^{1}

^{*}

^{2}

^{3}

^{2}

^{4}

The
α +
β ↔
β phase transformation kinetics of TC21 Ti-alloy during continuous heating and cooling were studied using a dilatometric technique. Dilatometric heating curve exhibited that two characteristic reflection points can be observed with increasing the heating temperature. T
_{s} referred to the initial transformation temperature of
α +
β →
β and T
_{f} referred to the final transformation temperature of
α +
β →
β. T
_{s} was reported at 720°C, whereas the corresponding T
_{f} was obtained at 950°C. The initial and final transforming temperatures by the first derivative curve were reported at 730°C and 955°C, respectively, which are close to the values obtained in the dilatometric heating curve. Dilatometric cooling curve showed that the starting temperature of
β →
β +
α phase transformation was 880°C; however, the corresponding finishing temperature was 670°C. The starting and finishing temperatures using the first derivative curve were obtained at 665°C and 885°C, respectively. The first derivative for the studied dilatometric heating and cooling curves showed that the starting and finishing temperatures of
α +
β ↔
β phase transformation were more accurate and objective. Results show the
α +
β →
β transformation heating curve exhibits a typical S-shaped pattern.

TC21 (Ti-6Al-2Sn-2Zr-3Mo-1Cr-2Nb-Si, wt.%) alloy was developed as a high strength, toughness, damage tolerance and low crack propagation rate and provides weight reduction, long service life, and high reliability in fabricated aircraft structural components such as frames and beams [

Dilatation behavior of metals depends on the thermal change of length as a function of temperature. When titanium alloys heated to β transus temperature, the change in volume will be affected by crystal structure changing from hexagonal close-packed to body-centered cubic as well as solute atom redistribution. In addition, titanium alloys possess apparent similarities to steels in terms of α + β → β phase transformation during continuous heating [

The dilatometric technique is commonly employed to analyze the kinetics of solid-state phase transformation [

TC21 Ti-alloy was received as bars of 7 mm diameter and 140 mm length. The chemical composition of the alloy analyzed by inductive coupled plasma-atomic emission spectrometry (ICP-AES) is listed in

A dilatometer attached with a computer-controlled horizontal pushrod dilatometer (LINSEIS DIL L76 instrument, Germany), was used for measuring the

Al | Mo | Nb | Sn | Zr | Cr | Si | Fe | C | N | H | O | Ti |
---|---|---|---|---|---|---|---|---|---|---|---|---|

6.5 | 3.0 | 1.9 | 2.2 | 2.2 | 1.5 | 0.09 | 0.05 | 0.01 | 0.01 | 0.001 | 0.07 | Bal. |

during continuous heating and cooling. Sample of diameter 4 mm and 20 mm length was precisely machined using wire electrical discharge machine (EDM). The sample was placed in contact with the pushrod and heated at a rate of 10˚C/min up to 1100˚C in static air, and then the sample cooled in air down to room temperature. Change in sample length with temperature was recorded using WIN-DIL software. The first derivative was determined using equation between temperature and length to determine the α + β ↔ β phase transformation temperature for the as-received material. The metallographic samples were heated at a normal heat treatment furnace at the heating rate of 10 k/min, and then water quenched at different temperature. The microstructures were observed by field emission scanning electron microscopy (FESEM). The samples for FESEM were prepared by rough and fine mechanical polishing followed by etching with a solution consisting of 3% HF, 30% HNO_{3} and 67% H_{2}O.

By heating the sample above 720˚C (Point A in

At 720˚C (Point A in

at this temperature. When the temperature obtains higher than 720˚C, the equilibrium will be broken, and α phase will be transformed to β phase. Then, the ratio of β to α increases and the expansion of the phase dominates the overall expansion of the alloy. As a result, a relative increase in sample length will be observed on the dilatometric curve until the β transus (950˚C). With further increase in temperature more than β transus, T_{β} (950˚C), a complete β phase will be formed. Two characteristic reflection points can be observed in the above mentioned dilatometric curve with increasing the heating temperature. They are defined as (T_{s}) which referred to starting transformation temperature of α + β → β and (T_{f}) finishing transformation temperature of α + β → β. T_{s} for the investigated TC21 Ti-alloy was reported at 720˚C, whereas the corresponding T_{f} was obtained at 950˚C.

In addition, the first derivative for the studied dilatometric heating curve showed that starting and finishing temperatures of the α + β → β phase transformation were more accurately and objectivity,

Studying phase transformation kinetics during continuous cooling from both α + β range and β range is essential if the material is to be properly processed [

cooling curve. The starting and finishing temperatures were obtained at 665˚C and 885˚C, respectively. It is found that the starting and finishing temperatures of β → β + α phase transformation were more accurately and objectivity.

The α + β → β phase transformation fraction is used to reflect the α + β → β phase kinetics during continuous heating. The dilatation characteristics of titanium alloy, which is a polycrystalline material, can be isotropic in the solid-state phase transformation [_{o}) and relative length (ΔL/L_{o}) in the solid-state phase transformation process can be expressed as [

( Δ V / V o ) = 3 ( Δ L / L o ) (1)

where L_{o} is the original sample length, V_{o} is the original sample volume, and ΔV and ΔL are the volume and length variations of the sample, respectively. Considering the dilatometric heating curve of the investigated TC21 sample, the degree of deviation degree of the dilatometric curve should be proportional to the α + β → β transformed volume fraction. Therefore, the level rule can be employed to analyze the dilatometric curve to explore the relationship between the α + β → β transformed volume fraction (ƒ) and heating temperature (

ƒ = L EO / L EF (2)

where L_{EO} and L_{EF} are the measuring lengths of EO and EF lines representing in

The α + β → β phase transformation in the TC21 Ti-alloy during continuous heating was identified by quenching the samples from different temperatures. Three samples were chosen based on the dilatometric curve measurements. The samples were heated continuously to 830˚C, 900˚C and 1000˚C and then immediately quenched in water.

When the temperature was above the β transus at 1000˚C (

From the results related to α + β ↔ β phase transformation of TC21 Ti-alloy during continuous heating and cooling, which were effectively studied by dilatometry technique, the following conclusions can be drawn.

1) Dilatometric heating curve showed that starting and finishing temperatures of the α + β → β phase were obtained at 720˚C and 950˚C, respectively. In addition, the first derivative for heating exhibited that starting and finishing temperatures were reported at 730˚C and 955˚C, respectively.

2) Dilatometric cooling curve exhibited that the starting temperature of β → β + α phase transformation was 880˚C; however, the finishing temperature was 670˚C. While, the starting and finishing temperatures using the first derivative curve were obtained at 665˚C and 885˚C, respectively.

3) The first derivative for the studied dilatometric heating and cooling curves showed that starting and finishing temperatures of the α + β ↔ β phase transformation were more accurately and objectivity.

4) The α + β → β transformation heating curve of the TC21 Ti-alloy presents a typical S-shaped pattern.

The authors declare no conflicts of interest regarding the publication of this paper.

Elshaer, R.N., Ibrahim, K.M., Barakat, A.F., Farahat, A.I. and Abbas, R.R. (2019) Determination of Phase Transformation for TC21 Ti-Alloy by Dilatometry Method. Open Journal of Metal, 9, 1-10. https://doi.org/10.4236/ojmetal.2019.91001^{ }