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Due to the excellent structural and functional properties, such as low density, high fracture toughness and fatigue strength, Al-Cu-Mg (2024 aluminum) alloy has become an attractive light weight material for inertial instruments and aerospace applications. However, it has relatively low dimensional stability. The aim of this investigation is to enhance the dimensional stability of 2024 aluminum alloy via two-stage aging method, and identify the precipitation behavior and the underlying stabilizing mechanisms of 2024 aluminum alloy. When the second aging time was 24 h, the uniformly distributed precipitates could effectively pin the dislocations and impede their movement. This leaded to a significant improvement of the micro-yield strength and the thermal strain in the 2024 aluminum alloy, and thus the dimensional stability of the alloy increased.

In this paper, the influent mechanism of nano-Y2O3 addition on the creep properties and fracture behavior of an as-cast near-α titanium alloy at 650 °C/150 MPa and 700 °C/150 MPa was discussed. The results showed that the steady state creep rate decreased and creep life increased by Y2O3 addition. The creep rupture life of Y2O3 addition alloy increased to three times at 700 °C/150 MPa. Due to the Y2O3 addition, on one hand, the refinement of β grains and α/β colonies can make effective slip distance of dislocations reduced and make the applied creep stress undertook by more grains, which makes the stress concentration of α/β boundaries reduced. On the other hand, the Y2O3 in grain interior played an important role in dislocation strengthening and load transfer strengthening effect, which demonstrates the positive effect of Y2O3 on the creep resistance. The fracture of Y2O3 particles, interface debonding and grain boundaries deformation all can make voids nucleated and cracks formed, therefore, the propagation and connection of voids and cracks caused the fracture of Y2O3/α-Ti alloy. The β phase dissolution and silicide precipitation indicated microstructure instability phenomenon presented within two alloys during creep exposure.

The high temperature creep behavior of a near-α Ti–6Al–4Sn–4Zr-0.8Mo–1Nb–1W-0.25Si alloy prepared by induction skull melting has been studied under the conditions of 625 °C-675 °C and 150 MPa–250 MPa using uniaxial creep tests in this work. Results indicated that the incoherent secondary phase particle, silicide, mainly precipitated from the α/β lath interface, which has a positive effect on hindering the movement of dislocations and pinning α/β lamellae boundaries during creep. The stress exponent and apparent activation energy was 7.2 at 650 °C and 417.6 kJ/mol at 200 MPa, respectively. The high stress exponent and apparent activation energy indicated that the creep process was controlled by dislocation climb and slip impeded by silicides. The main failure mechanism of the as-cast alloy during creep was the initiation, growth, and coalescence of cracks at the grain boundary, which was proven from the intergranular fracture feature.

The effect of nano-Y2O3 addition on microstructure and tensile properties of high-Al TiAl alloys prepared by induction skull melting is investigated. The result shows that the addition of nano-Y2O3 affords an obvious refinement in lamellar colony due to the heterogeneous nucleation. A crystallographic model is used to predict the α and γ nucleation potencies of Y2O3 particles during solidification. Besides, Y2O3 induces the solidification pathway of TiAl alloys into high-Al side, resulting in the increase of γ phase. Owing to fine grain strengthening and second phase strengthening, the tensile strength and elongation of TiAl–Y2O3 alloys are enhanced significantly at room temperature.

In this work, the β and (α+β) forging deformation were conducted on (TiB + TiC + Y2O3)/α-Ti composite to investigate the effect of reinforcement on dynamic recrystallization and globularization behavior. During β deformation, the reinforcements not only accelerated discontinuous dynamic recrystallization by particle stimulated nucleation mechanism, but also promoted the heterogeneous nucleation of α phase thus conducted the {0001}α pole figure of composite more widely distributed. However, during (α+β) deformation, the microstructure was characteristic of kinked α colonies morphology. Moreover, compared to matrix, the (TiB + TiC + Y2O3)/α-Ti composite depicted a more homogeneous microstructure and larger crystallography rotation both along or transverse to the α lamellar. The distribution of Schmid factor value indicated that the deformation behavior of matrix showed varies with the α colony while it was more homogeneous in composite. Reinforced with (TiB + TiC + Y2O3) reinforcements, on the one hand, the random SF value of composite conducted the incompatibility slip between adjacent α lamellar thus at last leaded the formation of HABs in inter-lamellar; on the other hand, the reinforcement itself could obstruct dislocation that accelerated the dynamic globularization behavior. Therefore, the (TiB + TiC + Y2O3)/α-Ti composite had a great advantage in optimizing microstructure especially globularization that made the composite of great application prospect.

In present work, a hybrid-reinforced titanium alloy has been β forged and heat treated successively. The microstructure evolution, deformation mechanism and fracture behavior were investigated to evaluate the contribution of HT on strength. The fine zone was formed after HT at the prior β grain boundary, and the (TiB + TiC + Y2O3) reinforcement could significantly enlarged the area of fine zone and homogenized the microstructure. Silicide precipitation was also observed in intra lamellar and α/β boundary after HT, thereby increasing the strength via Orowan strengthening mechanism. In addition, the deformation mechanism analysis indicated that the grain boundary slip contributed a great effort to the high temperature deformation during tensile testing. Due to the formation of fine zone and silicide precipitation, the grain boundary strength and intragranular strengths both improved. However, the equal-strength temperature improved more thus leaded to a high yield strength in (TiB + TiC + Y2O3)/α-Ti composite after HT. This result also had a substantial influence on fracture morphology, which achieved considerable strength in the matrix and composite at elevated temperature.

The as-extruded TiB whisker reinforced Ti6Al4V (TiBw/Ti6Al4V) composites with tailored architecture were fabricated by powder metallurgy followed by hot extrusion. High temperature tensile tests were conducted to study the superplastic deformation behavior at 900 °C, 925 °C, 950 °C and 975 °C with strain rates of 0.000316/s, 0.001/s and 0.00316/s, respectively. Microstructural observation showed that the fraction of β transformed microstructure increased with increasing strain, which indicated that the phase transformation was drastically affected by hot deformation. Voids next to the TiBw reinforcements were observed due to the incoordination deformation at small strain, but this kind of voids was refilled at large strain, which is attributed to the combined processes of recrystallization and coordination deformation. Furthermore, new voids were formed at the α/β interface at large strain, considered to be caused by the incoordination of phase boundary sliding. EBSD results implied that the texture intensity decreased during the superplastic tensile process due to recrystallization and grain rotation. TEM analysis revealed that the grains next to the TiBw reinforcements presented a higher dislocation density forming new boundaries by recrystallization. This effect led to grain refinement which was beneficial to grain boundary sliding, which resulted in the composites' high elongation.

The hot forging of the 2024Al/Al18B4O33w composite fabricated by the squeeze casting technique was carried out at 450 °C to improve the ductility. The microstructure and tensile properties of the as-cast and forged 2024Al/Al18B4O33w composites are studied. Non-homogeneous distribution of the reinforcement whiskers was revealed in the as-cast composite. The forging led to significant variations in the average length and distribution of the reinforcement whiskers in comparison with the as-cast composite. After forging, the tensile tests showed an increase in the ultimate tensile strength at room temperature and a decrease at elevated temperature. The forging highly increased the ductility both at room and elevated temperature.

The Ti3Al matrix composite sheet with a novel quasi-continuous chain reinforcement distribution was prepared by using roll bonding and subsequent reaction annealing with pure Ti and SiCp/Al foils. The (Ti3AlC + Ti5Si3) reinforcements were produced by in situ reaction of the as-rolled Ti–(SiCp/Al) laminated composite sheet after a two-stage annealing treatment including first annealing at 660 °C and second annealing at 1250 °C. The microstructure evolution during the reaction annealing was investigated. In the first reaction annealing, TiAl3 was formed, and the SiC particles were pushed together due to the Kirkendall effect, and after the second reaction annealing, Ti3Al, Ti3AlC and Ti5Si3 were synthesized. In addition, the reinforcement distribution presents a quasi-continuous chain microstructure in Ti3Al matrix composite sheet.

The Ti–(SiC p /Al) laminated composite was prepared by roll bonding of pure Ti and SiC p /Al foils. The microstructure analysis shows that the work-hardening of Ti reduces the deformation compatibility of the Ti and SiC p /Al layers with increasing the rolling reduction. A nano-sized TiAl 3 interfacial layer was formed at the interface between the Ti and SiC p /Al layers, which resulted in a good interlaminar bonding strength and an improved ductility of the laminated composite due to the nano-sized effect. A good strength–ductility balance, 307 MPa yield strength and 21.4% elongation to failure, was achieved. Combining with the low density of 3.62 g/cm 3 , 20% lower than that of Ti, this Ti–(SiC p /Al) laminated composite presents the potential application in the aerospace field.

In the present work, TiAl 3 layers were synthesized by reaction annealing of a multi-laminated Ti-(SiC P /Al) composite at 650 °C. It was found that Si dissolved in the TiAl 3 layers during annealing. The effect of Si in the TiAl 3 layers on mechanical properties was investigated both by experiment and theory. The experimental results show that after annealing at 650 °C for 3 h, the solid solubility of Si in the TiAl 3 layer is about 6 at%, which leads to the lattice distortion of TiAl 3 . Due to the solid solution of Si in TiAl 3 , the elastic modulus and nanohardness increased up to 280 Gpa and 10.7 Gpa, respectively. Furthermore, the mechanism of increased modulus due to the solid solution of Si is verified by first-principles calculation of the theoretical elastic properties and can be explained from the electronic structure: (i) a new hybridization between Si3s and Al3s; (ii) primarily covalent interactions along Al3Si bonds.

In this paper, WO3p and Al18B4O33w hybrid reinforced aluminum matrix composites were fabricated by squeeze casting. Heat-treatment of the composites was carried out at 500 °C for different times. Gibbs free energy calculation indicated that the reaction of WO3 with Al was spontaneous. DTA result showed that the reaction of WO3 with Al took place at 396 °C and 487 °C. XRD results showed that the interfacial reaction occurred after heat-treatment at 500 °C for 5 h and 10 h and the reaction products were WO2.90 and WO2.83, which resulted in the formation of micro-voids at the interface between particle and matrix and led to mechanical properties of the composites decreasing. However, mechanical properties of the composites heat-treated for 10 h did not decrease obviously compared with that of heat-treated for 5 h. The reason may be that the produced aluminum oxide film covered the WO3p and retarded progress of the chemical reaction.

Barium plumbate particle (BPOp)/2024Al was fabricated by ball milling and hot-pressing sintering. Nanosized Pb particles were formed from the in-situ reaction between BPOp and 2024Al matrix, and mainly distributed homogeneously inside grains. They have melting undercooling behavior due to the nanoscale effect. The hot extrusion deformation of the composite at different temperatures was performed to investigate the effect of Pb liquid phase on the deformation. The results show that the presence of the liquid reduces deformation resistance by decreasing dislocation pile-ups. In addition, the liquid acted as lubricant to accommodate deformation to increase the deformability.

In order to study the hybrid effect of in situ TiB whisker (TiBw) and TiC particle (TiCp) on the tensile properties of titanium matrix composites (TMCs), TMCs with various TiBw/TiCp volume ratios of 4:1, 1:1 and 1:4 were prepared by reactive hot pressing of blended powders of Ti, B4C, and graphite. Room tensile testing results exhibited that the composite with TiBw/TiCp volume ratio of 1:1 experienced an increase in the ultimate tensile strength of 25 and 50%, respectively, over the composite with ratio of 4:1 (based on Ti–B4C) and pure Ti. The hybrid strengthening effect still existed at elevated temperature but it was not as obvious as that of room-temperature. Meanwhile, the ductility of the composites was decreased with decreasing TiBw content. The results suggest that the hybrid effect exists both at room and at elevated temperatures and it should be taken into account to prepare TMCs with good mechanical properties.

The damping behavior of Al18B4O33 whisker-reinforced aluminum composites with and without an in situ reaction layer including nanosized Pb particles at the Al18B4O33–Al interface was studied. Experimental results indicate that Pb particles at the interface between Al18B4O33 and Al alter the dislocation status of the Al matrix adjacent to the interface, resulting in a change in damping properties of the Al18B4O33/Al composites. The damping peak at medium temperature (about 150 °C) in the Al18B4O33/Al composite was ascribed to a dislocation mechanism.

TiB clusters have been observed in the composite prepared by reactive processing using B4C powders of particle size 3.5 μm but were not found when using finer B4C powders. After hot extrusion these clusters were reduced. The size of the B4C particles affects the microstructure and the mechanical properties of the composite directly, and the utilization of finer B4C powders of a narrow size distribution can produce finer TiB whiskers.

Titanium matrix composite reinforced by in situ TiB whisker and TiC particle was prepared by reactive hot-pressing of the blended powder of Ti and B4C. The microstructure of the composite was studied by SEM and TEM. The symbiosis structure of TiC particle and TiB whisker was found apparently. The results showed that the position of TiC particle to TiB whisker was relatively fixed, and TiCp was more likely to grow up on the (101)TiB and (101(_))TiB planes but seldom on the (100)TiB plane. The fabrication method of the composite, reaction mechanism of Ti and B4C, and the surface microstructure of TiB whisker were taken into consideration to explain the phenomenon.