The Effect of Hot Deformation Parameters on Microstructure Evolution of the α-Phase in Ti-6Al-4V (original) (raw)
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Modification of alpha morphology in Ti-6Al-4V by thermomechanical processing
Metallurgical Transactions A, 1986
The modification of lamellar alpha phase in Ti-6AI-4V by hot working was investigated with the aim of controlling morphology (aspect ratio) and final grain size. The effect of strain was studied using forging at 955 ~ (1750 ~ followed by annealing at 925 ~ (1700 ~ to allow the alpha morphology to adjust. Increasing the deformation from 6.5 pct to 80 pct reduction caused the lamellar alpha morphology to become progressively more equiaxed upon annealing. TEM observations showed that annealing of material deformed to 6.5 pct resulted in recovery of the alpha, without a noticeable change in the morphology, while higher deformation resulted in plate sheafing and beta cusp formation. It was found that material with an initial thin alpha plate structure (thickness -~ 3.4/xm) breaks up at a lower critical strain than a material with a thicker plate morphology (thickness -6/xm). The material with thin alpha plates more rapidly forms equiaxed alpha grains separated by beta phase, while the material with a thicker plate structure exhibits more alpha/alpha boundaries after deformation and annealing. The morphology change from alpha lamellae into lower aspect ratio grains was identified to be by a break-up of the alpha lamellae, essentially by a two-step process: a formation of low and high angle alpha/alpha boundaries or shear bands across the alpha plates followed by penetration of beta phase to complete the separation. This break-up takes place during hot deformation and subsequent annealing.
The origins of heterogeneous deformation during primary hot working of Ti–6Al–4V
International Journal of Plasticity, 2002
A Ti-6Al-4V cylindrical specimen with a large grain colony microstructure was upset forged to 35% reduction at 815 C at a nominal strain rate of 0.1 s À1 . An orientation imaging microscopy (OIM) analysis was conducted in two representative areas, near the center with an estimated von Mises strain of 1.6, and 0.8 about midway from the center to the outer edge. The process of physically breaking up the lamellar microstructure (globularization) was examined, focusing on how the globularization efficiency was affected by the initial colony orientation. Microstructural maps based upon the electron backscattered pattern quality, crystal orientation, and an estimated Taylor factor (using a continuum assumption) were used to identify and quantify heterogeneous deformation phenomena. These analyses show that in regions where both prism and basal slip systems were not operational, macro shear bands developed, leading to kinked lamellar microstructural features. The shear bands concentrated shear in localized regions that were able to flow easily around remaining hard regions, leaving remnants of the hard regions intact. Also, development of large misorientations of 50-90 from the parent grain arising from a transformation from b to a are quantified and related to the globularization efficiency. #
Acta Materialia, 2003
The high-temperature deformation mechanisms of Ti-6Al-4V with a transformed microstructure were determined within the framework of inelastic-deformation theory. For this purpose, load-relaxation tests were conducted on samples with a lamellar structure containing different alpha-platelet thicknesses at temperatures of 715-900°C. The flow stressversus-strain rate curves for all the microstructures were well fit with an inelastic-deformation equation describing grain-matrix deformation (GMD) (dislocation glide + dislocation climb). However, for heavily pre-deformed specimens, grain-boundary sliding (GBS) as well as GMD was evident. The GBS rate was found to be most rapid for the microstructure with the thinnest alpha laths/platelets. Softening of heavily deformed material was attributed to a decrease in the internal-strength variable s * associated with reduced alpha-beta interface strength and to the occurrence of GBS.
Modification of Alpha Morphology in Ti-6AI-4V by Thermomechanical Processing
The modification of lamellar alpha phase in Ti-6AI-4V by hot working was investigated with the aim of controlling morphology (aspect ratio) and final grain size. The effect of strain was studied using forging at 955 ~ (1750 ~ followed by annealing at 925 ~ (1700 ~ to allow the alpha morphology to adjust. Increasing the deformation from 6.5 pct to 80 pct reduction caused the lamellar alpha morphology to become progressively more equiaxed upon annealing. TEM observations showed that annealing of material deformed to 6.5 pct resulted in recovery of the alpha, without a noticeable change in the morphology, while higher deformation resulted in plate sheafing and beta cusp formation. It was found that material with an initial thin alpha plate structure (thickness -~ 3.4/xm) breaks up at a lower critical strain than a material with a thicker plate morphology (thickness -6/xm). The material with thin alpha plates more rapidly forms equiaxed alpha grains separated by beta phase, while the material with a thicker plate structure exhibits more alpha/alpha boundaries after deformation and annealing. The morphology change from alpha lamellae into lower aspect ratio grains was identified to be by a break-up of the alpha lamellae, essentially by a two-step process: a formation of low and high angle alpha/alpha boundaries or shear bands across the alpha plates followed by penetration of beta phase to complete the separation. This break-up takes place during hot deformation and subsequent annealing.
Strain-path effects during hot working of Ti-6Al-4V with a colony-alpha microstructure
Metallurgical and Materials Transactions A, 2001
The that the successful application of the method requires an scanning electron microscopy was performed in the Brown increment of upsetting deformation prior to the shear that University Electron Microscopy Laboratory, which is supis imposed in the ECAE deformation zone. The efficacy ported by the National Science Foundation MRSEC. The of this approach was rationalized on the basis of the depenauthors thank Dr. Jenny Been, TIMET, for providing the dence of flow-softening rate on strain. At low strains ( Ͻ material and for many helpful discussions. 0.5), the flow-softening rate is high for Ti-6Al-4V (and similar alloys) with a colony microstructure, and the tendency for flow localization in shear is very high. At larger REFERENCES strains, at which flow softening is much lower, flow local-1. J. Been, D. Davis, and D. Aylor: Corrosion 99, Orlando, FL, 2000, ization is less likely. Hence, the use of an initial increment paper no. 641.
On the hot deformation behavior of Ti-6Al-4V made by additive manufacturing
Journal of Materials Processing Technology, 2021
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