Preparation of TiAl/Al_2O_3 composites by combustion synthesis

Studies the combustion synthesis of Al Ti TiO 2 system and concludes that, due to its low exothermic nature, a stable combustion wave can be maintained only when the system is ignited at a certain preheating temperature, and coupled with appropriate pseudo HIP process, dense TiAl/Al 2O 3 composites with density as high as 97% of the theoretical value can be produced, and points out. Microstructure observation shows in situ formed Al 2O 3 particles are of an average size smaller than one micron, and the hardness of TiAl matrix is enhanced by introduction of these particles.

Formation mechanism and growth kinetics of TiAl_(3) phase in cold-rolled Ti/Al laminated composites during annealing

Cold-rolled Ti/Al laminated composites were annealed at 525−625℃for 0−128 h,and the interfacial microstructure evolution was investigated.The results indicate that only the TiAl_(3) phase was formed at the Ti/Al interface;most of TiAl_(3) grains were fine equiaxed with average sizes ranging from hundreds of nanometers to several microns and the TiAl_(3) grain size increased with increasing annealing time and/or temperature,but the effect of annealing temperature on the TiAl_(3) grain size was far greater than that of annealing time.The growth of the TiAl_(3) phase consisted of two stages.The initial stage was governed by chemical reaction with a reaction activation energy of 195.75 kJ/mol,and the reaction rate constant of the TiAl_(3) phase was larger as the Ti/Al interface was bonded with fresh surfaces.At the second stage,the growth was governed by diffusion,the diffusion activation energy was 33.69 kJ/mol,and the diffusion growth rate constant of the TiAl_(3) phase was mainly determined by the grain boundary diffusion owing to the smaller TiAl_(3) grain size.

MICROSTRUCTURES AND MECHANICAL PROPERTIES OF IN-SITU Al_2O_3/TiAl COMPOSITES BY EXOTHERMIC DISPERSION METHOD

In-situ Al2O3/TiAl composites were fabricated by pressure-assisted exothermic dispersion （PAXD） method from elemental powder mixtures of Ti, Al, TiO2, and Nb2O5. The microstructures and mechanical properties of the as-sintered composites are investigated. The results show that the as-sintered products consist of γ-TiAl, α2-Ti3Al, Al2O3, and NbAl3 phases. Microstructure analysis indicates that Al2O3 particles tend to disperse on the grain boundaries. Application of a moderate pressure of 35 MPa at 1200℃ yields Al2O3/TiAl composites with fine Al2O3 reinforcement and a discontinuous network linking by Al2O3 particles. The aluminide component has a fine submicron γ ＋α2 lamellar microstructure. With increasing Nb2O5 content, Al2O3 particles are dispersed uniformly in the matrix. The hardness of the composites increases gradually, and the bending strength and fracture toughness of the composites reach to the maximum value, respectively.

ECAP工艺对TiAl_3-P/Al复合材料组织的影响

利用透射电子显微镜和光学显微镜研究了窀温下1～8道次等通道角挤压（ECAP）3工艺对TiAl3-P／Al复合材料组织的影响。结果表明，在ECAP挤压初期Al基体中的位错密度很高，在2道次后急剧降低；组织中位错墙比例在开始也呈现升高的趋势，随着应变量的增加，逐渐向小角度晶界转变；小角度晶界的出现比位错墙晚，晶内小角度晶界的比例变化趋势也是一个先增加后降低的过程，最终转变为大角晶界。ECAP过程中，TiAl3颗粒对Al基体组织变化的作用不明显。ECAP变形有效破碎了较大尺寸的TiAl3颗粒并改善了TiAl3颗粒在Al基体中分布的均匀度。板条状TiAl3在ECAP变形中不仅发生了脆性断裂，还发生了孪生变形，与基体金属的变形相互协调，使少量大尺寸TiAl3颗粒保留下来。

Fabrication and Mechanical Properties of Al_2O_3/TiAl Composites

Al2O3/TiAl composites were successfully fabricated by hot-press-assisted exothermic dispersion method with elemental powder mixtures of Ti, Al TiO2 and Nb2O5, and the microstructure and mechanical properties were investigated. The results indicate the fine Al2O3 particles tend to disperse on the grain boundaries. The grain size of TiAl matrix decreases and the hardness increases with increasing Nb2O5 content. The bending strength and fracture toughness reach to a maximum when Nb2O5 content is 6 wt%, under 642 MPa and 6.69 MPa·m^1/2, respectively. Based on the fractography and the observation of crack propagation path, it is concluded that the strengthening and toughening of such composites at room temperature can be attributed to the refinement of the TiAl matrix, the deflection behavior in the crack propagation and the dispersion of Al2O3 particles.

In-situ Preparation of Al/TiAl_3(P) Composite by Direct Reaction between Molten Al and TiO_2 Powder

TiAl3 particle reinforced pure Al composite has been made by direct reaction among molten Al, TiO2 and a flux. The composite exhibits high Synthetically properties. The strength and hardness are higher than those of pure Al matrix by 71.5% and 134% respectively However, the elongation is 20.36%, slightly lower than that of the Al matrix.

两种不同涂层的Ti纤维增强TiAl基复合材料

研究了涂覆单一涂层Ｙ２Ｏ３或Ａｌ２Ｏ３的Ｔｉ纤维增强γＴｉＡｌ基复合材料的界面和性能。运用物理气相沉积法在Ｔｉ纤维表面涂覆Ｙ２Ｏ３或Ａｌ２Ｏ３涂层后，然后用类似箔叠法制得ＴｉＹ２Ｏ３γＴｉＡｌ和ＴｉＡｌ２Ｏ３／γＴｉＡｌ复合材料。复合材料的弯曲强度从４４９ＭＰａ提高到５６２ＭＰａ（ＴｉＹ２Ｏ３／γＴｉＡｌ）或５７３ＭＰａ（ＴｉＡｌ２Ｏ３／γＴｉＡｌ），而且高于纯Ｔｉ纤维增强ＴｉＡｌ基复合材料的弯曲强度。纤维与基体间的界面反应层厚度由３０μｍ（Ｔｉ／γＴｉＡｌ）减少为１４μｍ（ＴｉＹ２Ｏ３／γＴｉＡｌ）或２０μｍ（ＴｉＡｌ２Ｏ３／γＴｉＡｌ）。

High-temperature oxidation behavior of Al_2O_3/TiAl matrix composite in air

The oxidation behavior of Al2O3/TiAl in situ composites fabricated by hot-pressing technology was in-vestigated at 900℃ in static air.The results indicate that the mass gains of the composites samples decrease gradually with increasing Nb2O5 content and the inert Al2O3 dispersoids effectively increase the oxidation resistance of the composites.The higher the Al2O3 dispersoids content,the more pro-nounced the effect.The primary oxidation precesses obey approximately the linear laws,and the cyclic oxidation precesses follow the parabolic laws.The oxidized sample containing Ti2AlN and TiAl phases in the scales exhibits excellent oxidation resistance.The oxide scale formed after exposure at 900 ℃ for 120 h is multiple-layered,consisting mainly of an outer TiO2 layer,an intermediate Al2O3 layer,and an inner TiO2+Al2O3 mixed layer.From the outer layer to the inner layer,TiO2+Al2O3 mixed layer presents the transit of Al-rich oxide to Ti-rich oxide mixed layer.Near the substrate,cross-section micrograph shows a relatively loose layer,and micro-and macro-pores remain on this layer,which is a transition layer and transferres from Al2O3+TiO2 scale to substrate.The thickness of oxide layer is about 20 μm.It is also found that continuous protective alumina scales can not be observed on the surface of oxida-tion scales.Ti ions diffuse outwardly to form the outer TiO2 layer,while oxygen ions transport inwardly to form the inner TiO2+Al2O3 mixed layer.Under long-time intensive oxidation exposure,the internal Al2O3 scale has a good adhesiveness with the outer TiO2 scale.No obvious spallation of the oxide scales occurs.The increased oxidation resistance by the presence of in situ Al2O3 particulates is at-tributed to the enhanced alumina-forming tendency and thin and dense scale formation.Al2O3 particu-lates enhance the potential barrier of Ti ions from M/MO interface to O/MO interface,thereby the TiO2 growth rate decreases,which is also beneficial to improve the oxidation resistance.Moreover,the multi-structure of the TiO2+Al2O3 mixed layer decreases the indiffusion of oxygen ions and also avails to improve the high temperature oxidation resistance of the as-sintered composites.