Papers by Keyword: Transformation Temperature

Abstract: In the present work, the effects of artificial aging treatment on the transformation temperatures and hardness of Cu-Al-Mn shape memory alloy have been investigated. The aging processes have been performed on the one-time re-melted and 90% rolled samples. Differential scanning calorimetry reveals that reverse transformation is present for the re-melted sample which is aged at 400°C. However, in 90% rolled condition, this transformation takes place at 200°C and 300°C. Hardness examination shows that the aged specimens possess higher values in hardness in comparison to un-aged samples at all studied temperatures. Although, the peak-aged condition was demonstrated at 300°C for the re-melted sample, the rolled sample displayed increased hardness levels up to 500°C. Based on the DSC measurements and microstructural observations, it can be asserted that the thermo-mechanical processing including rolling plus aging at 300°C provides favorable transformation characteristics for shape memory behavior.

Abstract: Shape memory materials are stimuli-responsive materials. They are widely used in military, medical, safety, and robotics applications. Until recently, only Ni-Ti based SMA’s are commercially used due to its relatively ease of manufacturing. However, the exorbitantly high cost of Ni-Ti based SMA limits its application to niche markets such as medical stents, aerospace and defence. Recently, it is found that Cu based alloys exhibit shape memory behavior. Out of which, Cu-Al-Be-Mn is most interesting SMA in terms of less process complexity and low cost. Cu–Al–Be-Mn shape memory alloys in the range of 09–15 wt.% of aluminium and 0.1-0.4 wt.% of Beryllium and 0.1 to 0.3 wt.% of Manganese, exhibiting β-phase at high temperatures and manifesting shape memory effect upon quenching to lower temperatures, were prepared through ingot metallurgy. The alloy ingots were homogenized followed by step quenching so as to obtain a structure that is completely martensitic. They were subsequently characterized by X-ray diffractogram (XRD), Differential Scanning Calorimetry (DSC) and Optical Microscope (OM). The shape memory properties of the alloys were studied by bend test. This paper emphasizes the synthesis and characterization of the Cu-Al-Be shape memory alloys.

Abstract: Thepaper deals with an experimental measurement of the transformation temperatures of Co-base alloy. Temperatures were determined by means of DTA-method during controlled heating and cooling. The samples in an as-received state were analysed at heating/cooling rates of 2, 5, 10 and 20 °C/min with the use of the equipment Setaram SETSYS 18TM (DTA-method). The samples after various heat treatments were analysed at heating/cooling rate of 5 °C/min by Setaram SETSYS 18TM (DTA-method). On the basis of evaluation of the results the influence of heating/cooling rate on shift of the transformation temperatures was determined. The influence of heat treatment on shift of the transformation temperatures was also studied. The samples in an as-received state and the samples after heat treatment were alsosubjected to the phase analysis by scanning electron microscopy using the microscope JEOL JSM-6490LV equipped with an energy dispersive analyser EDAX (EDS INCA x-act). The individual phases were identified by semi-quantitative X-ray microanalysis.

Abstract: Ti-50.7at.%Ni as one of Ni-rich TiNi shape memory alloy is very responsive to precipitation ageing treatment due to the formation of Ti₃Ni₄ precipitates. This study indicates that this alloy exhibits different transformation sequences under different ageing temperatures. Precipitation of Ti₃Ni₄ has introduced the formation of intermediate R-phase prior to martensitic transformation. The lack of 2-step R-phase transformation, B2→R1 and B2→R2 transformation in this alloy, suggested that Ti₃Ni₄ precipitation occurs homogeneously.

Abstract: This study investigated the influence of ⁶⁰Co-γ-irradiation on the structure and properties of Cu-Al-Ni shape memory alloys. The phase transformation temperatures were evaluated by differential scanning calorimeter (DSC). It was found that the γ-irradiation had a complex influence on the phase transformation parameters of Cu-Al-Ni SMAs. However, the transformation temperatures were shifted and a new curve was obtained after exposure to different irradiation doses. The thermodynamic parameters such as enthalpy and entropy tended to increase/decrease depending on the amount of the exposure. The structural properties of the exposed samples were studied by using optical microscopy and hardness measurements at room temperature. It was also found that the structural-properties of the Cu-Al-Ni SMAs were completely affected by the amount of the applied γ-irradiation dose.

Abstract: As a new class of shape memory alloy, Ni₂MnGa ferromagnetic shape memory alloy (FSMA) has both large reversible strain and higher response frequency, which has attracted considerable attention and has been widely investigated.The martensitic transformations temperatures of the as-cast Ni₂MnGa alloys are investigated by differential scanning calorimeter (DSC). Following conclusions were obtained from experimental results: By using DSC measurement, martensite transformation temperatures (including M_s, M_f, A_s and A_f) were obtained in two alloy series of Ni_51.5+xMn_23.5Ga_25-x and Ni_51.5Mn_23.5+yGa_25-y. Results showed that: martensite transformation temperatures enhanced with the Ni and Mn increasing respectively, and the effect is stronger for Ni than Mn. After 600°C vacuum annealing 6h using powder samples, the martensitic transformation temperature was slight lower than as-cast state.

Abstract: To predict phase equilibria of advanced high strength steels accurately, a new multicomponent thermodynamic database (FEDA) encompassing Fe, C, Mn, Si, Al and Ni is created according to relevant literatures. This study applies Thermo-Calc software to estimate phase transformation temperatures and phase fractions in ferrite and austenite two-phase region of test steels. Simultaneously, experiments are carried out with dilatometer and the quantitative metallographic analysis technology. It is found that the FEDA database is appropriate for predicting A₁ temperature and phase fractions of Al-containing steels at low temperatures, while the TCFE6 database can be used to estimate A₃ temperature. According to calculated phase fractions using FEDA database, a two-step heat treatment is made in order to achieve TRIP-aided steels. Samples annealed at 780°C for 3 min and held at 420°C for 5 min exhibit an optimum value of strength and ductility product up to 23690 MPa·%.

Abstract: The rhombohedral R-phase is an intermediate phase during the transformation from cubic austenite (B2) to monoclinic martensite (B19’). Compared to the martensitic transformation (B2 → B19’), the R-phase transformation (B2 → R) in NiTi based shape memory alloys (SMA) has a lot of advantages. The main advantages of the R-phase transformation are the very small temperature hysteresis (2-5K) and the high durability (> 100.000 cycles). The main disadvantage of actuators based on R-phase transformation is the small SMA effect (approx. 0,8%). Due to the small temperature hysteresis, the R-phase actuators are usable for temperature sensitive applications in heating technology or in air conditioning. The R-phase transformation doesn’t appear in each SMA alloy and needs a special thermo-mechanical treatment to be induced. Fuzzy logic is an approach to computing, based on degrees of truth, rather than the usual true or false (1 or 0) Boolean logic. It includes 0 and 1 as extreme cases of truth but also the various states of truth. The objective of this paper is to show the applicability of the approach of fuzzy logic in the field of SMA. To gain basic knowledge about influences, different binary NiTi alloys were investigated. The parameters annealing temperature, annealing time and Ni-concentration were varied and their influence on the transformation behavior of the R-phase was noted and added to the fuzzy logic knowledge base.

Abstract: The present research aims to understand the transformation temperatures and recovery strain of NiTi wires during heating and cooling under various constrained stresses. Both constrained stress and annealing temperature have significant effects on the shape memory characteristics. In general, increasing constrained stress causes an increase of the austenitic and martensitic transformation temperatures and a decrease of the recovery strain. This effect can be attributed to that the constrained stress inhibits the austenitic transformation, and thus more volume fraction of the martensite is retained during heating. The high annealing temperature leads to the decrease in the recovery strain.

Abstract: Abstract. To characterize the thermomechanical cyclic response, especially Pseudo-elastic behavior of NiTi shape-memory alloy (SMA) at different temperatures and different loading frequencies, a series of tests of both quasi-static and dynamic uniaxial compression cyclic loading have been performed on cylindrical samples, using an Instron servohydraulic testing machine and the Hopkinson technique. Transformation temperature, after dynamic loading is measured and compared with that of as-received state. The influence of the annealing temperature on the fatigue response is also examined. A few noteworthy conclusions are as follows: (1) Transformation temperature, can be changed under higher strain rates; (2) By changing the annealing temperature, the transformation stress and the dissipated energy of NiTi can also be obviously changed, so as it changes the transformation temperature, Af ;(3) In cyclic loading, the dissipated energy over cycles tends to be a minimum stable value, and it seems that cyclic loading leads to a stable Pseudo-elastic behavior of the alloy; (4) Repeated impact tests of the alloy produce slightly changes in the shape of the Pseudo-elastic loop and in the dissipated energy comparing with the quasi-static cyclic tests.