University of Zagreb Faculty of Metallurgy, Aleja Narodnih Heroja 3, 44103 Sisak, Croatia

International Conference on Innovative Technologies, IN-TECH 2014, Leiria, 10. - 13.09.2014

MICROSTRUCTURAL ANALYSIS AND VICKERS HARDNESS OF HEAT TREATED BIOMEDICAL Ti-Cr-Nb ALLOYS

Lj. Slokar, T. Matković, P. Matković University of Zagreb Faculty of Metallurgy, Aleja narodnih heroja 3, 44103 Sisak, Croatia

Keywords: Microstructure, Vickers hardness, heat treatment, biomedical alloys, Ti-Cr-Nb alloys

Abstract. The application of titanium-based alloys in biomedicine has been growing due to their favorable mechanical properties, biocompatibility and excellent corrosion resistance when compared to more conventional stainless steel and Co-Cr alloys. It is known that β-phase of titanium provides a better properties of titanium alloys for biomedical use then its other phases. Niobium is a biocompatible element which improves β-phase forming and it has beneficial effect on mechanical properties. In this work titanium alloys: Ti80Cr10Nb10, Ti70Cr10Nb20, Ti60Cr10Nb30 were obtained by melting and casting in a laboratory arc furnace in protective atmosphere. Phase analysis by X-ray diffraction showed that all alloys were two-phases (α+β). Heat treatment was performed to obtain β-single phase alloys with assuming of better properties when compared with α+β alloys. Heat treatment was performed at 950C for 3 hours followed by water quenching. Temperature of heat treatment was determined by differential scanning calorimetry. Microstructure of experimental alloys was analyzed by light and scanning electron microscope with energy-dispersive spectrometer. Microscopic observations and quantitative analysis clearly showed that phase transformations occurred. In alloys with lower niobium content α-phase was almost fully transformed to β-phase, but in alloy with higher niobium content martensitic α’’-phase was formed. This needle-like phase may be benefit on shape memory effect of alloy. Vickers hardness values of heat treated alloys were lower than those for as cast alloys and are more acceptable for biomedical application.

Introduction Materials to be used as permanent implants in the human body must be biocompatible, corrosion resistant, tissue compatible, vital and elastic. Also, the demand for metallic biomaterials is increasing rapidly because the world population is getting increasingly older, and elderly people have a higher risk of hard tissue failure. Among the metallic materials titanium and its alloys, due to their favorable properties, are the most suitable for biomedical applications. However, their biological and mechanical biocompatibility requires much improvement [1-3]. In this work niobium is selected as a biocompatible element which improves β-phase forming and it has beneficial effect on mechanical properties. Heat treatment of as-cast Ti-Cr-Nb alloys was performed with purpose of obtaining single-phase alloys with adequate Vickers hardness for biomedical applications.

Materials and methods Alloys chemical composition (Ti80Cr10Nb10, Ti70Cr10Nb20 and Ti60Cr10Nb30) was selected so as to see the impact of 10, 20 and 30 at.% of niobium addition to Ti-10Cr alloys. Experimental alloys were produced by melting pure elements (>99.9%) in a laboratory melting arc furnace under argon atmosphere. Melting was repeated for four times to ensure a chemical homogeneity. Obtained samples were casted in the same conditions into cylindrical shape (Ф8 mm x 25 mm). Cylinders were cut into several pieces for analysis by different methods. Phase analysis by X-ray diffraction (XRD) was performed on Philips PW3710 diffractometer using CuK α radiation. Microstructure of as-cast alloys was observed by light microscope Olympus GX 51 with digital camera. Quantitative analysis was performed using computer program UTHSCSA Image Tool. To obtain a temperature for heat treatment, alloy Ti 80Cr10Nb10 was analyzed using differential scanning calorimeter NETZSCH Jupiter STA 449 F1. Heat treatment was performed at 950 C for three hours followed by water quenching. For that purpose sample of each alloy was invested in vacuumed quartz ampoule. Heat treated alloys were metalographically prepared by grinding, polishing and etching in Kroll’s reagent at room temperature. Microstructure of samples was analyzed using Olympus light microscope. Detailed microstructural analysis was perfomed by scanning electron microscope (SEM) Tescan Vega TS 5136 MM equipped with Bruker energy-dispersive spectrometer (EDS). Vickers hardness of experimental alloys was determined by Vickers method by load of 19.60 N (HV2) during 10 s.

Results and discussion XRD phase analysis of as-cast alloys is performed by comparing the characteristic peaks with JCPDS files [4]. The results show that all alloys are two-phases (Table 1). So, for obtaining single-phase alloys heat treatment was performed on the basis of DSC results.

Table 1. XRD analysis of as-cast alloys

Alloy composition, at. Alloy No. Established phases %

1 Ti80Cr10Nb10 α+β

2 Ti70Cr10Nb20 α+β

3 Ti60Cr10Nb30 α+β International Conference on Innovative Technologies, IN-TECH 2014, Leiria, 10. - 13.09.2014

Sample of Ti80Cr10Nb10 alloy was analyzed by differential scanning calorimetry (DSC) and thermogravimetry (TG), and the result curves are in Figure 1. According to binary phase diagrams (Ti-Cr and Ti-Nb) [5], analyzed alloy has two-phase microstructure consisting of α and β phase. Therefore DSC curve shows that transformation from two-phase region (α+β) into single-phase (β) occurs at 830 C. On TG curve small increase of sample weight (0.17 %) as result of oxidation is evident. Since this value is acceptable, results of DSC analysis may be considered as a relevant. However, these are binary not ternary diagrams, so temperature of heat treatment was determined as 950 C.

Fig. 1. DSC curve for Ti80Cr10Nb10 alloy

Two-phase microstructure of as-cast alloys is obvious in light micrographs (Figs.2a-2c, upper corners). Namely, light micrographs of experimental alloys before (in upper corner) and after heat treatment are shown in Figures 2a, 2b and 2c. It can be seen that, after heat treatment, alloys 1 and 2 are nearly single-phase with very coarse grains, while in alloy 3 are clearly visible two phases in dendritic microstructure.

(a) Alloy 1, Ti80Cr10Nb10 (b) Alloy 2, Ti70Cr10Nb20 (c) Alloy 3, Ti60Cr10Nb30 Fig.2. Light micrographs of heat treated and as-cast Ti-Cr-Nb alloys

When these micrographs are compared with those of as-cast alloys, it can be seen that α/β transformation occurred. In that way, nearly single-phase alloys were obtained as a goal of heat treatment. However, it was not achieved in alloy 3, in which the percentage of α phase is increased for 16.49 %, and average area of grains decreased for 31.87 %. The results of quantitative metallography of experimental alloys (Table 2) show that in as-cast alloy 3, with the highest niobium content, was the highest amount of β phase. Average area of grains in alloys 1 and 2 were too big for measure by used program, while grains average area for alloy 3 were decreased by heat treatment (Table 2). International Conference on Innovative Technologies, IN-TECH 2014, Leiria, 10. - 13.09.2014

Table 2. Results of quantitative metallography of as-cast and heat treated experimental alloys

Average area of Alloy % β-phase % α-phase Alloy grains, μm2 composition, No. Heat Heat Heat at.% As-cast As-cast As-cast treated treated treated

1 Ti80Cr10Nb10 66.72 100.00 33.28 0.00 90.32 -

2 Ti70Cr10Nb20 59.31 100.00 40.69 0.00 232.64 -

3 Ti60Cr10Nb30 82.21 65.72 17.79 34.28 112.24 76.00

Microstructure of experimental alloys after heat treatment was analyzed in detail using SEM at magnification of 1500x (Figs. 3a-c). On SEM micrographs of alloys 1 and 2 (Figs. 3a and 3b) remains of α phase are visible since the α/β transformation was not complete. In microstructure of alloy 3 with maximum niobium content (30 at.%), besides α and β phases, needle-like martensitic α’’ phase is visible. It is caused due to fast cooling of alloy from β-phase area [6]. This needle-like phase may be benefit on shape memory effect of alloy in some biomedical applications.

(a) Alloy 1, Ti80Cr10Nb10 (b) Alloy 2, Ti70Cr10Nb20 (c) Alloy 3, Ti60Cr10Nb30 Fig. 3. SEM micrographs of heat treated alloys

Average chemical compositions of β and α phase in heat treated alloys (Table 3) were determined by energy-dispersive specrtometry (EDS). From the showed results it can be seen that β phase is a solid solution of chromium and niobium in β-titanium, and its composition corresponds to chemical composition of alloy. Similarly, α phase is a solid solution of niobium in α-titanium with negligible chromium content (<1 %).

Table 3. Average chemical composition of phases in heat treated alloys determined by EDS

Alloy Chemical composition of Alloy composition, at. Element phases No. % β phase, at.% α phase, at.% Ti 82 90

1 Ti80Cr10Nb10 Cr 10 1 Nb 8 9 Ti 71 91

2 Ti70Cr10Nb20 Cr 9 1 Nb 20 8 Ti 60 91

3 Ti60Cr10Nb30 Cr 11 1 Nb 29 8

EDS spectruum for Ti80Cr10Nb10 alloy is given in Figure 4. It shows characteristic peaks for titanium, chromium and niobium only. There is no evident presence of any other elements (e.g. oxygen) indicating the successful preparation of alloys. International Conference on Innovative Technologies, IN-TECH 2014, Leiria, 10. - 13.09.2014

Fig. 4. EDS spectrum of Ti80Cr10Nb10

The results of Vickers hardness measurements of heat treated alloys given in Table 4 show values from 312 to 465 HV. They are lower than HV2 values for as-cast alloys (Fig. 5). It can be explained by almost complete transformation of α phase into β phase. Namely, it is known that α phase reveals higher hardness values than β phase [3].

Table 4. Vickers hardness of as-cast and heat treated alloys

Alloy Alloy HV2 of as-cast HV2 of heat No. composition, at.% alloys treated alloys

1 Ti80Cr10Nb10 575 312

2 Ti70Cr10Nb20 412 321

3 Ti60Cr10Nb30 522 465

The largest decrease of hardness value is evident for alloy 1, which had the highest value in as-cast condition. Alloys 1 and 2 show similar hardness values (about 300 HV2) after heat treatment due to formation of nearly β single-phase microstructure. The smallest decrease of hardness was for alloy 3 which retained clearly defined three-phase microstructure. In addition, martensitic α’’ phase was formed due to rapid cooling and favorable composition.

600 500 400 HV2 300 as-cast 200 heat treated 100 0 1 2 3 Alloy No.

Fig. 5. HV2 values comparison of as-cast and heat treated alloys

Conclusions From the results obtained in this investigation it can be concluded as follow:  Microstructure of all as-cast alloys is two-phases. It consists of α and β phases.

 Heat treatment led to the α/β transformation in alloys Ti80Cr10Nb10 and Ti70Cr10Nb20, so they were nearly β single-phases.

 In alloy Ti60Cr10Nb30 third martensitic phase α’’ formed by heat treatment due to the highest niobium content (30 at.%).  Chemical composition of β phase corresponds to alloy composition.  EDS revealed that α phase is a solid solution of niobium in α-titanium with negligible chromium content (<1 %).  Vickers hardness values of heat treated alloys are lower than those for as-cast alloys due to phase transformation α into β. These values are more acceptable for biomedical application. International Conference on Innovative Technologies, IN-TECH 2014, Leiria, 10. - 13.09.2014

Finally, heat treatment of experimental Ti-Cr-Nb alloys, performed at 950 C during 3 hours followed by water quenching, led to favorable microstructure and Vickers hardness for their potential use in biomedicine.

Acknowledgements This work was supported by Ministry of Science, Education and Sports of the Republic of Croatia.

References

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