EVOLUTION OF THE MICROSTRUCTURAL PARAMETERS OF COLD WORK Ti-6- Al-4V ALLOY

The aim of this work is to investigate cold worked Ti-6Al-4V (α+β) alloy. The alloy was examined by X-ray diffraction using Rietveld refinement method. MAUD software (Materials Analysis Using Diffraction) was used to analyze the microstructural parameters evolution (crystallite size, root mean square strain (r.m.s) and dislocation density. The Crystallite size is smaller in the β-phase compared to the α-phase. Microstrain and dislocation density are higher in the α-phase than those found in the β-phasefor the as received material. The microstructural parameters of Ti-6Al-4V alloy exhibit typical values of cold deformation state. The results show that the deformation process reducesthe crystallite size (coherent diffraction domains) from 520 to 210 Ȧ in the α-phase. Consequently, the r.m.s increases from 5 E to 32 E and the dislocation density increases from 2.92 E to 4.6 Emafter 85 % thickness reduction. Keys words: Ti-6Al-4V alloy, cold working, crystallite size, dislocation density


Introduction
Ti-6Al-4V is one of the most used α+β titanium alloys, which is employed in many industries (aeronautic, aerospace, blades, discs, rings, airframes, fasteners, components, vessels, cases, hubs, forgings, biomedical implants), due to its attractive mechanical properties [1][2][3][4][5].The intensive utilization of this alloy requires the control of microstructural parameters during cold working.Nowadays, titanium alloyhave matured rapidly and it is moreinterestingthan other structural materials.Cold rolling is generally used as the plastic deformation processing of these alloys.Crystallite size, r.m.s strain and dislocation density are very important factors controlling the microstructure evolution during plastic deformation.Different X-ray Diffraction Line Profile Analysis (XRDLPA) techniques have been widely applied successfully for the evaluation of microstructural parameters in deformed metals and alloy systems [6][7][8][9][10][11][12][13][14].Recently, it was established that XRDLPA is a very useful technique to characterize deformed microstructures [15,16].In this work, the microstructural parameters evolution after cold rolling of Ti-6Al-4V was studied.Two cold plastic deformation levels, 50% and 85% (thickness reduction) were investigated.To achieve this work, XRDLPA was carried out using JAVA-based Materials Analysis Using Diffraction (MAUD) software [17−18].DOI

Results
Fig. 1 presents the XRD patterns obtained from the as received Ti-6Al-4V alloy with the presence of the characteristic diffraction peaks associated to the α-HCP and β-BCC phases.
Fig. 1 XRD obtained from Rietveld profile fitting of the as received Ti-6Al-4Valloy Fig. 2 and Fig. 3 show the XRD patterns of the cold rolled samples,the presence of the peaks associated to α-phase and the absence of all the peaks associated to the β-phase after cold rolling are clearly noticed.This can be explained by the low volume fraction of the β phase and the reduction in the sizeof coherent domains of diffraction.In addition, the crystalline anisotropy of the studied alloy contributes in the absence of peaks associated to the β-phase.The XRD patterns of all the specimens were subjected to detailed microstructural analysis using MAUD software as shown in Table 2.It is obvious that, β-phase at the as received state exhibit smallerdomain size than the α-phase.These observations are reminiscent to the low volume fraction of the β-phase.Moreover, the domain size for the α-phase are smaller than that of the corresponding α-phase in the case of the two deformed states (50 and 85 % thickness reduction) and decrease from 520 to 210 Ȧ after 85 % of thickness reduction.This is accompanied by an increase in the microstrain and dislocations density values with increasing the deformation amount.

Discussion
The GOF (Goodness Of Fit) value, which is an indicator of the fit quality strongly depends on all the values of the parameters obtained by the Rietveld analysis.This value oscillates, for the studied alloy between 0.99 and 0.85 with a mean value of 0.95, which is very close to 1.3 indicating the high quality of the fit according to the literature.The values of Rwp (%) and Rexp (%), which are indicators of the precision of the refinement, are very good (less than 10%).The lattice parameters of the deformed alloy appear to be stable.This stability of the lattice parameters is explained by the absence of any physical change (for instance an increase in the concentration of one of the solute atoms (Al or V)) that could modify the lattice parameters.Indeed, thermomechanical treatments (rolling and aging) have no effect on the lattice parameters, whose variation is only sensitive to the effects of electronic linkages of the chemical species.It is noted that thermal agitation also affects the lattice parameters.After deformation, the disappearance of the picks associated to the B-cc structureis observed.This can be explained by the small volume fraction of the  phase with strong deformation at the local scale.Consequently, small values of coherent domain size were obtained after deformation and cold working.Subsequently, only the evolution of the microstructural parameters of α phase was examinated (an overall decrease in the sizes of the domains as a function of the deformation rate is noticed).It is known that the higher the deformation, the lower the coherent diffraction domains size, consequently the micro distortions in the material increases as a function of the rolling rate.The immediate fact to note is the increase in the density of dislocations as a function of the rolling rate, which is explained by a mechanism of hardening by gathering the dislocations in walls and forests.Fig. 4 shows effect of the deformation on the crystallite sizes and the dislocations density; the higher the deformation, the lower the crystallite size and the higher the dislocations density.The same tendency was also observed in the literature [19][20].It was reported [21] that, some experimental observations from the X-ray analysis regarding the dislocation substructure evolution is explained by considering the basic deformation mechanism of the two-phase materials.During deformation, most of the dislocation assisted slip activities will occur in the α-phase.Thus, when the deformation rate increases, the activity of slip systems increases, and, consequently, dislocation density increases.An important finding of X-ray analysis is that in the deformed Ti-6Al-4V alloy, there is an increase of the dislocations density in the αphase.The plastic deformation could introduce much higher dislocations into the material and the substitution elements in the alloy seems to lead to drastic changes on post deformation [22].In this work, the dislocations density increases from 2.92 E +10 to 4.6 E +11 when the plastic deformation was more important.

DOI 10 .Fig. 4
Fig. 4 Evolution of the crystallite size and dislocation densityin the deformed samples

Table 1
Chemical composition of the as received alloy