Abstract
After homogenization, forging, cold rolling, and recrystallization of V-5Cr-5Ti alloy, universal testing machine, scanning electron microscopy, and transmission electron microscopy were used to study the effect of precipitates on the mechanical properties of the alloy and to estimate the strengthening effect. Results show that as-cast V-5Cr-5Ti alloy has a dendritic structure characterized by lamellar phase. After homogenization, the precipitates are transformed from a lamellar to a needle-like dendritic structure. The precipitates are broken into a short-bar or spherical phase during forging and cold rolling. The average tensile strength, yield strength, and elongation of the as-cast alloy are 505.0 MPa, 415.0 MPa, and 8.2%, respectively, with the brittle cleavage fracture as the dominant fracture mechanism. The fracture mechanism is transformed into a mixed fracturing mode of intergranular and quasi-dissociative fractures after homogenization. After 80% cold rolling and 1000 °C/1 h annealing, the average tensile strength, yield strength, and elongation of the alloy are 487.3 MPa, 382.7 MPa, and 26.2%. The alloy plasticity is greatly improved due to the refinement of the grain and precipitates. The fracture mechanism of the alloy after cold rolling and annealing is microporosity fracture. The precipitates enhance V-5Cr-5Ti alloy by Orowan strengthening mechanism. Taking the alloy after 80% cold rolling and annealing at 1000 °C/1 h as an example, the yield strength increment obtained by precipitate strengthening is about 50.1 MPa.
Science Press
As an important candidate structural material for fusion reactors, V-based alloy has low activation propertie
The mechanical properties of V-base alloys have been studied in detail. Most research is focused on the effects of alloying elements, irradiation, and the precipitates on the mechanical propertie
In the present study, the mechanical properties of V-5Cr-5Ti alloy were evaluated at room temperature via tensile test, and changes in the strength and plasticity of the alloy under different states were studied. Through microscopic observation, the influence of precipitates on the mechanical properties of V-5Cr-5Ti alloy was analyzed, and the strengthening effect of the precipitates was estimated.
V-5Cr-5Ti alloy was prepared by vacuum melting of high-purity V, Cr, and Ti. The V-5Cr-5Ti alloy used in this work was processed by the following steps: homogenization annealing, hot forging, and cold rolling. The homogenization was conducted at 1200 °C for 3 h in a vacuum (< 2×1
Fig.1 As-cast microstructure of V-5Cr-5Ti alloy
Tensile samples were cut from as-cast V-5Cr-5Ti alloy, V-5Cr-5Ti alloy plates were subjected to homogenization, hot forging+1020 °C/1 h annealing and 40%, 80% cold rolling. Part of the cold-rolling sheet was subjected to recrystallization annealing at 1000 °C/1 h in a vacuum (<2×1
Fig.2 Drawing of the tensile test sample
The microstructure and tensile fracture morphology of V-5Cr-5Ti alloy were observed using scanning electron microscope (SEM, model JSM-7001F). The sample for microstructure observation was subjected to electrolytic etching with solution HF:H2O=1:10 (volume ratio) under a voltage of 15 V and current of 0.6~1.0 A for 30 s.
The microstructure of the precipitates of V-5Cr-5Ti alloy was analyzed using FEI Tecnai F20 field transmission electron microscope (TEM). The MTP double-electrolytic jet apparatus was adopted for sample preparation, with 5vol% H2SO4+95vol% CH3OH as electrolyte, under a voltage of 10 V and electric current of ~20 mA at -10 °C.
V-5Cr-5Ti alloy ingots were prepared via secondary vacuum electron beam melting. Microsegregation of the ingots easily occurs due to the high temperature of vacuum electron beam melting, fast cooling rate, poor fluidity, and thermal conductivity of the V-5Cr-5Ti alloy.
Fig.3 Microstructures and precipitates of the V-5Cr-5Ti alloy in as-cast state (a, b) and after homogenization (c, d)
After forging and 1020 °C/1 h annealing, the grain of V-5Cr-5Ti alloy is a type of equiaxial grain structure, with a grain size of approximately 80 μm. The precipitates are in zonal distribution (
Fig.4 SEM images of microstructures and precipitates of V-5Cr-5Ti alloy after deformation processing: (a, b) hot forging+1020 °C/1 h; (c, d) 40% cold rolling; (e, f) 80% cold rolling
Fig.5 Microstructures and precipitates of cold-rolled V-5Cr-5Ti alloy after annealing: (a, b) 40% cold rolling+1000 °C/1 h; (c, d) 80% cold rolling+1000 °C/1 h
Fig.6 TEM images and SAED patterns of precipitates in V-5Cr-5Ti alloy: (a) as-cast; (b) homogenization; (c) forging+1020 °C/1 h; (d) 40% cold rolling; (e) 80% cold rolling; (f) EDS spectrum of the precipitate marked in Fig.6d
Fig.7 Fracture morphologies of V-5Cr-5Ti alloy in as-cast state (a, b) and after homogenization (c, d)
Fig.8 Fracture morphologies of V-5Cr-5Ti alloy after deformation processing: (a, b) forging+1020 °C/1 h; (c, d) 40% cold rolling; (e, f) 80% cold rolling
Fig.9 Fracture morphologies of cold-deformed V-5Cr-5Ti alloy after annealing: (a, b) 40% cold rolling+1000 °C/1 h; (c, d) 80% cold rolling+1000 ℃/1 h
Fig. 10 Tensile properties of V-5Cr-5Ti alloy at different states
After cold rolling, the precipitates of V-5Cr-5Ti alloy are broken into short bars or spheres, and the banded structures form. During cold rolling, slipping, dislocation, entanglement, and proliferation occur, resulting in grain elongation, fragmentation, and fibrosis. The strength increases dramatically, but the plasticity remains higher than that of the as-cast alloy. The average tensile strength and elongation of 80% cold-rolling V-5Cr-5Ti alloy are 752.3 MPa and 8.3%, respectively. After annealing at 1000 °C/1 h, the strength decreases sharply due to recrystallization, the plasticity increases rapidly, the strength is decreased by 35.2% and the elongation is increased by 215.7%. In contrast, the strength of alloy after 40% cold rolling+1000 °C/ 1 h annealing is not significantly increased, but the elongation is sharply increased. The strength and ductility of alloy after 80% cold rolling+1000 °C/1 h annealing are improved, exhibiting excellent comprehensive properties. The fracture mechanism of the alloy after cold rolling and annealing is microporosity fracture.
In comparison with as-cast V-5Cr-5Ti alloy, the alloy after cold rolling and annealing at 1000 °C/1 h has similar strength but considerably improved plasticity. Therefore, the dendritic structure of the precipitates deteriorates the plasticity. After 80% cold rolling, the precipitates are broken into a spherical phase. During the material deformation, the dislocation movement is blocked by the precipitates, enhancing the strength.
Fig.11 TEM images of microstructures of V-5Cr-5Ti alloy after cold rolling: (a) dislocation pileup and (b) precipitate
The precipitates adopt the Orowan mechanism to strengthen the alloy matrix. The critical shear stress can be obtained using the Orowan equatio
(1) |
where τ is the critical shear stress, b is the dislocation Burgers vector, G is the shear modulus of alloy, and λ is the second-phase particle spacing.
If the effective spacing, line tension, dislocation pair, and other influencing factors of the second-phase particles are considered, then a further accurate Orowan expression of shear stress τ can be obtained:
(2) |
On the basis of the relationship between shear and yield stresses in the reference,
(3) |
the incremental expression of the yield stress generated by the Orowan mechanism is as follows:
(4) |
where M is the Taylor factor (M=3.1), G is the shear modulus, ν is Poisson's ratio, b is the Burgers vector, fv is the volume fraction of the second phase, and r is the average radius of the second-phase particle.
The strengthening effect of the precipitates is calculated by using the 80% cold rolling+1000 °C/1 h annealing treated alloy as example. The interstitial atoms (C, O, N) in the alloy are supposed to form the precipitate (TiV)(CON), and the volume fraction fv of the precipitates can be calculated with a TiC density of 4.93 g/c
Fig.12 TEM image of precipitates in 80% cold-rolled V-5Cr-5Ti alloy
1) The as-cast V-5Cr-5Ti alloy has a dendritic structure characterized by lamellar second phase. After homogenization, the precipitates are transformed from a lamellar to a needle-like dendritic structure. The precipitates are broken into a short-bar, short-strip or spherical phase during forging and cold rolling.
2) The average tensile strength, yield strength, and elongation of the as-cast alloy are 505.0 MPa, 415.0 MPa, and 8.2%, respectively, with brittle cleavage fracture as the dominant fracture mechanism. After homogenization, the fracture mechanism is transformed into a mixed fracturing mode of intergranular and quasi-dissociative fractures. After 80% cold-rolling and 1000 °C/1 h annealing, the average tensile strength, yield strength, and elongation of the alloy are 487.3 MPa, 382.7 MPa, and 26.2%. The plasticity is greatly improved due to the morphological change in the grains and precipitates. After deformation processing, the fracture mechanism of the alloy after cold rolling and annealing is microporosity fracture.
3) During 80% cold rolling, the precipitates are broken into a spherical phase. The precipitates enhance V-5Cr-5Ti alloy by Orowan strengthening mechanism. Taking the alloy after 80% cold rolling and annealing at 1000 °C/ 1 h as an example, the yield strength increment obtained by precipitates strengthening is about 50.1 MPa.
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