Fundamental Study Of Key Technology For TiAl Intermetallic Compound Components Fabricated By Hot Isostatic Pressing

Hot isostatic pressing（HIP）technology,combined with tooling development,can realize the integration of the controlling of microstructure and mechanical properties and the compaction of components with complex geometry.As components fabricated by HIP technology,the mechanical properties or the shape accuracy can be comparable to those of components manufactured by forging technology or precision casting technology,respectively.Therefore,HIP technology is very suitable for the compaction of TiAl intermetallic compound components.However,when HIP technology was used to form TiAl intermetallic compound components,there are still some problems that need to be firstly solved,such as the problem in regard to the preparation of TiAl powders with high-performances,the problem with respect to the evolution mechanism of microstructure and mechanical properties during HIP processes,the problem regarding the high compaction temperature of HIP and the problem respecting the surface oxide film of TiAl powders.Therefore,in order to solve the above-mentioned technical bottlenecks,Ti-47Al-2Cr-2Nb（TiAl）powders were chosen as the raw-material and the five aspects of the properties of powders,the interface evolution,the low-temperature first HIP process,the high-temperature secondary HIP process and a new multiple-process of thermal hydrogen treatment（THT）/HIP were systematically investigated in this dissertation.The main conclusions are as follows:（1）A combined method of two vacuum-arc remelting methods and rotating electrode process were used to prepare TiAl powders.The properties of powders were systematically studied.The evolution mechanism of the microstructure of powders was clarified.The results suggest that the TiAl powders,prepared by this combined method,have the uniform composition and meet the design requirements.The microstructure of powders consists ofα₂ matrix phase andγsecond phase.The microstructure and the oxygen content are closely related to the particle size for powders.The larger the particle size of powders is,the more the content ofγphase in powders is,the coarser and more diverse the morphology ofγphase in powders is and the more the shrinkage cavities in powders are.Moreover,the larger the particle size of powders is,the smaller the specific surface area of powders is and the lower the oxygen content of powders is.（2）A two-step HIP process was studied,combining with the traditional low-cost control die material of soft-steel.The effects of HIP processes on the elemental diffusion and the phase formation near the interface between TiAl intermetallic compound and soft-steel were studied.The evolution mechanism of the interface was clarified.The operating temperature range of HIP of the soft-steel control die was defined.The results suggest that the interface contains three layers.As the temperature of HIP increases from950°C to 1150°C,the width of the interface also increases from²0μm to³00μm and the width of the surface contamination layer on the TiAl intermetallic compound also increases from⁷μm to¹50μm.As the HIP temperature rises to 1200°C,the interface has severely melted.The mechanism is that the elements strongly diffuse,so that the chemical composition near the interface reaches the eutectic composition,which leads to form the liquid phase and then the interface melts.（3）The effects of low-temperature first HIP processes on the evolution of microstructures and mechanical properties of TiAl intermetallic compound were investigated.The results suggest that the microstructure of the samples of HIP consists ofγmatrix phase andα₂ second phase.And the content ofα₂ phase decreases with the increase of the temperature of HIP until it reaches the state of equilibrium.The density of the samples of HIP increases with the increase of the temperature of HIP.As the temperature of HIP reaches 1150°C and the powders undergo the large plastic deformation,the samples of HIP are completely densified and the prior particle boundaries（PPBs）disappear,but a large number of microcracks generate in the samples of HIP.The grain size of the samples of HIP increases with the increase of the particle size of powders that are used to compact the samples of HIP,but the change of the grain size of the samples of HIP is not obvious with the increase of the temperature of HIP.Nb is substantially uniformly distributed betweenγphase andα₂ phase.Cr is segregated inα₂ phase.The room-temperature mechanical properties are closely related to the density,the PPBs,the content ofα₂ phase,the grain size and microcracks for the samples of HIP.（4）The elemental diffusion and the formation of B₂ phase in TiAl intermetallic compound were studied during high-temperature secondary HIP processes.A new mechanism of B₂ phase on mechanical properties of TiAl intermetallic compound was proposed.The results suggest that the distribution of Cr element and the formation of B₂phase were closely related to the temperature of HIP and the keeping time of HIP.With the increase of the temperature of HIP and the keeping time of HIP,the diffusion and segregation of Cr element gradually reaches the state of equilibrium inα₂ phase.As the temperature of HIP rises to 1250°C,the diffusion and segregation of Cr element continue doing and finally lead to the formation of B₂ phase betweenγphase andα₂ phase.In addition,if the keeping time of the HIP process of 1150°C is too long,a small amount of B₂ phase can also form betweenγphase andα₂ phase.The content of Cr element in B₂phase can reach⁸.20 at.%,while the content of Cr element inα₂ phase always maintains in the state of equilibrium（³.25 at.%）,so the content of Cr element inγphase will decrease,which finally leads to that the room-temperature plasticity of TiAl intermetallic compound deteriorates.（5）The control of crystal orientation and microcracks in TiAl intermetallic compound were studied during high-temperature secondary HIP processes.A new method for suppressing microcracks in structural materials was proposed.The results suggest that in the nearlyγmicrostructure（NG-microstructure）,the crystallographic-orientation betweenα₂ phase andγphase is that the{0002}α₂ is approximately perpendicular to the{111}γ,which is adverse to the glide of the unit dislocations on{111}γunder the condition of the phase stress betweenα₂ phase andγphase.γphase of NG-microstructure exhibits low ductility and many microcracks generates inγphase.However,in the fully lamellar microstructure（FL-microstructure）,the crystallographic-orientation betweenα₂ phase andγphase becomes that the{0002}α₂ have intermediate angles with the{111}γ.Under the condition of the phase stress,the intermediate angles make it easier for the unit dislocations to move.γphase of FL-microstructure exhibits good ductility and many dislocations and twinning are found inγphase.Microcracks much decrease inγphase of FL-microstructure and mechanical properties of TiAl intermetallic compound with FL-microstructure can be remarkably improved.（6）A new multiple-process of THT/HIP was investigated.The main characteristic of this process is that TiAl powders were firstly hydrogenated and then were dehydrogenated in two steps.During the process of the hydrogenation and then the dehydrogenation,the effects of hydrogen-atoms on the surface oxide film of TiAl powders and the high-temperature formability of TiAl intermetallic compound can be utilized.For TiAl intermetallic compound prepared by the THT/HIP process,the oxygen content decreased by 130 ppm and the compaction temperature of HIP reduced by 50¹00°C,comparing with TiAl intermetallic compound prepared by the traditional HIP process.The compressive fracture strength and ratio of TiAl intermetallic compound prepared by the THT/HIP process is as high as 2830 MPa and 34.4%.The mechanism that the surface oxide film is removed from the surface of the powders of THT is that during the first step of dehydrogenation,the hydrides of TiAlHx and a part of the hydrides of TiHx decompose into a large number of hydrogen-atoms（These hydrogen-atoms can be considered as the active hydrogen）and then these hydrogen-atoms go out and go across the surface oxide film of the powders of THT,which leads to that the surface oxide film can be reduced and removed.In addition,the surface tension from the decomposition of hydrides can also remove the surface oxide film of the powders of THT.The reasons that the compaction temperature of HIP decreased during the THT/HIP process have three aspects.The one is that during the THT/HIP process,the residual hydrides of TiHx will also decompose into hydrogen-atoms and those hydrogen-atoms make the creep stress of TiAl intermetallic compound decrease and then make the large plastic deformation occur at a lower-temperature.The one is that the high-temperature compaction property of the powders of THT can be also improved and then the compaction temperature of HIP can be also lowered.The other is that the grain refinement due to the decomposition of hydrides can also improve the high-temperature formability of the powders of THT.Finally,under the condition of the three aspects,the compaction temperature of HIP can be remarkably reduced during the THT/HIP process.