Abstract
Fe50Ni28Co17Al11.5Ta2.5 single crystals oriented along the [001] direction were investigated in order to establish the influence of two-step aging conditions on superelastic properties. The homogenized and quenched single crystalline material was subjected to a combination of high-temperature and low-temperature heat treatment at 973 K for 0.5 h and at 723 K for various aging times, respectively. As a result, fine and coherent γ’ precipitates were formed. Using diffraction of high energy synchrotron radiation, the volume fraction of γ’ precipitates was computed while their size was determined by high resolution TEM analysis. Compared with one-step heat treatment, the two-step aging process enables control of the precipitate size in a more accurate way. Moreover, it allows one to obtain a higher volume fraction of precipitates without increasing their size significantly. The obtained coherent γ’ precipitates ranged in size from 5 to 8 nm; that considerably improved mechanical properties. The highest superelastic response was obtained for single crystals aged at 973 K for 0.5 h followed by aging at 723 K for 3 h. The single crystals treated with such conditions exhibited a superelastic strain of 15% in which the mechanical martensite stabilization was substantially suppressed.
Highlights
Over the past twenty years there has been a growing interest in new intelligent materials; among them, shape memory alloys (SMA), in which shape change occurs by applying an external thermal, magnetic or mechanical field, have received a great deal of attention
They are practically undeformable and the main strengthening mechanism is related to their size and distribution due to dislocation bypass [46]
It can be concluded that two-step aging, which combines short-time high-temperature and longer time low-temperature aging, effectively decreases the stress for the onset of martensitic transformation
Summary
Over the past twenty years there has been a growing interest in new intelligent materials; among them, shape memory alloys (SMA), in which shape change occurs by applying an external thermal, magnetic or mechanical field, have received a great deal of attention. These include Fe-based SMAs that are characterized by low material cost and good formability which may result in broad application [1,2,3,4,5]. It is worth noting that this effect only appears in Fe-based
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