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

Ladislav Straka, Jan Drahokoupil+5 more|arXiv (Cornell University)|Nov 6, 2014

Shape Memory Alloy TransformationsMaterials Science41 references16 citations

TL;DR

This study investigates thermally induced structural changes in Ni50Mn25+xGa25−x magnetic shape memory single crystals with very low twinning stress, using in situ X-ray diffraction to track lattice parameter evolution from 173 K to 343 K. The key finding is that twinning stress in Type 2 twins correlates strongly with the monoclinic angle γ and c/a ratio, but not with a−b differences, indicating that lattice distortion, particularly monoclinic distortion and shear, governs low twinning stress, while a/b lamination plays no significant role.

ABSTRACT

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 $x\leq3.5$, 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.

Motivation & Objective

• To understand the origin of exceptionally low twinning stress in 10M Ni-Mn-Ga magnetic shape memory alloys.
• To investigate how temperature-dependent structural changes in lattice parameters influence twinning stress.
• To determine whether lattice parameters a, b, c, and γ correlate with twinning stress in Type 1 and Type 2 twin boundaries.
• To assess the role of nanoscale twin structure refinement and intermartensite transformations (10M ↔ 14M ↔ NM) in twinning stress behavior.
• To test the predictions of microstructural models linking lattice distortion to twin boundary motion.

Proposed method

• In situ X-ray diffraction was used to measure lattice parameters a, b, c, and monoclinic angle γ in Ni50Mn25+xGa25−x single crystals across 173–343 K.
• The 10M martensite structure was modeled as monoclinic, with unit cell derived from the cubic L21 parent phase.
• Twinning stress was measured and correlated with temperature-dependent lattice parameters to test microstructural models.
• The study compared alloys with x = 2.7–3.9 to identify compositional and structural trends in lattice behavior and twinning stress.
• Sudden changes in lattice parameters were analyzed to detect intermartensite transformations and nanoscale structural refinements.
• Statistical comparison of twinning stress with γ, c/a, and (a−b) was performed to assess their influence on twin boundary motion.

Experimental results

Research questions

• RQ1 Does the monoclinic angle γ correlate with the temperature dependence of twinning stress in Type 2 twins?
• RQ2 Is there a significant correlation between the a−b lattice parameter difference and twinning stress in Type 1 or Type 2 twins?
• RQ3 How do changes in lattice parameters c and c/a ratio relate to twinning stress evolution with decreasing temperature?
• RQ4 What causes the sudden rise in twinning stress below a certain temperature, and is it linked to lattice parameter anomalies or phase transformation embryos?
• RQ5 To what extent does nanoscale refinement of the 10M martensite twin structure, indicated by small lattice parameter changes, influence twinning stress?

Key findings

• With decreasing temperature, lattice parameters a and γ increased, c decreased, and b remained nearly constant in all samples.
• For x ≤ 3.5, sudden, sharp changes in a and b parameters occurred, leading to a = b in some regions, suggesting nanoscale twin structure refinement.
• The temperature dependence of γ and c/a ratio showed a strong correlation with twinning stress, supporting their role in controlling twin boundary motion.
• No significant correlation was found between twinning stress and the (a−b) parameter, indicating that a/b lamination does not play a major role in low twinning stress.
• The rise in twinning stress below ~T0 was not consistently linked to lattice parameter changes, suggesting the emergence of 14M phase embryos as the most likely cause.
• The results support the microstructural model that monoclinic distortion (γ) and shear (c/a) are key factors in enabling very low twinning stress in Type 2 twins.