Shingo Kurosu

Microstructures and mechanical properties of Co-29Cr-6Mo alloy fabricated by selective laser melting process for dental applications

The selective laser melting (SLM) process was applied to a Co-29Cr-6Mo alloy, and its microstructure, mechanical properties, and metal elution were investigated to determine whether the fabrication process is suitable for dental applications. The microstructure was evaluated using scanning electron microscopy with energy-dispersed X-ray spectroscopy (SEM-EDS), X-ray diffractometry (XRD), and electron back-scattered diffraction pattern analysis. The mechanical properties were evaluated using a tensile test. Dense builds were obtained when the input energy of the laser scan was higher than 400 J mm⁻³, whereas porous builds were formed when the input energy was lower than 150 J mm⁻³. The microstructure obtained was unique with fine cellular dendrites in the elongated grains parallel to the building direction. The γ phase was dominant in the build and its preferential <001> orientation was confirmed along the building direction, which was clearly observed for the builds fabricated at lower input energy. Although the mechanical anisotropy was confirmed in the SLM builds due to the unique microstructure, the yield strength, UTS, and elongation were higher than those of the as-cast alloy and satisfied the type 5 criteria in ISO22764. Metal elution from the SLM build was smaller than that of the as-cast alloy, and thus, the SLM process for the Co-29Cr-6Mo alloy is a promising candidate for fabricating dental devices.

Effects of chromium and nitrogen content on the microstructures and mechanical properties of as-cast Co-Cr-Mo alloys for dental applications.

The microstructure and mechanical properties of as-cast Co-(20-33)Cr-5Mo-N alloys were investigated to develop ductile Co-Cr-Mo alloys without Ni addition for dental applications that satisfy the requirements of the type 5 criteria in ISO 22674. The effects of the Cr and N contents on the microstructure and mechanical properties are discussed. The microstructures were evaluated using scanning electron microscopy with energy-dispersive X-ray spectroscopy (EDS), X-ray diffractometry (XRD), and electron back-scattered diffraction pattern analysis. The mechanical properties were evaluated using tensile testing. The proof strength and elongation of N-containing 33Cr satisfied the type 5 criteria in ISO 22674. ε-phase with striations was formed in the N-free (20-29)Cr alloys, while there was slight formation of ε-phase in the N-containing (20-29)Cr alloys, which disappeared in N-containing 33Cr. The lattice parameter of the γ-phase increased with increasing Cr content (i.e. N content) in the N-containing alloys, although the lattice parameter remained almost the same in the N-free alloys because of the small atomic radius difference between Co and Cr. Compositional analyses by EDS and XRD revealed that in the N-containing alloys Cr and Mo were concentrated in the cell boundary, which became enriched in N, stabilizing the γ-phase. The mechanical properties of the N-free alloys were independent of the Cr content and showed low strength and limited elongation. Strain-induced martensite was formed in all the N-free alloys after tensile testing. On the other hand, the proof strength, ultimate tensile strength, and elongation of the N-containing alloys increased with increasing Cr content (i.e. N content). Since formation of ε-phase after tensile testing was confirmed in the N-containing alloys the deformation mechanism may change from strain-induced martensite transformation to another form, such as twinning or dislocation slip, as the N content increases. Thus the N-containing 33Cr alloy with large elongation is promising for use in dentures with adjustable clasps through one piece casting.

Microstructure and Mechanical Properties of α’ Martensite Type Ti-V-Al Alloy after Cold- or Hot Working Process

Ti alloys are widely utilized for industrial applications due to their excellent mechanical properties combined with low density. In general, Ti alloys are classified as , + and  alloys, with further subdivision into near  and metastable  alloys. Quite recently, we have presented new type structural ’ martensite (H.C.P.) Ti alloys with low Young’s modulus, high strength and excellent ductility at room temperature. In this work, we examined the microstructure and mechanical properties of ’ martensite type Ti-V-Al alloy after cold- or hot working process. Then, we found that deformation behavior of ’ initial microstructure as compared with (+) initial microstructure was different based on the results of stress-strain curves and Processing Maps under the hot working process. Further, cold rolled ’ martensite microstructure exhibited the refined equiaxed dislocation cell structure, thereby resulting in high strength. This result suggests the new type deformation processing (for both cold- and hot work processing) utilizing ’ martensite in industrial Ti alloys.

Grain Refining Technique and Mechanical Properties of the Biomedical Co-Cr-Mo Alloy

Thermomechanical processes combined with reverse transformation from -hcp and M2N) dual-phase to -fcc phase of biomedical Co-Cr-Mo-N alloys were examined. According to XRD analysis, the reverse transformation from  to  is completed after heat treating at 1223 to 1373 K without deformation, while after heat treating at 1223 and 1273 K contained residual M2N particles in the  matrix. After the subsequent hot compression at condition at 1273 K and strain rate less than 1 s-1, and 1223 K and strain rate less than 10 s-1, ultra fine grains less than 1 m with residual M2N were obtained. Therefore, it is suggested that the residual M2N suppressed further growing of DRXed grain. The obtained microstructure with ultra fine grain dispersed with residual M2N was obtained to demonstrate both grain refining strengthening and precipitation strengthening.

Dynamic Recrystallization Behavior of Biomedical CCM Alloys in Hot Compression Process

Dynamic recrystallization behavior of Co-29Cr-6Mo-0.16N alloy was analyzed in details. Compression tests were carried out in a computer aided Thermecmaster- Z hot forging simulator. The results showed that uniformly distributed superfine grain size could be obtained by continuous dynamic recrystallization (DRX) process; Texture-free microstructure with uniformly distributed equiaxed fine grains was obtained. The formation of profuse stacking faults and their subsequent intersections are considered to be the principle mechanisms of DRX.

Effect of Sigma Phase in Co-29Cr-6Mo Alloy on Corrosion and Mechanical Properties

Effect of the sigma (σ) phase in Co-29Cr-6Mo alloy on corrosion and mechanical behavior has been investigated. The area fraction of the σ phase varies depending on the aging time at 1023 K. The area fraction of the sigma phase increases with increasing aging time and reaches 0.6 % after aging at 1023 K for 21.6 ks. Inductively coupled plasma atomic emission spectrometer (ICP-AES) analysis revealed the quantity of released Co ion shows almost the same values regardless of different area fraction of the σ phase. In addition, plastic elongation and 0.2% proof strength exhibit almost the same values, although the alloys have different area fraction of the σ phase. These results suggest that a small amount of the σ phase (<0.6%) hardly affect the corrosion and mechanical properties in the Co-29Cr-6Mo alloy.