Yuanfei Han

Effect of solid carburization on surface microstructure and hardness of Ti-6Al-4V alloy and (TiB+La2O3)/Ti-6Al-4V composite

Abstract Solid carburization was employed to improve the hardness of Ti-6Al-4V alloy and (TiB+La 2 O 3 )/Ti composite. The samples wrapped in graphite powder were placed in sealed quartz tubes, followed by solid carburization at 1227 K for 24 h. Microstructure and phase analysis indicated that TiC reinforcements and Ti-C solid solutions were introduced after solid carburization. Moreover, the volume fraction of equiaxed α-Ti phase in diffusion layer decreased obviously with increasing sample depth. Hardness testing results indicated that both the carburized surfaces performed significant improvement of about 100% in micro-hardness compared with untreated materials. The variation of carbon contents with increasing sample depth resulted in a hardened layer of 300 μm in the carburized samples. Meanwhile, slight influence on the internal microstructure and hardness indicated that solid carburization was an effective method in strengthening the surface of titanium alloy and titanium matrix composite.

Effect of Microstructures on Fatigue Crack Growth Behavior of Friction Stir Processed NiAl Bronze Alloy

Abstract:Friction stir processed (FSPed) NAB alloy exhibits inhomogeneous microstructures that can be divided into three subregions from the top surface to the bottom according to α phase morphologies: Widmanstatten α subregion, banded α colonies, and stream-like α colonies. In this study, a constant stress intensity range (ΔK) was used for each sample to study the effect of microstructures on the fatigue crack growth rate (FCGR) of FSPed NAB alloy. The results show that α phase in banded and stream-like α colonies experiences completely dynamic recrystallization and forms equiaxed α grains during FSP. The FCGR of FSPed NAB alloy continuously decreases from the top surface to the bottom. In the subregion with stream-like α colonies, the alloy containing a higher content of equiaxed α grains and fine κiv phase, and less retained β (β′) phase exhibits the best FCG resistance. The equiaxed α grains deflect the main crack and increase crack tortuosity effect, which make a main contribution to FCG resistance of FSPed NAB alloy, while martensite β′ phase produced during FSP accelerates its fatigue crack growth. Compared to matrix alloy, FSPed NAB alloy exhibits better FCG resistance only at high ΔK levels. At low ΔK levels, the crack deflection effect caused by coarser κ phase in the matrix alloy obviously improves its FCG resistance. With the increasing ΔK, the aforementioned crack deflection effect gradually diminishes and fatigue crack prefers to propagate in a flat way, resulting in higher FCGR of matrix alloy.

Study on squeeze casting of an in situ 5 vol.-% TiB2/2014 Al composite

Abstract An in situ 5 vol.-% TiB2/2014 composite was prepared by an exothermic reaction of K2TiF6, KBF4 and Al melts. The effect of introduction of in situ formed TiB2 particles on the squeeze-casting formability of the composite was discussed. The microstructural evolution and changes in the mechanical properties of the composite at different squeeze pressures were investigated. The results showed that a pouring temperature of 710°C, a die temperature of 200°C and a squeeze pressure of 90 MPa were found to be sufficient to get the qualified squeeze cast and maximum mechanical properties for an Al 2014 alloy. However, the pouring temperature, die temperature and squeeze pressure need to be increased to 780°C, 250°C and 120 MPa for the composite to get the qualified squeeze cast and maximum mechanical properties as a result of the effect of introduction of in situ formed TiB2 particles on the solidification process, plasticity and fluidity of the composite. The microstructural refinement, elimination of casting defects such as shrinkage porosities and gas porosities and improved distribution of TiB2 particles in the case of the composite result when pressure was applied during solidification. Compared with the gravity-cast composite, the tensile strength, yield strength and elongation of the squeeze-cast composite at 120 MPa increased by 21%, 16% and 200%.

CeO2 induced dispersive distribution of TiB2 particles in in situ TiB2/Al composite

Abstract The roles of CeO2 additive during preparation of in situ TiB2/Al composite, alleviating particle settlement in composite melt and significantly improving particle dispersion in final microstructure, are studied in this paper. It is evidenced that the CeO2 additive reacts with Al melts to release Ce solute into the melts, and the released surface active Ce is absorbed in the Al/TiB2 interfaces without any other reaction products. First principles calculations show that the interfacial energy of Al/TiB2 interfaces is reduced owing to the presence of Ce in Al/TiB2 interfacial area. Therefore, the wettability of molten Al on TiB2 surface is increased and the dispersion of TiB2 particles in Al matrix is eventually improved.

Study on the castability and mechanical properties of thin-walled nickel-base superalloy

The castability and mechanical properties of thin-walled nickel-base superalloy castings fabricated by gravity casting and centrifugal casting were investigated. It is shown that, despite its microporosity has slight change, the casting fabricated by centrifugal casting presents fewer misruns and less Laves phase than that of gravity casting. The ultimate tensile strength, yield strength and stress rupture lives of centrifugal casting are improved by about 2.2, 7.4 and 41.1%, respectively. However, the elongation (EL) and reduction in area are sharply decreased by about 10.9 and 25.0%, respectively. Paradoxically, the stress ELs of both castings are similar. The seemingly contradictory results in mechanical properties stem from the opposite effects of centrifugal force on hardening elements macrosegregation and melt turbulence.