Research On The Fe-Ni-Al-Ta Based Polycrystalline Shape Memory Alloys

Ferrous polycrystalline shape memory alloys on account of their low price,processing plastic, and high strength, are expected to become a class of functionalmaterials with good prospects, but the research on the ferrous polycrystalline shapememory alloys was rarely reported, and their practical applications have not yet beenfully developed. So, it is very useful to investigate the ferrous polycrystalline shapememory alloys.In this paper, the effects of Ta additions on microstructure and properties in theFe60.5-xNi28Al11.5Taxalloys are investigated by metallurgical microscope, X-raydiffraction, SEM, EDS, high-temperature dilatometer spectrometer and pressure testmachine. In the meantime, the effects of aging and addition of Co element on themicrostructure and properties of Fe-Ni-Al-Ta shape memory alloy have been alsoexplored. The results show that:(1) With the increment of Ta additions in the Fe60.5-xNi28Al11.5Taxalloys, the γ’phase content increases which strengthened the austenitic matrix, meanwhile thecompressive strength, the recoverable strain and the maximum strain of the agingstate alloys decrease first and then increase, and the alloys’ residual strain firstlydecreases and then increases. When the Ta content is1at.%, the alloy’s compressivestrength, recoverable strain and plastic deformation strain reach its maximum value.Thus, the alloy has the best pseudo-elastic at this time. The mean thermal expansioncoefficient of the alloys decreases with Ta additions, when the Ta content is1at.%,the mean thermal expansion coefficient is at its minimum.(2) The microstructure and properties ofFe59.5Ni28Al11.5Ta1shape memoryalloy significantly are affected by the aging process. Because of aging treatment, γ’phase and α’ phase precipitation, strengthens the austenitic matrix. The properties ofshape memory alloy are influenced by the particle size, the distribution, the volumefraction of precipitate phases. With the increase of aging time, at873K the stressinduced martensite critical stress of the aging state alloy decreases first and thenincreases, the alloys’ compressive strength, the recoverable strain and the hardness increases first and then decreases, besides, the alloy’s residual the strain firstlydecreases then increases, when the aging time is60h, the alloy’s recoverabledeformation is maximum, reaching14.9%, the alloy’s residual strain relative isminimum. At this point the alloy exhibits good nonlinear pseudo-elasticity,compressive strength and hardness. In this case the alloy precipitation strengtheningphase γ’ phase is more than other aging state, the distribution is relatively uniform.(3) The microstructure and properties ofFe42.5Ni28Co17Al11.5T a1memory alloysignificantly are affected by the aging temperature, the precipitation of different phase,the distribution of the precipitated phase. When the alloy ages60h at723K, much βphase precipitates in the alloy, and mainly gathers at the grain boundaries, whichreduces the strength of the matrix；at873K, the precipitation of the γ’ phase, whichdisperses in the matrix and plays for strengthening the role of the matrix, thenprecipitation of the β phase is relatively small；at923K, the number of β phaseprecipitation in the alloy increases；at1023K, a large amount of β phase precipitate inthe alloy, and mainly gather at the grain boundaries. Besides, the γ’ phase also growsup together, which lose its dispersion strengthening effect. With the increase of agingtemperature, the recoverable strain first increases and then decreases, the residualstrain of the alloy decreases and then increases, the aging temperature is873K, thealloy’s compressive strength is maximum, reaching2535Mpa, the alloy’s recoverabledeformation is maximum, reaching15.2%, the residual strain of the alloy is relativelyminimal. At this point the alloy exhibits good pseudo-elasticity. Cobalt as solute atomgives rise to lattice deformation, and it can make the γ’ precipitation strengtheningphase increasing, which can increase the strength of the alloy, but it is not helpful forthe pseudo-elasticity.