L. C. Zhang

Spectroscopic properties and energy transfer in Yb3+/Er3+-doped phosphate glasses

Abstract The phosphate glasses with high Yb3+/Er3+ dopant were prepared. The absorption and fluorescence emission spectra were measured, and the emission cross-section for 4 I 13/2 → 4 I 15/2 transition of Er3+ was calculated on the basis of McCumber theory. The energy transfer (ET) and the effect of changing concentrations of Er3+ and Yb3+ ions on spectroscopic properties were investigated, pumping at 970 nm. The optimizations of Yb2O3 and Er2O3 concentration were found in our glasses. The influence of –OH groups on the ET and the emission at 1.54 μm were also discussed.

Deformation mechanism at large strains in a high-Nb-containing TiAl at room temperature

Abstract This paper investigates the deformation behavior and nano-scale structural characteristics after large deformation (29%) at room temperature of a Ti–45Al–9Nb–2.5Mn (at.%) alloy with duplex microstructure. Dislocation glide is the main deformation mode at low strains (such as 2%). The interaction of moving dislocations reduces the mobility of dislocations by pinning, and when deformation strain reachs 5%, both dislocation glide and mechanical twining are active. With further increase in strain, deformation twining becomes the dominant deformation mode, dividing the original γ grains and lamellar structure into nano-scale. The characteristic feature of the deformation twin in γ grains after large deformation at room temperature is the bent deformation twin (DT or DT′). There are many sub-structures in the DT, which makes the (111)T not exactly straight. High resolution TEM observations showed that (111)DL plane of the DT twins exhibits a large misorientation angle with the (111)M plane of the γ matrix. The orientation relationship between the matrix and the DT or DT′ is actually not the expected true twin relationship. The existence of numerous 1/3[111] Frank partial dislocations was regarded as accommodating the bending of the boundaries of DT. Local zigzag bending of the barrier twin boundaries can be observed due to twin intersections. Intersection induced lattice distortion in the intersection area for type I twin intersection is more severe than that for type II intersection. As results of the relaxation of the heavy lattice distortion, nano-scale subgrains boundaries may form together with a rectangular region near the intersection area. Nanotwins could be observed in the rectangular region for type I intersection. The nanotwins are formed with a homogeneous 1/6[11 2 ] twinning dislocation glide mechanism. For type II twin intersection, the intersection induces the formation of secondary twinning ST in the barrier twin. No region with nanotwins can be observed near the intersection area. The local stress concentration in type II twin intersection may be relaxed by the emission of dislocations from the intersection area.

Emission properties of highly doped Er3+ fluoroaluminate glass

Abstract Er 3+ -doped fluoroaluminate (AYF) glass was compared with fluorozirconate (ZBLAN) and tetraphosphate (PE) glass as a host material for 1.54-μm emission. Experimental results show that the Er 3+ :AYF glass has a smaller concentration quenching and much stronger intensity for the 1.54-μm emission. In high dopant, the 1.54-μm emission is two times stronger in Er 3+ /AYF glass than in ZBLAN glass, and 10 times stronger than in PE glass.

TEM study on lamellar microstructure and α2/γ interfacial structure in a hot-deformed two-phase γ-TiAl-based alloy

Abstract The hot deformation behavior of the lamellar microstructure in a Ti–45Al–8Nb–2.5Mn–0.05B alloy has been studied by transmission electron microscopy (TEM). Numerous subgrain boundaries and two types of deformation twins were frequently observed in the hot-deformation-induced lamellae. The occurrence of nonequilibrium semi-coherent α 2 / γ interfaces is an important microstructural feature of hot-deformed lamellar microstructure in a similar Ti–45Al–10Nb–2.5Mn–0.05B alloy. The distinctiveness of these α 2 / γ interfaces is the misorientation from the conventional α 2 / γ orientation relationship {111} γ ‖{0001} α 2 , 〈110〈 γ ‖〈1120〉 α 2 , as well as the asymmetry in the sense of interfacial boundary planes deviating from the equilibrium atomic planes (111) γ or (0001) α 2 . Numerous interfacial ledges containing 1/3[111] Frank partial dislocations exist in this nonequilibrium semi-coherent α 2 / γ interface. The analyses point out that, in the hot-deformation-induced bent lamellae, the 1/3[111] Frank partial dislocations in the ledges of α 2 / γ interfaces can be formed by reactions between the matrix and interfacial dislocations. This type of semi-coherent α 2 / γ interface is formed in order to accommodate the relative rotation of α 2 and γ plates resulting from their heterogeneous deformation behavior. In the sharply bent lamellae, there even occur largely misoriented, non-coherent α 2 / γ interfaces with the {111} γ plane no longer being parallel to the {0001} α 2 plane, resulting from the lamellae bending/kinking during heavy deformation. At these nonequilibrium, non-coherent α 2 / γ interfaces with high interfacial strain energy, the T(Q) deformation twins, being inclined to the lamellar interface, are observed to be preferentially formed. The process is favored by the localized stress field of the interfacial misfit dislocations.

Study on the stress-induced γ → α2 transformation in a hot-deformed Ti-45Al-10Nb alloy by high-resolution transmission electron microscopy

A stress-induced gamma --> alpha(2) transformation phenomenon in a hot-deformed Ti-45Al-10Nb alloy was investigated by high-resolution transmission electron microscopy (HREM). This deformation-induced gamma --> alpha(2) phase transformation is an interface related process. The interfacial superdislocations emitted from the deviating semi-coherent alpha(2)/gamma interface react with each other or with the moving dislocations in the gamma-phase resulting in the formation of the stress-induced alpha(2)-phase. Meanwhile, the orientation of the gamma-phase, of which interface the interfacial superdislocation swept, was also changed. The structure of the dislocation ledges plays an important role in the formation of the deformation-induced alpha(2)-phase.

Nonequilibrium α2γ interfacial structures in a γ-TiAl-based two phase alloy after hot-working and annealing

Abstract A transmission electron microscope investigation has been performed on the thermal instability of the nonequilibrium α 2 γ interfacial structure in a hot-forged Ti-45Al-10Nb-2.5Mn-0.05B alloy. The distinctiveness of these nonequilibrium α 2 γ interfaces in the hot-deformed sample is the misorientation from the conventional orientation relationship {111}γ//{0001}α2, 〈 110 〉γ// 〈 1120 〉α2. The structure of this nonequilibrium interface boundary is characterized by numerous interfacial ledges containing 1 3 [111] Frank partial dislocations. The analysis points towards the formation of this type of nonequilibrium α 2 γ interface being caused by the relative rotation of α2 and γ plates in the lamellar structure. Further TEM studies of microstructural changes during annealing for 4 min/1000 °C of the nonequilibrium α 2 γ lamellar structure generated by hot-forging revealed that many low-angle grain boundaries are formed in the γ plates of lamellar colonies, resulting in a γ subgrain microstructure; necking of α2 plates occurs preferentially at α2 subgrain boundaries; subsequently disintegration of the original α2 plates takes place resulting in spheroidized α2 particles which are aligned at the prior α 2 γ boundaries.

Structural change of deformation twin boundaries in a heavily deformed γ-TiAl-based alloy

Structural change of deformation twin boundaries in gamma phase of a Ti-45Al-10Nb-2.5Mn alloy with similar to 29% deformation strain has been investigated by TEM. We found numerous bent twins propagating in the gamma matrix, which were inclined at angles of a few degrees to the (111)(M) plane in the gamma matrix. HREM observations showed that (111)(DT) plane in the twins exhibited a large misorientation angle with the (111)(M) plane, and many 1/3[111] Frank partial dislocations were observed in the incoherent twin boundaries. These 1/3[111] Frank partials, which were possible to form through dissociations of matrix dislocations at the twin boundaries, can accommodate the bending of the twins and the misorientation from the true-twin orientation relationship

Hot-deformation-induced α2/γ interfacial structure in a two-phase γ-TiAl-based alloy

Abstract The occurrence of nonequilibrium semi-coherent α 2 ⧸ γ interfaces is an important microstructural feature of hot-deformed Ti–45Al–10Nb–2.5Mn–0.05B alloys. The feature of the α 2 ⧸ γ interfaces is the misorientation from the conventional orientation relationship{111} γ {0001} α 2 , 〈110〉 γ 〈1120〉 α 2 , as well as the asymmetry in the sense of interfacial boundary planes deviating from the equilibrium atomic planes (111) γ or (0001) α 2 . Numerous interfacial ledges containing 1/3[111] Frank partial dislocations exist in this nonequilibrium semi-coherent α 2 ⧸ γ interface. The analysis points toward the formation of this type of nonequilibrium α 2 ⧸ γ interface accommodating the relative rotation of α 2 and γ plates in the hot-deformation-induced bent lamellar structure. In the sharply bent lamellae, there even occur largely misoriented non-coherent α 2 ⧸ γ interfaces with the {111} γ plane no longer being parallel to the {0001} α 2 plane. These interfaces result from the lamellae bending/kinking during heavy deformation.

TEM investigation of twin intersection in a Ti-45Al-9Nb-2.5Mn alloy deformed at room temperature

Three penetration mechanisms of the type-I twin intersection with the dislocation gliding atomic planes of (11(1) over bar)(TB), (001)(TB), and (115)(TB) respectively have been observed in a high-Nb containing TiAl base alloy by high resolution TEM. It was found that the active mode of those intersection mechanisms was related to the thickness of the incident twin (TI). When the TI is very thin such as 2-3 nm, the active intersection mode usually is the intersection mechanism with the dislocation gliding atomic planes of (11(1) over bar)TB; when the TI gets thick, the active intersection mode moves to the intersection mechanism with the gliding atomic planes of (001)TB; as the TI becomes thick enough such as 15 nm which is similar to the thickness of the barrier twin (TB), the active intersection mode changes to the intersection mechanism with the gliding atomic plane of (115)TB The observed thickness dependence indicates that active different twin intersection mechanisms need different local stress concentrations near the intersection boundary, which increases with increasing TI thickness. A twin intersection mainly includes two successive processes that are dislocation dissociation on the intersection boundary and the dislocation glides in the barrier twin. The local stress concentration needed to activate those intersection mechanisms is related to the difficulties of these two successive processes. The observed sequence in thickness dependence is consistent with the sequence in difficulty of those intersection mechanisms based on a detailed analysis of those two successive processes

Formation of a triangular striated structure in the twin intersection area in γ-TiAl during room-temperature deformation

A triangular striated region, which is related to the type-I twin intersection in gamma-TiAl during room-temperature deformation, has been investigated by high-resolution transmission electron microscopy (HREM). The triangular structure around the intersection area is supposed to be a result of the heavy lattice distortion in the case of the type-I twin intersection. Under a local stress concentration, some nanotwins were observed to nucleate at the ledges on the incoherent twin boundary of the incident twin tip. Some dislocation dissociations at the ledges may be responsible for nucleation of these nanotwins. These nanotwins within the triangular striated region propagate with a homogeneous 1/6 [11 (2) over bar] twinning dislocation glide mechanism. The propagation of nanotwins can accommodate the lattice distortion of the triangular striated region without any crack at the nanotwin boundaries