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Shape Memory and Superelasticity

, Volume 4, Issue 4, pp 450–461 | Cite as

Influence of Micro-EDM on the Phase Transformation Behaviour of Medical-Grade Nitinol

  • James Wamai Mwangi
  • Linda Weisheit
  • Viet Duc Bui
  • Matin Yahyavi Zanjani
  • Andreas Schubert
Article

Abstract

Most nitinol medical applications are hinged on its superelasticity and shape memory—two unique properties that are dependent on nitinol’s phase transformation between a martensitic phase and an austenitic phase. Since these transformations are thermomechanical in nature, establishing the influence of thermal processing on nitinol’s phase-transformation behaviour is vital as this can help in predicting changes and/or tuning its mechanical properties to fit specific applications. This study uses differential scanning calorimetry to investigate the influence of micro-electrical discharge machining (micro-EDM) on nitinol’s phase-transformation behaviour. For conclusive analysis, a relatively athermal Jet-ECM process is used as a reference for the as-received material, whereas Laser, a more commercially established medical-grade nitinol machining process, is used to provide comparative analytical aid. From the results, it can be clearly shown that high discharge energies in micro-EDM do indeed have the potential to significantly alter nitinol’s transformation behaviour, including reducing thermal hysteresis and resulting in the occurrence of an unusual three-peak phenomenon in the endothermic reverse transformation on heating.

Keywords

Micro-EDM Nitinol Phase-transformation behaviour Shape memory Superelasticity Differential scanning calorimetry Laser Jet-ECM 

List of Symbols

Transformation Temperature Variables

\(A_{\text {f}}\)

Austenite finish if one austenite phase exists (\(^{\circ }{\text {C}}\))

\(A_{{\text {f}}1}\)

Austenite finish for the first austenitic phase (\(^{\circ }{\text {C}}\))

\(A_{{\text {f}}2}\)

Austenite finish for the second austenitic phase (\(^{\circ }{\text {C}}\))

\(A_{\text {p}}\)

Austenite peak if one austenite phase exists (\(^{\circ }{\text {C}}\))

\({A}_{{\text {p}}1}\)

Austenite peak for the first austenitic phase (\(^{\circ }{\text {C}}\))

\(A_{{\text {p}}2}\)

Austenite peak for the second austenitic phase (\(^{\circ }{\text {C}}\))

\(A_{\text {s}}\)

Austenite start if one austenite phase exists (\(^{\circ }{\text {C}}\))

\(A_{{\text {s}}1}\)

Austenite start for the first austenitic phase (\(^{\circ }{\text {C}}\))

\(A_{{\text {s}}2}\)

Austenite start for the second austenitic phase (\(^{\circ }{\text {C}}\))

\(M_{\text {f}}\)

Martenite finish if one martensite phase exists (\(^{\circ }{\text {C}}\))

\(M_{{\text {f}}1}\)

Martenite finish for the first martensitic phase (\(^{\circ }{\text {C}}\))

\(M_{{\text {f}}2}\)

Martenite finish for the second martensitic phase (\(^{\circ }{\text {C}}\))

\(M_{\text {p}}\)

Martenite peak if one martensite phase exists (\(^{\circ }{\text {C}}\))

\(M_{{\text {p}}1}\)

Martenite peak for the first martensitic phase (\(^{\circ }{\text {C}}\))

\(M_{{\text {p}}2}\)

Martenite peak for the second martensitic phase (\(^{\circ }{\text {C}}\)]

\(M_{\text {s}}\)

Martenite start if one martensite phase exists (\(^{\circ }{\text {C}}\))

\(M_{{\text {s}}1}\)

Martenite start for the first martensitic phase (\(^{\circ }{\text {C}}\))

\(M_{{\text {s}}2}\)

Martenite start for the second martensitic phase (\(^{\circ }{\text {C}}\))

\(R_{\text {cf}}\)

R-phase finish while cooling (\(^{\circ }{\text {C}}\))

\(R_{\text {cp}}\)

R-phase peak while cooling (\(^{\circ }{\text {C}}\))

\(R_{\text {cs}}\)

R-phase start while cooling (\(^{\circ }{\text {C}}\))

\(R_{\text {hf}}\)

R-phase finish while heating (\(^{\circ }{\text {C}}\))

\(R_{\text {hp}}\)

R-phase peak while heating (\(^{\circ }{\text {C}}\))

\(R_{\text {hs}}\)

R-phase start while heating (\(^{\circ }{\text {C}}\))

Other Variables

\(\Delta H\)

Transformation enthalpy per unit volume (J/kg)

\(\Delta H_{\text {A}}\)

Enthalpy if one austenite phase exists (J/kg)

\(\Delta H_{{\text {A}}1}\)

Enthalpy for the first austenitic phase (J/kg)

\(\Delta H_{{\text {A}}2}\)

Enthalpy for the second austenitic phase (J/kg)

\(\Delta H_{\text {M}}\)

Martensite enthalpy (J/kg)

\(\Delta H_{\text {RC}}\)

R-phase enthalpy on cooling (J/kg)

\(\Delta H_{\text {RH}}\)

R-phase enthalpy on heating (J/kg)

\(i_{\text {e}}\)

Discharge current (A)

T

Test temperature (\(^{\circ }{\text {C}}\))

\(t_{\text {e}}\)

Pulse duration (s)

\(u_{\text {e}}\)

Discharge voltage (V)

\(E_{\text {d}}\)

Discharge energy (J)

\(\sigma \)

Stress (N/m\(^{2}\))

\(\epsilon \)

Strain

Notes

Acknowledgements

The authors are thankful to the German Academic Exchange Service (DAAD), Technische Universitaet Chemnitz, and NACOSTI, Kenya for facilitating this research. Special thanks are offered to Johnson Matthey Medical Components Company for providing the nitinol sheet, and to Fraunhofer IWU Dresden and Chemnitz for the DSC measurements and SEM measurements, respectively.

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Copyright information

© ASM International 2018

Authors and Affiliations

  • James Wamai Mwangi
    • 1
  • Linda Weisheit
    • 2
  • Viet Duc Bui
    • 1
  • Matin Yahyavi Zanjani
    • 1
  • Andreas Schubert
    • 1
  1. 1.Professorship of Micromanufacturing TechnologyChemnitz University of TechnologyChemnitzGermany
  2. 2.Fraunhofer Institute for Machine Tools and Forming TechnologyDresdenGermany

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