| Due to its small density,excellent specific strength and modulus,high ductility and fracture toughness,good corrosion resistance and biocompatibility,Ti-6Al-4V alloy becomes a workhorse used in various fields such as military,aerospace,automotive,and biomedical industries.However,owing to the high production costs of Ti-6Al-4V components and structural members,this alloy is unaffordable for numerous cost-sensitive applications.To solve these problems,this study directly fabricates fully dense Ti-6Al-4V rods by powder compact extrusion of cost-effective Ti H2/Al60V40 powder blend.Futhermore,this study also fabricates harmonically structured Ti-6Al-4V alloy with enhanced strain hardening capacity through powder milling and hydrogen-enabled eutectoid transformation.This study systematically elucidates the correlations of processing parameters/structure/mechanical properties of Ti-6Al-4V,and supplies advices for producing titanium alloys with optimized strength and toughness.Firstly,fully lamellar Ti-6Al-4V rods are fabricated through hot extrusion of coarse-grained Ti H2/Al60V40 powder.Dehydrogenation of Ti H2 is nearly completed during induction heating,alloying and fully consolidation of powder compact is achieved during extrusion,which indicates the feasibility of this procedure.An extrusion temperature of1200℃is sufficient to dissolve V-rich particles,and with increasing the extrusion temperature from 1100℃to 1400℃,priorβgrain sizes increase dramatically from 54 to 175μm,while the change ofαlamella thickness is negligible.Analysis reveal thatα/βlamella interfaces are mainly responsible for grain strengthening.Priorβgrains whose sizes reach 150μm have a deleterious effect on the tensile ductility due to localized deformation,as evidenced by the intergranular fracture.Therefore,the rod extruded at 1200℃have the best elongation to fracture of 9.3%.A nanocrystalline Ti H2/Al60V40 composite powder is prepared by high enegy mechanical milling.By hot extrusion of compacts of the nanocrytalline Ti H2/Al60V40 composite powder,harmonically structured Ti-6Al-4V alloy samples with ultrathin（α-Ti+β-Ti+σ-Ti H）lamellar structured cores embedded in a skeleton ofα-Ti coarse-grained plates are fabricated.The formation of the harmonic structure can be attributed to the stabilization effect of the residual hydrogen onβ-Ti phase and the suppressed eutectoid transformation.In terms of the mechanical properties,the ultrahigh fracture stress（1555 MPa）of harmonically structured samples,which is the highest reported value so far,is ascribed to the interface strengthening of ultrathin lamellar structured cores（398 MPa）together with a high content of oxygen（280 MPa）.However,the stress concentration and plastic instability of ultrathin lamellar structured cores restrict the elongation to fracture to be 3.5%.Part of the as-extruded harmonic rod is vacuum annealed at 700℃.Results show that vacuum annealing below theβ-transus temperature does not change the prior-βgrain structures;instead,it causes a clear coarsening of the ultrathin-lamellar-structured cores.Benefiting from the optimized microstructural parameters,the 700°C annealed sample experiences improved plastic deformation capacity and relieved stress concentration,showing a better strength-ductility combination（a yield strength of 1218 MPa,a fracture strength of 1431 MPa,and a total elongation of 4.5%）than the as-extruded sample.Changing harmonic structure into a regular Widmanstatten structure causes not only a significant decrease of strength but also a clear drop in tensile ductility.It is found that the transgranular fracture mode,multiple crystallographic orientations ofαlamellae,and improved strain hardening capacity are ductility-improvement mechanisms for harmonic structure.Finally,the particle size and oxygen content of as-milled powder is controlled to reduce the oxygen content in the samples and modify hydrogen-enabled solid-state transformation.Through suppressing the stabilization effect of the residual hydrogen onβ-Ti phase,a dual harmonic structure,consisting of coarse-grainedα-Ti shells at priorβgrain boundaries and ultrafineα-Ti skeletons and nanoscaleα+β+δcores inside priorβgrains,is attained.This dual harmonic structure is beneficial in alleviating mechanical mismatch between hetero-domains and fully exploiting the hetero-deformation induced hardening,thereby leading to a high strain hardening rate and large uniform elongation.As a result,the material exhibits a better strength-ductility synergy（a yield strength of1095 MPa,an ultimate tensile strength of 1338 MPa and a uniform elongation of 7.7%）than the one-level harmonic structure.Our results demonstrate that creating heterogeneous structures at multiple length scales helps pursuing strength-ductility synergy of titanium alloys. |
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