Search

Martensitic transformations enable various emerging applications like the shape memory effect and elastocaloric applications in NiTi. Increasing the speed of this transformation can shorten the response time for actuation and increase the power density of caloric cooling systems. Up to now, research on the speed and possible time limits of the martensitic transformation in NiTi has been limited to milli- and microsecond experiments. The dynamics of the transformation for shorter time scales are therefore unknown. Here, we report the fastest transformations in NiTi so far by heating an epitaxial NiTi film with a ns laser pulse and tracking the martensitic transition with in-situ synchrotron X-ray diffraction. We find that the martensite to austenite transition upon heating can proceed within the 7 ns pulse duration of the laser, but it requires substantial overheating as the rate of the transformation increases with the driving energy. The austenite to martensite transition is slower because cooling proceeds by conductive heat transfer, but with appropriate undercooling, the complete transformation from martensite to austenite and back only takes 200 ns. We compare our results to previous experiments on the Heusler alloy Ni-Mn-Ga and (K,Na)NbO3 and find very similar trends, which reveal that fast martensitic transformations in general follow a universal scaling law.

Raw data for the publication titled 'Formation of martensitic microstructure in epitaxial Ni-Mn-Ga films after fast cooling' done by Yuru Ge (FWIN-HZDR) and the colleagues. The order of the figures follows the latest manuscript version before submission, labeled "v13".

This dataset belongs to the paper "Transforming martensite in NiTi within nanoseconds" and contains all raw data used for the paper. It includes raw data of reciprocal space maps taken at P23 Petra III DESY Synchrotron. It also contains the Python code used to analyze the raw data and the martensite intensities extracted from the raw data. Information about sample, measurement techniques and further data description can be found in README.txt.

During cooling, conventional martensitic transformation can only be realized from austenite to martensite. Recently, a so-called reentrant martensitic transformation obtained much interest due to an additional transformation from martensite to austenite during further cooling. Obviously, materials with this reentrant transformation will increase the number of physical effects and possible applications. However, until now, only bulk samples are reported available, which are not suitable for applications in micro-devices. In this work, ferromagnetic Co-Cr-Ga-Si films were selected as a model system to explore the reentrant transformation behavior in thin films. We observed that the films grow epitaxially on MgO (100) substrates and exhibit a martensitic transformation if deposited at a sufficiently high temperature or with an additional heat treatment. Film within the austenite state are ferromagnetic while films within the martensitic state just exhibit a very low ferromagnetism order.

Raw data of a paper done by Yuru Ge. The paper title is finally signed as: Growth and Martensitic Transformation of Ferromagnetic Co-Cr-Ga-Si Epitaxial Films. The authors are Yuru Ge, Klara Lünser, Fabian Ganss, Peter Gaal, Lukas Fink,and Sebastian Fähler. This is the final version which is accepted by the journal Science and Technology of Advanced Materials on 07.08.2023

Martensitic transformations are important for many materials exhibiting magneto-, elasto- and barcaloric effects and often the functional properties benefit from additional magnetic transformations. For caloric applications, thin films are of particular interest due to their fast heat exchange, which promises a high cooling power. In particular epitaxial films are a model system to understand the formation of the martensitic microstructure.[1] However, commonly transformations from austenite to martensite only occur during cooling. The recent observation of a so-called reentrant transformation obtained much interest, as an additional transformation from martensite to austenite was observed during further cooling.[2,3] A reentrant transition increases both, the number of new physical effects and the possible applications. However, reports are currently very few and only on bulk materials. Will this phenomenon occur also in thin films? Will the martensitic transformation and microstructure differ when induced by heating or cooling? Also for caloric applications, reentrant martensite is interesting as it may enable bidirectional caloric effects driven by deformation and recovery of the ferromagnetic shape memory alloys (FSMA) through both, heating and cooling. As a model system, epitaxial Co-Cr-Ga-Si ferromagnetic shape memory thin films were grown by DC magnetron sputtering. To obtain epitaxial growth, films were deposited on different substrates as well as orientations. We identify MgO(100) as the optimum substrate. Films were grown at different deposition temperatures and the influence of an additional postannealing process was examined to understand the influence on elemental composition, structure, microstructure and the transformation behavior. As a kind of summary, Fig.1-a depicts a phase diagram of all samples prepared. Under optimum conditions, we obtain epitaxial growth, as evident from pole figure measurements of an austenitic film (Fig. 1-b). When films are

Martensitic transformations are important for many materials exhibiting magneto-, elasto- and barcaloric effects and often the functional properties benefit from additional magnetic transformations. For caloric applications, thin films are of particular interest due to their fast heat exchange, which promises a high cooling power. In particular epitaxial films are a model system to understand the formation of the martensitic microstructure.[1] However, commonly transformations from austenite to martensite only occur during cooling. The recent observation of a so-called reentrant transformation obtained much interest, as an additional transformation from martensite to austenite was observed during further cooling.[2,3] A reentrant transition increases both, the number of new physical effects and the possible applications. However, reports are currently very few and only on bulk materials. Will this phenomenon occur also in thin films? Will the martensitic transformation and microstructure differ when induced by heating or cooling? Also for caloric applications, reentrant martensite is interesting as it may enable bidirectional caloric effects driven by deformation and recovery of the ferromagnetic shape memory alloys (FSMA) through both, heating and cooling. As a model system, epitaxial Co-Cr-Ga-Si ferromagnetic shape memory thin films were grown by DC magnetron sputtering. To obtain epitaxial growth, films were deposited on different substrates as well as orientations. We identify MgO(100) as the optimum substrate. Films were grown at different deposition temperatures and the influence of an additional postannealing process was examined to understand the influence on elemental composition, structure, microstructure and the transformation behavior. As a kind of summary, Fig.1-a depicts a phase diagram of all samples prepared. Under optimum conditions, we obtain epitaxial growth, as evident from pole figure measurements of an austenitic film (Fig. 1-b). When films are