The 12th Global Conference on Materials Science and Engineering (CMSE 2023)

Abstract: Some materials take place in class of advanced smart materials, by exhibiting a peculiar property called shape memory effect. These alloys have dual characteristics called thermoelasticity and superelasticity, from viewpoint of memory behavior. Shape memory effect is initiated on cooling and deformation processes and performed thermally on heating and cooling, with which shape of materials cycles between original and deformed shapes in reversible way. Therefore, this behavior can be called thermal memory or thermoelasticity. Superelasticity is performed with stressing and releasing the material in elasticity limit at a constant temperature in the parent phase region, and shape recovery occurs instantly upon releasing. These phenomena are result of crystallographic transformations called martensitic transformation.

Thermoelasticity is governed by the thermal and stress induced martensitic transformation. Thermal induced martensite occurs on cooling along with lattice twinning and ordered parent phase structures turn into twinned martensite structures by means of lattice invariant shears, and these structures turn into detwinned martensitic structures with deformation by means of stress induced transformation. Lattice twinning occurs in <110 > -type directions on the {110}-type plane of austenite matrix in self-accommodating manner. Superelasticity is also governed by stress induced martensitic transformation and ordered parent phase structures turn into detwinned martensite structures with stressing.

Copper based alloys exhibit this property in metastable β-phase region, which has bcc-based structures. Lattice invariant shear and twinning is not uniform in these alloys and gives rise to the formation of complex layered structures, depending on the stacking sequences on the close-packed planes of the ordered parent phase lattice. The layered structures can be described by different unit cells as 3R, 9R or 18R depending on the stacking sequences on the close-packed planes of the ordered lattice. The unit cell and periodicity are completed through 18 layers in direction z, in case of 18R martensite, and unit cells are not periodic in short range in direction z.

In the present contribution, x-ray diffraction and transmission electron microscopy (TEM) studies were carried out on copper based CuAlMn and CuZnAl alloys. X-ray diffraction profiles and electron diffraction patterns exhibit super lattice reflections. X-ray diffractograms taken in a long-time interval show that diffraction angles and intensities of diffraction peaks change with the aging duration at room temperature. This result refers to the rearrangement of atoms in diffusive manner.

Keywords: Shape memory effect, martensitic transformation, thermoelasticity, superelasticity, lattice twinning, detwinning.

Abstract: The direct joining of Ta with steel leads to several problems inherent in the formation of brittle reaction regions and complexities associated with the formation of butt joints during further processing of the composite. To overcome these complications, an intermediate layer made of a soft material with high thermal conductivity, such as Cu, can be used. This work presents a comprehensive study of recent advances in understanding of the microstructure-property relationships in a tantalum-stainless steel (SS) composite fabricated by explosive welding with copper interlayers, both during composite formation and post-processing annealing. Using scanning (SEM) and transmission electron microscopy (TEM), interfacial microstructures were investigated, thus providing guidelines for the design of materials. To support the microstructural findings, the evolution of the structure and properties of the interfacial layers was investigated using X-ray synchrotron radiation and nanohardness tests, respectively. Particular attention was paid to the description of the reaction regions and the competition between the strain hardening and softening processes occurring during the formation of the clad and further heat treatment. It was observed that the composite interfacial layers exhibited a complex and hierarchical microstructure. Analyses of the solidified melt regions using various SEM and TEM techniques revealed areas with different morphology and chemical composition. The reaction regions located near the Ta/Cu interface consisted only of a mixture of Cu and Ta particles (ultrafine grains and small dendrites), while the areas near the Cu/SS interface consisted of nanograins with a more complex chemical composition, containing elements from bonded sheets. No significant influence of the annealing process (under the applied conditions - at 750 °C for times up to 103 h) on changes in the microstructure of the solidified melt was observed, apart from a slight increase in grain size. The solidified melt regions, in the as-welded and annealed states were basically 2-3 times harder than the strain hardened steel layers.

Keywords: Explosive welding, Ta/Cu/steel clads, SEM & TEM, Deformation

Acknowledgements: This research was funded in part by National Science Centre (NCN) of Poland, within the project no.: 2018/31/B/ST8/00942.