Introduction: Co-based alloys for biomedical applications are usually fabricated by investment casting method and exhibits inherent defects that are cause of concern and can lead to early implant failure[1]. In these alloys, the FCC and the HCP phases co-exist. Thus, there are efforts to obtain a single HCP phase Co-based alloys to exploit its mechanical properties[2]-[4]. The investigated Co- 20 wt. %Cr alloy was as-cast segregation-free and has 88 wt. % and 91 wt. % of ε-martensite in the as-cast and heat treated condition, respectively. The resultant alloy microstructures were characterized and tested for determinations of mechanical properties.
Materials and Methods: Co-20 wt. %Cr alloy was prepared in a vacuum induction furnace with argon atmosphere. The alloy was cast into a copper mold to achieve a cooling rate of 230K/s. The as-cast ingots were mechanically polished and etched using a 60 vol. % HNO3 + 40 vol. % H2O solution. Microstructures were observed with a scanning and transmission electron microscopes (JEOL 7600 and 1200 EX, respectively). X-ray diffraction was carried out using a Siemens D-5000 diffractometer with a Kα-Cu (1.5418 Å) radiation. Ingots sections were heat-treated at 1023.15 K (750°C) for 30, 60 and 180 minutes[5]. Mechanical properties were measured with an Instron 1210 at a travel speed of 0.5 mm/min.
Results: Microstructural features in the as-cast and heat treated condition (1023.15K for 60 min.) are shown in Figure 1a-b. Strain-stress curves and mechanical properties of alloy in both conditions are shown in Figure 2a and Table 1. The amount of ε-martensite was obtained from diffraction patterns employing the equation proposed by Sage and Guillaud[6] (Figure 2b).
Discussion:
Figure 1a-(a) shown directionally solidified columnar dendrites in the as-cast condition with 88 wt.% of athermal ε-HCP phase. Fine plates of athermal ε-martensite inside columnar dendrites are shown in TEM micrographs (Fig. 1a-(b)) that are consistent with the HCP1 martensite reported by K. Rajan[7] its corresponding pattern is in Fig 1a-(c). For aged alloy, SEM and TEM micrographs (Fig. 1b-(a) and (b)) reveals a kind of recrystallization caused by excess vacancies movement, consequence of rapid solidification[8].
As-cast alloy exhibits yield and ultimate tensile strengths of 183 and 332, respectively, but an elongation of 4.7%. After isothermal alloy aging at 1023.15 K for 60 minutes, the mechanical properties exhibit a dramatic improvement (i.e. 280 MPa yield strength and a UTS of 618 MPa). This confirms that an increases of ε-martensite (from 88 to 91 wt. %) transformed isothermally and recrystallization (grain formation) after aging promotes an increment on the alloy mechanical properties[9]-[11]. Moreover, in this condition the balance between UTS, Young’s modulus and elongation percentage are better than the same properties for the alloy heat-treated at 1023.15K for 30 and 180 minutes as shown in Table 1 and Figure 2a.
Conclusions: In this work, an innovative solidification process is proposed to produce Co-Cr alloys with a high percentage of ε-martensite (HCP phase). In addition, we demonstrated that a heat treatment below martensitic transformation temperature improves significantly its mechanical properties.
A. Tejeda; C. Flores-Morales; C. Zorrilla; CONACYT
References:
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