Ch. Somsen

Influence of thermal annealing on the martensitic transitions in Ni–Ti shape memory alloys

We report mainly on resistance R(T) measurements between 4.2 and 1300 K on NixTi100−x alloys, with 51<x<54.5at%, quenched from different annealing temperatures TA to room temperature. When quenched from the B2-phase stability range (TA=1273 K), alloys with 51<x<54 show an increase in R(T) with decreasing T below 300 K. Subsequent annealing at TA=653 K (1 h) and quenching leads to a reduction of the R(T) anomaly below 320 K and the occurence of a martensitic transition (MT) from the B2- to the R-phase, with TR=310 K, rather independent of x. After annealing at 723 K (1 h) and 823 K (1 h), respectively, two-step MTs occur from B2 to R and subsequently to B19′, with MS(B19′) depending on x and TA. After annealing at 923 K or higher TA, MTs cannot be found anymore, and the R(T) behaviour is similar to that after quenching from 1273 K. Studies of R(T) at high T on samples quenched from 1273 K reveal the occurence of mainly two annealing stages. The first one at around 500 K marks structural changes inducing the martensitic phases at lower temperatures. The second one at about 900 K marks the formation of the B2-phase and the disappearance of other phases triggering the MT. The R(T) results are compared with the thermal expansion a(T) and X-ray investigations. The structural phase diagram of Ni–Ti around NiTi is discussed.

Small-angle neutron scattering of precipitates in Ni-Ti shape memory alloys

Abstract Small-angle neutron scattering was performed on polycrystals of Ni–(46–49) at.% Ti quenched in ice water from the solid solution. The presence of small precipitates of a radius of about 1 nm was found for Ni–(46, 47 and 48) at.% Ti. Assuming a composition of Ni 4 Ti 3 of the precipitates, their volume fraction varies from 7% to 0.3%. No precipitates are found if the Ti content is closer to stoichiometric NiTi. The formation of these precipitates already during quenching seems to suppress the formation of martensite. Ni–(47.9 and 48.5) at.% Ti were further aged for 1 h at 553 K, and small-angle scattering shows a fully established precipitate microstructure. The particles have a radius of about 1.5 nm and a mean interparticle distance of 4.8–5.8 nm. From the integrated small-angle scattering curves, a volume fraction of Ni 4 Ti 3 particles of about 20% is obtained.

A transmission electron microscopy procedure for in-situ straining of miniature pseudoelastic NiTi specimens

Abstract The in-situ transmission electron microscopy technique allows direct observations of formation and growth of stress-induced martensite in pseudoelastic NiTi shape memory alloys. The present paper reports on the development of a miniature test procedure for in-situ straining experiments with specimens taken from small components. The deformation of an ultra-fine grained NiTi specimen is characterised by transmission electron microscopy (at early loading stages) and by optical microscopy (at larger strains). A complementary finite element analysis of the complex strain state in the specimen rationalises why the stress-induced martensitic transformation first occurs in the thin foil region of the specimen before spreading towards the outer rim of the specimen.

Elemental Mapping of NiTi with EFTEM

Martensitic transformations in Ni-rich NiTi shape memory alloys take place as multistage transformations. In Ni-rich alloys with an austenitic B2 matrix, coherent Ni{sub 4}Ti{sub 3} precipitates form from thermo-mechanical processing and affect the sequence of the martensitic transformation. Any composition inhomogenieties that develop during the evolution of the Ni{sub 4}Ti{sub 3} precipitates will have a large influence on the multistage martensitic transformations, since the martensite start temperature, M{sub s}, is strongly dependent on the Ni concentration of the matrix. Since concentration differences on the order of 0.5 at% are sufficient to influence the transformation, providing sufficiently accurate concentration profiles for meaningful structure-property correlations is a challenging experiment. This investigation employs elemental mapping by energy-filtered transmission electron microscopy (EFTEM) to attempt to measure the concentration profiles at these precipitate-matrix interfaces.