| There is a considerable interest in the development of β titanium alloys due to their high strength to weight proportion and the perfect combination of strength compared with other titanium alloys. Also, β titanium can be strengthened via precipitation hardening. All these characteristics make the β titanium alloys to the attractive alloys for applications under critical high stress. Therefore, they have significant applications in aircraft, automotive and even military industries. T he processing of β alloys usually consists of a hot working operation followed by heat treatment. T he final hot working step is normally performed in the α+β field. Solution treating at α+β field(below β transus) leads to the precipitation of the primary α. T he heat treatment temperature can control volume fraction of the primary α, and forging deformation can effect on the primary α shape. T he increase of deformation leads to a globular primary α. Performances of the β titanium alloys are very sensitive to their microstructure. The grain size, grain boundary α phase, primary α phase and secondary α phase morphology strongly affect their properties. A β high strength titanium alloy(Ti-3.5Al-5Mo-4V-2Cr-2Sn-2Zr-1Fe),which is relatively similar to the Ti B20 alloy nominal composition is used in the present study. Reinforced the titanium alloys by hard precipitates such as T i C and T i B will demonstrate the possibility of virtually enhancing the tensile properties of composites. T hrough many reinforcement compounds, Ti B and Ti C are the most favorable due to their low density, high elastic modulus, and good interfacial debonding with titanium alloyThis dissertation firstly presents the effe ct of the heat treatment of asforged on the microstructures and tensile properties of the alloy, the influence of(Ti C+Ti B) on the microstructure and tensile properties of alloy and finally a study on the microstructures and mechanical properties of Ti-3.5Al-5Mo-4V-2Cr-2Sn-2Zr-1Fe and(Ti B+Ti C)/Ti-3.5Al-5Mo-4V-2Cr-2Sn-2Zr-1Fe composite after rolling process is carried out.Ti-3.5Al-5Mo-4V-2Cr-2Sn-2Zr-1Fe alloy is produced by non-consumable arc-melting technology and hot-forging. T he microstructure and ten sile properties of a β high strength titanium alloy Ti-3.5Al-5Mo-4V-2Cr-2Sn-2Zr-1Fe were investigated. Microstructure observations indicated that the primary α phase, formed during soluti on treatment, could limit the growth of the β grains. The fine β grains could be related to the primary α phase located at the boundaries of β grain, because was limmted by pinning effects. T he grain boundary tends to thicken with the increase in aging time, which would have negative effects on the mechanical properties of the alloy including the strength and ductility. Obviously, the content of the primary α significantly decreases when the solution temperature increases to the β transus soluted. The strength is significantly higher when the alloy is solution treated above the β transus temperature than below β transition temperature, and the ductility remarkably increases in the(α/β) region. After the alloy is soluted at 780°C followed by aged at 500°C, the elongation increases to about 14% with a yield strength near 1400 MPa, and the elongation measures 21% after aging at 600°C. T hese results suggest that the ductility of the alloy could be further enhanced by increasing the aging temperatures. Hence, the microstructure of this alloy soluted below the β transus is beneficial to the ductility.A hybrid of Ti B+Ti C reinforced Ti-3.5Al-5Mo-4V-2Cr-2Sn-2Zr-1Fe alloy was also produced by non-consumable arc-remelting followed by hot-forging. The average grain size of the as-cast alloy is about be 750±100μm measured by image software analysis of several SEM photomicrograph and the as-cast composite is 150±20μm. T he results show that the Carbide particles and Boride whiskers are segregated at the boundaries of β grains of as-cast and distributed uniformly in accordance with forging. After solution treatment at 860oC for 0.5h the size of the β grains in the composite is about 55±7.6μm and the size of the β grains in the matrix alloy is about 99.3±6.9μm. The composite treated bellow the β field showed smaller grain sizes than the alloy. the grain size of the composite treated below the β transus smaller than that of the matrix. Ultimate tensile strength and yield strength is about 1625 MPa and 1500 MPa respectively, and the ductility is 7% after the composite treated at 780oC puls aged at 500oC. T he ductility of(Ti B+Ti C)/(Ti-3.5Al-5Mo-4V-2Cr-2Sn-2Zr-1Fe) composite could be ameliorate by increasing the aging temperatures. T he composite treated at 780oC puls aged at 570oC, adequate strength and good ductility were obtained(tensile strength of the alloy is 1350 MPa with elongation of about 18% and tensile strength of composite was 1500 MPa with elongation of about 13%).As far as the elastic modulus is concerned, the elastic modulus of most β-Ti alloys ranged from 30 to 90 GPa. T here is great interest in the development of β titanium alloys because of their stellar formability. The reinforcements were distributed uniformly in the alloy and the elastic modulus was improved about 25 GPa. In general, β titanium alloys can obtain good tensile properties by solution treatment followed by aged treatment after rolling. T he Ti-3.5Al-5Mo-4V-2Cr-2Sn-1Fe alloy is a high-strength β titanium alloy that shows excellent tensile properties after forging. T he last chapter of this study presents the effect of heat treatment on the microstructure and tensile properties of both the alloy and the composite after hot rolling. Microstructure observation shows that the alloy rolled in the α+β field leads to smaller β grain size than the alloy rolled in β field in all heat treatment conditions. The alloy soluted treated at α+β field has smaller β grain size than the alloy soluted treated at β field. Tensile test results referred that the strength of the alloy was improve d significanting by an aging treatment which was appropriate to the alloy shows secondary α phase. Rolling and solution treatment in(α+β) field result in better st rength and ductility than of the β field rolling and treatment. T herefore, in order to obtai n high strength and ductility, alloy rolled in the α+β field followed by soluted treated at α+β field plus aged should be chosen as long as possible before aging heat treatment. However, the ductility of the composite was sufficiently high when compared to that of the matrix alloy because of the presence a small amount of the reinforcements.Comparisons between effects of forging and hot rolling on the composite it can be seen from mechanical properties of the composite that the tensile strength and elongation of composite material after rolled in the α+β field followed by solution at 780oC plus aging 500oC are 1732 MPa. The tensile strength was large compared with those in the forging, but the elongation is smaller then that of forging case at the same conditions. |
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