Commercially pure titanium (ASTM B 348 Gr. 1), hereafter called Ti 12 and Ti-6Al-4V (ASTM B 348 Gr. 5), hereafter called Ti 31 were obtained from Mishra Dhatu Nigam Ltd., Hyderabad, India in the form of rods 8 mm diameter. The microstructures of the as-received Ti 12 and Ti 31 samples are shown in Figs. 1 and 2 respectively. Table 1 shows the chemical composition as obtained from the manufacturer.

Fig. 1.

Microstructure of as-received Ti 12.

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Fig. 2.

Microstructure of as-received Ti 31.

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In the present study, the Ti 12 and Ti 31 grade alloys were subjected to annealing heat treatment at 600, 700, 800 and 900 °C in an electric furnace for 1 h followed by air-cooling. Samples were protected from oxidation by coating with borax and placing in an airtight container during annealing. Microstructures, as well as mechanical properties were studied after the heat treatment to understand the influence of heat on substrate metals during plasma coating and ceramic firing.

The sample surface was polished by initially grinding with successively finer emery papers and finally polished on a disc using lavigated alumina. After this, the samples were etched with Kroll's reagent (2% hydrofluoric acid + 8% nitric acid + 90% water) and viewed under an optical microscope.

Grain size in the present study was measured by the linear intercept method using Filar eyepiece attached to the optical microscope. A known linear length calculated for particular magnification was projected on the microstructure using Filar eyepiece. The number of grains intersected was counted for the known length. The grain size was determined in four different directions and repeated in ten different locations of the microstructure. The grains intersected each time were counted. The grain size was calculated by dividing the line length projected on the respective microstructure divided by the average of the number of grains intersected.

Tensile test was performed using Instron machine of model 4206 at a crosshead speed of 1 mm/min. The dimensions of the samples tested are shown in Fig. 3. Ultimate tensile strength, 0.2% offset yield strength, and percentage elongations were estimated from the load versus displacement plot. Five samples were tested for each treatment condition and the average values were reported.

Fig. 3.

Dimension of the specimen used for tensile strength measurement (in mm).

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Figs. 4 and 5 present a set of microstructures of samples of commercially pure titanium (Ti 12) and Ti-6Al-4V (Ti 31) heated at different temperatures.

Fig. 4.

Microstructure of Ti 12 annealed at a) 600 °C, b) 700 °C, c) 800 °C, d) 900 °C.

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Fig. 5.

Microstructure of Ti 31 annealed at a) 600 °C, b) 700 °C, c) 800 °C, d) 900 °C.

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In the case of Ti 12 (Table 2) on annealing at 600 °C, no change in microstructure was observed. Alpha (α) grains were similar to those of as-received material with an average grain size of 35 microns. On annealing at 700 °C, initiation of grain growth with variation in the grain size was seen with an average grain size of 44 microns. At 800 °C, a considerable amount of grain coarsening was seen with grain size 58 microns. Extensive grain twinning was also seen (Fig. 4c). Annealing at 900 °C showed colonies of serrated bright etching α plates and particles of dark etching retained β (Fig. 4d). The microstructure was stable till temperature up to 700 °C (Figs. 4a, b).

Annealing of Ti 31 at 600, 700, and 800 °C showed fine dark-etching β phase in a matrix of bright-etching α (Fig. 5a–c). The microstructure remained the same as that of as-received material. On annealing, at 900 °C, the microstructure showed (Fig. 5d) coarser β phase as dark-etching inter-granular constituent between bright- etching α phases.

Table 2 represents the grain size of Ti 12. The grain size was found to increase with the increase in annealing temperature for Ti 12. Annealing at 600 °C showed the same average grain size like that of as-received material. At 800 °C, a considerable increase in grain size with twin formation was observed.

Typical stress-strain diagrams are presented in Figs. 6 and 7 for Ti 12 and Ti 31, respectively. Tables 3 and 4 shows the mechanical properties obtained. Ti 12 showed a significant reduction in the yield strength and tensile strength at 800 and 900 °C whereas Ti 31 showed reduction at 900 °C.

Fig. 6.

Tensile curves of Ti 12 in the as received conditions and after different heat treatment temperatures.

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Fig. 7.

Tensile curves of Ti 31 in the as received conditions and after different heat treatment temperatures.

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The response of titanium and titanium alloys to heat treatment depends on the composition of the metal and the effects of alloying elements on the structural transformation of titanium. It has been reported [4,21,22] that a temperature change of roughly 600–800 °C takes place in the substrate metal during plasma coating and ceramic firing. These temperature changes are likely to influence the microstructural and mechanical properties of the substrate metal [4].

During plasma coating, the temperature of plasma flame is estimated to be in the order of 10,000 °C. The suspended powder, softened by plasma flame, is propelled by the stream of ionized gas on to the substrate metal. The particles striking the substrate metal flattens to form thin platelets and adheres to the surface [2]. Previous investigations [21,22] have shown that the substrate on which coating is done may be raised to the temperature of the order 600 °C. This is primarily because of the short duration of exposure to a high temperature of plasma and also because of the rapid heat conduction. Similarly, the temperature of 600–700 °C is attained during the ceramic firing of metal-ceramic restorations [4]. Temperatures of this order are sufficient to bring about microstructural changes and consequently physical and mechanical changes in the substrate metal [4,21]. To check any possible changes in the microstructure and mechanical properties of substrate metals Ti 12 and Ti 31, it was decided to perform heat treatment close to the average temperature the substrate metals attain during plasma coating and ceramic firing.

On occasions, it is necessary to examine the structural features of metals like grain and phase that influence the properties of materials. An optical microscopic examination is a handy tool in the study and characterization of materials. The microstructural analysis helps to find the association between properties and structure.

Fig. 4a, b shows the microstructures of samples annealed at 600 and 700 °C respectively. These microstructures are very similar to that of as-received material presented in Fig. 1. At a temperature of 800 °C, grain coarsening was observed together with twin formation (Fig. 4c). At the highest temperature of 900 °C, the presence of β phase was observed as shown in Fig. 4d. This is to be expected since the β transformation occurs at 882 °C [19,20]. Table 2 shows slight grain coarsening, which was observed as the temperature was raised to 700 and 800 °C. Considerable grain coarsening was observed at 900 °C. Since the material was heated above its β transformation temperature of 882 °C, the expected plate like α and inter-granular β was seen [19,20].

Ti 31 annealed at 600, 700, 800 °C showed similar microstructures as that of as received material (Fig. 2 and Fig. 5a–c). Fine dark-etching β phase can be observed in a matrix of bright-etching α phase. The actual practice of heat treatment varies with the alloy producer and user. To place the alloy in a soft, relatively machinable condition, the alloy is heated to about 700 °C in the lower range of α + β region, held for 1 h, then furnace cooled. This heat treatment, called mill annealing, produces a microstructure of globular crystals of β in α matrix [19,20]. A typical microstructure obtained is shown in Fig. 5b.

The changes in the microstructure because of exposure to a higher temperature of annealing has led to a change in mechanical properties of Ti 12 and Ti 31 (Figs. 4–7). Table 3 illustrates that the mechanical properties were similar to those of the as-received material. However, at 700 and 800 °C because of the grain coarsening, a marginal decrease in yield strength and ultimate tensile strength was observed. At 900 °C, there was a considerable coarsening of the microstructure. Yield strength was observed to drop to 20 kg/mm2. Similarly, in the case of Ti 31 properties after annealing at 600, 700, and 800 °C were very similar to those of the as-received material. Grain twinning at 800 °C (Fig. 4c) and β precipitation at 900 °C (Fig. 4d) led to the decrease in mechanical properties of Ti 12. Similarly coarsening of α and β phases of Ti 31 at 900 °C was responsible for reduction in mechanical properties [19,20].