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SPECIAL SECTION ON DAMPING OF SHAPE MEMORY ALLOYS, COMPOSITES, AND FOAMS

# Stress-Strain-Temperature Behavior Due to $B2-R-B19′$ Transformation in $NiTi$ Polycrystals

[+] Author and Article Information
P. Šittner1

Institute of Physics, ASCR, Na Slovance 2, Prague, 18221, Czech Republicsittner@fzu.cz

V. Novák

Institute of Physics, ASCR, Na Slovance 2, Prague, 18221, Czech Republic

P. Lukáš

Nuclear Physics Institute, ASCR, 250 68 Řež, Czech Republic

M. Landa

Institute of Thermomechanics, ASCR, Dolejškova 5, Prague 8, 18000, Czech Republic

1

Corresponding author.

J. Eng. Mater. Technol 128(3), 268-278 (Feb 26, 2006) (11 pages) doi:10.1115/1.2204945 History: Received August 26, 2005; Revised February 26, 2006

## Abstract

Thermomechanical behavior of superelastic $NiTi$ wires undergoing sequential $B2-R-B19′$ martensitic transformation was investigated by two recently developed in-situ experimental methods (in-situ neutron diffraction and combined ultrasonic and electric resistance measurements) capable of detecting and recognizing the activity of various deformation/transformation processes in $NiTi$ and theoretically by micromechanical modeling. An earlier model of SMA polycrystal transformation is further developed, so it accounts for the strains due to the R-phase related deformation processes in activated $NiTi$. A continuous variation of the rhombohedral distortion angle $α$ of the R-phase structure with temperature and stress is newly introduced as a legitimate deformation mechanism. Simulation results for $NiTi$ bars and wires exposed to three types of thermomechanical tests—mechanical loading at constant temperature, cooling under constant stress, and recovery stress tests are presented and confronted with results.

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## Figures

Figure 1

In-situ neutron diffraction measurement on polycrystalline SMAs. A polycrystalline specimen is loaded thermomechanically using deformation machine and furnace installed at the neutron diffractometer. Integrated intensities and positions of the reflections of the austenite and martensite phases are evaluated from diffraction patterns recorded in-situ during the thermomechanical test in stopovers at preselected [σ,ε,T] states (8).

Figure 2

In-situ UltraSonic and Electric Resistance measurements /USERIST/ on an activated NiTi wire. The speed CL and attenuation αL of longitudinal ultrasonic waves propagating in NiTi wires and its electric resistance ρ are simultaneously measured during thermomechanical cyclic loads.

Figure 11

Simulation of thermomechanical recovery stress tests on NiTi in tension (a),(b) and compression (c),(d). The individual graphs show (a),(c) macroscopic stress-temperature responses and (b),(d) phase fraction—temperature responses. Corresponding stress-strain responses are given in Fig. 1.

Figure 3

An example of tensile nonequilibrium stress-temperature diagram of Ni-Ti crystal undergoing B2-R-B19′ transformations (solid lines denote the transformation conditions). The dashed lines correspond to “dynamic” R-M transformation lines shifting to the lower temperature (T0,R-M decreases), the slope of which (sR-MT) increases with decreasing temperature due to the continuous variation of the rhombohedral distortion of the R-phase.

Figure 4

Experimental stress-strain curves of the NiTi alloy undergoing B2-R-B19′ transformation (bar specimen Rs=25°C, Ms=−13°C, position control, strain rate 5×10−4): (a) compression tests at various constant temperatures; (b) tension/compression test at T=22°C

Figure 5

Results of the in-situ neutron diffraction experiment on NiTi undergoing stress induced martensitic transformation B2→R→B19′ in a compression test at T=20°C (position control, strain rate 5×10−4). Diffraction patterns (b) are recorded during stopovers (a) along the compression test. The evolution of integral intensities of individual diffraction peaks (c) reflects the volume fractions of phases 4-21R(RC). 003R(RT), and 120M(MC). The evolution of lattice strains (d) reflects the stresses in the R and M phases (Eq. 1) during the compression test.

Figure 6

Simulation of stress-strain behavior of NiTi due to B2→R→B19′ transformation in (a) tension and (b) compression at various temperatures. Evolution of phase fractions with total strain (c) and component stress-strain curves (d) for the compression test at T=20°C.

Figure 7

Results of two USERIST experiments on two different NiTi wires showing the R-phase deformed in tension (position control, strain rate 1×10−4) starting from the R-phase (a) and starting from the austenite phase (b). The set of six graphs shows the in-situ measured variation of the electrical resistance ρ (top), attenuation αL (middle), and wave speed CL (bottom) of the longitudinal acoustic wave propagating through the wire.

Figure 8

Simulation of the strain-temperature responses upon cooling NiTi wire under various constant applied stresses in tension (a) and compression (b)

Figure 9

Simulation of cooling NiTi wire under various constant stresses in tension (a)50MPa, (b)100MPa, (c)300MPa. Evolution of phase fractions 1 and some hkl component variables (2),(3) during cooling.

Figure 10

Simulation of thermomechanical recovery stress tests on NiTi in tension and compression—stress-strain response (stress-temperature responses are shown in Figs.  1111

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