Effect Of Cold-rolling Strain On The Structure,Mechanical Property And Corrosion Resistance Of Ta-4W Alloy

Tantalum alloys are typical refractory metals with a body centered cubic(BCC)crystal structure.Due to their excellent mechanical properties,corrosion resistance and good biocompatibility,tantalum alloys have been widely used in electronics,aerospace,high temperature applications,chemical engineering,nuclear engineering and biomedicine engineering.To produce high-quanlity components of tantalum alloys,it is important to study the microstructures,mechanical and corrosion properties of the alloys during their thermomechanical processing.In the past three decades,extensive studies on the characterization and quantification of deformation microstructures of face centered cubic(FCC)metals such as Al,Cu and Ni and BCC metals such as interstitial free(IF)steels have been done by using the transmission electron microscope(TEM),whereas investigations of refractory metals were limited.The study on the deformation mechanism and strengthening mechanism of BCC refractory metals is still lacking.For this reason,a refractory alloy Ta-4W was chosen in the present study to investigate its microstructural evolution during cold rolling from small to large strains.Multil-scaled characterization techniques such as X-ray diffraction(XRD),electron backscatter diffraction(EBSD)and TEM were applied to quantitatively characterize the microstructural parameters of the alloy during rolling,and a quantitative model between the microstructural parameters and the mechanical properties of the alloy was thus established.The effect of rolling reductions on the corrosion behavior of the alloy in sulphuric acid were also studied by using potentiodynamic polarization and electrochemical impedance spectroscopy.The following conclusions were thus drawn from the studies of the present study:(1)The microstructural evolution of Ta-4W during cold rolling from small to large von Mises strains(?_{v M}=0.12 to 2.7)was quantitatively studied using TEM.A link between the microstructural evolution and the measured high strength as well as the high strain-hardening rate in the alloy was observed.The microstructural evolution of Ta-4W during cold rolling was similar to that of FCC metals,i.e.grain subdivision by geometrically necessary boundaries(GNBs)and incidental dislocation boundaries(IDBs)on two size scales.Besides,Taylor lattice was also formed in the alloy.Volume elements defined by GNBs,which were nearly parallel to slip planes,were further divided by diffuse cell structures and by remnant Taylor lattices.With increasing strain,the diffuse cells evolved into clear IDBs enclosing cells,while the Taylor lattices disappeared.S-bands perturbed pre-existing microbands(MBs,also one form of GNBs)and transformed the MBs into lamellar boundaries(LBs)at shallow angles to the rolling direction.Grain subdivision was thus intermediate between those observed in cell forming alloys and in non-cell forming alloys.(2)With increasing strain,the average misorientation angles across GNBs and IDBs increased while the average boundary spacings decreased.All boundaries were generally of mixed character with different Burgers vectors and these boundaries belonged to the low energy dislocation structure(LEDS).In general,distributions of the microstructural parameters at each strain level were found to exhibit a universal scaling law.The only exception was that the GNBs misorientation angle distributions after 90%cold rolling(?_{v M}=2.7)formed a second peak at high misorientation angles,which consistented with the delayed textural development of the alloy until high strain rolling.(3)Based on the measured microstructural parameters,a model for the flow stress was derived by utilizing linearly additive contributions from solute strengthening,dislocation strengthening(Taylor)by IDBs and boundary strengthening(Hall-Petch equation)by GNBs,respectively.The model showed that the relative contributions of the strength mechanisms evolved with increasing strain and microstructural evolution:solutes and friction stress dominated at small strains while boundary strengthening dominated at larger strains.Such a behavior was distinctly different from that of FCC metals and IF steels,in which dislocation strengthening by IDBs dominated at all strains.Calculated flow stresses based on the microstructural model were in close agreement with the tension tests determined values.The stress strain curves of tensile tests also showed a continuous parabolic hardening of stage III across the entire strain range up to?_{v M}=2.7,without either saturation or transition to the low hardening rate of stage IV of work hardening.(4)A total of 68 grains were analyzed for the structural morphology and the crystallography of GNBs planes after cold rolling from low to moderate strains(?_{v M}?0.8).The morphology of grain subdivision could be classified into three types:diffuse cells/Taylor lattices,one set of GNBs and two sets of GNBs.The GNBs were parallel(within 10°)to the{110}or{112}slip planes.Schmid factor analysis demonstrated that the grain oirentation played a determinative role in the microstructural evolution where GNBs tended to form on the slip plane with the largest Schmid factors and the smallest trace angles to the transverse direction on the transverse section.(5)The corrosion current density and polarization resistance of the alloy cold rolled to different reductions in sulphic acid were measured.The results demonstrated that the cold rolling deformation has a two-side effect on the corrosion resistance of the alloy.Low to intermediate rolling deformation(?_{v M}?0.8)was detrimental to the corrosion resistance due to the increased dislocations density during the rolling process;while high strain rolling deformation(1.4??_{v M}?2.7)enhanced the corrosion resistance caused by the formation of strong textures(preferential crystallographic orientations).The 90%(?_{v M}=2.7)cold rolled sample showed a greatly enhanced corrosion resistance,even better than the initial annealed sample.