Incommensurate–commensurate transition and nanoscale domain-like structure in iron doped Ti–Ni shape memory alloys

Abstract

Precursor phenomena of displacive transformation have been studied by optical and transmission electron microscope observation and X-ray diffraction of Ti-(50 À x)Ni-xFe (x ¼ 2, 4, 6, 8 in at.%) alloys. We found that a Ti-44Ni-6Fe alloy exhibits a second-order-like incommensurate-commensurate transition without latent heat and discontinuity in lattice parameters. In other words, diffuse scatterings appear in an electron diffraction pattern at an incommensurate position on cooling; they move gradually towards 1/3h110i as the temperature decreases and lock into the commensurate position at 180 K. The commensurate phase is not expanded along one of the h111i directions, unlike the R-phase formed by a first-order transformation in Ti-48Ni-2Fe and Ti-46Ni-4Fe alloys. In addition, the commensurate phase shows a nanoscale domain-like structure, which is inherited from the incommensurate state of the parent phase. Thus, the anomalies in physical properties observed in the incommensurate state are most likely the precursor phenomena of the commensurate phase in the Ti-44Ni-6Fe alloy. In the case of a Ti-42Ni-8Fe alloy, the incommensurate state remains even at 19 K.

Position of diffuse satellites appearing in Ti-44Ni-6Fe shape memory alloy

“…The intensity of diffuse satellites increase and the position gradually shift toward commensurate position of g B2 +1/3 0> * with decreasing temperature. These results are consistent with the previous study [8].…”

“…Based on these results, they concluded that a microdomain is not a small R-phase but a single transverse atomic displacement. We also investigated this behavior and reported in the paper [8]. That is we investigated the composition and temperature dependences of the diffuse satellites appearing in electron diffraction patterns of Ti-(50-x)Ni-xFe alloys (6 x 10), which exhibit a negative temperature coefficient of resistivity below a temperature T min as shown in Fig.…”

Stress-induced strain glass to martensite (R) transition in a Ti50Ni44.5Zhang1, Wang2, Ding3 et al. 2011Phys. Rev. B281190View full textAdd to dashboardCite

“…5(a)) showed a normal B2 spectrum as expected by a frozen STG. As the stress increased to about 80 MPa, the intensity of the 110 B2 peak decreased and two new peaks appeared, which can be indexed as 112 R and 300 R , being the same as those of the normal R martensite in Ti 50 Ni 48 Fe 2 alloys 21 . When the stress reached 180 MPa [the maximum stress point in Fig.…”

“…Thus it seems that the local strain order in the nano-domains is an R-like structure, being the same as the binary Ti-Ni STG 10 . The above TEM observation demonstrates during a STG transition there is a gradual increase in the size and in volume fraction of the nano-domains, which have an R-like structure 21,22 . It is the origin of the gradual broadening of the 110 B2 peak during the cooling as shown in Fig.…”

Electronic Structure of B2-Type Ti–Ni–Fe Alloys Exhibiting Second-Order-Like Structural Transformation

“…14,15) Recently, we measured electrical resistivity, magnetic susceptibility and specific heat for a series of Ti-(50 À x)Ni-xFe alloys with Fe content up to 20 at%, and found that these physical properties exhibit anomalies in the parent phase of the alloys which show the R-phase transformation and also in the alloys with Fe content more than 6 at% which do not show apparent Rphase transformation. 16,17) In addition, these anomalies are observed most in Ti-44Ni-6Fe and Ti-42Ni-8Fe alloys and gradually suppressed as Fe content increases. We believe that these anomalies can be so-called precursor phenomena.…”