Fabrication Of TiNb-based Composites With High Strength And Plasticity Based On Crystallization Theory

With the development of modern industry, pursuing higher strength and plasticity intitanium alloy is the main aim for the researcher in order to meet the stringent requirementsfor new application. Recently, although a series of high-performance titanium alloys withmicrosturues of ductile β-Ti dendrites embedded in a nanostructured matrix are fabricated byrapid solidification, the small size of these alloys limit its application as a kind of importantengineering materials. It was well accepted that equiaxed-grained materials also have highmechcaial property. Crystallization of amorphous phase, as an important way to fabricatenanocrystalline/ultrafine-grained materials, also is used to prepare equiaxed-grained materials.However, most of equiaxed-grained materials syhthesized by crystallization of amorphousphase have relative low strength and plasticity. In this paper, based on related cryatallizationtheories, a series of TiNb-based amorphous/nanocrystalline alloy powders were fabricated bymechanical alloying, and their densification behaviors under spark plasma sintering (SPS),the intrinsic relationship between crystallization mechanism of the fabricated amorphousalloy powders and the microstructure and mechanical property of the sintered and crystallizedbulk alloys, the effect of fabrication parameters on the microstructure and mechanicalproperty of the sintered and crystallized bulk alloys, composition design rules of titaniumalloys were carried out to confirm the advantages of this new way to fabricatehigh-performance titanium alloys. The results obtained can provide a design basis forfabricating bulk titanium alloys with excellent mechanical property.Firstly, amorphous Ti40.6Zr9.4Cu37.5Ni9.4Al3.1alloy powder with wide supercooled liquidregion of98K was synthesized by mechanical alloying. Crystalline Ti40.6Zr9.4Cu37.5Ni9.4Al3.1alloy powder was obtained by annealing of the synthesized amorphous alloy powder. Thedenisification behavior of the amorphous and crystalline alloy powders with the same SPSsintering parameter indicate that the viscous flow of the amorphous powder can acceleratedensification process and the denisification behavior has two stages, whereas thisphenomenon disappears in the crystalline powder. In addition, high heating rate can promotedensification process of the amorphous powder by reducing the viscous flow activationenergy of the amorphous powder, but it has little effect on the crystalline powder. Hence, it provides theoretical support for the fabrication of the high-densitynanocrystalline/ultrafine-grained titanium alloys by viscous flow of the amorphous powder atrelative low sintering temperature.Subsequently, amorphous Ti64Nb12Cu11.2Ni9.6Sn3.2alloy powder was prepared bymechanical alloying. Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation was employed toreveal the relelationship between crystallization mechanism of metallic glass powder andmicrostructure of bulk alloys fabricated by powder consolidation and crystallization ofamorphous phase, Avrami exponent n show that different crytallization stages have differentcrystallization mechanism. In addition, with increasing of heating rate from10K/min to40K/min, the Avrami exponent n are1.9,2.0,2.5, and2.9, repectively. This indicates thathigher heating rate can improve nucleation rate of the amorphous powder duringcrystallization process, and thus reduce the grain size in the fabricated bulk alloys. It was thefirst time from the perspective of the crystallization kinetics to reveal the growth mechanismof nuclei during crystallization process, providing theoretical support for fabricationnanocrystalline/ultrafine-grained titanium alloys.Thirdly, bulk alloys was fabricated by SPS of the Ti66Nb13Cu8Ni6.8Al6.2alloy powderswith different milling times. Results show that the as-fabricated bulk alloys consist of the twocrystallized phases of β-Ti and (Cu,Ni)-Ti2. With increasing of milling time, the volumefraction of the Widmanst tten structure-like matrix and the equiaxed-grained matrix decresesand increases, respectively. Meanwhile, the yield strength of the as-fabricated bulk alloys hasno obvious change, but the fracture strength and the fracture strain increase with increasing ofmilling time. The bulk alloy fabricated by crystallization of fully amorphous alloy powderdisplays the highest fracture strength of2350MPa, and the largest fracture strain of28.5%.The research results further comfirm the promising way for fabricating ultrafine-grained andequiaxed-grained bulk titanium alloys with excellent mechanical property by powdermetallurgy.Finally, based on the composition of Ti70Nb30, a series of amorphous alloy powders wereprepared by adding element of Fe and Co or Cu and Ni. All the bulk alloys fabricated bycrystallinzation of amorphous powder display high fracture strength and fracrue strain. TheTi65Nb22.5Co6.25Fe6.25and Ti66Nb13Fe8Co6.8Al6.2bulk alloys present the same crystallized phases of β-Ti and CoTi2, but their yleid strengths increase and fracture strength decreasewith adding of Al element. For the TiNbFeCoAl alloy composition, changing the relativecontent of Fe and Co will not change their crystallized phases. With increasing of Fe content,the yield strength of the as-fabricated bulk alloys increases, while the fracture strain decreases.For the as-fabricated Ti71.6Nb15.8Cu4.8Ni4Al3.8bulk alloys, they display a microstructure of(Cu,Ni)Ti2and AlNbTi2phases surrounded by β-Ti matrix. Meanwhile, for some amorphousalloy powder with the same composition consolidated by different sintering parameters, theyield strength and fracture strain of the obtained bulk alloys decreases and increases withincreasing of sintering temperature, respectively. The fracture strength of2850MPa for theTi66Nb13Fe8Co6.8Al6.2bulk alloy and the fracture strain of40.5%for theTi71.6Nb15.8Cu4.8Ni4Al3.8bulk alloy are higher than those of other titanium alloys reported sofar. In addition, bulk alloys fabricated by SPS of amorphous alloy powder with higher glassforming ability usually exhibit lower strength and plasticity.In summary, amorphous powder can promote densification process by viscous flow, andhigh heating rate can promote the nucleation rate of amorphous powder during SPS process,and ultrafine-grained and equiaxed-grained bulk alloys fabricated by SPS of amorphous alloypowder possess higher mechanical propert. Therefore, the method of powder consolidationcoupling crystallization of amorphous phase is a very promising method for frabricating bulkalloys with high density and excellent mechanical property.