of the MSM parameters during the long-term use is recorded and used as an input data for the models developed in the European MAFESMA co-operation. The search for alloys with wide stable thermal property range showing MSM effect has continued and alloys that are stable down to 4 K have been established. Modeling based on Ginsburg-Landau theory has been applied to evaluate aging and thermal fluctuations in the modulated Ni-Mn-Ga structures. As a commercial target, AdaptaMat Ltd. develops technology to produce Ni-Mn-Ga magnetic shape memory material with improved quality, lower twinning stress, longer fatigue life as well as lower cost and better availability for use in research and development.

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A New Diffraction Approach To Crystal Structure Determination of Nano-twined Martensites

Cited by 3 publications

References 8 publications

The relation between lattice parameters and very low twinning stress in Ni₅₀Mn_25+xGa_25−xmagnetic shape memory alloys

The relation between lattice parameters and very low twinning stress in Ni₅₀Mn_25+xGa_25−xmagnetic shape memory alloys

Thermally induced changes of structure in Ni$_{50}$Mn$_{25+x}$Ga$_{25-x}$ magnetic shape memory single crystals with very low twinning stress

Recent Development of the Magnetic Shape Memory Materials Research in Finland

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A New Diffraction Approach To Crystal Structure Determination of Nano-twined Martensites

Cited by 3 publications

References 8 publications

The relation between lattice parameters and very low twinning stress in Ni50Mn25+xGa25−xmagnetic shape memory alloys

The relation between lattice parameters and very low twinning stress in Ni50Mn25+xGa25−xmagnetic shape memory alloys

Thermally induced changes of structure in Ni$_{50}$Mn$_{25+x}$Ga$_{25-x}$ magnetic shape memory single crystals with very low twinning stress

Recent Development of the Magnetic Shape Memory Materials Research in Finland

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Product

Resources

About

The relation between lattice parameters and very low twinning stress in Ni₅₀Mn_25+xGa_25−xmagnetic shape memory alloys

The relation between lattice parameters and very low twinning stress in Ni₅₀Mn_25+xGa_25−xmagnetic shape memory alloys