Elastic modulus of biomedical titanium alloys by nano ... - Sistemas

424 P. Majumdar et al. / Materials Science and Engineering A 489 (2008) 419–425The elastic modulus of the cp-Ti obtained by both the techniquesis close to the standard value, which is 105 GPa [37]. Theelastic modulus of near TZN alloy is less than that of cp-Ti.In case of TNTZ alloy, the elastic modulus is about half of theelastic modulus of cp-Ti. Compared with cp-Ti and TZN samples,the elastic modulus of TNTZ alloys is closer to that of thebone (10–40 GPa).The elastic modulus of stainless steel and Co-based alloysis 206 and 240 GPa, respectively, which is significantly higherthan that of bone tissue (∼10–40 GPa). The elastic modulus ofcp-Ti, near and Ti alloys vary between 121 and 57 GPa. Theconstituent phases of the Ti alloys have different elastic modulusvalues. It has been reported that the phase exhibits a muchhigher elastic modulus as compared with the phase and theelastic modulus of the phases of titanium alloys increase in thesequence < ′′ < < [1,38,39]. Hon et al. [39] have reportedthat different phases (, and ) have different elastic modulusand they were found to be related by E = 1.5E and E = 2.0E .Moreover, Kim et al. [40] have reported that the metastable phase has lower elastic modulus than stable phase.In the present investigation, the cp-Ti consisted of singlephase (Fig. 1a) and thus, showed higher elastic modulus valuethan other investigated samples. The microstructure of the TNTZalloy is mainly single-phase equiaxed grains (Fig. 2a) andhence it gave the lowest elastic modulus value. The microstructureof TZN alloy water quenched from 800 ◦ C is mainlymartensite with some amount of phase in the matrix (Fig. 3b).The elastic modulus of this sample is 61 GPa (measured by ultrasonicmethod), which is close to that of Ti alloy. The otherinvestigated TZN alloys consisted of and phases with differentmorphology and distribution and hence their elastic modulusfalls between that of cp-Ti and Ti alloy.In nano-indentation technique, the accuracy of the elasticmodulus measurement depends on whether the projected areaof the indentation is a sufficient sampling area to represent anaverage elastic modulus. An indentation depth of 2 m correspondsto a projected triangular area of side ∼14 m [15]. Thenano-indentation technique should give accurate estimation ofthe elastic modulus for single-phase materials or for multiphasematerials with very fine precipitates distributed homogeneouslyin the matrix. The large deviation from the average values can beattributed to inhomogeneity in the microstructure on nanoscale.Also, the deviation can be influenced by the position of theindentation. The scatter in elastic modulus measurement withnano-indentation method seems to be inherent in the techniquewith the scatter being more apparent in case of multiphasematerial than in the single-phase material. In this study, bothcp-Ti and TNTZ consisted of single-phase grains, and hencethey showed less scatter in the measured elastic modulus valuesthan multiphase TZN alloy. This is simply because theelastic modulus measurement of a multiphase material by nanoindentationmethod is strongly influenced by the selection ofthe location of the indentation in the microstructure, the size ofthe microstructural constituents and the elastic modulus of theindividual phases.In case of ultrasonic technique, the velocity of ultrasound inmaterials is generally obtained from the time of flight of ultrasonicwaves through the known thickness of the sample andit is directly related to the elastic property and density of thesolid. Hence, the elastic modulus of the material measured byultrasonic technique is a better representation of the bulk materialproperty. In case of cp-Ti, TNTZ, and water quenched TZNalloys, the elastic modulus values obtained by nano-indentationand ultrasonic techniques are comparable. At room temperature,the pure cp-Ti exhibits single-phase structure (Fig. 1a). TNTZ800 WQ alloy consists of mainly matrix (Fig. 2a). The overallmicrostructure of TZN 800 WQ is martensite with a smallamount of phase (Fig. 3b). In case of two-phase / TZN alloywater quenched from 700 ◦ C(Fig. 4b), a fine and uniform distributionof and throughout the microstructure was observedresulting in more or less similar elastic modulus values by boththe techniques.However, in case of TZN samples furnace cooled from 800and 700 ◦ C, a large difference was observed in the elastic modulusmeasured by nano-indentation and ultrasonic techniques.This can be attributed to the non-uniform distribution of thephases in the microstructure. Moreover, the microstructure wasrelatively coarse in these cases. The TZN sample furnace cooledfrom 800 ◦ C produced basket-wave type structure characteristicof Widmanstätten structure. Lamellar packets of grainsof different orientations were observed within prior grains.Presence of primary phase of globular morphology and transformed was observed in the TZN alloy furnace cooled from700 ◦ C. In both these samples, grains were bigger comparedwith the indentation area; accordingly, the indentations weremainly taken on the matrix and the large grains were deliberatelyavoided in order to obtain the average property of thematerial. In this process, the elastic modulus of the lamellar andglobular phase, which is higher than that of phase, was notincluded into the average elastic modulus value. Hence, the elasticmodulus measured by nano-indentation technique is lowerthan that of ultrasonic method.4. Summary and conclusions(I) The elastic modulus of the investigated Ti and its alloysvaries between 60 and 120 GPa. The TNTZ alloy showslowest elastic modulus whereas cp-Ti with grains giveshighest elastic modulus among the investigated samples.The elastic modulus of the near TZN alloy falls inbetween Ti and TNTZ alloy. Heat treatment of the TZNalloy changes its constituent phases and their size, distributionand morphology. Hence, the elastic modulus alsochanges accordingly. In general, elastic modulus of thesamples water quenched from single-phase or two-phaseregion is lower than samples furnace cooled from sametemperature.(II) Elastic modulus can be measured more accurately by ultrasonicmethod, as it is directly related to the elastic propertyand density of the material.(III) The accuracy of elastic modulus measurement using nanoindentationtechnique depends on the projected area ofindentation. The measurements are expected to be accurateif the indentation samples sufficiently large area to