Cyclic deformation and austenite stabilization in Co35Ni35Al30 single crystalline high-temperature shape memory alloys

doi:10.1016/j.actamat.2009.08.038

Acta Materialia

Volume 57, Issue 20, December 2009, Pages 6123-6134

https://doi.org/10.1016/j.actamat.2009.08.038 Get rights and content

Abstract

This study reports on the role of repeated stress-induced martensite (SIM) transformations on the pseudoelastic (PE) behavior of solutionized Co₃₅Ni₃₅Al₃₀ [0 0 1]-oriented shape memory single crystals under both isothermal and non-isothermal conditions (referred to as “training”). It is demonstrated that training results in austenite stabilization and strengthening, consequently increasing the critical transformation stress levels for the SIM ( $σ_{crit}^{SIM}$ ), and promoting excellent cyclic stability and reproducibility of the PE response. This is attributed to the formation of dense dislocation arrangements and fine coherent sub-nanometer hexagonal close-packed Co and γ′ (Ni₃Al:L1₂) precipitates during training. The training that involved cyclic loading conditions was more effective than the monotonic stress–strain tests at different temperatures in modifying SIM characteristics bringing about (i) a large pseudoelastic temperature range of 350 °C with $σ_{crit}^{SIM}$ levels reaching 1 GPa with complete recoverable strains of 2%, and (ii) excellent stable cyclic PE response at temperatures as high as 250 °C.

Section snippets

Motivation and background

In recent years, considerable attention has been devoted to the development of high-temperature shape memory alloys (HTSMAs) as the demand from robotic, automotive, aerospace, turbine engine and air-conditioning industries has grown significantly, where the operating temperatures are often higher than 100 °C. It is common practice to alloy binary systems in order to achieve higher martensitic start (M_s) transformation temperatures and/or to improve the ductility of the alloys; for instance, (i)

Material and experimental procedures

An ingot of CoNiAl with a nominal composition of 35Co–35Ni–30Al (in at.%) was prepared using vacuum induction melting. The single crystals were grown using the Bridgman technique in a He environment. Specimens with dimensions of 4 × 4 × 8 mm³ were electro-discharge-machined from the bulk single crystals such that their longer, i.e. compression axis was along the [0 0 1] orientation. The samples were solutionized at 1350 °C for 24 h followed by quenching in water. For the heat-treatments, the samples

Influence of thermomechanical history on mechanical behavior

Fig. 2 shows the Type 1 training (cyclic stress–strain response) of an as-solutionized Co₃₅Ni₃₅Al₃₀ [0 0 1]-oriented single crystal at 40 °C with a constant strain range of 2.5% under compressive loading conditions. It is apparent from the figure that no significant cyclic degradation has occurred in terms of accumulation of residual strains and stress hysteresis, where the latter remains constant at 35 MPa throughout the cycling as indicated for the first and the 1000th cycle in the figure. The

Discussion

It is clear from both the thermomechanical and TEM results that the MT behavior in Co₃₅Ni₃₅Al₃₀ alloys is strongly affected by the microstructure that was developed during training. This is clarified with the help of data obtained from the PE curves at 120 and 220 °C shown in Fig. 3b for the samples under different trained conditions as shown in Table 1. Table 1 shows that $σ_{crit}^{SIM}$ and Δσ values increase, and ɛ_tr values decrease with temperature. The raise in Δσ indicates the high dissipation

Summary

In the present study, the effect of repeated stress-induced phase transformation at different temperatures on the pseudoelastic behavior of solutionized Co₃₅Ni₃₅Al₃₀ [0 0 1]-oriented shape memory single crystals was analyzed as a function of temperature under compressive loading conditions. The results revealed that the significant changes in the microstructure that occur during cyclic loading (referred to as “training”) govern the macroscopic stress–strain response. The key findings of the

Acknowledgements

The present study was supported by Deutsche Forschungsgemeinschaft (DFG), US Army Research Office, Contract No. W911NF-06-1-0319, the US National Science Foundation – Division of Materials Research, Grant No. 0805293 and by the Russian Foundation for Basic Research, Project No. 08-08-91952 NNIO-a.

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Cyclic deformation and austenite stabilization in Co35Ni35Al30 single crystalline high-temperature shape memory alloys

Abstract

Section snippets

Motivation and background

Material and experimental procedures

Influence of thermomechanical history on mechanical behavior

Discussion

Summary

Acknowledgements

Mater Sci Eng A

Scripta Mater

Acta Mater

Scripta Mater

Scripta Mater

Intermetallics

Intermetallics

Scripta Mater

Scripta Mater

Scripta Mater

Mater Sci Eng A

Acta Mater

Mater Sci Eng A

Acta Mater

Mater Sci Eng A

Scripta Mater

Scripta Mater

Acta Mater

Acta Mater

Scripta Metall

Scripta Metall

J Mech Phys Solids

Scripta Metall

Cyclic deformation and austenite stabilization in Co₃₅Ni₃₅Al₃₀ single crystalline high-temperature shape memory alloys