Aiming at overcoming the strength-ductility trade-off in structural Ti-alloys, a new family of TRIP/TWIP Ti-alloys was developed in the past decade (TWIP: twinning-induced plasticity; TRIP: transformation-induced plasticity). Herein, we study the tunable nature of deformation mechanisms with various TWIP and TRIP contributions by fine adjustment of the Zr content on ternary Ti-12Mo-xZr (x = 3, 6, 10) alloys. The microstructure and deformation mechanisms of the Ti-Mo-Zr alloys are explored by using in-situ electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The results show that a transition of the dominant deformation mode occurred, going from TRIP to TWIP major mechanism with increasing Zr content. In the Ti-12Mo-3Zr alloy, the stress-induced martensitic transformation (SIM) is the major deformation mode which accommodates the plastic flow. Regarding the Ti-12Mo-6Zr alloy, the combined deformation twinning (DT) and SIM modes both contribute to the overall plasticity with enhanced strain-hardening rate and subsequent large uniform ductility. Further increase of the Zr content in Ti-12Mo-10Zr alloy leads to an improved yield stress involving single DT mode as a dominant deformation mechanism throughout the plastic regime. In the present work, a set of comprehensive in-situ and ex-situ microstructural investigations clarify the evolution of deformation microstructures during tensile loading and unloading processes.

The prominent comprehensive properties of solid-solution- and intermetallic-based Ti alloys are derived from their diverse microstructures induced by multi-component alloying, which results in a chemical composition complexity. A cluster-plus-glue-atom model, characterizing the chemical short-range orders, was introduced to explore the relationships among the local atomic distributions of alloying elements in different phase structures of Ti alloys, including α-Ti, β-Ti, ω-Ti, α-TiAl, γ-TiAl, O-TiAlNb, and B2-Ti(Al,Nb). Specific cluster structural units, i.e., cluster formulas, for these phases were determined with the guide of the Friedel oscillation theory for electron-structure stabilization. It is due to the change of cluster structural units that induces the phase transformation, which is attributed to the amounts of primary alloying elements of Al and Nb. The total atom number (Z) values in these cluster structural units, calculated by the Fermi vector k, are all very close to the integer of Z = 16. Furthermore, the composition rules of industrial multi-component Ti alloys based on these phases were generalized in light of the cluster formula approach, which will open up a new route towards designing high-performance Ti alloys with complex compositions.

Especially in the past decades, Ti-6Al-4V alloy has received much attention, not only due to its high melting temperature, good corrosion resistance, low density and high hardness, but also because of the diverse and complicated microstructures formed under different conditions. This makes Ti-6Al-4V a potential candidate in both aerospace industries and fundamental research. It is well known that the solidified microstructures of alloy have a great influence on their mechanical properties. Therefore, it is crucial to investigate the mechanical properties of Ti-6Al-4V solidified under different conditions, in particular in the undercooling conditions. However, it is noted that most research on the solidification of Ti-6Al-4V alloy was carried out under equilibrium condition. With respect to Ti-6Al-4V alloy solidified under substantial undercooling conditions, few studies could be found. Thus, it is interesting to study two points: (1) the feature of the microstructure of Ti-6Al-4V alloy solidified under highly undercooled conditions and large cooling rate, (2) the influence of undercooling and cooling rate on the mechanical property of Ti-6Al-4V alloy. To address these two problems, Ti-6Al-4V alloy was rapidly solidified in a drop tube. The main results are summarized as follows. The microstructure of the Ti-6Al-4V alloy solidified under free fall condition displays "lamellar α+β→α dendrites→basket-weave α'+β→ needle-like α'→ needle-like α'+ anomalous β " transformation with decreasing the droplets diameter. And the needle-like α' phase in the original boundaries of equiaxed β grains is transformed into a continuous distribution and anomalous structure of β phase when the droplet size is less than about 400 μm. The microhardness of this alloy ranges from 506 kg/mm2 to 785 kg/mm2 when the droplet diameter decreases from 1420 μm to 88 μm, which is much higher than that of the master alloy. For "lamellar structure of α+β phases", "needle-like α' phase" and "needle-like α' phase+ anomalous β phase", the microhardness increases with the decrease of droplet diameter. But for 'basket-weave' microstructure, the microhardness diminishes with the decrease of droplet diameter.

The so-called bimodal microstructure of Ti-6Al-4V alloy, composed of primary α grains (αp) and transformed β areas (βtrans), can be regarded as a “dual-phase” structure to some extent, the mechanical properties of which are closely related to the sizes, volume fractions, distributions as well as nano-hardness of the two constituents. In this study, the volume fractions of primary α grains (vol.%(αp)) were systematically modified in three series of bimodal microstructures with fixed primary α grain sizes (0.8 μm, 2.4 μm and 5.0 μm), by changing the intercritical annealing temperature (Tint). By evaluating the tensile properties at room temperature, it was found that with increasing Tint (decreasing vol.%(αp)), the yield strength of bimodal microstructures monotonically increased, while the uniform elongation firstly increased with Tint until 910 °C and then drastically decreased afterwards, thereby dividing the Tint into two regions, namely region I (830-910 °C) and region II (910-970 °C). The detailed deformation behaviors within the two regions were studied and compared, from the perspectives of strain distribution analysis, slip system analysis as well as dislocation analysis. For bimodal microstructures in region I, due to the much lower nano-hardness of βtrans than αp, there was a clear strain partitioning between the two constituents as well as a strain gradient from the αp/βtrans interface to the grain interior of αp. This activated a large number of geometrically necessary dislocations (GNDs) near the interface, mostly with <c+a> components, which contributed greatly to the extraordinary work-hardening abilities of bimodal microstructures in region I. With increasing Tint, the αp/βtrans interface length density gradually increased and so was the density of GNDs with <c+a> components, which explained the continuous increase of uniform elongation with Tint in this region. For bimodal microstructures in region II, where the nano-hardness of βtrans and αp were comparable, neither a clear strain-partitioning tendency nor a strain gradient across the αp/βtrans interface was observed. Consequently, only statistically stored dislocations (SSDs) with <a> component were activated inside αp. The absence of <c+a> dislocations together with a decreased volume fraction of αp resulted into a dramatic loss of uniform elongation for bimodal microstructures in region II.

为了提高Ti-6Al-4V合金的加工硬化率和塑性，基于其团簇成分式12[Al-Ti₁₂](AlTi₂)+5[Al-Ti₁₄](V₂Ti)设计成分式为4[Al-Ti₁₂](AlTi₂)+12[Al-Ti₁₄](V₂Ti)的(Ti-4.13Al-9.36V, %)合金，采用激光立体成形工艺制备Ti-4.13Al-9.36V和Ti-6.05Al-3.94V(对比合金)，研究了沉积态和固溶温度对其显微组织和力学性能的影响。结果表明，沉积态Ti-4.13Al-9.36V和Ti-6.05Al-3.94V合金的显微组织均由基体外延生长的初生β柱状晶和晶内细小的网篮α板条组成。Ti-6.05Al-3.94V合金的初生β柱状晶的宽度约为770 μm，α板条的宽度约为0.71 μm；而Ti-4.13Al-9.36V合金的初生β柱状晶的宽度显著减小到606 μm，α板条的宽度约为0.48 μm。经920℃固溶-淬火处理后Ti-6.05Al-3.94V样品的显微组织为α'+α相，其室温拉伸屈服强度约为893 MPa，抗拉强度约为1071 MPa，延伸率约为3%。经750℃固溶-淬火处理后Ti-4.13Al-9.36V样品的显微组织为α''+α相，与α'马氏体相比，应力诱发的α''马氏体能显著地提高合金的加工硬化能力，其室温拉伸屈服强度约为383 MPa，抗拉强度约为 989 MPa，延伸率达到了17%。这表明，根据团簇理论模型调控α''+α的显微组织能有效提高激光立体成形Ti合金的加工硬化能力和塑性。

在Ti-6Al-4V合金锻造成形双态组织基材上采用激光立体成形方法(送粉式激光增材制造)沉积块体试样，研究了不同线能量密度输入下基材与增材结合区的微观组织特征及形成机制。结果表明，结合区内不同高度部位由于热源影响程度存在差异，形成了从下到上的非均匀组织。其中下部区域由于峰值温度较低，仍保持初始双态组织形貌，但发生一定粗化；中部区域随着温度升高以及保温时间延长，形成等轴α相、层片α相及大量次生α相的混合组织；而上方靠近增材区的峰值温度超过β相转变温度，完全转变为由层片α相形成的魏氏体组织，并伴随着由于元素扩散不充分而形成的阴影结构。对包含基材区和增材区的结合试样进行拉伸测试发现，在设定的能量密度范围内，断裂位置均远离结合区，表明增材区与基材区结合良好，结合区强度超过基材区及增材区强度。此外，对比不同能量密度复合制造Ti-6Al-4V试样的拉伸测试结果发现，线能量密度为100 J/mm时，结合区以及增材区α相特征尺寸较小，复合制造试样的屈服强度和抗拉强度最大。随线能量密度的增大，复合制造试样屈服强度和抗拉强度均减小，而延伸率增加。