Abstract
In some types of surgical implants, such as bone screws and plates, Grade 2 Ti is seriously considered as a replacement for the Ti-6Al-4V alloy. Advantages are lower cost and the absence of Al and V, which have been identified as potentially harmful to human health. The present paper shows that the lower strength of the commercially pure metal can be enhanced by Severe Plastic Deformation followed by conventional cold rolling, so as to reach a strength level higher than the technical requirements applicable to the alloy. This was ascertained by tensile and Vickers hardness tests from which it was concluded that the best combination of properties are obtained by submitting the metal to Equal Channel Angular Pressing (four passes at 300 °C) followed by a 70% thickness reduction by cold rolling. Although the present results are valid for the material only, and not for the product considered, that is, bone screws, it appears that this solution is a step towards the replacement of the Ti6-4 alloy by Grade 2 Ti, at least for some types of metallic medical implants.
Highlights
Surgical implants make intensive use of the well known Ti-6Al-4V alloy
Comparison of samples 1X and 1XH shows that, yield and maximum tensile strength are almost identical, the hot deformed sample exhibits a much higher ductility, viz 22% strain to fracture against 11%. This suggests that room temperature ECAP may not be ideal when compared with warm ECAP, provided that in this latter case deformation temperature is kept below levels which would promote softening mechanisms
On this respect 300 °C seems to be ideal since analysis of the tensile behavior of the present samples suggests only a small amount of recovery[6], but the residual stresses appear to be reduced and as a result strength is unchanged whilst elongation is considerably increased
Summary
Surgical implants make intensive use of the well known Ti-6Al-4V alloy. Cost consideration and concerns regarding the effect of Al and V on human health constitute a strong motivation for the replacement of said alloy[1], and a viable candidate is the lower cost Ti Grade 2, from which a better corrosion resistance is expected, its tensile strength compares unfavorably with that of the alloy, viz. 350 MPa against 1000 MPa. A recent paper by Zhao[4] shows that, contrarily to previous assumptions[5], Ti can be deformed at room temperature, and eight passes in a 120° die were carried out This process reduced the grain size from 23 to 0.2 μm and as a consequence the yield strength was raised to 710 MPa, that is, a 160% increase over that of annealed Grade 1 Ti. The authors emphasize that reduction of pressing speed is a key detail to successfully deform Ti at room temperature. The basic model describes the formation of a banded structure of elongated subgrains, which under increasing strain transform into an equiaxed array of grains[10] This transition from subgrains to grains is in actual fact a transition from low angle boundaries (2 ≤ ξ < 15°) to high angle boundaries (ξ ≥ 15°) a phenomenon which is currently under intense study. After a preliminary discussion regarding the effect of such process on Grade 2 Ti tensile behavior, the feasibility of using that material in the manufacture of cortical screws will be analyzed
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