High temperature copper-based shape memory alloys with primary particles

In order to lower the transition temperatures and increase the resistivity against thermal decomposition of the Cu-Al-Nb alloys, the effect of Ni, Co and Cr addition on this behaviour was studied The fourth element in the ternary alloy changes the chemical composition of the matrix and the particles of the Nb (Cu, Al)$_2$ phase and decreases the plasticity of the quateniary alloys. To explain this behaviour the nano-indentation measurements have been carried out and changes ofthe elastic modulusand nino-hardness across the interface have been evaluated

High temperature Cu-Al-Nb - based shape memory alloys

New type of Cu-Al-Nb - base alloys containing nickel, cobalt and chromium with high martensitic transformation temperatures have been developed. In this paper the mechanical properties, shape memory characteristics, the temperatures of reversible martensitic transformation and thermal stability of these alloys are presented. Introducing chromium and specially nickel and cobalt to the Cu-Al-Nb alloys gives a possibility to decrease the characteristic temperatures. However, these elements create in the martensitic matrix primary particles of intermetallic phases such as AlCo and NiAl and change the chemical composition of the Nb(,Al,Cu)2 - the main phase in the ternary Cu-Al-Nb alloy. Presence of precipitates results in decrease of the elongation of these alloys. Nickel and cobalt improve the thermal stability and resistance to decomposition

Nanocrystalline platinum layer deposited on NiTiCu shape memory strip

The Ni25Ti50Cu25 shape memory strip was covered by thin nanocrystalline platinum layer. Structure of the layer was studied by means of X-ray diffraction and transmission electron microscopy. Transformation temperatures were determined using differential scanning calorymetry (DSC). The studies show that the layer reveals nanocrystalline structure with average crystalline size of 44 nm. Lattice distortion was relatively low –0.19%. Almost 25% of total grains are oriented along 〈111〉 direction. It was stated that the nanocrystalline platinum layer does not limit martensitic transformation in the covered strip, which reveals one step reversible martensitic transformation from the parent B2 phase to the orthorhombic martensite B19 phase. Also no influence of the layers on the shape recovery was noticed

TWSME in Cu-Al-Ni Alloys Obtained by Stabilised Stress Induced Martensite

The effect of deformation degree, temperature and time of ageing on the TWSME has been studied in polycrystalline and single-crystalline Cu-Al-Ni alloys. The results confirm that to obtain the best TWSME one needs to select the training parameters. The trained single crystals exhibit a lower TWSME than the polycrystalline alloys for the same training conditions. In the trained single crystals a single martensite variant has not been observed, but they showed a fewer variants than the untrained samples. The TEM studies of the trained samples exhibit a complex dislocation arrangements which interact with very thin induced plates of martensite. Ageing of the one of polycrystalline alloys caused precipitation of coherent γ2-phase particles. No precipitates were found in the second one and in the similarly trained single crystals although in these alloys a significant TWSME was observed. This means that the role of the γ2-phase particles presence to obtain the TWSME by ageing under stress is not that important as it has been suggested

Martensitic transformation in NiTiCo strip covered by nanocrystalline platinum layer

The surface modifications and their effects on the transformation characteristics of shape memory alloys are of interest. The NiTiCo strip was produced applying twin roll casting technique and after that covered by platinum layer using magnetron sputtering. Structure of the layer was studied by means of X-ray diffraction and electron microscopy. Transformation temperatures were determined using differential scanning calorimeter (DSC). The investigations show that in the NiTiCo strip occurs two-steps (by the R-phase) reversible martensitic transformation between the parent B2 phase and the monoclinic martensite B19’. The platinum layer reveals nanocrystalline structure with average grain diameter of 41 nm. It was stated that the nanocrystalline platinum layer does not limit martensitic transformation in the covered strip and can be used as a protective layer

Effect of deformation on structure and mechanical behavior of polycrystalline Ni-Mn-Ga alloys

Deformation of polycrystalline Ni-Mn-Ga alloys was studied for both the L21 parent phase and 10 M martensitic state. The effect of deformation by compression up to the fracture on mechanical behavior and structure change was studied for the inhomogeneous as-cast state and after annealing at 900○C for 50 hours. The structure, after deformation of the parent phase, observed by TEM reveals the presence of 10 M and 14 M martensite in the matrix, whereas the deformation of 10 M martensite does not show the change of the structure type. The stress-strain curves were analyzed and compared with the earlier published results

Martensitic transformation in melt-spun Ni-Mn-Ga ribbons

Off-stoichiometric Ni-Mn-Ga alloys, with various chemical compositions, were produced by melt-spinning technique. Transformation behavior was studied by means of differential scanning calorimetery (DSC). Ribbons and bulks reveal reversible martensitic transformation which occurs, dependently on chemical composition, in the range of temperatures: between 100°C and -150°C. Transmission electron microscope and X-ray diffractometer were used for structure examination. Dependently on chemical composition five-layered (10 M) or seven-layered (14 M) martensite was observed, whereas the parent phase shows the B2 structure. Due to high cooling rate realized in melt-spinning technique some amount of amorphous phase was found. For all ribbons magnetization was measured versus change of temperature as well as magnetic field. Due to relatively high density of dislocations lower value of magnetization were obtained