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GRAIN GROWTH KINETICS OF PURE TITANIUM

F. X. Gil, D. Rodriguez and J. A. Planell Dept. Ciencia de 10s Materiales e Ingenierta Metaltigica.

E.T.S. Enginyers Industrials, Universitat Polit&mica de Catahmya. Barcelona, Spain.

(Received April 28,1995) (Revised June 26,1995)

Introduction

Grain growth is the pmcess by which the average grain size of an aggregate of crystals increases. The driving force for this process results from the decrease in free energy which is due to the reduction in total gram boundary area (I).

Grain gtowth takes place by difhtsion when the temperature is high enough and the time of heat treatment is long enough This means that the number of grams/unit volume decreases, the size of the grams increases, and both the gram boundary area/unit volume, and the of stored energy/unit volume, decrease. Consequently a state of higher thermodynamic stability is reached.

The microstructure of a polycrystalline material is a key factor in its technology, as it determines a wide range of properties including mechanical strength, toughness, corrosion resistance, hardness, electrical conductivity and magnetic susceptibility. Consequently, knowledge about grain growth is of fimdamental importance, not only for its intrinsic interest, but also for its technological significance (1,2).

In the present work experimental research on gram growth kinetics at different temperatures and times of heat treatment for titanium in a and g phases has been carried out. The aim of the present study was to quantify the progress of gram growth and the activation energy for a g titanium.

Emerimental Procedure

A commerciallypure titanium used in the present work has been kindly donated by Klockner S.A., which came in cylindrical rods of 6 mm. in diameter and 200 mm in length. Disks 6 mm. high were cut out and 38 specimens were prepared. The chemical impurities in this alloy are shown in Table 1. The microstructure of the as received material consisted of a phase equiaxed grains as shown in Figure 1.

TABLE 1 Chemical Impurities in Weight Percent

Nitrogen E 0.05

Carbon Hydrogen

0.10 0.12

Iron

0.30

*gen

0.35

1361

1362 GRAIN GROWTH KINETICS Vol. 33, No. 8

Figure 1. Microstructure of the received material.

Two samples were used as reference, while the others were subjected to d&rent heat treatments at either 700,75Oor800 ‘Ttostudytheccphasegraingrowthkineticsandat900, IOOOor 1100”Ctostudythea-P transition which occurs above 882 “C by applying heating times of either 3,5,10,15,60 or 120 minutes at each temperature.

A set of specimens was placed into the furnace at the fixed temperature for each experiment and then taken from the furnace after the appropriate time of heat treatment and quenched into water at 20 “C.

D (rm)

Figure 2. Results of grain measurement versus time for tempatures below fi-trams.

Vol. 33, No. 8 GRAlN GROWTH KINETICS 1363

D (pm) XXI-

450_ 1x . 4CD-

. 350-

XXI-

ZO-

X0-

150-

loo-

50-”

O! 1 I I I I 1 0 20 40 60 80 100 120

t w3

Figure3.Redtsofgrainmeasurementverswtimefortemperatures above p-transw.

Subsequently, the specimens were metallographically polished and etched with a solution containing both HF and HNO,. Metallographical observation was csrried out by optical microscopy. The area of the grain was obtained on Image Analysis Technique with the Omnimet 3 program.

Results and Discussion

The measured grain size as a function of the time and temperature of heat treatment is plotted in Figure 2 for the a phase and in Figure 3 for the g phase.

Thegrainsizeincreases when the heat treatment temperature and the heat treatment time increase. Taking into account that the initial average grain size of the alloy represents an area of 23 12 p2. the final average grain size atter 12.0 minutes of heat treatment increases to 3411 pm2 at 700°C to 6721 pm2 at 750°C and to 23316 q2 at 800 “C. The g phase areas were found to be 100520 p2 atIer heating at 1000°C and of 144623 q2 at 1100°C. The final average gram size for the specimens treated at 900°C showed no increase of area for the a phase, due to the proximity of the temperature to transition point.. Apparently the thennal energy was used mainly for the g phase nucleation in a grain boundaries. Consequently, the diffusion process that produces the grain growth is inhibited. Figure 4 shows the initial phase transformation in the form of a‘ needles due to water quench, mainly on the grain boundaries.

From Figures 2 and 3 it is possible to notice that the grain growth takes place at a very fast rate in the tirst 10 to 15 minutes of the heat treatment and for such times onwards the growth rate decreases. This rate of decreasecanbeexplainedbytakingintoaccountthegrainsizeincreasewhichproducesadecreaseinthegrain boundary area per unit volume ratio. This might mean that the grain boundary interfacial energy per unit volume decreases and therefore, the driving force for grain growth is lower.

Figure 5 shows a sample heat treated at 700°C for 120 minutes. The equiaxed grains with twins of a phase are clearly observable. Since the grain size in the longitudmal and in the transverse direction appeared to be thesamefbrthergrainsize measumments were performed only at the cross-section of the extruded bars. This

1364 GRAIN GROWTH KINETICS Vol. 33, No. 8

Figure 4. Growth of a ‘ needles on the grain boundary.

means that the texture effect is probably not significant because the microstructure of extn&d samples showed complete recrystallization. Figure 6 shows the sample heat treated at 1100°C for 120 minutes. Considerable grain growth can be observed. The microstructure corresponds to a’ martensite with an acicular morphology.

Kinetic grain growth follows I-I&t’s distribution since the maximum radius is 1.8 times higher than the average radius value. This means that un%orm growth occurs in the whole sample and size distribution obeys an asymptotic law typical for a state of equilibrium (6). When the grain size data were plotted as a function of log D versus log t a straight line resulted. This means that the general expression for grain growth follows the equation (3).

D - D, - Kt”

where D is the mean grain size, D, is the initial grain size, K is a temperature dependent constant, t is the time and n is the time exponent obtained from the slope of the log D Vs log t plot.

Moreover, ifthe assumption would be correct that atomic diffusion across the grain boundary is a simple activated process, and when n is independent of the temperature, it follows that K can be written as (4,5):

where & is the activation energy, T is the temperature in Kelvin K, is the pre-exponential rate constant and Risthegasconstant. This would explain why a plot of Ln(D-D,) Vs 1 /T produces a straight line. In that case the slope should be -EJR.

The kinetic behaviour is in agreement with these assumptions, since after taking logarithms, linear equations are obtained with good correlation coefficients. The growth order n for each heat treatment temperature is shown in Table 2. From these data, it can be derived that the growth exponent increases with temperature. However these values are higher than those found for other metals and alloys (7,8,9). In the

Vol. 33, No. 8 GRAJN GROWTH KINETICS 1365

Figure 5. Microsi~cture of a sample heat treated at 700°C Figure 6. Microshucture of a sample heat treated at 1100°C for 120 minutes. for 120 minutes.

ideal case the growth exponent would be 0.5 but generahy n is observed to be less than 0.5 due to the role played by diBerent gram growth parameters such as impurity-drag, tke surface effect, texture, dislocation substructure, heterogeneities.. . (10,ll). In our case, the titanium studied presents high purity and very low dislocations density because it is a completely recrystaked metal. Probably for that reason the growth exponent is around 0.5.

The following activation energies for grain growth for the difkrent heat treatments were: 100 KJ/mol for a phase and 20 KJ/mol for g phase. These activation energies ditkr very much. The difl’erence might be due to the difference in crystal structure. The a phase has a hexagonal structure and the g phase has a body centered cubic structure. The latter structure is more open than the hexagonal and prone to diffusion and therefore expected to result in a higher growth rate.

The effect of solute additions on the recrystallization of titanium has been studied by Reinbach and Nowikow (12). Of the solutes added (iron, ahnninium, tantahun, tin and chromium) chromium was found to have the greatest effect in reducing the recrystallization rate. This fact explains the low values of activation energy on commercially pure titanium. Reinbach and Nowikow found for the g phase of T&AU-4V alloy an activation energy of 95 KJ/mol (13), for Ti-6Al-5Zr-O.SMo-0.25Si 203 KJ/mol(14), and for the Ti-5.6Al- 3.5Zr-lNb-0.25Mo-0.3Si 211 Urn01 (14).

Good agreement is observed between the Semiatin’s model (15) for beta gram growth in titanium alloys aud our rest&s fa grain growth kinetics. This is due to the good accuracy of the Analysis hnage technique and the data analysis by computer. Others studies (12,16) reported deviations from the theoretical model using a line intercept technique for de&mining the grain growth kinetic.

TABLE 2 K, n Values for Different Treatment Temperatures

T log K n

700 0.03 0.30

750 0.98 0.35

800 0.98 0.50

1000 3.41 0.60

1100 4.2 0.50

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(1993). 14. S. P. Fox, Tiimn 92. Science and Techology, Ed. F. H. Frees and I. Captan TMS, pp. 769-776 (1993). 15. S. L. Semiatin, J. C. Soper and I. M. Sukonnik, Scripta Met. et Mat., vol. 30, no 7, pp. 95 l-955 (1994). 16. E. P. Abrahamson, Trans. Met. Sot. AIME, 221,1193 (1961).