Dental Materials Journal 2010; 29(5): 570–574

Effect of chromium content on mechanical properties of casting Ti-Cr alloys Masayuki HATTORI, Shinji TAKEMOTO, Masao YOSHINARI, Eiji KAWADA and Yutaka ODA

Department of Dental Materials Science, Tokyo Dental College, 1-2-2 Masago, Mihama-ku, Chiba 261-8502, Japan Corresponding author, Masayuki HATTORI; E-mail: [email protected]

The mechanical properties of a series of binary Ti-Cr alloys were investigated. Chromium content ranged from 5 to 20 mass%. Dumbbell- and disk-shaped specimens of each alloy were obtained by casting for mechanical testing and microstructural observation. Yield strength (YS) at 0.2%, tensile strength (TS), elongation (EL) and Vickers hardness (Hv) were determined. The TS and YS of Ti-15Cr were similar to those of Ti-20Cr at approximately 880 or 900 MPa and higher than those of cp-Ti by nearly 55%. Among all Ti-Cr alloys, Ti-10Cr showed the lowest EL. At 50 µm below the surface, Hv ranged from 370 to 420. Addition of 15 or 20 mass% chromium to titanium yielded sufficient strength and relatively high elongation values. Judging from the results of the mechanical properties, the suitability of Ti-Cr alloys with 15 or 20 mass% chromium for use in dental prostheses.

Keywords: Mechanical property, Titanium alloys, Titanium casting

the mechanical properties of a series of cast Ti-Cr INTRODUCTION alloys with a chromium content of up to 20 mass% with Titanium has seen increased use in fixed and the aim of determining their suitability as titanium removable prosthetics due to its excellent alloys for dental protheses. biocompatibility, high corrosion resistance and favorable mechanical properties1-3). Despite these MATERIALS AND METHODS advantages, however, titanium and titanium alloys are inherently difficult to cast as they have a high melting Preparation of alloys point and show strong reactivity to elements such as Experimental titanium alloys with a chromium content oxygen at such temperatures4-7). β-titanium alloys have of 5, 10, 15, and 20 mass% (denoted as CR5, CR10, been suggested as a solution to these problems as they CR15, and CR20, respectively) were made by melting offer certain benefits such as a lower melting sponge titanium (>99.8 mass%, Osaka Titanium temperature, solid-solution hardening and high Technologies, Amagasaki, Japan) and pure chromium ductility with a body-centered cubic structure8-13). (99.99 mass%, Japan Metals & Chemicals Co., Ltd., In recent years, titanium implants and prostheses Tokyo, Japan) in an argon-arc melting furnace (ACM- have come into wide use in clinical dentistry. However, 01, Diavac Limited, Yachiyo, Japan). To make some studies have noted pigmentation and discoloration homogeneous 30-g alloy buttons, the metal was turned occurring with titanium implants and denture over 6 times during melting. A single dumbbell-shaped bases14-16). Recently, in other experiments on titanium acrylic pattern for tensile testing (15-mm gauge length alloys, we found that an experimental Ti-20Cr alloy and 3-mm diameter) and a disk-type wax pattern for containing 20 mass% chromium showed the least hardness testing (14-mm diameter and 1.4-mm discoloration and superior corrosion resistance in a thickness) were directly attached to the sprue-former fluoride-containing saline solution17,18). In addition, and invested in a mold ring with alumina/magnesia- Takemoto et al. found that addition of chromium to based investment materials (Titavest CB, J. Morita, titanium in the 5 to 20 mass% range was effective in Kyoto, Japan). Each invested mold was burned out in enhancing resistance to corrosion by fluoride19). accordance with the manufacturer’s recommendations On the basis of the Ti-Cr phase diagram20), the by heating to 900°C at 15 °C/min and holding at this eutectoid composition is approximately 13.5 mass%. temperature for 50 min. The mold was then furnace- The Ti-5Cr and Ti-10Cr alloys correspond to the cooled to 650°C and subsequently placed in the casting hypoeutectoid compositions, whereas, the Ti-15Cr and machine. Ti-20Cr alloys are in the hypereutectoid composition An argon-arc melting/pressure difference casting range. Alloying chromium to titanium had a marked unit (Cyclarc II, J. Morita, Kyoto, Japan) was used to effect on the mechanical properties of Ti-Cr systems, fabricate cast specimens. For each casting, the casting suggesting a potential advantage8,13). However, the chamber was filled with argon gas and pressure set at mechanical properties of casting Ti-Cr alloys have been 0.03 MPa. To melt the buttons, an argon-arc was still uncertain because the properties changed by generated for 40 sec with an electric current at 150 A. processing methods, i.e. by using an investment After bench-cooling, the castings were divested and materials9). sandblasted with glass beads (80 micron, Jelenlko, The purpose of the present study was to evaluate Armonk, USA) from a distance of approximately 10 mm

Received Nov 13, 2009: Accepted May 24, 2010 doi:10.4012/dmj.2009-118 JOI JST.JSTAGE/dmj/2009-118 Dent Mater J 2010; 29(5): 570–574 571 at a pressure of 0.5 MPa for 15 sec to remove any adhering investment using a sandblaster (Whirlwind, Jelenko, Armonk, USA). Castings in which no porosity was recognized in radiographs (DCX-100, Asahi Roentgen, Kyoto, Japan) were subjected to mechanical testing. Commercially pure titanium specimens (cp-Ti, JIS Grade 2, J. Morita, Kyoto, Japan) were also fabricated as a control.

Mechanical properties Tensile tests (n=5) were performed at a crosshead speed of 0.5 mm/min at room temperature using a universal testing machine (Autograph AG-I, Shimadzu, Kyoto, Japan) equipped with a strain gauge Fig. 1 Yield strength and tensile strength of casting extensometer. Yield strength (YS) at 0.2% offset, metals tested. ultimate tensile strength (TS) and elongation (EL) to fracture were obtained from a stress-strain curve plot. The Vickers microhardness (Hv) of each alloy was determined using a microhardness tester with a load of 2.94 N and 15 sec duration time (HMV-1, Shimadzu, Kyoto, Japan) on polished cross-sections of the casting disks. Measurements were made at 50 µm, 100 µm, and thereafter at 100 µm intervals up to 500 µm below the surface of the casts. Five specimens for each alloy were used and three indentations were made for each specimen at each location.

Fractography and Metallography After tensile testing, surfaces were examined to determine fracture pattern under a scanning electron microscope (JSM 6340F, JEOL, Akishima, Japan). Fig. 2 Elongation of casting metals tested. Metallographic observation was performed as follows: the cast disk was polished with abrasive paper and then in a slurry of suspended 0.05 µm colloidal silica; polished specimens were etched in Keller’s reagent21); etched surfaces were observed under an optical microscope (Metaphot, Nikon, Tokyo, Japan).

X-ray diffraction X-ray diffraction for phase analysis was performed for the cast specimens of each metal using Cu Kα radiation by using a diffractometer (RINT2500, Rigaku, Akishima, Japan) operating at 30 kV and 300 mA. The phases were identified by matching each characteristic peak with the JCPDS files.

Statistical analyses Fig. 3 Changes in Vickers hardness at various depths The mechanical properties (YS, TS, EL, and Hv) were from surface of casting metals tested. statistically analyzed using a one-way analysis of variance (ANOVA) at a significance level of p=0.05. Specimens were then compared using the Scheffé test at a significance level of p=0.05. cp-Ti or titanium alloy with 5 mass% chromium (CR5). On the other hand, CR10 exhibited the lowest EL. The RESULTS CR5, CR15, and CR20 specimens showed higher Mechanical properties ductility. No statistically significant differences were The results for YS and TS are shown in Fig. 1 and observed in EL between cp-Ti and the various titanium those for EL in Fig. 2. The YS and TS of titanium alloys, except for with CR10. alloys with 10 mass% chromium (CR10) or more (CR15, The results of the hardness test for the Ti-Cr alloys CR20) were significantly greater (p<0.05) than those of are shown in Fig. 3. In all the alloys tested, hardness 572 Dent Mater J 2010; 29(5): 570–574

Fig. 4 SEM micrographs of casting metals after tensile testing.

at 50 µm below the surface ranged from 370 to 420. In DISCUSSION CR5, CR15 and CR20, hardness gradually decreased with increase in depth up to 100–150 µm, after which Alloying chromium to titanium offers a number of it leveled off. In CR10, however, hardness gradually advantages such as increased mechanical strength, increased with increase in depth up to 100 µm. corrosion resistance to fluoride and a lower melting point20). In the present study, we evaluated the Fractography and Metallography mechanical properties of a series of Ti-Cr alloys with Scanning electron micrographs of the typical fracture four different levels of chromium content ranging from surfaces of the Ti-Cr alloy castings after tensile testing 5 to 20 mass% in 5% increments. Titanium alloys with are shown in Fig. 4. All Ti-Cr alloys, except 10 mass% 10 mass% chromium or more showed a higher YS and chromium, exhibited dimple-like structures in the TS than cp-Ti or titanium alloy with 5 mass% center of the fractured surfaces. In CR10, the fractured chromium. Among all Ti-Cr alloys, titanium alloys with surfaces consisted of dendrite structures (arrowhead) 10 mass% chromium exhibited the highest strength. and cleavage-mode predominated in the center of the Some disagreement exists with regard to the castings. The typical microstructure of each alloy relationship between chromium content and strength. casting is shown in Fig. 5. CR5 showed an acicular Shimizu reported an increase in tensile strength of structure, with a large grain size of 200 µm. CR10 binary Ti-Cr alloys with an increase in chromium appeared to have an inhomogeneous microstructure. content ranged from 10–20 mass%, however, the The fine acicular structure of CR15 was similar to that strength was reduced addition of chromium further of CR20. more9). Ikeda et al. reported that while the microstructure of binary Ti-Cr alloy fundamentally X-ray diffraction exhibited β-phase, when the chromium content ranged The typical XRD patterns of the series of binary Ti-Cr from 7–13 mass%, the alloy exhibited ω-phase, alloys are shown in Fig. 6. α-Ti peaks were found for depending on cooling conditions, resulting in increased the CR5 and CR10 patterns. β-Ti peaks were found in strength22). We believe that one possible explanation for the diffraction patterns of all Ti-Cr alloys. Apparently, the disparity between our results and those of these more than 10 mass% chromium, the retained β was earlier studies may lie in the casting process used. observed in the XRD patterns. The ω peak was found Although we used alumina/magnesia-based investment, in CR10, this minimal broadened peak was identified Shimizu used a phosphate-bonded mold, which induces as the likely ω-phase of the 2θ at 79.55 or 79.75°. a reaction in the titanium alloy that results in highly Dent Mater J 2010; 29(5): 570–574 573

Fig. 5 Typical microstructures of casting metals.

than that in an earlier study9). Again, this may be explained by the use of a phosphate-bonded mold by Shimizu9). These result suggested that one of the reason why difference of the reaction between the alloys and the molds in the present study, because of cast the alloys into phosphate-bonded mold. Furthermore, the diameter of their tensile test specimens was 2 mm, while that of this study was 3 mm. It is possible that the smaller diameter used in their study was the cause of the lower EL value obtained, even though the thickness of the reacted layer was similar to that in the present study. On the other hand, in this study, Ti-Cr alloy with 10 mass% chromium exhibited the lowest EL among all alloys tested. As described above, this agrees with the results of an earlier study9) showing lower ductility in Ti-Cr alloy with 10 mass% chromium than other Ti-Cr alloys. A decrease in ductility was also observed in the fractography. Cleaved grains, which are characteristic of decreased ductility, were observed in CR10. On the other hand, dimple ruptures, indicative of a typical ductile fracture, were observed in the fractured interior structures of CR15 and CR20. Fig. 6 XRD patterns of casting metals. Hardness near the surface in all the alloys was almost the same. In CR5, CR15, and CR20, hardness gradually decreased with increase in depth up to 100– brittle specimens. Therefore, they obtained a lower 150 µm, in the control specimens showed the same tensile strength at 10 mass% chromium content than tendency in depth up to 200 µm. However, in CR10, the Ti-15Cr and Ti-20Cr. hardness gradually increased with increase in depth up The EL of each Ti-Cr alloy in this study was higher to 100 µm. Note that the interior hardness of CR10 was 574 Dent Mater J 2010; 29(5): 570–574 the highest. 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