Effect of Post Heat Treatment on the Microstructure and Microhardness of Friction Stir Processed NiAl Bronze (NAB) Alloy
NiAl bronze (NAB) alloy is prepared by using friction stir processing (FSP) technique at a tool rotation rate of 1200 rpm and a traverse speed of 150 mm/min. A post heat treatment is performed at the temperature of 675 °C. The effect of heat treatment on the microstructure and microhardness is studied. The results show that the microstructure of the FSP NAB alloy consists of high density dislocations, retained β phase (β′ phase) and recrystallized grains. When annealed at 675 °C, discontinuous static recrystallization (DSRX) takes place. The content of β′ phase gradually decreases and fine κ phase is precipitated. After annealing for 2 h, both the microhardness of the FSP sample in the stir zone (SZ) and the difference in hardness between the SZ and base metal decrease due to the reduction of the dislocation density and β′ phase, accompanying recrystallized grain coarsening. With further increasing of the annealing time to 4 h, the aforementioned difference in hardness nearly disappears.
Microstructural development in complex nickel-aluminum bronzes
The development of microstructure in complex nickel-aluminum bronzes of nominal composition Cu-10 wt pet Al-5 wt pct Ni-5 wt pct Fe, during continuous cooling from temperatures approaching the solidus, has been investigated using optical and electron microscopic techniques. At high temperatures the alloy consists of bccβ phase, and as the temperature is lowered the fcc copper-rich α phase is precipitated with a Widmanstätten morphology. As the temperature continues to decrease, Fe3Al precipitates form in theβ phase and this is followed by the formation of Fe3Al precipitates in the a phase. On further cooling theβ phase decomposes eutectoidally into ga+ NiAl; the eutectoid reaction initiates at thea/β interface and grows into theβ regions. Under normal cooling conditions the decomposition ofβ into α+ NiAl does not reach completion and the remnantβ phase transforms to martensite.
Comparison of Corrosion and Cavitation Erosion Behaviors Between the As-Cast and Friction-Stir-Processed Nickel Aluminum Bronze
An as-cast nickel aluminum bronze (NAB) was treated by friction-stir processing (FSP). Immersion test and electrochemical measurements under cavitation erosion condition were carried out to investigate the long-term and short-term corrosion behaviors of the as-cast and as-FSP NAB. Cavitation erosion tests were conducted in both distilled water and 3.5 wt% sodium chloride (NaCl) solution. The immersion test indicated that the electrochemical impedance of the as-FSP NAB was much higher, while there was little difference between them in the short-term test. The cumulative mass loss of the as-cast NAB was about 1.5 and 2 times as large as that of the as-FSP in distilled water and 3.5 wt% NaCl, respectively. The higher corrosion resistance of the as-FSP was a result of the refined and homogenized microstructure. Improved mechanical properties and fewer galvanic corrosion sites are attributed to the higher cavitation erosion resistance of the as-FSP NAB.