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Two samples of an α + β Ti-4Al-4Mo-4Sn-0.5Si (wt%) alloy, one with fine prior β grains and a heterogeneous microstructure and the other with coarse prior β grains and a homogeneous microstructure were prepared by extrusion of compacts of TiH 2 /Al/Mo/Sn/Si powder blend at 1200 and 1300 °C respectively. The heterogeneous microstructure is manifested by softer Mo-rich regions with a full α/β lamellar structure containing a high number density of parallel and nanometer sized FCC δ (TiH) needles in a harder matrix with a microstructure consisting of interweaved α plates and β transformed structure (β t) domains and grain boundary α (GB α) layers. The matrix also contains a small fraction of Sn-rich regions which have the same microstructure as the matrix. The Mo-rich and Sn-rich regions form due to the limited diffusion of Mo and Sn alloying elements. This heterogeneous microstructure sustains a high tensile yield strength of 1199 MPa, a high strain hardening rate, and an excellent tensile ductility of 12.0%. In contrast, the homogeneous microstructure comprising of interweaved α plates and β t domains together with GB α layers sustains a similar high strength, but a clearly lower strain hardening rate and a significantly lower tensile ductility of 3.5%. Non-homogeneous deformation capacity, the release of energy by forming numerous microcracks and the intermittent growth of microcracks of the heterogeneous microstructure suggest that this microstructure has a high plasticity and high resistance to the growth of microcracks. • The α + β titanium alloy fabricated has a heterogeneous microstructure and fine prior β grains. • The heterogeneous microstructure encompasses softer Mo-rich regions in a matrix of α/β t microstructure. • This alloy exhibits a superior combination of high strength (YS: 1199 MPa and excellent ductility (12.0%). • This is attributed to the compatible deformation and high microcrack growth resistance of the microstructure. [ABSTRACT FROM AUTHOR]

• With an addition of 0.2 wt%Si, the Si atoms are equally in α and β phases. • Si solute atoms retard the growth of α phase, causing refinement of the α/β t lamellar structure. • Si atoms enhance solid solution strengthening and Si atoms enhanced α/β t interface strengthening. • Increasing the Si content to 0.5 wt% cause a further increase of the yield strength by 50 MPa. • With a high strength, the tensile ductility deteriorates, due to the premature fracture of grain boundary α layers. Ti-4Al-4Mo-4Sn-xSi (wt%) (x = 0, 0.2, 0.5) alloys were fabricated by hot extrusion of compacts of blends of TiH 2 , AlMo60 master alloy and other required elemental powders followed by heat treatments involving vacuum annealing, solution treatment, air cooling and aging. The microstructure of the alloys consists of α plates, lamellae of α and transformed β structure (β t), β t blocks as well as grain boundary α layers. With the addition of 0.2 wt%Si, the Si atoms exist in equal concentration in both α and β t regions as solute atoms, and they retard the growth of α phase, causing a decrease of the average thickness of α plates and lamellae and slight increase of the volume fraction of β t lamellae and blocks. The solid solution strengthening and enhanced α/β interface hardening brought by the 0.2 wt%Si cause the yield strength of the Ti-4Al-4Mo-4Sn alloy to increase significantly from 1030 to 1130 MPa with a slight decrease of the tensile ductility (7.4% vs 6.8%) and without changing the fracture behavior. With the addition of 0.5 wt%Si, the extra Si causes nanometer sized silicide (Ti 5 Si 3) precipitates to form in both α and β t regions and further microstructural refinement. The additional α/β interface hardening and precipitation hardening cause the yield strength of the alloy to increase further to 1177 MPa, but the tensile ductility deteriorates clearly with the elongation to fracture decreasing to 4.9%. The main reason for this ductility decrease is the premature fracture of grain boundary α layers caused by the higher flow stress which can induce more strain localization at the grain boundary α layers. [ABSTRACT FROM AUTHOR]

A powder metallurgy (PM) near α Ti–6Al–2Sn–4Zr–2Mo−0.1Si (wt.%) alloy was fabricated by in-situ dehydrogenation and hot extrusion of a TiH 2 -based powder compact and heat treatments. Three types of microstructures were obtained in the as-consolidated state and after different heat treatments. They are a basketweave microstructure consisting of a network of inter-penetrating α plates filled with domains of ultrafine β transformed structure (β t) (Type I microstructure), an α/β lamellar microstructure with most β layers being thin and discontinuous plus α layers at prior β grain boundaries (Type II microstructure) and an α/β t lamellar microstructure (Type III microstructure). Although the three types of microstructures render the alloy with a similar yield strength (1085–1109 MPa) and ultimate tensile strength (1217–1239 MPa) due to the balanced effects of various strengthening mechanisms, the Type II microstructure exhibits a significantly lower tensile ductility than the other two types of microstructures (elongation to fracture: 5.5% vs. 13–14%). Examination of dislocations in the tensile deformed specimens reveals that dislocations cutting through the thin β layers in the Type II microstructure during deformation. Such interactions between moving dislocations and thin β layers in the Type II microstructure are expected to be the primary reason for the clearly lower strain hardening rate, premature fracture of grain boundary α layers and low tolerance to strain localization observed in the tensile tests, fracture surface examination and DIC analysis. This correlation between the microstructures and tensile deformation behavior strongly suggests that preventing moving dislocations cutting through thin β layers by turning them into thicker and ultrafine structure strengthened β t lamellae or blocks is critically important in ensuring high tensile ductility of high strength PM near α titanium alloys. • Three types of microstructures are obtained in a PM near α titanium alloy. • All three types of microstructures render the alloy with a similar high strength. • Type II microstructure gives a much lower tensile ductility than other two types of microstructures. • The lower ductility is attributed to dislocation cutting of thin β layers in Type II microstructure. • Preventing the dislocation cutting through is key for both high ductility and high strength. [ABSTRACT FROM AUTHOR]