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Investigation of thermomechanical properties of ferromagnetic NiFega shape memory alloy subjected to pseudoelastic compression test

Pieczyska E., Dutkiewicz J., Masdeu F., Luckner J., Maciak R.

Archives of Metallurgy and Materials

2011

Vol. 56, iss. 2

401-408

In the study stress-induced reversible phase transformation in NiFeGa magnetically controlled shape memory alloy subjected to pseudoelastic compression test was investigated. The specimen's mechanical characteristics and temperature changes related to the exothermic martensite transformation and endothermic reverse transformation were measured in contact-less way by using a fast and sensitive infrared camera (IR). It was found that the stress-induced phase transformation process occurs in this alloy in heterogeneous way, since the observed specimen's temperature distribution was not uniform. Stress-strain curves obtained for the first, as well as for the subsequent six loading-unloading compression cycles and their related temperature changes, elaborated as average from the specimen's surface, were analyzed. It was concluded that the stress and the temperature changes developing in the subsequent cycles depend on the applied test conditions, however the highest discrepancies were observed between the first and the second cycles of the compression loading.

W pracy przedstawiono wyniki badań zmian parametrów mechanicznych oraz temperatury stopu Ni54Fe19Ga27, wykazującego magnetyczną pamięć kształtu. Próbki stopu poddawano procesowi pseudosprężystego ściskania. Temperaturę mierzono za pomocą szybkiej kamery termowizyjnej. Stwierdzono, że indukowana naprężeniem przemiana fazowa zachodzi w tym stopie w sposób niejednorodny, a przebieg charakterystyk mechanicznych i zmian temperatury w kolejnych cyklach obciążania i odciążania prówbki zależy od zastosowanej metodyki badawczej.

Microstructure and properties of α + β brass after ECAP processing

Dutkiewicz J., Masdeu F., Malczewski P., Kukuła A.

Archives of Materials Science and Engineering

2009

Vol. 39, nr 2

80-83

Purpose: Purpose of this paper is to determine the effect of Equal Channel Angular Pressing (ECAP) processing on the microstructure and hardness of α + β ?brasses. The effect of deformation temperature and number of passes was investigated particularly on the shape and size of grains of both phases. Design/methodology/approach: The specially constructed channel with 90° pressing angle, allowing heating of the tool with the sample was used for ECAP processing. The grain size was investigated using optical and transmission electron microscopy. The hardness and measurements microhardness were used to determine the effect of ECAP on the hardness of both phases. Findings: Significant grain refinement down to 300 nm from the initial 20 mm was observed after ECAP processing at 300°C. At 400°C grain refinement occurred down to 1-3 mm. Frequent microtwins were observed within aphase. The microhardness of the αphase was higher than that of αphase, 235 HV and 173 HV respectively. Research limitations/implications: The limitation is a size of the sample which makes difficult future applications. Another one is elevated temperature (minimum 300°C) otherwise the samples forms crack. This limits also the grain refinement which is above the range of nanomaterials. Practical implications: Significant grain refinement allows to increase the hardness and strength of the sample preserving a good plasticity. The limitation is the size of the channel what limits the application. The material could be used in such cases when high strength of brasses is needed with sufficient plasticity and good conductivity. Originality/value: In this paper detailed TEM studies were performed for α + β brasses showing high density of microtwins and higher density of dislocations within ? phase, than in the bphase. Higher hardness of the bphase results from the ordering, which hinder deformation of this phase.