Microstructural and Hardness Evaluation of Ni4Mo Alloy with Aluminum Additions

The Ni₄Mo at.% Mo alloy, here referred to as Ni₄Mo, is a single phase face-centered cubic structure (α) above 868^oC; below this temperature it becomes long-range ordered (β). The conversion of α to βordering reaction) on cooling or by other heat treatment causes a marked increase (e.g., doubling) in the hardness and yield strength. However, extreme extreme embrittlement also occurs. A major purpose of the present study was to see if the addition of moderate amount of aluminum would prevent the embrittlement yet retain the strengthening during ordering. Aluminum was chosen because it might allow the alloys to retain the good corrosion resistance of Ni₄Mo.

The alloys studied were the base Ni₄Mo alloy and Ni₄Mo with addition of 2, 5, 7, and 9 at.% aluminum. All samples were small arc melted buttons. They were solution heat treated at 1280^oC for 50 hours in quartz tubes filled with argon, then quenched by breaking the tube under water. Samples were then aged at 600, 650, 700, 750, 850, and 950^oC for up to 100 hours. Microhardness was measured and ductility was examined by bending thin slices. The microstructures were evaluated using optical, scanning electron and transmission electron microscopy. Corrosion resistance was evaluated using cyclic anodic polarization and polarization resistance testing.

A major finding was that aluminum increased considerably the hardness for all aging treatment and for all alloys (a maximum of about 650 HV after aging the 5, 7, and 9 at.% A1 alloys for 100 hours at 700^oC), but did not prevent embrittlement. However, in the solution heat treated condition the hardness of the 9 at.% A1 alloy (440 HV) was approximately double that of the Ni₄Mo alloy (220 HV), and good ductility was retained.