Investigation of Microstructure Evolution and Phase Selection of Peritectic Cuce Alloy During High-Temperature Gradient Directional Solidification

In this work, a CuCe alloy was prepared using a directional solidification method at a series of withdrawal rates of 100, 25, 10, 8, and 5 μm/s. We found that the primary phase microstructure transforms from cellular crystals to cellular peritectic coupled growth and eventually, changes into dendrites as the withdrawal rate increases. The phase constituents in the directionally solidified samples were confirmed to be Cu2Ce, CuCe, and CuCe + Ce eutectics. The primary dendrite spacing was significantly refined with an increasing withdrawal rate, resulting in higher compressive strength and strain. Moreover, the cellular peritectic coupled growth at 10 μm/s further strengthened the alloy, with its compressive property reaching the maximum value of 266 MPa. Directional solidification was proven to be an impactful method to enhance the mechanical properties and produce well-aligned in situ composites in peritectic systems

Tribology and corrosion properties investigation of a pulse electrodeposition duplex hard-particle-reinforced Ni Mo nanocomposite coating

In this work, Ni-Mo-SiC-TiN composite coatings were prepared by pulse electrodeposition. Through analyzing the effect of particle content on the phase structure and morphology of NiMo coatings, the relationship between nanoparticles co-deposition amount and the mechanical properties and corrosion resistance of Ni matrix composite coatings was evaluated. The results show that the coating prepared at an electrolyte concentration of 20 g/L is flat and dense presenting the highest particle content. The crystallite size ranging from 28.27 nm to 11.85 nm is affected by different nanoparticle concentrations. As shown by the Tafel polarization and wear test, the incorporation of two hard particles improved the coating performance, and the corrosion current density was reduced by 74% to 1.84 μA/cm2. The wear rate decreased from 10.196 × 10−4 mm3/N·m to 2.65 × 10−4 mm3/N·m, and the average friction coefficient decreased to 0.11. The duplex hard particles play a bearing and hindering role during friction and corrosion. Based on the co-deposition kinetic model, a five-step pulse deposition model referring to nanoparticles was established. It is found that SiC/TiN particles co-deposited to the preferential Ni (111) crystal face and subordinate Ni (200) face. And the good binding ability between the matrix and the nanoparticles results in a high load transfer effect, thus enhancing the tribological properties of the coating. Moreover, the interaction between the duplex nanoparticles was further discussed

Microstructure Evolution and Mechanical Properties of FeCoCrNiCuTi0.8 High-Entropy Alloy Prepared by Directional Solidification

A CoCrCuFeNiTi0.8 high-entropy alloy was prepared using directional solidification techniques at different withdrawal rates (50 μm/s, 100 μm/s, 500 μm/s). The results showed that the microstructure was dendritic at all withdrawal rates. As the withdrawal rate increased, the dendrite orientation become uniform. Additionally, the accumulation of Cr and Ti elements at the solid/liquid interface caused the formation of dendrites. Through the measurement of the primary dendrite spacing (λ1) and the secondary dendrite spacing (λ2), it was concluded that the dendrite structure was obviously refined with the increase in the withdrawal rate to 500 μm/s. The maximum compressive strength reached 1449.8 MPa, and the maximum hardness was 520 HV. Moreover, the plastic strain of the alloy without directional solidification was 2.11%, while the plastic strain of directional solidification was 12.57% at 500 μm/s. It has been proved that directional solidification technology can effectively improve the mechanical properties of the CoCrCuFeNiTi0.8 high-entropy alloy

The Effect of Copper Content on the Mechanical and Tribological Properties of Hypo-, Hyper- and Eutectoid Ti-Cu Alloys

Titanium alloys are widely used in aerospace, chemical, biomedical and other important fields due to outstanding properties. The mechanical behavior of Ti alloys depends on microstructural characteristics and type of alloying elements. The purpose of this study was to investigate the effects of different Cu contents (2.5 wt.%, 7 wt.% and 14 wt.%) on mechanical and frictional properties of titanium alloys. The properties of titanium alloy were characterized by tensile test, electron microscope, X-ray diffraction, differential scanning calorimetry, reciprocating friction and wear test. The results show that the intermediate phase that forms the eutectoid structure with α-Ti was identified as FCC Ti2Cu, and no primary β phase was formed. With the increase of Cu content, the Ti2Cu phase precipitation in the alloy increases. Ti2Cu particles with needle structure increase the dislocation pinning effect on grain boundary and improve the strength and hardness of titanium alloy. Thus, Ti-14Cu shows the lowest elongation, the best friction and wear resistance, which is caused by the existence of Ti2Cu phases. It has been proved that the mechanical and frictional properties of Ti-Cu alloys can be adjusted by changing the Cu content, so as to better meet its application in the medical field

Influences of processing parameters on microstructure during investment casting of nickel-base single crystal superalloy DD3

The effects of solidification variables on the as-cast microstructures of nickel-base single crystal superalloy DD3 have been investigated by using the modified Bridgman apparatus. The experiments were performed under a thermal gradient of approximately 45 K·cm-1 and at withdrawal rates ranging from 30 to 200 m·s-1. The experimental results show that the primary and secondary dendritic arm spacings (PDAS and SDAS) decrease when the withdrawal rate is increased. Compared with the theoretical models of PDAS, the results are in good agreement with Trivedi’s model. The relationships of PDAS and SDAS with withdrawal rates can be described as l1 = 649.7V -0.24±0.02 and l2 = 281V -0.32±0.03, respectively. In addition, the size of the γ′ phase significantly decreases with increasing withdrawal rate

Cost-affordable and qualified powder metallurgy metastable beta titanium alloy by designing short-process consolidation and processing

Short-time processing route has been designed to manufacture cost-affordable and high-quality powder metallurgy (PM) metastable β titanium alloy, containing rapid powder consolidation (modified thermomechanical pressing), one-step thermomechanical processing (simple open die uniaxial hot forging by industrial press) and fast heat treatment (one-step annealing at various temperatures for only one hour). Based on comprehensive microstructure characterizations and mechanical property examinations, underlying microstructural evolution mechanism and microstructure-property relationship of the produced alloys were uncovered and elucidated thoroughly. Homogeneous macrostructure and fine-grain microstructure without undissolved particles and large pores are obtained for the alloy after thermomechanical powder consolidation as a result of the concurrent effect of external deformation and high-temperature diffusion. One-step open-die forging is verified to produce full-dense and sound PM alloy pancake with large-scale and high strength. Attributed to the harmonious concurrence of hierarchical α precipitation and heterogeneous grain structure, synergistic strength-ductility combinations are achieved for the alloy after specific processing and heat treatment with the tensile strength and strain at failure values of 1386.5 MPa/7.3% and 1252.3 MPa/9.0%, respectively. These strength-ductility combinations are comparable and/or even better than other metastable β titanium alloys prepared by some PM and ingot metallurgy approaches with relatively high cost and time consumption

Achieving enhanced toughness of a nanocomposite coating by lattice distortion at the variable metallic oxide interface

Ceramic coatings are in general a kind of brittle material because they are predominantly made up of ionic crystals that avoid dislocation motion caused by lattice distortion. In this regard, a remarkable toughened ZrO2/MgO nanocomposite coating is obtained by the plasma electrolytic oxidation (PEO) process and in-situ synthesized ZrO2 with quantitative control approach. It is revealed that the toughening behavior of the ZrO2/MgO coating is related to the coordination and diversion of lattice distortion at the metallic oxide interface, which induces distinct dislocation motion at the interface. The semicoherent interface between m-ZrO2 and MgO is verified to act as a buffer to realize toughening of the nanocomposite coating through dislocation slipping induced by lattice coordinated distortion. Simultaneously, significant interfacial lattice distortion transfer and dislocation pinning are discovered at the semicoherent interface between t-ZrO2 and MgO, which are beneficial to toughness enhancement of the nanocomposite coating. The results indicate that the toughening effect occurs along with dislocation slipping and pinning caused by lattice distortion of the ZrO2/MgO semicoherent interface, which enables the toughness of novel nanocomposite coating to reach 2.7 times of the traditional PEO coating.</p

Multiscale exploit the role of copper on the burn resistant behavior of Ti-Cu alloy

“Titanium fire” can cause catastrophic consequences for machinery components made of titanium alloys, especially in the case of aero-engines. As such, there is continuing interests in developing burn resistant titanium alloys. The burn resistant functionality and mechanical strength of titanium alloys rely on their microstructural characteristics that are significantly determined by the alloying components. This work investigated the burn resistant performance of Ti-Cu alloy and analyzed the role of alloying Cu. With sufficient content of Cu in the system, the formation of Cu rich layer cuts off the oxygen transfer pathway and hinders the burning process. Meanwhile, the addition of Cu results in the reduction of chemical kinetics. For the Ti-Cu alloy with insufficient Cu content, the burn resistance is sacrificed by the discontinuous Cu-rich clusters, among which oxygen penetration prolongs the burning process. Apart from the Cu-rich protective layer, various burning processes accompanied with oxidation products are detected via reduced area X-ray diffraction along the burnt cross section. This work provides a comprehensive understanding of the impacts of Cu content on the burn resistant characteristics of Ti-Cu alloy, which offers theoretical foundation of burn resistant mechanism to the design and fabrication of high-performance titanium alloys.</p