Titanium with gradient nano-to-micron scale grains from surface to matrix was fabricated by surface mechanical grinding treatment(SMGT) at room temperature.The SMGT-treated titanium shows higher strength than that of as-received one,but moderate ductility between those of ultra-fine grained(UFG) and coarse-grained titanium.Tensile stress-strain curves of SMGT-treated titanium show double strain hardening regimes.The strain hardening rate(dσ/dε) decreases with increasing strain in tensile deformation.The high strain hardening rate at initial yielding is attributed to nano-to-micron-grained surface layer.The low strain hardening rate at large plastic strain regime primarily results from coarse-grained matrix.The SMGT-treated titanium shows a ductile fracture mode with a large number of dimples.The small size of dimples in the treated surface layer is due to the combination of the high strength and strain hardening exponent.The difference between dimple size in nano-to-micron-grained surface layer and coarse-grained matrix is discussed in terms of plastic zone size at the tip of crack in the SMGT-treated titanium.
Materials that undergo a reversible change of crystal structure through martensitic transformation （MT） possess unusual functionalities including shape memory, superelasticity, and low/negative thermal ex- pansion. These properties have many advanced applications, such as actuators, sensors, and energy conversion, but are limited typically in a narrow temperature range of tens of Kelvin. Here we report that, by creating a nano-scale concentration modulation via phase separation, the MT can be rendered continuous by an in-situ elastic confinement mechanism. Through a model titanium alloy, we demon- strate that the elastically confined continuous MT has unprecedented properties, such as superelasticity from below 4.2 K to 500 K, fully tunable and stable thermal expansion, from positive, through zero, to negative, from below 4.2 K to 573 K, and high strength-to-modulus ratio across a wide temperature range. The elastic tuning on the MT, together with a significant extension of the crystal stability limit, provides new opportunities to explore advanced materials.
Ferroic glasses(strain glass,relaxor and cluster spin glass)refer to frozen disordered states in ferroic systems;they are conjugate states to the long-range ordered ferroic states—the ferroic crystals.Ferroic glasses exhibit unusual properties that are absent in ferroic crystals,such as slim hysteresis and gradual property changes over a wide temperature range.In addition to ferroic glasses and ferroic crystals,a third ferroic state,a glass-ferroic(i.e.,a composite of ferroic glass and ferroic crystal),can be produced by the crystallization transition of ferroic glasses.It can have a superior property not possessed by its two components.These three classes of ferroic materials(ferroic crystal,ferroic glass and glass-ferroic)correspond to three transitions(ferroic phase transition,ferroic glass transition and crystallization transition of ferroic glass,respectively),as demonstrated in a generic temperature vs.defectconcentration phase diagram.Moreover,through constructing a phase field model,the microstructure evolution of three transitions and the phase diagram can be reproduced,which reveals the important role of point defects in the formation of ferroic glass and glass-ferroic.The phase diagram can be used to design various ferroic glasses and glass-ferroics that may exhibit unusual properties.
Ti5553-xFe (x=0.4, 1.2, 2.0, wt.%) alloys have been designed and fabricated through BE (blended element) sintering to investigate the effect of Fe-addition on athermal ω-phase transformation, α-phase evolution and age hardening behavior. The results show that the formation of athermal ω-phase is fully suppressed in water-quenched specimens when Fe-addition is up to 2 wt.%. The relevant timescales of α formation during initial stages of aging indicate that incubation time increases with Fe-addition. Further aging results in continuous nucleation and growth of α-phase but finer intragranular α lamellae exhibit in Ti5553-2Fe alloy. In addition, the width and extent of grain boundary α-film increase slightly with incremental Fe-addition, especially in furnace cooling condition. Result of Vickers hardness manifests that Fe-addition leads to a strong hardening effect in both solution and aging treatment. The solid solution strengthening is quantitatively estimated by ab initio calculation based on the Labusch?Nabarro model. The evolution of α-precipitate is rationalized by Gibbs free energy. The prominent hardening effect of Ti5553?2Fe alloy is attributed to both large lattice misfit of β-matrix and fine α-precipitate distribution.
Wen-guang ZHUPei LIXun SUNWei CHENHua-lei ZHANGQiao-yan SUNBin LIULin XIAOJun SUN
