Issue 35

A. Nikitin et alii, Frattura ed Integrità Strutturale, 35 (2016) 213-222; DOI: 10.3221/IGF-ESIS.35.25 215 Fe C Si Cr Mo N Al Zr O H Titanium 0.2 - 07 < 0.1 0.15 – 0.4 0.8 - 2 2 - 3 < 0.05 5.5 - 7 < 0.5 < 0.15 < 0.015 Balance Table 1 : Chemical composition of VT3-1 titanium alloy, weight %. The second set of specimens was machined from virgin extruded bars, produced by All Russian Institution of Light Alloys with respect to state standards [13]. Heat treatment was performed to get a fully lamellar micro-structure and mechanical properties close to forged VT3-1. Mechanical properties of both materials were obtained on tensile specimens of 75 mm in length under displacement rate of 0.075 mm/min. The result of tensile test and obtained mechanical properties are listed in Tab 2. Process Young's modulus, GPa UTS, MPa Yield Stress, MPa Deformation at rupture, % Mass density Dynamic Young's modulus (GPa) Forged 114 989 960 6 4500 116 Extruded 106 1107 1050 13 4500 110 Table 2 : Mechanical properties of forged and extruded VT3-1 titanium alloys The extruded titanium alloy has mechanical properties a little bit higher than the forged one, except its elongation at rupture that is significantly different. It is more than two times higher for extruded titanium that is, therefore, more ductile. Furthermore their micro-hardness is approximately the same: 364 HV 500 for forged and 373 VH 500 for extruded VT3-1. Measurement was realized in the plane of maximum shear stress of specimens along a line through a cross-section of the specimen. Results shows a pronounced scatter for the forged titanium alloy (the difference of 44 HV 500 was found for points spaced by 1.2 mm), while for extruded VT3-1 it is less than 5 HV 500 for the same distance. Such difference may be explained by features of micro-structures for these alloys. The 3D mapping of the micro-structure is shown on Fig. 2 for forged and extruded alloys. (a) (b) Figure 2 : The 3D mapping of micro-structure for (a) forged and (b) extruded VT3-1. Both alloys have fully lamellar micro-structure with elongated alpha-platelets. In the case of forged VT3-1 platelet size is larger (about 2 µm in width and 10-15 µm in length), while w = 1 µm by l = 2-3 µm for the extruded one. Moreover, micro-structure of extruded titanium alloy is more homogeneous (except some zones in the core of the bar), while the micro-structure of forged titanium is processed in macroscopically large zones with similar platelets morphology. These zones called 'macro-zones' [14] and characterized by similar crystallographic orientations of the platelets within each macro-zone. The existence of such specific of micro-structure explains the large scatter obtained in micro-hardness values for forged VT3-1.