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Vitel G.,Axinte Uricariul School | Paraschiv A.L.,Technical University Gheorghe Asachi | Suru M.G.,Technical University Gheorghe Asachi | Cimpoesu N.,Technical University Gheorghe Asachi | Bujoreanu L.-G.,Technical University Gheorghe Asachi
Optoelectronics and Advanced Materials, Rapid Communications | Year: 2011

Temperature memory effect (TME) is usually manifested by a kinetic stop during reverse transformation of thermally induced martensite to parent phase, after performing an incomplete heating in the previous thermal cycle, applied to a shape memory alloy (SMA). Technically TME causes the splitting of the endothermic peak corresponding to martensite reversion, on differential scanning calorimetry (DSC) thermograms. The present paper illustrates new manifestations of TME on DSC thermographs of hot rolled Cu-Zn-Al SMAs and discusses the accompanying structural changes observed by optical and scanning electron microscopy.


Vitel G.,Axinte Uricariul School | Suru M.G.,Technical University Gheorghe Asachi | Paraschiv A.L.,Technical University Gheorghe Asachi | Lohan N.M.,Technical University Gheorghe Asachi | And 3 more authors.
Materials and Manufacturing Processes | Year: 2013

Lamellar thermal actuators, for temperature control in hydraulic systems, were manufactured from a Cu-Zn-Al shape memory alloy and trained in bending, between 100 and 500 cycles. The cycles were performed automatically and comprised electrical heating and ventilated air cooling during which a load, fastened at actuator's free end, was lifted by shape memory effect and lowered by martensite-formation softening, respectively. The structural effects of training, evaluated by differential scanning calorimetry (DSC), optical and atomic force microscopy, consisted in a shifting tendency to higher values of critical transformation temperatures of martensite reversion to parent phase and in a noticeable decrease of surface micro-relief accompanying the occurrence of martensite plates oriented according to a single direction. The critical temperature of the start and finish of martensite reversion to parent phase (As and Af, respectively) were determined both by means of the deflectograms (curves of specimens free end displacement vs. temperature) and of the DSC charts. The structural effects of training cycles were evaluated, as well, by AFM measurements of martensite plates. The average measured widths and heights of martensite plate profiles decreased almost seven and almost four times, respectively, after 500 training cycles. Copyright © 2013 Taylor and Francis Group, LLC.


Vitel G.,Axinte Uricariul School | Paraschiv A.L.,Technical University Gheorghe Asachi | Suru M.G.,Technical University Gheorghe Asachi | Cimpoesu N.,Technical University Gheorghe Asachi | Bujoreanu L.-G.,Technical University Gheorghe Asachi
Optoelectronics and Advanced Materials, Rapid Communications | Year: 2012

A normalized hot forged Cu-15Zn-6Al (mass. %) shape memory alloy (SMA) was subjected to tempering heat treatments meant to enhance the reversion to parent phase (austenite) of thermally induced martensite. By means of differential scanning calorimetry (DSC) the reversibility of martensitic transformation was verified during constant-rate heating-cooling cycles. The results have showed that 373 K- tempering destabilized martensite and enabled its reversion at lower temperatures, on heating. Conversely, 473 K-tempering caused the disappearance of martensite phase. The structural features, observed both by optical microscopy (OM) and by scanning electron microscopy (SEM), at the phase transformation product obtained after 473 K-tempering suggest the presence transitory bainite, since its nucleation is diffusionless while its growth is diffusion controlled, which could explain the lack of reversibility.

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