The effect of heat treatment on microstructure and tensile properties as well as wear behavior on TC21 (Ti-6Al-2Sn-2Zr-3Mo-1Cr-2Nb-Si, wt.%) Ti-alloy was investigated. The samples were solution treated at 900°C for 15 min followed by furnace cooling to 800°C with a cooling rate 1°C/min and holding for 20 min, then the samples cooled down to room temperature either using water quenching (WQ) or air cooling (AC). Consequently, aging treatment was applied at 575°C for 4 hr. The microstructure feature showed a secondary α phase ( α s) precipitated in residual β phase due to the step cooling from 900°C to 800°C inside furnace as well as the aging treatment. The highest wear rate was obtained for WQ samples due to increasing in volume fraction of α p (58%). However, the lowest wear rate was reported for WQ + Aging samples due to the high hardness. Optimum mechanical properties of the studied TC21 Ti-alloy were obtained for AC + Aging condition. A better combination of hardness, tensile properties, and wear resistance was achieved for AC + Aging samples, although their wear resistance was found to be slightly lower than that of WQ + Aging samples.
Titanium alloys, especially α + β titanium alloys, are widely used in advanced aerospace applications, aero-engines and chemical industries. The combination of high strength-to-weight ratio, excellent mechanical properties and good corrosion resistance makes them the best material choices for some critical applications [
TC21 (Ti-6Al-2Sn-2Zr-3Mo-1Cr-2Nb-Si, wt.%) alloy is a new category of (α + β) titanium alloys with high strength, toughness, and damage-tolerance properties. It belongs to eight-component system, which develops for structural application in advanced aircraft and aerospace [
By applying a heat treatment technique, TC21 alloy can obtain a better combination of tensile properties, fracture toughness, and low fatigue crack growth rate. In such case, the performance and engineering application value will be better than the widely used conventional Ti6Al4V alloy [
Recently, a variety of studies investigated the effect of thermomechanical treatment [
In this research, TC21 samples were received as bars with a diameter of 7 mm and 140 mm long. The β transus temperature was experimentally determined that was approximately 960˚C. The chemical composition of the investigated TC21 alloy is given in
Al | Mo | Nb | Sn | Zr | Cr | Si | |||||
---|---|---|---|---|---|---|---|---|---|---|---|
6.5 | 3.0 | 1.9 | 2.2 | 2.2 | 1.5 | 0.09 | |||||
Fe | C | N | H | O | Ti | ||||||
0.05 | 0.01 | 0.01 | 0.001 | 0.07 | Bal. | ||||||
The samples were solution treated at 900˚C for 15 min to achieve equilibrium followed by furnace cooled to 800˚C with a cooling rate 1˚C/min and holding for 20 min. Then the samples cooled either using water quenching or air cooling. Consequently, aging treatment was applied at 575˚C for 4 hr. The experimental procedure for the heat treatment process is shown in
Dry sliding wear test was conducted using pin-on-ring tribometer testing machine in accordance to ASTM G99-05 standard. The wear sample with a cylindrical shape of 7 mm in diameter and 12 mm long was fixed against a rotating hardened ring (outer diameter of 73 mm and surface hardness of 63 HRC). Prior to each test, sample and counterpart surfaces were ground with different emery papers up to 1000 grit and cleaned ultrasonically in acetone to avoid the presence of humidity and non-desirable deposits. The ring was rotated at different speeds, which yielded linear sliding speeds of 1, 1.25, 1.5, 1.75 and 2 m/s. An applied constant normal load of 20 N was used for a duration of 15 min. During testing, a jet of compressed air was pointed at the edge of the ring to avoid accumulation of wearing debris on the ring. The weights of samples were measured before and after testing using electronic scales with 0.1 mg accuracy. The test results were evaluated according to the loss in weight. The mean value of results from three experiments has been reported. Worn surface of some selected samples was examined and analyzed using XRD and FESEM in order to determine the post-experimental wear mechanisms.
The microstructure of as-received TC21 alloy consists mainly of primary equiaxed α phase and transformed β matrix, as shown in
diameter of α phase was approximately 2.5 µm and its volume fraction approached to 65%. The equiaxed α phase is distributed homogeneously in the entire field of view. The XRD pattern confirmed the presence of α and β phase in the as-received TC21 alloy,
The microstructure of solution treated TC21 samples followed by either WQ or AC is shown in
primary α phase and precipitation of secondary α phase. For WQ condition, the grain size of primary α phase was in the range of 3.05 µm and the volume fraction was about 58%, and those in AC condition increased to 4.08 µm and decreased to 52%, respectively. Precipitation of secondary α phase at AC condition is due to increasing in cooling time. Therefore, it could be concluded that precipitation of secondary α phase could be obtained in a large amount by lower the cooling rate (AC) or applying a step cooling during solution treatment. Hence, a proper supersaturation could be considered as a critical factor for precipitation of secondary α phase in heat treating of TC21 alloy.
Based on the above discussion, the microstructure after WQ and AC conditions consists of primary equiaxed α phase, secondary α phase and transformed β phase. Each phase has been identified by X-Ray diffraction analysis, as shown in
Solution treatment and aging is considered one of the processes usually applied on TC21 alloy [
also slow cooling rate as well as aging treatment (
Vickers hardness measurements were carried out to investigate the influence of solution and aging treatments on the as-received TC21 alloy,
(HV348) compared to WQ samples due to existing of a lower amount of αp phase (52%). AC + Aging samples showed a hardness value of HV395. Moreover, WQ + Aging samples obtained the highest hardness value (HV438) due to the presence of a lower amount of αp phase (40%) compared to the others. It is also noticed that cooling rate has a relatively small effect on the hardness of TC21 alloy. The variation in hardness with aging treatment is more pronounced. There is an increase in hardness of 29% in case of WQ + Aging condition compared to WQ condition. Based on hardness results, it can be expected that WQ + Aging condition will show better tribological performance than other conditions.
Room temperature tensile properties of TC21 alloy after solution treatment with different cooling rates and aging treatment are shown in
and AC conditions, where the ultimate tensile strength increased from 1036 MPa to 1295 MPa and from 1061 MPa to 1239 MPa, respectively. On the other hand, the elongation decreased from 19% to 9% and from 14% to 11%, respectively.
As expected, WQ + Aging condition revealed the lowest elongation (9%) and reduction of area (32%), (
In spite of applying the same solution treatment and aging, the residual β matrix exhibited different contents due to the difference in cooling rates. The content of residual β matrix has a great influence on tensile strength. As can be seen in
showed a rising trend as increasing in content of residual β matrix. This phenomenon was also found by Shi et al. [
σ u + 337.37 + 15.881 R β (1)
σ y + 353.34 + 14.765 R β (2)
where σu and σy represent ultimate tensile strength, and yield strength, respectively, and Rβ represents the content of residual β matrix. The correlation coefficients for ultimate strength and yield strength are 0.943 and 0.949, respectively, which indicate that both ultimate strength and yield strength have good linear correlations with the content of residual β matrix.
The content of residual β matrix has also an influence on tensile ductility. As shown in
E l + 41.845 − 0.5552 R β (3)
R A + 68.014 − 0.6022 R β (4)
The correlation coefficients for elongation and reduction of area are 0.956 and 0.958, respectively, which indicated that ductility has good linear correlations with the content of residual β matrix. This indicated that the content of residual β matrix was one of the factors that influence the ductility of TC21 alloy.
The fracture surface of some selected tensile samples of WQ and AC conditions was examined using field emission scanning electron microscope (FESEM). Fracture surface of AC composed mainly of equiaxed ductile dimples and no flat areas were found (
The fracture mechanisms of AC + Aging and WQ + Aging conditions were studied,
areas. These dimples are relatively smaller and shallower than those existing in cases of AC and WQ conditions. However, fracture surface of WQ + Aging condition (
The effect of sliding speed on wear rate of the studied TC21 alloy in all conditions is presented in
rate of WQ samples was approximately one and half times higher than WQ + Aging samples. This may be expected for the as-aged samples, irrespective of microstructure, where they exhibited higher hardness as compared to the samples without aging.
The wear rate of all conditions significantly can be classified into three stages with increasing sliding speed,
reach a steady state which is denoted as Stage ΙΙ. With increasing sliding speed to 2 m/s, the temperature of contact surface will be increased. Then, a relative thermal hardening effect will be happened and the wear rate will be decreased. This phase is defined as Stage ΙΙΙ.
When the sample surface is first brought into contact with the rotating ring, the wear occurs by removal of high amount of asperities and then initial oxide layer will be formed over the surface. In general, a harder material is able to hold a thicker oxide layer more firmly as compared to a softer one [
The FESEM micrographs presented in
seen on wear tracks independently of sliding speed. Extent of plastic deformation or “ploughing” is found to be smaller in case of WQ + Aging condition. Layers with plastic deformation were found over the worn surfaces with relatively smooth areas for all sliding speeds.
Shallow grooves resulted from penetrating of hard abrasive particles were observed over the worn surfaces. The penetration depth depends mainly on the relative hardness of abrasive particles with respect to the sample surface hardness. As the hardness of WQ + Aging samples is getting higher than WQ samples, it is expected that depth of penetration in WQ + Aging condition surface became less. In such case, it will produce less material removal due to ploughing action and smaller extent of plastic flow in case of WQ + Aging condition. Thus, WQ + Aging condition exhibited significantly less wear rate as compared to WQ condition.
FESEM examination for WQ showed that at least two wear mechanisms were existing in those conditions. Existence of flakes removed from contact surface by delamination mechanism (
flakes of material were detached from the surface by adhesion to the ring surface and forming a transfer layer. Some of the transferred material is lost, but some was still re-embedded and smeared over the contact surface. For this theory of delamination, successive discussion and implementation by numerous authors [
The worn surfaces of WQ condition (
The worn surface morphology of WQ + Aging condition is shown in
From the experimental results related to microstructure, hardness, tensile properties and wear characteristics of TC21 alloy subjected to different heat treatment processes, the following conclusions can be drawn:
1) Duplex stage solution treatment with two different cooling rates and aging treatment, produced a structure consisted of secondary α phase precipitated in a residual β phase due slow cooling rate from 900˚C to 800˚C inside furnace or the final slow cooling rate by air as well as the aging treatment. Fine secondary α platelets and residual β phase formed a residual β matrix;
2) Cooling rate has a relatively small effect on hardness of the studied TC21 alloy. The variation in hardness with aging treatment was more pronounced;
3) Microstructure of TC21 alloy composed mainly of α phase and residual β matrix strengthened by secondary α platelets;
4) Optimal hardness, tensile properties and wear resistance can be achieved by AC + Aging condition, however wear resistance is found to be slightly lower for WQ + Aging condition;
5) Tensile fractured surface morphologies for WQ condition showed equiaxed dimple feature with good ductility, and WQ + Aging condition obtained intergranular and equiaxed dimple fractured mechanism with poor ductility;
6) The highest wear rate was obtained for WQ condition. However, the lowest wear rate was given for WQ + Aging condition due to its high hardness. In addition, wear rate of all conditions increased with increasing sliding speed until 1.75 m/s and then decreased with increasing sliding speed of 2 m/s.
7) Abrasion wear mechanism was noticed in WQ + Aging samples. Adhesion, transfer layer formation and cracking wear mechanisms were observed in case of WQ samples. At low sliding speed, micro fragmentation wear mechanism was observed, and brittle detachment of large particles wear mechanism was obtained at high sliding speed.
Elshaer, R.N., Ibrahim, K.M., Barakat, A.F. and Abbas, R.R. (2017) Effect of Heat Treatment Processes on Microstructure and Mechanical Behavior of TC21 Titanium Alloy. Open Journal of Metal, 7, 39-57. https://doi.org/10.4236/ojmetal.2017.73004