Effects of alloying and high-temperature heat treatment on the microstructure of Nb–Ti–Si based ultrahigh temperatur e alloys

The phase co mpositions, microstructure and especi ally phase i nterfaces in the as-cast and heat-treated Nb– Ti–Si based ultrahigh temperature alloys have been investigated. It is shown that β(Nb,X)5Si3 and γ(Nb,X)5Si3 are the primary p hase s in the Nb–22Ti–16Si–5Cr–5Al (S1) (at%) and Nb–20Ti–16Si–6C r–4Al–5Hf–2B–0.06Y (S2) (at% ) alloys, respectively. The Nb solid solution (Nbss) is the primary phase in Nb–22Ti–14Si–5Hf–3Al–1. 5B –0.0 6Y (S3) (at%) alloy . An orientation relationship between Nbss and γ(Nb,X)5Si3 was determine d to be (1-10)Nb//(101-0)γ and [111]Nb//[0001]γ in the as-cast S2 and S3 alloys. Some original β(Nb,X)5Si3 transfor med into α(Nb,X)5Si3 because Al and Cr diffused from the β(Nb,X)5Si3 to Nbss during heattreatment at 1500 °C for 50 h in the S1 alloy. Mean while, Ti diffused from Nbss to β(Nb,X)5Si3, which induced a Ti to generate near the interface between Nbss and Ti-rich β(Nb,X)5Si3. The orientation relationship between the newl y-formed a Ti and previous Nbss was (110 )Nb//(1-10-1) αTi and [001]Nb//(12-3-1)αTi. Among the ( Nb,X)5Si3 phases , the contents o f Cr and Al in β(Nb,X)5Si3 are n earl y the same as those in γ(Nb,X)5Si3 but obviously hi gher than those in the α(Nb,X)5Si3, where as the content of Si in α(Nb,X)5Si3 is nearly the same a s that in γ(Nb,X)5Si3 but higher than that in the β(Nb,X)5Si3     

Investigation on the homogenization treatment and element segregation on the microstructure of a γ/γ'-cobalt-based superalloy

The aim of the present study was to investigate the effect of element segregation on the microstructure and γ' phase in a γ/γ' cobalt-based superalloy. Several samples were prepared from a cast alloy and homogenized at 1300℃ for different times, with a maximum of 24 h. A microstructural study of the cast alloy using wavelength-dispersive spectroscopic analysis revealed that elements such as Al, Ti, and Ni segregated mostly within interdendritic regions, whereas W atoms were segregated within dendrite cores. With an increase in homogenization time, segregation decreased and the initial dendritic structure was eliminated. Field-emission scanning electron microscopy micrographs showed that the γ' phases in the cores and interdendritic regions of the as-cast alloy were 392 and 124 nm, respectively. The size difference of γ' was found to be due to the different segregation behaviors of constituent elements during solidification. After homogenization, particularly after 16 h, segregation decreased; thus, the size, chemical composition, and hardness of the precipitated γ' phase was mostly uniform throughout the samples.     

Effect of pickling processes on the microstructure and properties of electroless Ni–P coating on Mg–7.5Li–2Zn–1Y alloy

The electroless plating Ni–P is prepared on the surface of Mg–7.5Li–2Zn–1Y alloys with different pickling processes.The microstructure and properties of Ni–P coating are investigated.The results show that the Ni–P coatings deposited using the different pickling processes have a different high phosphorus content amorphous Ni–P solid solution structure,and the Ni–P coatings exhibit higher hardness.There is higher phosphorus content of Ni–P amorphous coating using 125 g/L Cr O3and 110 ml/L HNO3(w68%)than using 180 g/L Cr O3and 1 g/L KF during pre-treatment,and the coating structure is more compact,and the Ni–P coatings exhibit more excellent adhesion with substrate(Fcup to22 N).The corrosion potential of Ni–P coating is improved and exhibits good corrosion resistance.As a result,Mg-7.5Li-2Zn-1Y alloy is remarkably protected by the Ni–P coating.     

The precipitation strengthening behavior of Cu-rich phase in Nb contained advanced Fe–Cr–Ni type austenitic heat resistant steel for USC power plant application

Copper has been used as a strengthening element in newly developed Fe–Cr–Ni type austenitic heat resistant steel for inducing Cu-rich phase precipitation to meet high temperature strength requirement for 60°C Ultra Super-Critical (USC) coal fired power plants for many years. However, the precipitation behavior and strengthening mechanism of Cu-rich phase in these advanced austenitic heat resistant steels is still unclear. In order to understand the precipitation strengthening behavior of Cu-rich phase and to promote high strength austenitic heat resistant steel development, 18Cr9 NiCuNb steel which is a Cu-added Nb contained advanced Fe–Cr–Ni type austenitic heat resistant steel has been selected for this study to be aged at 650°C till to 10,000 h. Micro-hardness and room temperature tensile test were conducted after long-time aging. SEM,TEM, HRTEM and three dimensional atom probe (3DAP) technology accompanying with thermodynamic calculation have been used to investigate the Cu-rich phase precipitation behavior during 650°C aging. The experimental results showed that Cu atoms can quickly concentrate in clusters at very early precipitation stage to form the fine nano-size Cu-rich ‘‘segregation areas’within less than 1 h at 650°C. With increasing aging time at 650°C Cu atoms continuously concentrate to Cu-rich segregation areas (clusters) and simultaneously other kinds of atoms such as Fe, Cr and Ni diffuse away from Cu-rich segregation areas to austenitic matrix, and finally to complete the transformation from Cu-rich segregation areas to Cu-rich phase. However, there is only Cu atoms concentration but not crystallographic transformation from early stage of Cu-rich clusters forming to the final Cu-rich phase formation. Even the Cu atom becomes the main composed element after 500 h aging at 650°C the Cu-rich phase still keeps coherent relationship with austenitic matrix. According the experimental results in this study, Cu-rich phase precipitation sequence which starts from the Cu atom segregation followed by the Cu diffusing from matrix to segregation areas and Fe, Cr and Ni atoms diffuse out from Cu-rich areas to matrix without crystallographic transformation is proposed. The Cu-rich phase is the most dispersed phase and contributes the most important strengthening effect among all precipitated phases (M23C6, MX and Cu-rich phase). It has been found that Cu-rich phase is very stable and still keeps in nano-size even for 10,000 h aging at 650°C. The unique precipitation strengthening of Cu-rich phase in combination with nano-size Nb-rich MX phase and grain-boundary M23C6carbide contributes excellent strengthening effect to 18Cr9 NiCuNb austenitic heat resistant steel.