Research Of Twinning Mechanism For Polycrystal Pure Titanium Deformed At Room Temperature At Various Strain Rates

Rolled high purity titanium (99.995%) plate with a classic rolling texture wasemployed in the present thesis. Cylindrical samples were cut along the normal direction(ND) compressed along the ND. Uniaxial compression tests were carried out at roomtemperature at various strain rates from10-3s-1to102s-1. X-ray diffraction and EBSDtechniques were employed to determine the initial and deformed textures as well as tofollow the microstructure evolution. In addition, the twinning variants duringdeformation were detected and characterized quantitatively. Three generation of twinswere detected in the present study: primary {1122}contraction (CT) and {1012}extension (ET) twins, secondary {1012}and {1122}twins and tertiary {1122}twins.The twinning shear expressed in the twinning reference frame was rotated into theneighbour crystallographic reference frame. Finally, the accommodation strainsrequired in the neighbour reference frame were used to analyze the process of variantselection during primary, secondary and tertiary twinning, in association with theSchmid law. The flow stress curves were fitted with a9th. order polynomial, some casesthe order was higher. In addition, the second derivative method was also employed todetermine the critical stresses and strains for twinning and dynamic recrystallization(DRX).The main conclusions are summarized below:1. There are three clear stages in the plot of strain hardening rate () as a functionof true stress (). This is made evident by smoothing the flow curves usingpolynomials of order9. Some cases require increasing the order to10.2. There is a point of inflection in the slope of the flow curve, which correspondsto a minimum in the curve of the negative of the second derivative of the flow stress.Large amounts of twins were detected beyond this point, which was identified as thefirst (twinning) critical strain. A second critical point was identified as the strain atwhich the negative of the second derivative of the flow stress attains a second minimum.Fine, new, equiaxed grains were detected beyond the second critical strain by means ofmicrostructure analysis. The second critical point was associated with the initiation ofdynamic recrystallization. The critical strains were determined to be-0.24and-0.67,respectively, for samples deformed at a strain rate of0.01s-1.3. There are two main nucleation sites for DRX: in the vicinity of initial grain boundaries and at twin boundaries. The newly formed equiaxed grains were alignedalong these boundaries. High densities of dislocations were accumulated around theinitial grain and twin boundaries; these became more entangled as the strain wasincreased. These dislocations began to rearrange and cross-slip to form dislocation-freesub-boundaries. Migration and rotation took place to convert these boundaries into highangle boundaries, which were then the boundaries of the new DRX grains.4. Both twinning and DRX rotated and reoriented the matrix crystal. The textureproduced by twinning deviated from ND towards TD. DRX grains can also nucleatedinside of the twins. On the other hand, the DRX grains formed along the initial grainboundaries were randomly oriented. Both kinds of DRX grains had the effect ofrandomizing and weakenning the initial texture.5. The three generations of twins were detected and characterized quantitativelyby using stereographic projection: CT and ET primary twins, ET and CT secondarytwins and CT tertiary twins. Some low Schmid factor twins were observed to formduring deformation.6. The secondary twins were classified into groups A, B and C: A-41.3°1543,B-48.4°5503, C-87.9°4730. The frequency of B detected in the current studyis higher than the other two.7. In the case of the primary twins, variants requiring large amounts of exyand eyxglide were detected even when they were associated with low SFs, as they required littleexpenditure of work. Conversely, the tensor components exzand eyzand ezxand ezywereconsidered to involve deformation modes that were relatively difficult to activate, asthey involved the activation of basal slip (for the former pair) and twinning orpyramidal glide (for the latter pair).8. In the case of secondary twins, high SF secondary twins that requireaccommodation by pyramidal glide cannot form. Conversely, the observed high SFsecondary twins were the ones that can be accommodated either by prismatic or bybasal glide. Low SF secondary twins can only form when their accommodation requiresprismatic or basal glide. Accommodation by pyramidal glide was never observed.9. With respect to tertiary twinning, all the tertiary twins detected in the presentstudy were associated with high SFs. The missing high SFs variants would haverequired the activation of pyramidal glide, the most difficult deformation mode, whereasthe observed tertiary twins were accommodated either by prismatic or by basal glide.10. Secondary twins with high SFs grow to larger sizes in their host primary twins than low SF twins within the same host, which means that the growth of secondarytwins follow Schmid law.11. The critical stress for DRX increases with strain rate. The critical strain fortwinning is relatively insensitive to strain rate as this mechanism is athermally activated.Conversely, as DRX is thermally activated, the critical strain for DRX decreases as thestrain rate is decreased or the temperature is increased. The density of dislocationincrease with strain rate; this provides the driving force for DRX. The strain ratesensitivity for high purity titanium deformed at room temperature was calculated and isexpressed as m=0.02.12. The CRSS’s of the various slip systems differ for Ti and Mg. The “easiest” slipfor Ti is the prismatic with a CRSSPr=181MPa, and the most “difficult” slip system isthe pyramidal with a CRSSPyr=494MPa. Conversely, the most “easy” slip for Mg is onthe basal plane, CRSSBas=0.7MPa. The most “difficult” slip is on the prismatic plane,CRSSPr=40MPa. The esiest-to-most difficult CRSS ratios for the two metals are1:2.7(Ti) and1:57(Mg). These ratios differ by a factor of about21times. Therefore, theprocess of twin variant selection differs considerably in the two metals. For Mg, variantformation requires basal slip in the neighbour while the variant requiring prismatic slipwill be absent. By comparison, twin variant selection is more complicated in Ti than inMg because of the large differences in the CRSS’s: i) primary twins with low SF requireprismatic glide, while variants with high SF’s that require basal or pyramidal glide willbe impeded. ii) secondary and tertiary twin variants with low SFs can form if theyrequire prismatic or basal glide in the neighbour; conversely, high SF variants requiringpyramidal glide will be absent.