| Ti and its alloys exhibit combinations of mechanical and chemical properties, such as high specific strength, excellent corrosion resistance and biocompatibility that render them attractive for applications in a variety of industries such as biomedical, aerospace and transportation. Recently, with the advent of nanostructuring(decreasing the size scale of the structure down to the nm range) numerous investigators have studied the behavior of nanostructured(NS; grain size < 100 nm) or ultrafine-grained(UFG; grain size in the range of 100–1000 nm) Ti produced by severe plastic deformation(SPD) techniques and reported increases in mechanical properties over those of conventional coarse-grained Ti. At the present time, equal channel angular pressing(ECAP) and high-pressure torsion(HPT) are most commonly used SPD techniques for producing NS or UFG Ti. However, they are not convenient to process and produce materials in the form of plates and sheets and the sizes of NS or UFG Ti samples prepared by these techniques are very limited.In the present work, NS commercially pure(CP, grade 2) Ti with average grain size of 80 nm was produced by asymmetric plus symmetric rolling(ASR plus SR) at room temperature. To understand the nanomechanical properties of this NS Ti, nanoindentation hardness(Hn) and elastic modulus(En) of different planes within the NS Ti specimens were measured using continuous stiffness measurement mode at room temperature. For comparison, the nanomechanical properties of the as-received hot-rolled coarse-grained(CG) Ti and UFG) Ti with only asymmetric rolling process were also investigated. It was found that Hn of the Ti samples increased significantly with the decrease of grain sizes, while En exhibited a slight decrease as the grain sizes decreased from CG to NS regime. The increase of Hn was expected to be caused by higher density of dislocations and finer grains attained by severer plastic deformation, while the slight decrease of En was considered as a result of the increased density of lattice defects and volume fraction of the grain boundary atoms. Furthermore, the nanomechanical properties of different planes of the Ti specimen exhibited a little difference which can be expressed as Hn(RD-TD) > Hn(ND-RD) > Hn(TD-ND) and En(RD-TD) > En(ND-RD) > En(TD-ND). These differences were ascribed to crystallographic textures formed by rolling deformation.The effects of low temperature annealing(≤400 oC) on microstructure and mechanical behavior of the NS Ti produce by ASR plus SR were investigated systematically. The annealed Ti samples possessed UFG structure in which grain size was below 200 nm even though annealed at 400 oC for 30 min, which suggested considerable thermal stability of the microstructure. It was found that annealing of this NG Ti with appropriate temperature and time resulted in the abnormal effect that the microhardness, strength and ductility were simultaneously enhanced as compared to those corresponding to the as-processed state. Further increasing of the annealing temperature and time leads to the decrease of microhardness and strength. Strain rate jump tests revealed that the strain-rate sensitivity(SRS) parameter m of the NS Ti was slightly enhanced with the increase of annealing temperature. The mechanisms corresponding to the variations of mechanical behavior were analyzed based on microstructure evolution of the annealed NS Ti.Textures in the NS Ti produced by ASR plus SR were studied by X-ray diffraction method and the relationships between textures and mechanical properties were also investigated. The main textures in the NS Ti are basal plane texture ?0001? formed by deformation and a multiple recrystallization textures, such as ?1213? ?10 10?, ?1213? ?10 11? and ?1122? ?1 100?. The orientation density of deformation textures are higher than that of the recrystallization textures in the NS Ti. The higher volume fraction of recrystallization textures in NS Ti indicates that in situ recrystallization occurred during the severe plastic deformation process. The density of deformation textures in this NG Ti decreased after annealing while the volume fraction of recrystallization textures reduced after annealing at relatively higher temperature due to the normal growth of grains. The deformation resistance in normal direction of the plates was increased by the basal plane texture ?0001?, leading to the highest hardness of RD-TD plane. Meanwhile, The deformation resistance in transversal direction of the plates increased by the texture of ?1122? ?1 100?, ?1213? ?10 10? and other recrystallization textures, therefore the hardness of ND-RD plane is higher than that of TD-ND plane.Fine-grained(FG) Ti with average grain size smaller than 5 μm was produced by annealing NS Ti at the temperature of 500~700 oC for different time. The tensile deformation behavior of this FG Ti were investigated. The phenomenon of yield point elongation(YPE), namely, a plateau following yielding in the stress-strain curve, was observed in FG Ti. It was found that, with increasing the grain size, the YPE in FG Ti appeared and increased to a maximum value, and then decreased, finally disappeared. Microstructures of FG Ti samples before and after the YPE were investigated. The results show that this YPE phenomenon is attributed to the dislocation behavior and characteristics of grain size in the FG Ti. Face-centered tetragonal(fct) titanium hydride was observed by transmission electron microscopy in the FG Ti. This γ-fct titanium hydride has a c/a ratio of 1.08. It was found that the γ-fct hydride possesses inherent orientation relationships with the FG hexagonal close-packed(hcp) α-Ti matrix, namely ? ? ? ?γ α110 10 10, ? ? ? ?γ α001 0001, γ α?114 ? ?12 16?? ? ? ? and ? ?γ112 ?1213?α? ? ? ?. The fct titanium hydride plates within hcp matrix have the thickness of 10 – 150 nm and length of 50 – 3000 nm while the matrix grains have the size of 50 – 3000 nm. The fct titanium hydride was found to be very scarce in this Ti and the relationship between the length of γ-fct hydride and grain size of the hcp matrix has also been discussed.Bulk NS Ti with porous structure was synthesized by cold gas dynamic spraying(cold spraying) of Ti particles to thick coatings(e.g., > 10 mm in thickness). Accordingly, the grain size, lattice parameter, lattice strain, porosity, microhardness, tensile and compressive behavior of the bulk Ti deposits before and after annealing were comparatively analyzed. The results show that the microstructure of the as-sprayed bulk Ti was characterized by a grain size of ~ 60 nm, lattice expansion(~ 2% for a and ~ 3% for c), lattice strain(~ 1.65×10-5) and residual compressive stress(~ 53 MPa). Moreover, annealing of the as-deposited bulk Ti led to a significant decrease in lattice expansion, lattice strain and residual stress, whereas porosity remained unchanged(~11%). The mechanisms of grain growth, as well as the evolution of particle interfaces during annealing, were also investigated. In terms of mechanical behavior, the as-deposited bulk Ti exhibited a very low modulus(52 GPa) with relatively high tensile and compressive strength values(180 and 850 MPa, respectively). Annealing in the temperature range of 750 to 900 oC led to a significant increase of tensile and compressive strength(to 380 MPa and more than 1200 MPa, respectively). Finally, annealing resulted in a slight increase of elastic modulus, which was rationalized on the basis of changes in pore geometry in the bulk Ti deposits. |
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