Tingfeng Song

Ferroelectric hafnia is one of the most promising materials for next generation of non‐volatile memory devices. Several strategies have demonstrated to be of interest to improve its functional properties. Interface engineering, realized by the introduction of additional layer in the capacitor structure, is demonstrated as a promising strategy. However, interface layers can have multiple implications, such as changes in the chemistry of the interfaces and an increase of depolarization field, whose effects are difficult to discriminate. The role of HfO₂ and ZrO₂ capping is explored on polarization, retention, endurance, and leakage properties of Hf_0.5Zr_0.5O₂ epitaxial films. In HfO₂ capped films, lower polarization is observed, and endurance and retention are also comparably worse than in ZrO₂ capped films. Complementary under illumination ferroelectric characterization and capacitance measurements indicate a reduction of defects and interface capacitance contribution in ZrO₂ capped films. For both cappings, the interfaces with the Hf_0.5Zr_0.5O₂ layer are shown to be compositionally sharp and the phase of Hf_0.5Zr_0.5O₂ (HZO) grains is replicated on the capping layer, indicating that electrostatic effects prevail and that the use of interface layers with high permittivity, here ZrO₂, is crucial to favor good functional properties.

Ferroelectric HfO2 films are usually polycrystalline and contain a mixture of polar and nonpolar phases. This challenges the understanding and control of polar phase stabilization and ferroelectric properties. Several factors, such as dopants, oxygen vacancies, or stress, among others, have been investigated and shown to have a crucial role on optimizing the ferroelectric response. Stress generated during deposition or annealing of thin films is a main factor determining the formed crystal phases and influences the lattice strain of the polar orthorhombic phase. It is difficult to discriminate between stress and strain effects on polycrystalline ferroelectric HfO2 films, and the direct impact of orthorhombic lattice strain on ferroelectric polarization has yet to be determined experimentally. Here, we analyze the crystalline phases and lattice strain of several series of doped HfO2 epitaxial films. We conclude that stress has a critical influence on metastable orthorhombic phase stabilization and ferroelectric polarization. On the contrary, the lattice deformation effects are much smaller than those caused by variations in the orthorhombic phase content. The experimental results are confirmed by density functional theory calculations on HfO2 and Hf0.5Zr0.5O2 ferroelectric phases.

Dissimilar metal joining between titanium and kovar alloys was conducted using electron beam welding. Metallurgical bonding of titanium alloys and kovar alloys was achieved by using a Cu/Nb multi-interlayer. The effects of welding speed on weld appearance, microstructure and mechanical properties of welded joints were investigated. The microstructure of welded joints was characterized by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive spectroscopy (EDS). The mechanical properties of welded joints were investigated by tensile strength and micro-hardness tests. The results showed that welding speed had great effects on the weld appearance, microstructure, and mechanical properties of electron beam-welded joints. With an increase of welding speed, at the titanium alloy side, the amount of (Nb,Ti) solid solution was increased, while the formation of brittle FeTi was effectively suppressed. At the kovar alloy side, microstructure was mainly composed of soft Cu solid solution and some α-Fe + γ phases. In addition, higher welding speeds within a certain range was beneficial for eliminating the formation of cracks, and inhibiting the embrittlement of welded joints. Therefore, the tensile strength of welded joints was increased to about 120 MPa for a welding speed of 10 mm/s. Furthermore, the bonding mechanism of TC4/Nb/Cu/4J29 dissimilar welded joints had been investigated and detailed.

High‐quality joints between titanium alloys and stainless steels have found applications for nuclear, petrochemical, cryogenic, and aerospace industries due to their relatively low cost, lightweight, high corrosion resistance, and appreciable mechanical properties. This article reviews diffusion bonding between titanium alloys and stainless steels with or without interlayers. For diffusion bonding of a titanium alloy and a stainless steel without an interlayer, the optimized temperature is in the range of 800–950°C for a period of 60–120 min. Sound joint can be obtained, but brittle FeTi and Fe‐Cr‐Ti phases are formed at the interface. The development process of a joint mainly includes three steps: matching surface closure, growth of brittle intermetallic compounds, and formation of the Kirkendall voids. Growth kinetics of interfacial phases needs further clarification in terms of growth velocity of the reacting layer, moving speed of the phase interface, and the order for a new phase appears. The influence of Cu, Ni (or nickel alloy), and Ag interlayers on the microstructures and mechanical properties of the joints is systematically summarized. The content of FeTi and Fe‐Cr‐Ti phases at the interface can be declined significantly by the addition of an interlayer. Application of multi‐interlayer well prevents the formation of intermetallic phases by forming solid solution at the interface, and parameters can be predicted by using a parabolic diffusion law. The selection of multi‐interlayer was done based on two principles: no formation of brittle intermetallic phases and transitional physical properties between titanium alloy and stainless steel.

Electron beam welding of a titanium alloy (Ti‐6Al‐4V) and a kovar alloy (Fe‐29Ni‐17Co) was performed by using a Cu/Nb multi‐interlayer between them. Microstructure and composition of welded joints were analyzed by means of optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, and X‐ray diffraction. Mechanical properties of welded joints were evaluated by microhardness and tensile strength tests. Results indicated that in case of 0.22 mm thickness of Nb foil, microstructure of the titanium alloy side was mainly composed of Ti solid solution and some intermetallic compounds such as FeTi and CuTi₂, whereas in case of 0.40 mm thickness of Nb foil, the appearance of weld was more uniform and hardness of the weld zone decreased sharply. However, tensile strength of welded joints was increased from 88.1 MPa for the 0.22 mm Nb foil to 150 MPa for the 0.40 mm Nb foil. It was found that thicker Nb foil could inhibit diffusion of Fe atoms towards the titanium alloy side, thus promoting the formation of Ti solid solution and a small amount of CuTi₂ and eliminating FeTi. In addition, in both cases, Cu_0.5Fe_0.5Ti was found in the fusion zone of the titanium alloy side, which had an adverse effect on mechanical properties of welded joints.

Multi-walled carbon nanotubes (MWCNTs) and graphenes have been taken for novel reinforcements due to their unique structure and performance. However, MWCNTs or graphenes reinforced copper matrix composites could not catch up with ideal value due to reinforcement dispersion in metal matrix, wettability to metal matrix, and composite material interface. Taking advantage of the superior properties of one-dimensional MWCNTs and two-dimensional graphenes, complementary performance and structure are constructed to create a high contact area between MWCNTs and graphenes to the Cu matrix. Mechanical alloying, hot pressing, and hot isostatic pressing techniques are used to fabricate Cu matrix self-lubricating nanocomposites. Effects of MWCNTs and graphenes on mechanical properties and microstructures of Cu/Ti3SiC2/C nanocomposites are studied. The fracture and strengthening mechanisms of Cu/Ti3SiC2/C nanocomposites are explored on the basis of structure and composition of Cu/Ti3SiC2/C nanocomposites with formation and function of interface.

This research investigates effects of contact pressure on multiaxial fretting fatigue behavior of Al-Zn-Mg alloy which has a wide variety of applications in automobile and rail transit engineering. Al-Zn-Mg alloy is exposed to cyclic loadings and contact pressure. The cyclic stress-strain curve and displacement amplitude curve under different contact pressure of Al-Zn-Mg alloy were investigated using point contact multiaxial fretting fatigue test equipment. Based on cyclic stress-strain curve and displacement amplitude curve under different contact pressure, fretting wear characteristics, fretting fatigue fracture properties and fretting fatigue lives as well as the effects of contact pressure on multiaxial fretting fatigue damage and fretting fatigue fracture mechanisms of Al-Zn-Mg alloy were extensively analyzed.