The non-ferrous SMAs (shape memory alloys) have, normally, two problems that hinder the use in industrial scale: the natural aging and grain growth. The first degrades the memory effect, while the second, observed during the alloy's mechanical processing, modifies the phase transformation temperatures. Thus, the study of recrystallization kinetics is important for enabling the control of hardened state as a function of treatment time without allowing the exaggerated grain growth. The objective of this study is to determine the recrystallization kinetics in different SMAs (Cu-14Al-4Ni, Cu-12Al-0.5Be and Ni-42Ti), based on an empirical law of J-M-A (Johnson-Mehl-Avrami), as well as their activation energies for grain growth process according to the empirical Arrhenius law.Quantitative evaluations of the grain growth kinetics over a wide range of indicated DSC (differential scanning calorimetry) temperatures have been performed. The results show that the alloy less susceptible to aging in temperatures below the recrystallization peak is the Ni-42Ti, because it presented the highest activation energy, followed by the Cu-14Al-4Ni. The equations that describe the recrystallization kinetics follow the empirical law of J-M-A. The recrystallization kinetics accompanied by hardness variation was an important tool, working as an advisor for selection of treatment time as a function of temperature.
DCT (deep cryogenic treatment) is commonly used in industry to improve the wear resistance characteristics of steels, especially. However, there are just a few researches about the effects on non-ferrous metals. The purpose of this work was to investigate how DCT affects the properties of Cu-14Al-4Ni alloy treated at different soak time and submitted to thermomechanical cycling. A comparative experimental analysis was performed of the thermal properties of alloys obtained on a vacuum furnace, treated by DCT and thermomechanically cyclized. The results indicates that thermomechanical cycling promoted the appearance and growth of the martensitic phase γ' 1 , less ductile than the martensitic phase β' 1 , which together with the induced hardening produced an increase in transformation temperatures and microhardness. The higher the number of cycles, the greater these effects. The DCT promoted an increase in the intensity of the diffraction peaks corresponding to the phase β' 1 and the maintenance of them during the thermomechanical cycling of the material, which indicates that the DCT stabilizes the martensitic phase β' 1 and, consequently, caused a reduction and stabilization of the martensitic transformation temperatures and the microhardness, when compared to the untreated material. The longer the soaking time of DCT, the greater these effects.
The non-ferrous shape memory alloys have, normally, two problems that hinder its use in industrial scale: the natural aging and grains growth. The first degrades the memory effect, while the second, observed during the processing of alloy, modifies the temperatures which the transformations occur. Thus, the study of kynetic of recrystallization is important for enabling the control of hardened state in function of treatment time, without causing excessive grain growth. Therefore, the objective of this study is to determine the kinetics of recrystallization of Cu-14Al-4Ni shape memory alloy, based on an empirical law of the formation of Jonhson-Mehl-Avrami, as well as their activation energies for grain growth process according to the empirical Arrhenius law. The alloy was vacuum melted in an induction furnace. After casting, the bulk samples of the alloy were homogenized for 24 hours, solubilized and hot rolled followed by water-quenching to initiate the recrystallization. Then, different samples were annealed at temperatures close to the peak, start and end of the DSC curve. Following the heat treatments, the samples were submitted to mechanical tests and the values of the properties were correlated to the fraction transformed for determination of recrystallization’s kinetic. For the characterization of the grain growth process, analyses in optical microscopy were accomplished and all annealed samples were examined by statistical metallography and the grain sizes were measured. After measurements, the ln[-ln(1-Yrec)] x ln(t) and the ln [D-Do] x 1/T diagrams were plotted to determine the parameters of Jonhson-Mehl-Avrami equation and the activation energy of the process, respectively. The results showed that the equation of the recrystallized fraction follows the empiric law of the formation of Jonhson-Mehl-Avrami for the considered property, as well as, also showed that the alloy Cu-14Al-4Ni is extremely sensitive to temperature variation in which the alloy is treated, having a dual kinetics of grain growth. In the first domain, between 670 and 710°C, the diagram provides a value for the activation energy equal to 39.32 KJ/mol, in the second domain, between 710 and 790°C, the diagram provides a value for the activation energy equal to 9.01 KJ/mol.
This paper aims to verify the Cu 9 Al 4 phase influence on the nanomechanical behavior of the Cu-14Al-4Ni-xTi alloy obtained by rapid solidification with addition of different amounts of Ti. Using the Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Energy Dispersion Spectroscopy (EDS) and X-Ray Diffraction (XRD), it was possible to perform the samples' microstructural characterization. In addition, the reduction of the Cu 9 Al 4 phase precipitation and the X-phase appearance were verified according to the increase of the titanium percentage added. The nanomechanical behavior was evaluated by nanoindentation tests, which showed a significant decrease of the elastic modules and an increase of the Poisson coefficient's according to the titanium amount. This research establishes that the reduction of Cu 9 Al 4 phase implies on the increase of the capacity to dissipate energy. Therefore, the high damping capacity combined with the X-phase presence increases the super elasticity and the alloy ductility.
The present research aimed to analyze the influence that different contents of titanium (x = 0.5, 0.6 and 0.7 wt.%) have on the martensitic transformation temperature of a Cu-14Al-4Ni (wt.%) SMA (shape memory alloy). The Cu-14Al-4Ni-xTi samples were casted in an arc-melting furnace and rapidly solidified. All samples underwent heat treatment in a tubular furnace at a temperature of 1,100 °C for 30 min and water quenched at 25 °C. Subsequently, samples were analyzed by SEM (scanning electron microscopy) with EDS (energy dispersive spectroscopy), XRD (X-ray diffraction) and DSC (differential scanning calorimeter). SEM images and XRD patterns showed that the presence of titanium modified the alloy's microstructure, induced the formation of three titanium rich phases called "X" phase (CuNi 2 Ti, Cu 3 Ti and AlCu 2 Ti) and reduced the presence of the brittle phase γ 2 (Cu 9 Al 4) for samples with 0.6 and 0.7 wt.% Ti. The titanium added to the copper based SMA also functioned as a refiner, reducing GS (grain size) up to approximately 80% with the increase of Ti content. DSC results exhibited low enthalpy levels, hysteresis, as well as low start martensitic transformation temperatures.
Deep cryogenic treatment (DCT) is industrially applied to improve the wear resistance characteristics of tool steels. However, on non-ferrous metals, the knowledge about the obtained characteristics after DCT is limited. The purpose of this work was to investigate how DCT affects the properties and the behavior of the Cu-14Al-4Ni alloy treated at different times and after thermomechanical cycling was performed. In the present investigation, there was performed a comparative experimental analysis of the transformation temperatures, microhardness and shape recovery capacity of the alloy obtained by smelting, treated by DCT and thermomechanically cycled. The DCT provided the stabilization of the martensitic phase β'1 and, consequently, the stabilization of the phase transformation temperatures and the shape recovery capacity of the shape memory effect of the alloy, increasing the alloy life.
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