Coupled stress-strain and electrical resistivity measurements on copper based shape memory single crystals

Abstract

Recently, electrical resistivity (ER) measurements have been done during some thermomechanical tests in copper based shape memory alloys (SMA's). In this work, single crystals of Cu-based SMA's have been studied at different temperatures to analyse the relationship between stress (σ) and ER changes as a function of the strain (ε). A good consistency between ER change values is observed in different experiments: thermal martensitic transformation, stress induced martensitic transformation and stress induced reorientation of martensite variants. During stress induced martensitic transformation (superelastic behaviour) and stress induced reorientation of martensite variants, a linear relationship is obtained between ER and strain as well as the absence of hys teresis. In conclusion, the present results show a direct evidence of martensite electrical resistivity anisotropy.

Electro-thermomechanical properties of superelasticity in single crystals shape memory alloys

“…In this method, a well-stabilized current passes across the sample and potential difference (∆V) is measured thought the useful length. Gonzalez et al present details of the coupling ER measurements in tensile tests experimental procedures 5 . Equation 1 represents electric resistivity changes where Rε and ρε are respectively the electrical resistance and electrical resistivity for a given strain, Rεo and ρεo corresponding to the properties just before test (ε null at test temperature).…”

“…In this case, martensitic phase (β 1 ' martensite) was initially deformed, differently of previous tests. During AB interval occurs β 1 ' occurs martensite variants reorientation process where variants that has the highest Schmidt factor increase in the expense of other variants 5 . In the B point was formed a single variant martensite.…”

“…For Cu-Zn-Al alloy, the difference between each sample was 4 degrees and 9 degrees for Cu-Al-Be alloy. Changes in the crystallographic orientation of the tensile axis influences critical stress of the martensite induction, constant level of stress for martensite formation (plateau without inclination in the stress curve) and maximum transformation strain to obtain the martensite single crystal 2,5 . In the superelastic test, only one of the 24 martensite variants is favored by the crystallographic relations with tensile axis direction.…”

“…In the superelastic test, only one of the 24 martensite variants is favored by the crystallographic relations with tensile axis direction. Single variant induced will be the one which leads to the highest Schmidt factor 5 . In the Figure 8 is shown micrographs of the b and c CAB12 tensile samples by means of optical microscopy where only one variant was induced in each sample.…”

DESENVOLVIMENTO DE DISPOSITIVO PARA AVALIAR A FADIGA FUNCIONAL DE LIGAS DE Ti-Ni COM MEMÓRIA DE FORMA

“…A fadiga das LMF, por exemplo, possui dois aspectos: mecânico e fenomenológico. As ligas podem falhar por ruptura mecânica (mecanismos da fratura) ou podem deixar de executar o EMF (perda do efeito memória) devido ao processo de estabilização martensítica [14]. Esta pesquisa faz o estudo do comportamento da fadiga funcional em atuadores de Ti-Ni com a forma de molas helicoidais.…”