Semi-solid Sintering Fabrication And Mechanism Of Strengthening And Toughening Of Newly Bimodal Structural Ti-based Alloys

Optimizing or introducing new fabrication process and controlling micro structures (type/grain size/morphology/distribution of phases) are considered as the most effective approaches to strengthen and toughen titanium alloys. Recently, various strategies have been established to address the low plasticity that accompanies fine grains. Especially, introducing a bimodal or multi-modal grains distribution has been reported to be broadly applicable in many material systems. In the case of elevated temperature materials, such as Ti, it has been developed for many years to fabricate high-performance titanium alloys with a micro structure of micron-sized β-Ti dendrites embedded in nanostructured matrix by rapid solidification. This approach, however, is inherently limited to certain alloy compositions and hence is not widely applicable to other alloy systems that are of interest for engineering applications. In recent years, our research group has successfully fabricated Ti-based alloys with excellent combinations of mechanical properties by sintering and crystallizing amorphous powders with the similar compositions as those by rapid solidification approach. These fabricated alloys consisting of ultrafine grained equiaxed bcc β-Ti matrix+ultrafine grained equiaxed fee phase can be obtained with large size for industrial application. However, bimodal structure in Ti-based alloys has never been obtained by this powder metallurgic method, thus making it difficult to further improve the mechanical properties of Ti-based alloys. Motivated by the aforementioned questions, in this paper, a relative new approach, coupling sintering of nanocrystalline/amorphous composite powders with subsequent semi-solid treatment, referred to hereafter as semi-solid sintering, is introduced to fabricate bimodal structural Ti68.8Nb13.6Co6Cr(Cu)5.1Al6.5 alloys in an effort to achieve both high strength and large plasticity. The main research priorities in our study are the microstructural evolution and strengthening and toughening mechanism of the newly bimodal Ti-based alloys and the effect of milling time on the semi-solid sintered Ti-based alloys.As a first step, nanocrystalline/amorphous Ti68.8Nb13.6Co6Cr5.1Al6.5 alloy powder was synthesized by mechanical alloying after 90h milling. The DSC curve and in-situ high temperature XRD of the as-milled powder shows that 1250℃ is within the semi-solid temperature range. By optimizating semi-solid sintering parameters, bimodal structural Ti68.8Nb13.6Co6Cr5.1Al6.5 alloy consisting of micron-sized equiaxed β-Ti surrounded by ultrafine lath-shaped CoTi2 phase was successfully fabricated. The formation of this fabricated bimodal microstructure ia related to distinct four stages during sintering:1) Spatial and chemical rearrangement of nanocrystalline/amorphous composite powders; 2) nucleation and growth of β-Ti and CoTi2 by crystallization of amporhous powder; 3) formation of a liquid-solid mixture, or semi-solid state containing melt CoTi2 and solid β-Ti phase; and 4) final evolution into a bimodal micro structure containing micron-grained β-Ti matrix and solidified ultrafine CoTi2 after rapid cooling. The bimodal Ti68.8Nb13.6Co6Cr5.1Al6.5 alloy shows excellent combination of yield stress of 1562MPa, fracture stress of 3011MPa and fracture strain of 40.1%, surpassing any titanium alloys fabricated by other methods so far. The large plasticity is attributed to propagation and multiplication of dislocations in the ductile micron-sized β-Ti. The ultrahigh strength results from the pinning effect against to dislocations in β-Ti by CoTi2 phases, thus improving the capability of strain hardening.Subsequently, considering that compared to the element Cr, the element Cu has some similarities in formed phase and differences in thermal properties of formed phase in multicomponent composition design, aiming at fabricating new micro structure and optimizing mechanical properties, Ti68.gNb13.6Co6Cu5.1Al6.5 alloy was choosed and semi-solid sintered after 90h milling of its elemental blended powders. By optimizating the semi-solid sintering parameters, bimodal and multi-modal structural Ti68.8Nb13.6Co6Cu5.1Al6.5 alloys were successfully fabricated. Interestingly, the multi-modal structural alloy consists of nano-sized acicular a’martensite precipitated from micron-grained β-Ti matrix and ultrafine lath-shaped CoTi2 (partially twins). The multi-modal structure is completely different from the aforementioned bimodal Ti68.8Nb13.6Co6Cr5.1Al6.5s alloys or other bimodal Ti-based alloys by rapid solidification. This micro structure evolution is similar to the four stages as-mentioned before but in the final stage α’martensite precipitates from micron-grained β-Ti matrix during the rapid cooling process. Interestingly, the multi-modal structural Ti68.sNb13.6Co6Cu5.1Al6.5 alloy shows excellent properties of yield stress of 1611MPa, fracture stress of 3139MPa and fracture strain of 40.1%, little higher than those of bimodal Ti68.8Nb13.6Co6Cr5.1Al6.5 alloy. The ultrahigh strength and large plasticity was ascribed to improved capability of strain hardening by propagation and multiplication of dislocations in the ductile micron-sized β-Ti and the restricting of dislocations by CoTi2 phases and α’phases.Finally, to illuminate the influence of milling time on semi-solid sintered Ti-based alloys, conventional sintering and semi-solid sintering were utilized to sinter Ti68.8Nb13.6Co6Cu5.1Al6.5 alloy powders with different milling times. With increasing milling time, the content of amorphous phase increases in the as-milled powder. Meanwhile, the onsets of the semi-solid temperature intervals of the as-milled powders with different milling times are below 1250℃. The heterogeneous microstructure of conventionally sintered alloys from Oh-milled powder consists of coarse-grained β-Ti (Nb), AlNbTi2 and tiny CoTi2. All the conventionally sintered alloys from the as-milled powders are composed of ultrafine equiaxed β-Ti and ultrafine equiaxed CoTi2; grain sizes of phases decrease gradually to nano-size with increasing milling time. The yield stress and the work hardening capacity of the conventionally sintered alloys increase and decrease with the increasing milling times, respectively. Besides, semi-solid sintered alloy from the Oh-milled powder consists of coarse equiaxed β-Ti and tiny amounts of nano-sized lath-shaped CoTi2 and all of the semi-solid sintered alloys from the as-milled powders are composed of bimodal structures with micron-sized β-Ti matrix and ultrafine lath-shaped CoTi2 phase. Notablely, α’ appears in the semi-solid sintered alloy from the 90h-milled powder. With increasing milling time, the distribution of lath-shaped CoTi2 phases tends to be ordered and the skewed CoTi2 phase embeded into the β-Ti matrix disappears gradually. Moreover, the grain size of β-Ti decreases with increasing milling time. The yield stress of the semi-solid sintered alloys had no obvious change with the change in the milling times but the fracture stress and fracture strain of the semi-solid sintered alloys escalate with the increasing milling time. The mechanical properties of all semi-solid sintered Ti68.8Nb136Co6Cu5.1Al6.5 alloy are superior to those of the conventionally sintered alloys for the same allo powders with the same milling time.