|dc.description.abstract||The rapid development of various high-precision technologies in the aerospace, military and civil fields has put forward higher and higher requirements for the performance of materials. Traditional titanium alloys have become inadequate in the face of the comprehensive requirements of the application environment, so composite materials have increasingly become the focus of material researchers. titanium boride (TiB) is the most attractive reinforcement because of the balance and stability of its thermal, physical, and mechanical properties. Boron is essentially insoluble in Ti and its alloys, so it does not cause lattice embrittlement. In this thesis, a Ti-6Al-4V matrix composite reinforced by in-situ synthesized TiB whiskers were successfully fabricated by hot pressing of a mixture of Ti-6Al-4V alloy swarf and boron powder.
Hot pressing temperature significantly affects the densification and properties of the fabricated titanium alloy matrix composites. With the increase of hot processing temperature from 1050ºC to 1250ºC, the porosity of the fabricated composites was decreased from 12.8% to 2.7%. When the hot-pressing temperature was reached to 1250ºC, the Ti-6Al-4V-3TiB composite could be successfully consolidated with a well-bonded prior swarf boundaries (no obvious voids were appeared at the prior swarf boundary) and less TiBx phases (most are TiB), leading to a reasonable ultimate tensile strength (1121MPa) and yield stress (1028MPa) were achieved.
The amount of boron addition (1vol%, 3vol%, 5vol%, 7vol%) influences the phase constitutions, microstructure, and mechanical properties of the fabricated swarf Ti-6Al-4V matrix composites hot-pressed at 1250°C. The UTS and YS of the fabricated composite was first increased when the boron addition was increased from 1vol% to 3vol%, and then decreased with the boron addition was further increased to 7vol%. However, the ductility of the fabricated composites was continually decreased with increasing the boron addition amount, and this is attributed to the increase amount of agglomerated TiB reinforcement phases. The Ti-6Al-4V-3TiB (adding 3vol% of boron) had the optimal mechanical properties among all of the fabricated composites, having a UTS of 1108MPa-1127MPa, YS of 1025MPa-1034MPa, and a strain to fracture of 3.45% - 3.98%.
Comparing to the hot-pressed Ti-6Al-4V-3TiB composite, The post-processed (extrusion at 950°C) composite had finer grain size and smaller lamellar spacing, which contributed to slightly increase the composite’s ductility to 4.18% - 5.19%, but the strengthen effect of TiB reinforcement effect was reduced due to the formation of voids between the reinforcing phase and the matrix phase introduced by severe plastic deformation.
Various heat treatments were carried out for the hot-pressed and hot extruded Ti-6Al-4V-3TiB composites to tailor the composite’s microstructure for achieving desired strength-ductility balance. After STA-955 (955℃ 1 hr water quenching + 550℃ 6hrs air cooling) and STA-855 (855℃ 1 hr water quenching + 550℃ 6hrs air cooling), the strength of hot-pressed and hot extruded composites were improved, due to the acicular secondary α phase and nano-size secondary α phase, respectively. The hot extruded composite had the best mechanical properties after STA-955, with a UTS of 1259MPa - 1263MPa, YS of 1165MPa - 1170MPa, and a strain to fracture of 3.15% - 3.28%.||