Wyniki 1-3 spośród 3 dla zapytania: authorDesc:"Dariusz Chrobak"

The effect of lattice distortion on the course of the martensitic transformations in NiTi shape memory alloys

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In the Ni-Ti system it is the B2 intermetalic NiTi phase that undergoes the reversible martensitic transformation to the B19′ monoclinic phase. For an alloy of any composition (provided it ensures the B2 phase presence) cooled down very slowly the transformation occurs always at the same temperature i.e. about 60°C. In the Ni-rich alloys the precipitation process may take place that changes the transformation characteristic temperatures and/or its sequence. There are several variants of this process depending on the ageing temperature [1], these are: β0 → β1 + Ni4Ti3 → β2 + Ti2Ni3 → β3 + TiNi3, Ta < 680 ± 10°C β0 → β′1 + Ni3Ti2 → β′2 + TiNi3, 680 ± 10°C < Ta < 750 ± 10°C β0 → β″1 + Ni3Ti for Ta > 750 ± 10°C. However, the most significant influence on the course of the martensitic transformation in the NiTi alloy have the Ni4Ti3 particles. The strain fields around these coherent precipitates as well as the decrease of the Ni concentration in the matrix change the characteristic transformation temperatures and cause occurrence of the R-phase transition preceding the B19′ martensite formation. Similar effects take place in the NiTi equiatomic alloys deformed and then annealed at temperatures below the recrystallisation temperature. In both cases additional effects in form of a multistage transformation were often observed. The first data on the multistage martensitic transformation were given by Todoroki and Tamura [2], Stróż et al. [3] and Zhu et al. [4]. It was found that depending on the applied thermal treatment there exist three or even four more or less overlapping peaks on the DSC cooling curves. The occurrence of the R-phase transition in these alloys is understandable as this transition causes less lattice distortions and thus is favoured when the internal stresses exist in the sample. Howeve[...]

Influence of alloying additions on magnetostriction and Young's modulus of nanoperm type amorphous alloys

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In the present paper magnetostriction, Young's modulus and primary crystallization were examined for a group of amorphous alloys of nanoperm family i.e. Fe78B22, Fe76B22X2 (X=Cr, Zr, Nb). It was shown that the alloying additions cause a correlated decrease of Young’s modulus (over 2 times) and magnetostriction coefficients - parallel (about 40%) and spontaneous (about 30 %) in rel[...]

Characterization of low-temperature nitrided NiTi shape memory alloy DOI:10.15199/28.2019.3.3

  1. INTRODUCTION Shape memory alloys (SMAs) belong to a class of the shape memory materials, which posses ability to “memorise" or retain their previous form when subjected to certain stimulus such as thermomechanical or magnetic variations. Due to their unique and superior properties, SMAs have drowned significant attention and interest in recent years in broad range of commercial applications. Especially, well-known in this field is nickel-titanium alloy (NiTi), which is also known as NiTiNOL. Invented by William Buehler and Frederick Wang in 1962 NiTi alloy has found wide engineering and technical applications in numerous commercial fields, such as: consumer products, automotive, aerospace, mini actuators, micromechanical systems, robotics and biomedical [1, 2]. In field of aerospace, a family of high force release devices called the “Frangibolt¨, an array of fast acting Pinpullers, a range of innovative Ejector Release Mechanisms (ERMs), and a host of other SMA Actuators have been known [3]. The NiTi-based shape memory alloys are preferable for most of these applications due to their high mechanical and corrosion properties as well as the stability of the effect. However, there are some application areas that require improvement in surface hardness as well as an abrasion resistance. Therefore, studies were done in order to improve these properties through the use of various method of surface treatment, which form e.g. layers of titanium oxide, titanium nitride, carbon coatings such as DLC (diamond like carbon) or NCD (nanocrystalline diamond) and ceramic layers. The methods include: electrochemical oxidation, ion implantation, RFCVD, glow discharge and laser treatment [4÷11]. The limitation in the application of these surface treatments is the temperature of the process, which may negatively affect the shape memory effects [5, 11]. The most of these methods, e.g. nitriding, have been carried out at high tempera[...]

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