Microstructure of a V-Containing Cobalt Based Alloy Prepared by Mechanical Alloying and Hot Pressed Sintering

In this paper, a bulk V-containing cobalt-based alloy with high chromium and tungsten contents was prepared by mechanical alloying and hot pressed sintering using Co, Cr, W, Ni, V and C pure element powders. XRD, SEM, TEM and Vickers hardness tests were employed to characterize the microstructure and mechanical properties of the mechanical alloyed powders and hot pressed bulk cobalt-based alloy. The results show that all elements can be mixed uniformly and that the Co, Cr, and Ni elements were made into an amorphous state after 10 h ball milling in a high energy ball miller. The microstructure of the prepared bulk alloy was composed of a γ-Co matrix with a large number of nano-twins and fine M23C6 and M12C carbide particles well-distributed in the alloy. The V element was mainly distributed in M23C6-type carbide and no V-rich MC-type carbide was found in the microstructure. The prepared alloy had a high hardness of 960 ± 9.2 HV and good a fracture toughness KIc of about 10.5 ± 0.46 MPa·m1/2. The microstructure formation and strengthening mechanisms of the prepared cobalt-based alloy are discussed.

Resistance of Thermally Aged DSS 2304 against Localized Corrosion Attack

In this paper, the effects of thermal aging in the 650–850 °C range on the localized corrosion behaviour of duplex stainless steel (DSS) 2304 was investigated. Pitting corrosion resistance was assessed by pitting potential (Epitt) and critical pitting temperature (CPT) determination, while the degree of sensitisation (DOS) to intergranular corrosion (IGC) was evaluated by double loop electrochemical potentiokinetic reactivation (DL-EPR). The susceptibility to stress corrosion cracking (SCC), evaluated in standard NACE TM-0177 solution at pH 2.7 and 25 °C, with the addition of S2O32− at 10−3 M, resulted in general good agreement with pitting and IGC behaviour. In fact, as-received DSS 2304 aged for 5 min at 650 °C or 750 °C presented a high resistance to localized corrosion. The alloy corrosion behaviour was severely impaired with an aging time of 60 min at 650 °C and of 10 or 60 min at 750 °C, due to the precipitation of finely distributed M23C6-type chromium carbides at ferrite/austenite interphases, which determined the formation of chromium and molybdenum depleted areas. The behaviour of samples aged at 850 °C also depended on the aging time, but, at 60 min, the rediffusion of passivating elements produced a recovery of the alloy resistance to pitting, IGC and SCC.

Strengthening of a CoCrFeNiMn-Type High Entropy Alloy by Regular Arrays of Nanoprecipitates

In this paper, we report microstructure and mechanical properties evolution of the CoCrFeNiMn-type high entropy alloy, containing small amounts of Al and C, during cold rolling and subsequent annealing at 700-1100°C. In the initial as-cast condition the alloy has coarse-grained single face-centered cubic (fcc) phase structure. Cold rolling and annealing substantially refine fcc grains; in addition M23C6 type carbides appear. After annealing at relatively low temperatures (≤900°C), these particles are arranged in characteristic arrays aligned with rolling directions. The specific microstructure of the thermomechanically processed alloy is suggested to be the reason of the balanced combination of tensile strength and ductility.

Precipitation of Nano-Sized Z-Phase in HR3C Austenitic Heat Resistant Steel

Effect of heat treatment on the precipitation behavior of secondary phases in a HR3C austenitic heat resistant steel was investigated. The microstructure of the steel in solution-treated state consists of austenitic matrix and coarse Z-phase particles. After aging treatment at 650–950°C for 1h, M23C6-type carbide precipitates along random grain boundaries. Dense and homogeneous nanosized Z-phase precipitates within austenite grains are obtained by an aging treatment at a temperature between 800 and 900°C for 1h. The high density of dislocation walls produced during the water-cooling process after solution treatment facilitate the precipitation of the nanosized Z-phase. With increasing the aging temperature, the hardness initially drops, then increases and reaches a peak when the aging temperature is at 850°C due to the precipitation of the nanosized Z-phase.

Microstructural Changes of FSX-414 Superalloy during TLP Bonding

The as-cast FSX-414 Co-based superalloy samples were solution treated at 1150oC for 4h and then aged at 980oC for 4h. Specimens for joining were cut from the as-cast ingot and TLP bonding carried out at the same conditions as for the solution and solution+aging treatment, using MBF-30 interlayer. Microstructures were studied for as-cast, heat treated and TLP bonded specimens. These studies showed that the ununiform distributed carbides of MC type in the as-cast specimens replaced by M23C6 type carbides with uniform distribution in the heat treated microstructure. Due to complete isothermal solidification, no eutectic structure in the bond region were wasobserved, but some intermetallics in the diffusion affected zone (DAZ) were observed. Microhardness tests were used to compare the hardness of age hardened specimens with bonded specimens at the same heat treatment condition. Hardness profile also showed a peak in DAZ region in spite of complete isothermal solidification.