Ti-2007 Science and Technology, edited by M. Ninomi, S. Akiyama, M. Ikeda, M. Hagiwara, K. Maruyama The Japan Institute of Metals (2007) Mechanical Properties of Ti-Cr System Alloys Prepared by Powder Metallurgy Yonosuke Murayama1 , Akira Okubo2 , Hisamichi Kimura2 1Niigata Institute of Technology, Kashiwazaki 945-1195, Japan 2Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Metastable β titanium alloys consisting of non-cytotoxic elements are very promising as biocompatible implant materials, because the low elastic modulus is expected. Ti-Cr system alloys have been investigated as a dental alloy because of the excellent biocompatibility. In this experiment, Ti-Cr alloys of the composition range from Ti-17mass%Cr to Ti-26mass%Cr were prepared by spark plasma sintering method. Microstructure of Ti-Cr alloy after homogenization heat treatment consists of metastable β phase and the plate-shaped pre-eutectoid α precipitation. The yield stress of the Ti-Cr alloy is over 950MPa depending on the amount of chromium. The elastic modulus of the Ti-Cr alloy changes also with the amount of chromium. Ti-Cr alloy containing chromium around 20 mass% has low elastic modulus in comparison with pure titanium. Keyword: metastable β titanium, powder metallurgy, biocompatibility, spark plasma sintering, elastic modulus, mechanical properties 1. Introduction human body. Sometimes powder metallurgy is better to get It is predicted that a demand for an artificial implant material alloy without segregation. We aim to investigate the like artificial bones or artificial teeth will increase in future due mechanical properties, especially elastic modulus, of Ti-Cr to the arrival of aging society and medical development. system alloys in relation with the composition and the Therefore, a development of titanium alloy with low elastic microstructure. modulus and high strength is desired, because titanium shows excellent biocompatibility and non-toxicity for a tissue of 2. Experimental Procedure human body. In typical alloying elements of titanium alloy, The starting materials are high-purity commercial powders of niobium and tantalum of β-isomorphous element and 99.9% pure titanium powder with the size under 46μm and 99. chromium of β-eutectoid alloy element is recognized as non- or 9% pure chromium powder with the size under 63μm, which minimal-toxic element against the human cell. It is known that were manufactured by Kojundo Chemical Laboratory Co., Ltd. the elastic modulus of metastable β titanium alloy containing The titanium and chromium powders are blended in the five β-isomorphous elements of niobium or tantalum is low, though different compositions from Ti-17mass%Cr to Ti-26mass%Cr it depends on amounts of alloying elements1). This variation of and are mixed for 1.8ks by a cross-rotor mixer of MEIWA Co., elastic modulus of metastable β titanium alloy is recognized to Ltd. (CM-3). The blended powders were sintered in a carbon depend on the balance of phase stability of β phase and ω container by spark plasma sintering method (SPS sintering), phase. Recently, Ti-Nb-Sn alloy with the low elastic modulus using the equipment of SPS SYNTEX Inc. (SPS-1050). The and high strength was developed caused by restraining ω phase SPS sintering was conducted under the pressure of 40MPa at by adding tin2). Active research of metastable β titanium alloy 1473K for 600s. Because some SPS sintered alloys show based on Ti-Nb system is being conducted3-5). The metastable compositional segregation, the homogenization heat treatment β phase has the possibility to decrease the elastic modulus of was conducted at 1473K for 21.6ks in vacuum. The cooling titanium alloy. rate after a homogenization heat treatment is 50K/min from By the way, chromium is β-eutectoid element to stabilize the β 1473K to 673K and 11K/min from 673K to 473K. The phase of titanium. Chromium has the advantage that the effect microstructures were identified by optical microscope and a as a β stabilizer is higher than a β-isomorphous element and scanning electron microscope. The local compositions were chromium is cheaper than niobium or tantalum. It is regarded analyzed using electron-prove micro analyzer (EPMA) that Ti-Cr alloy shows excellent corrosion resistance in human manufactured by JEOL Ltd. (JXA-8600 SX/MX). The body6). Ti-Cr alloy around the composition of Ti-20mas%Cr crystallographic structure was identified by X-ray diffraction has been investigated as a dental material because Ti- analysis (Shimadzu Co., XD-D1). 20mass%Cr shows not only good corrosion resistance but also The mechanical properties were evaluated by a compression ductility. In this experiment, Ti-Cr system alloys are produced test using rectangular samples with the dimension of 2.5 x 2.5 by powder metallurgy, because the powder metallurgy has sev- x 7mm3. The compression test was conducted using the eral possible advantages. As for the powder, a cost decreases. autograph of Shimadzu Co. (AG-IS10kN). The crosshead The powder metallurgy is better to get desired surface structure speed was constant, which became an initial strain rate of 1 x for 10-4s-1 when the plastic deformation behavior was evaluated. The elastic 1489 modulus was evaluated by a strain gauge (Kyowa Co., Ltd.) analysis. The total of the analysis value of titanium and the pasted on the rectangular test piece. Load was increased chromium is being put as 100% in Table 1. An average of gradually and strain under the constant applied load was oxygen content was 0.26% in the range from 0.239% to 0. measured and the elastic modulus was calculated. The elastic 277%. Carbon content was less than 0.05% though carbon modulus of some sample was measured also by the free was measured only about Ti-20Cr alloy. Oxygen and carbon resonance vibration method (JE-RT, Nihon Techno-Plus Co., seemed to be mixed during powder processing, since carbon Ltd) using a sample with dimensions 1.5mm (thickness) x container was used for SPS sintering. The composition ratio 8mm (width) x 49mm or 24mm (length). All elastic modulus of titanium and chromium was almost as expected. was measured at room temperature. Fig. 1 shows optical micrographs of the Ti-Cr alloys after Ti-Cr alloys in this experiment are called according to the SPS sintering. Ti-17Cr shows comparatively uniform balanced compositions like Ti-20Cr hereafter. The microstructure, and fine precipitation disperses in the composition is indicated by the mass percent unless the unit is equiaxed grains. Ti-20Cr showed the same microstructure as specially written. Ti-17Cr. There is unevenness in light and shade contrast in Ti-14Cr (a). A lath martensite transformation microstructure 3. Results and Discussion was observed at some grains of Ti-14Cr as shown in Ti-14Cr (b), which the grains look dark in light and shade contrast in 3.1 Microstructure Ti-14Cr (a). According to the EPMA analysis, the amount of The results of the chemical compositions of the Ti-Cr alloys chromium at such area was about 6.5%. Remained to evaluate the mechanical properties in this experiment are chromium was observed in the microstructure when the summarized in Table 1. Titanium and chromium was amount of chromium beyond 23%. Ti-26Cr (b) shows the analyzed by the method of inductively coupled plasma results of EPMA analysis at the circumference of remained spectrometry. Oxygen and carbon was analyzed by the chromium. Cr2Ti compound is formed around the remained method of emission spectrochemical chromium. The amount of chromium of matrix is higher than the balanced composition. Because some SPS sintered alloys show compositional segregation, the homogenization heat treatment was conducted at 1473K for 21.6ks in vacuum. Table 1. Chemical compositions of Ti-Cr alloys (mass%) Fig. 2 shows optical microstructure after the homogenization heat treatment. All alloy shows equiaxed grain structure in which plate-shaped precipitation dispersed as shown in Ti- 14Cr (b) of Fig. 2., Fig. 3 and Table 2 are results of Figure 1. Optical Micrographs of Ti-14Cr, Ti-17Cr, Ti-23Cr and Figure 2. Optical Micrographs of Ti-14Cr, Ti-17Cr, Ti-20Cr, Ti-26Cr (a) after SPS sintering and SEM micrograph of Ti-23Cr Ti-23Cr and Ti-26Cr after homogenization heat treatment. (b) with results of EPMA analysis. 1490 Fig. 4 shows X-ray diffraction patterns of the SPS treatment. Only the peaks of bcc structure can be seen in sintered specimen after the homogenization heat treatment though the composition is different in any specimen. Though the separation of the peak in X-ray diffraction pattern is distinguished in specimen just after SPS sintering, the separated peak becomes a sharp single peak after the homogenization heat treatment. The position of the peak deviated on the high angle side with the increase in the amount of chromium. The lattice parameter calculated from the (110) peak changed from about 3.225nm of Ti-14Cr to about 3.200nm of Ti-26Cr. The Ti-Cr alloy of this Figure 3. SEM micrograph with points analyzed with EPMA. experiment consists of retained β phase as shown in the X- ray diffraction pattern. It is known that the metastable β phase decomposes into β1+β2 when the hypereutectoid Ti-Cr alloy is cooled rapidly and then aged7). The plate-shaped Table 2. Results of EPMA analysis of marked points in Fig. 3 (mass%) precipitation observed in Fig. 3 disappeared, when the specimen is quenched into ice water after heat treatment at 1323K for 3.6ks. Though the specimen of this experiment was cooled in furnace, a cooling speed was comparatively fast, that is, the cooling rate was 50K/min from 1473K to 673K and 11K/min from 673K to 473K. Narayanan et al. found the precipitation of β2 phase before the precipitation of α phase in Ti-15Cr alloy step-quenched in 723K which is high temperature than occurrence of an athermal ω phase8).