In the present work, titanium alloy with a composition of Ti-6.5Al-3Mo-1.9Nb-2.2Sn-2.2Zr-1.5Cr (TC21) was subjected to plastic deformation and aging processes. A Plastic deformation at room temperature with 2%, 3% and 4% stroke strain was applied on the studied samples. Then, the samples aged at 575 °C for 4 hr. By applying different plastic deformation ratios, the structure revealed an elongated and thin β-phase embedded in an α-phase. Secondary α-platelets were precipitated in the residual β-phase. Maximum hardness (HV440) was obtained for 4% deformed + aged samples. Minimum hardness (HV320) was recorded for the as-cast samples without deformation. The highest ultimate tensile strength of 1311 MPa was obtained for 4% deformed + aged samples due to presence of high amount of dislocation density as well as precipitation of secondary α-platelets in the residual β-phase. The lowest ultimate tensile strength of 1020 MPa was reported for as-cast samples. Maximum elongation of 14% was registered for 4% deformed + aged samples and minimum one of 3% was obtained for as-cast samples. Hence, strain hardening + aging can enhance considerably the elongation of TC21 Ti-alloy up to 366% and 133% in case of applying 4% deformation + aged compared to as-cast and aged samples without applying plastic deformation, respectively.
Titanium alloys, especially α + β, exhibited a combination of high strength-to-weight ratio, good fatigue performance, excellent corrosion resistance that make them the best material choice for some critical applications such as advanced aerospace applications, petroleum sector and chemical industries [
TC21 Ti-alloy samples were cast as ingots using a vacuum induction skull melting (ISM) furnace in a graphite mould. Before melting, the graphite mould was preheated in the heating chamber inside the ISM furnace to 900˚C. Hereafter, the raw TC21 Ti-material is heated to reach 1700˚C to melt it under vacuum that reached to 4 × 10−2 mbar. The samples cast in a graphite mould as 30 mm in diameter and 300 mm long. Then, they were machined to reduce the bar size into 25 mm diameter and 250 mm long. Hot swaging process was applied at 700˚C to reduce the diameter to 8 mm at 12 steps. Annealing process was applied at 550˚C for 2 hr followed by furnace cooling to reduce residual stress resulting from swaging process. The chemical composition of the investigated TC21 Ti-alloy is given in
The samples were classified into four groups, where the first group was as-cast samples, the second group was swaged + annealed samples, and the third group was aged samples at 575˚C/4hr after swaging and annealing processes. Plastic deformation (strain hardening) process was applied at room temperature with 2%, 3% and 4% strain fourth group using a universal tensile testing machine Hereafter, aging treatment was applied at 575˚C at 4 hr. The samples were prepared for metallography work by grinding, polishing and etching using a solution consisting of 3% HF, 30% HNO3 and 67% H2O. The samples were metallography examined using field emission scanning electron microscope (FESEM). Vickers hardness was carried out in accordance to ASTM E92-16 Standard. The samples were machined to 4 mm diameter with a gage length of 20 mm to determine the tensile properties. Tensile testing was carried out according to ASTM E8/E8M-16 Standard at room temperature using a strain rate of 1 mm/min. Fractography of some selected fracture tensile samples was analyzed and examined using FESEM.
Microstructure of as-cast TC21 Ti-alloy showed a matrix consisting of equiaxed β-phase and various morphologies of α-phase. The α-phase was located between the β-phase in the matrix and also at the grain boundaries. The average β-grain
Al | Mo | Nb | Sn | Zr | Cr | Si | Fe | C | N | H | O | Ti |
---|---|---|---|---|---|---|---|---|---|---|---|---|
6.10 | 2.96 | 2.11 | 1.98 | 2.05 | 1.44 | 0.10 | 0.07 | 0.01 | 0.02 | 0.003 | 0.10 | Bal |
size was in the range of 150 - 250 µm as shown in
63%, 31% and 6%, respectively. These fine αs were precipitated from the supersaturated β-phase during aging process. The precipitated αs platelets on β-phase have a big role in increasing the strength of the studied TC21 alloy. The last group of the samples was subjected to plastic deformation by tension + aging at 575˚C/4hr. The microstructure of this group showed also equiaxed structure that composed of αp, αs and βtrans phases, Figures 1(d)-(f). This structure is similar to the aged samples, but the difference was located in the β-phase. The transformed β-phase after applying plastic deformation obtained an elongated shape that depends on the percentage of the plastic deformation (2%, 3% &4%). The percentage of elongation of transformed β-phase increases with increasing the percentage of deformation 2% to 4%. By elongating the transformed β-phase, the matrix can be described as isolated islands of β-phase in α-matrix as comparing to the swaged or aged samples. The XRD pattern confirmed the presence of αp, αs and βr phases in the aging only and 4% Def + aging conditions,
Vickers hardness was carried out to evaluate the hardness of as-cast, swaged, aged samples and also to investigate the influence of strain hardening on TC21 Ti-alloy,
to the as-cast samples. Therefore, it can be concluded here that 4% deformation + aging samples showed the highest hardness compared to the other conditions.
The tensile properties of TC21 Ti-alloy for as-cast, swaged, aged, and deformed + aging samples are shown in
The fracture surface of some selected tensile samples (swaged, aged, 2% Def + aging, and 4% Def + aging) was examined using FESEM,
1) As-cast microstructure showed a matrix consisting of β-grains and various morphologies of α-phase. The average β-grain size was in the range of 150 - 250 µm.
2) Microstructure of swaged samples consisted of primary equiaxed α phase and transformed β phase. The average grain size of α phase was in the range of 2.3 µm and its volume fraction approached 63%.
3) Mechanical properties increased with increasing the percentage of deformation from 2% to 3% to 4% deformation.
4) Optimum ultimate tensile strength of 1311 MPa was obtained for 4% deformed + aged samples due to presence of high amount of dislocation density as well as precipitation of secondary α-platelets in the residual β-phase.
5) The lowest ultimate tensile strength of 1020 MPa was reported for as-cast samples.
6) Maximum elongation of 14% was observed for 4% deformed + aged samples and minimum one of 3% was obtained for as-cast samples.
The authors declare no conflicts of interest regarding the publication of this paper.
El-Deeb, M.S.S., Ibrahim, K.M., Mohamed, S.S. and Elshaer, R.N. (2021) Influence of Plastic Deformation and Aging Process on Microstructure and Tensile Properties of Cast Ti-6Al-2Sn-2Zr-2Mo-1.5Cr-2Nb-0.1Si Alloy. Open Journal of Metal, 11, 11-20. https://doi.org/10.4236/ojmetal.2021.112002