The effects of Zr additions on 650℃ tensile deformation and fracture mechanisms were investigated in Ti-1100 alloy. During 650℃ tensile test for Ti-1100 alloy with 4wt.% Zr, the following results were obtained: 1) grain boundary sliding, long range motion of dislocations in α phase and α/β interface sliding are the main deformation modes; 2) with increase of test temperature, prism slip, basal slip and pyramidal slip were more easily to be observed as a result of decrease of critical shear stress; 3) distinct α/β interface sliding was found to be on {10-10} α // {1-12} β terraces, with sliding direction along [0002] α //[110]β crystal vector ; 4) sliding of α platelet in α colonies and GB α layer leads to creation of mocropores along grain boundary, while orientation relationship difference between α colony and GB α may lead to different extent of micropore propagation . The observation and analyses demonstrate that both lamella sliding and GB sliding control the processes of micropore initiation and intergranular fracture formation.
Difference of deformation and fracture mechanisms between 600℃and 650℃tensile tests in Ti-1100 alloy with normal Zr addition was also studied. It was found that Grain boundary sliding, dislocation long range motion in α phase and α/β interface sliding exist in both 600℃and 650℃tensile deformation in Ti-1100 alloy. As the equicohesive temperature of the Ti-1100 alloy is about 600℃, the fracture surface of 600℃tensile sample shows transgranular fracture mode, while the fracture surface of 650℃tensile sample shows intergranular fracture mode.
With the increase of Zr content, the dissolution temperature of S2 type silicide increase. If the alloy was heat treated at 1050℃, at this temperature the alloy was in β plus S2 two-phase region. In the subsequent cooling, the silicides were preserved. These residual silicides prevent migration of grain boundary , which result in smaller grain size than Ti-1100 with 4 wt. % Zr. In addition, the residual silicides improve the nohomogeneous nucleation relatively, and result in the transition of the microstructure from lamellar to a mixed one with colony α plus basket weave α morphology.
The changes of silicide precipitation process and microstructure lead to changes of deformation and fracture mechanisms at high temperatures:
Firstly, the residual silicides block long-range slip of dislocation. Dislocations are unable to round the silicide through Orowan mechanism. In order to continue the deformation process, cross slip of dislocation was necessary;
Secondly, decrease of the grain size and colony size shortens the α/β interface sliding distance significantly. Moreover, the basket weave α morphology can also restrict α/β interface slip;
Thirdly, with the increase of Zr content, the equicohesive temperatures of the alloys were increased. If the alloys were subjected to tensile tests at 650℃, plastic deformation of the alloys were dominated by deformation within grains because of the strengthened GB and the resultant decrease of deformation from GB sliding. Consequently the fracture mode also changed from intergranular fracture for 1# alloy with normal Zr addition to transgranular fracture for 2#, 3# and 4# alloy with increased Zr addition.
These above-mentioned changes of deformation mode and fracture mechanisms lead to increased high-temperature strength. It illustrates that high-temperature tensile properties of Ti-1100 alloy may be improved effectively by increase of Zr content.
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