Study of the transitory formation of α 1 bainite, as a precursor of α-phase in tempered SMAs

Abstract

Bainite transitory formation, during the heating of a martensitic copper base SMA, was predicted by DSC and DTA and confirmed by XRD, OM and TEM only in hot pressed air cooled state homogenized and tempered at 573 K. In this heat treatment state, due to the pre-existence of α-phase, bainite amount is expected to be too low, therefore nondistinguishable, for higher tempering temperatures, where it is suggested that transitory α 1 bainite turned to equilibrium α-phase. The results suggest that bainite formation could be considered as a precursor of α-phase during the heating of Cu-Zn-Al martensitic alloys, with no pre-existing α.

Precipitation hardening in the Cu–11wt.%Al–10wt.%Mn alloy with Ag addition

“…The exothermic event E 1 is associated with a phase precipitation [5] and it was not detected in the curve obtained for the Cu-11 wt.%Al-10 wt.%Mn alloy. The event E 2 was attributed to the bainite formation [6]. With increase of ageing time these thermal events disappear and a new peak E 3 is verified from 30 min and it becomes more intense with increase of ageing time for the Cu-11 wt.%Al-10 wt.%Mn-3 wt.%Ag alloy.…”

On role of atomic migration in amnesia occurrence during complex thermal cycling of Cu–Zn–Al shape memory alloy

“…Lamellas of a Cu–15Zn–6Al (mass-, as all compositions will be expressed hereinafter) SMA, with typical thickness of 10 −3 m, were obtained by induction melting, casting, hot rolling, instant water quenching, machining and homogenisation (1070 K/18 ks/water), according to a previously detailed procedure 19. After this thermomechanical processing, the structure of the alloy under study was fully martensitic and contained no noticeable precipitates, neither by optical13 or transmission electron microscopy nor by X-ray diffraction 16.…”

“…machining and homogenisation (1070 K/18 ks/water), according to a previously detailed procedure. 19 After this thermomechanical processing, the structure of the alloy under study was fully martensitic and contained no noticeable precipitates, neither by optical 13 or transmission electron microscopy nor by X-ray diffraction. 16 From such a martensitic lamella, after careful mechanical removal under water cooling of any marks of superficial corrosion, three fragments weighing less than 50610 26 kg each were cut, with appropriate caution in order not to alter their thermomechanical history.…”

“…(i) to shift to lower temperatures becoming less intense, up to total extinction (ii) to slow down more than 12 times in the first part and almost four times at the end of the process (iii) to form lath martensite disposed as interlocking needles. 13 The present paper aims to further analyse the behaviour, during complex thermal cycling, of a Cu-Zn-Al SMA which was developed as a potential candidate for electric actuators, under the form of either bending lamellas 14 or long stroke lamellar helical springs. 15 Considering that, due to diffusion controlled phenomena either intermediate or equilibrium phases, such as bainite 16 or aphase with specific orientation 17 and morphology, 18 were formed during martensite aging, the present experiments will comprise thermal cycles intercalated with prolonged natural aging at room temperature (RT).…”

Comparative Studies of Work-developing Capacity of Shape Memory Alloys (SMA) and Polymers

“…Thermoelastic martensite is softer than austenite and the SMA is able to develop work-generating SME. After a number of cooling-deformation-heating cycles, in the framework of a thermomechanical treatment called training, the SMA is able to instantly recover its hot and cold shapes during heating and cooling, respectively, by twoway shape memory effect (TWSME) [10,11].…”