Mechanical Properties and Mechanisms of Fatigue Crack Nucleation of Two Austenitic Stainless Steels Used as Biomaterials

7th World Biomaterials Congress, Sydney, Australia, May 2004

Mazzei, A. C. A.1 2, Giordani, E. J. 1, Almeida, F.C.1, Ferreira, I.1.
1 University of Campinas, Campinas, SP, Brazil
2 GM Reis Implantes, Campinas, Brazil

Abstract

The ASTM F 138 stainless steel is currently the most used material to produce orthopedic implants and a considerable number of failures of temporary or permanent orthopedic implants occur. Since some years the austenitic ISO 5832-9 stainless steel is also being used in implants fabrication as a replacement to the ASTM F 138 without a consistent research of its performance. The objectives of the this work were to characterize and analyze the microstructure, the basic mechanical properties (tensile and hardness), the localized corrosion resistance, and fatigue properties in neutral and artificial physiological solution (0.9% NaCl solution) of the ASTM F 138 and ISO 5832-9 stainless steels that are being used as orthopedic implant materials. Moreover a study of the fatigue and corrosion fatigue crack nucleation mechanisms of these materials is cared out. Thereby, this study is one of the bases to analyze the replacement of ASTM F 138 by ISO 5832-9 in orthopedic applications.

Microstructural analysis revealed that the presence of particles of precipitates, characterized as Z-phase (NbCrN), is responsible for considerable austenitic grain refinement of ISO 5832-9 steel (average diameter » 15 m m) when compared to ASTM F 138 steel (average diameter » 60 m m). The high levels of strength of ISO 5832-9 steel (Sy » 500 MPa and Su » 860 MPa) when compared to ASTM F 138 steel (Sy » 250 MPa and Su » 590 MPa) are mainly attributed to grain refinement and nitrogen presence in solid solution in ISO 5832-9 steel [1].

The localized corrosion tests in 0.9% NaCl aqueous solution revealed that the nitrogen in solid solutions also confers the better performance of ISO 5832-9 steel when compared to ASTM F 138 steel.

Despite the combination of better strength and better corrosion resistance, the fatigue tests to high stress levels revealed that the presence of artificial aggressive environment has stronger influence on the fatigue life of ISO 5832-9 than on the ASTM F 138 steel. The fatigue and corrosion fatigue crack nucleation studies [2, 3] showed the greater influence due to establishment of favorable conditions to crevice corrosion within the geometrical discontinuities formed from rupture of many particles of Z-phase precipitates still in first cycle of stress under conditions of plastic deformation, as shown in figure 1.

In this way, if the material does not undergoes cold plastic deformation and so the integrity of precipitates was maintained, the combination of better strength and better corrosion resistance suggests that the ISO 5832-9 steel is a very promising material to orthopedic implants producing, mainly in more severe applications involving longer times and higher stress levels.

Keywords

stainless steel, mechanical properties, fatigue.

Acknowledgments

The authors would like to thank FAPESP for the financial support.

figure 1

(a) Analysis of the gage length of the ISO 5832-9 sample used in the fatigue test in aggressive environment (0.9% NaCl solution). The crack growth mechanism is the microcracking coalescence in the Z phase precipitates.

(b) Fracture surface obtained from a fatigue test in aggressive environment. Observe the crack nucleation at the elongated Z phase precipitate.

References

[1] Degallaix, S., Foct, J., Hendry, A. Mechanical behavior of high-nitrogen stainless steels Materials Science and Technology, v.2, p.946-950, September 1986.
[2] Dickson, J. I., Shiqiong, L., Baïlon, J.-P. Microstructural and fractographic aspects of corrosion fatigue, Materials Characterization, v.28, p.327-347, 1992.
[3] Suresh, S.. Fatigue of materials. 2th Edition, Cambridge University Press, 1998.