Effect of α+β phase rolling and aging treatment on laminated bimodal structure in high temperature titanium alloy

In this work, we have prepared a novel laminated bimodal structure with layer-by-layer distribution of αp and αs phase in high temperature titanium alloy by α+β phase rolling and aging treatment. The lamellar bimodal microstructure is comprised of long-primary αp and secondary αs phase in the Ti-6.0Al-3Sn-5Zr-0.5Mo-1.0Nb-1.0Ta-0.4Si-0.2Er alloy. During α+β phase rolling at 990 °C, the primary αp grain deformed to elongated layer structure, while the lamella secondary αs deformed to kinked and broken chaos morphology. After 800 °C for 1h stabilization and 700 °C for 5h aging treatment, the thickness of the elongated αp layer slightly grew, while the kinked and broken αs were recovered and partially recrystallized to form a layered fine grain structure. The lamellar bimodal microstructure, i.e., elongated primary αp with layered bimodal αs phase has enhanced strength and ductility of the high temperature titanium alloy. This enhanced strength and toughness is mainly attributed to the lamellae αp structure and layered fine αs grain formed by hot rolling and aging treatment.


Introduction
Near-α titanium alloys have become an important research issue, due to their excellent specific strength, high temperature properties and corrosion resistance [1,2].Numerous Ti-Al-Sn-Zr-Mo-Nb-Si based alloys, i.e.IMI834 [3] and Ti-1100 [4], have been used to manufacture high-temperature parts of aeroengines such as compressor disks and blades.
To achieve synergy enhancement of strength and ductility, it is necessary to develop new microstructures.In recent years, heterostructured materials have recently been attracted extensive attention from the materials community [5,10].Xiaolei Wu [6] et al. prepared the heterogeneous lamella structure from commercially pure titanium, it opens a new frontier toward high tensile ductility without sacrificing the high strength of metals, back stresses are primarily responsible for the observed high strength.Back stress is not one of the four strengthening mechanisms of metals, because, in traditional homogeneous polycrystalline materials, back stress is very small, while for heterostructured materials,

Material and Experimental Methods
The Ti-5.5Al-3Sn-5Zr-0.5Mo-1.0Nb-1.0Ta-0.4Si-0.2Eralloy ingot with a diameter of 200 mm and length of 400 mm was produced by triple vacuum arc melting to ensure the homogeneity of the chemical composite.The β/(α + β) transit-temperature of the titanium alloy was measured as 1000 °C by differential thermal analysis and metallographic method [7].Then, the cubic billets were forged at initial temperatures of 980 °C to obtain two bars with the same diameter of 30 mm [8].
Olympus BX51M optical microscope (OM), electron backscatter diffraction (EBSD) and JEM-2010F transmission electron microscope (TEM) were used to observe the metallographic structure and detailed microstructure.The scanning step of EBSD was 0.14 µm.The metallographic samples were finely ground with sandpaper, mechanically polished and etched.The etching solution is 3%HF+5%HNO3+ 92%H2O.The TEM Samples were ground mechanically to a thickness of 50 μm and punched into the disc with 3 mm diameter, then twin-jet electro-polished in a 60% HClO4 50 mL-CH3(CH2)3OH 350 mL-CH3OH 600 mL solution at -25 °C and 20 V followed by ion beam thinning process.
Tensile testing was performed at room temperature using a universal testing machine AG-250KNIC, which was according to GB/T 228-2002 (eqv ISO 6892:1998) [9].

Results
The microstructure of the as-forged alloy sample is shown in figure .1.It can be seen from figure.1, that the microstructure that was mainly composed of elongated α phase and the original β grains are completely broken after the forging process at 970 °C.It is also shown a part of α plate that had been recrystallized to equiaxed α grains, and another non-recrystallized α phase reveals strips.To analyze the grain size and crystal orientation of the layered structure, the detailed microstructures were observed with EBSD.It can be seen from figure .3(a) that the long-αp phase is a single phase including low misorientation cell structure, with a length of 10-15 μm and a width of about 5 μm, and was surrounded by fine αs.In figure .3(b), red represents 37.1% of the deformed, yellow represents 59.2% of the substructured, and blue represents 3.7% of the deformed.It can be seen from figure .3(b) that αp reveals some substructure, indicating that dynamic recovery has occurred during the hot rolling process, and αs has undergone obvious grain refinement during the hot rolling process, the grain size is very small, it is mainly comprised of dynamically recovered αp grains inside substructure and recrystallized fine grains of secondary αs phase.To analyze the morphology of silicide and Ti3Al (α2) precipitated in the as-aged alloy, the detailed microstructures were observed with TEM.As can be seen from figure.4 (a) and (b), fine silicide is distributed at the interface of secondary αs lath, the precipitation of silicide along the secondary α phase was attributed to the solution concentration gradient of Si between α phase and β phase.In αp phase and αs phase, silicides with the size of 0.5 μm were distributed.Furthermore, the selected area diffraction pattern showed a super-lattice pattern, which indicated that the ordered α2 phase precipitated from α matrix.The corresponding dark field image presented the precipitation and distribution of α2 phase.higher yield strength, and 27% higher fracture elongation.

Discussion
Based on the above microstructure analysis and tensile property testing, the superior mechanical properties in laminated bimodal structures could be interpreted based on the microstructure evolution and deformation mechanism.During hot rolling, the αp grain was elongated, while the αs grain was subjected to obvious grain refinement, as shown in figure .2(a).The αs grain accounts for 80% of the alloy, so grain boundary hardening of αs is the main reason to improve the strength and elongation.
Xiaolei Wu [6] reported that a previously unidentified heterogeneous lamella structure possesses both the UFG strength and the CG ductility.Under tensile loading, the soft domain will start plastic deformation first, but is constrained by the surrounding hard domain.It will cause dislocations piled up in such soft grains and blocked at interfaces, which produces long-range internal stress, i.e. back stress in the soft domain.This back stress is believed to strengthen the materials with heterostructures.

Summary
In this work, we have prepared a novel laminated bimodal structure with layer-by-layer distribution of αp and αs phase in high temperature titanium alloy by α+β phase rolling and aging treatment.The lamellar bimodal microstructure, i.e., elongated primary αp with layered bimodal αs phase has enhanced strength and ductility of the high temperature titanium alloy.This enhanced strength and toughness is mainly attributed to the lamellae αp structure and layered fine αs grain formed by hot rolling and aging treatment.

Figure. 1
Figure.1 Microstructure of high temperature titanium alloy after forging at 970 °C.The microstructure of the alloy samples is shown in figure.2(a) was heat treated at 990 °C/1 h/AC (air cooling) + 700 °C/5 h/AC, the bimodal microstructure consists of lamellar primary αp grains (area fraction 20%) and transformed β (βtrans ) matrix composed of secondary αs colony and residual β films.Then, the sample with bimodal microstructure was hot rolled at 990 °C.The volume reduction of each rolling step was about 15%, the holding time was 5 min, and the final deformation reduction was 70%.

Figure. 4
Figure.4 Detail microstructure of laminated bimodal structure (a) αs phase, (b) (c) the precipitation of silicide, and (d) dark field image of α2 phase.

Figure. 5
Figure.5 The tensile stress-strain curves of bimodal structure and laminated bimodal structure.

Figure. 5
Figure.5 shows the tensile stress-strain curves for bimodal structure and laminated bimodal structure.Compared with the bimodal structure, the exhibits a higher yield stress (YS) of 973 MPa and higher ultimate tensile stress (UTS) of 1050 MPa, and a higher fracture elongation (EL) of 14%.Compared with the bimodal structure, the laminated bimodal structure has 11.8% higher tensile strength, 21.6%