Formation of metastable phases of Ni-C

Mechanical alloying (MA) of nickel and graphite powders was performed in the composition range Ni_1-xC_x (x = 0.10 to 0.90) by use of a conventional ball mill. The structure of me-chanically alloyed samples was examined by X-ray diffraction, transmission electron micros-copy (TEM), scanning electron microscopy (SEM), and differential scanning calorimetry. A remarkable supersaturation of carbon in face-centered cubic (fcc) nickel phase was observed. A metastable phase Ni₃C was formed by a prolonged MA treatment. For the purpose of com-parative study, MA of cobalt and graphite powders was also performed in composition Co_1-xC_x (x = 0.10, 0.15, and 0.30). The supersaturation of carbon in fcc cobalt and formation of a metastable carbide Co₃C were confirmed.     

Dynamic consolidation of metastable nanocrystalline powders

Nanocrystalline metal powders synthesized by mechanical alloying in a ball mill resulted in micron-sized powder particles with a nanosized (5 to 25 nm) substructure. Conventional consolidation methods resulted in considerable coarsening of the metastable nanometer crystallites, but dynamic consolidation of these powders using explosive techniques produced fully dense monoliths while retaining the 5- to 25-nm substructure. Numerical modeling, used to guide the experimental phase, revealed that the compression wave necessary for suitable consolidation was of the order of 10 GPa for a few tenths of a microsecond. The consolidation process is described, and the retention of the metastable nanostructure is illustrated. This article is based on a presentation made in the symposium “Dynamic Behavior of Materials,” presented at the 1994 Fall Meeting of TMS/ASM in Rosemont, Illinois, October 3-5, 1994, under the auspices of the TMS-SMD Mechanical Metallurgy Committee and the ASM-MSD Flow and Fracture Committee.     

Fracture and fractography of metastable austenites

External test variables such as rate and temperature, and changes in alloy composition are shown to have a number of effects on the fracture of high-strength, metastable austenitic steels. One rate-dependent phenomenon is an unusual fracture mode transition wherein a flat mode changes to a shear mode when the amount of transformation product in the vicinity of the crack tip is reduced by adiabatic heating. The point at which this happens in any one test is dependent upon the velocity of the slowly growing crack which in turn is dependent upon the crosshead rate. Because of this rate effect, the plane stress fracture toughness decreases by as much as 30 pct at higher crosshead rates. Fractographically, it was ascertained that at room temperature, both phases failed in a ductile manner, but at ?196°C, martensite containing greater than about 0.27 wt pct C would cleave. This resulted in a “ductile-brittle” transition in metastable austenites at ?196°C as a function of carbon content. Other compositional variations change the austenite stability which controls the amount of strain-induced marteniste occurring at the crack tip. It is shown that a plane stress fracture toughness (K _C) approaching 500,000 psi-in.^1/2 may be achieved by decreasing the stability of the austenite. The variation ofK _c with austenite stability agrees qualitatively with a theoretical model for the invariant shear contribution to the fracture toughness of metastable austenites.     

Cavitation behaviour of a metastable Cr-Mn-austenite

The microstructures of three steels with metastable austenite with mass contents of 10-14 % Cr, 10-15 % Mn and O.1-0.35 % C were optimized by a simulation of the cavitation load. Results of cavitation tests show that the microstructure should be fine grained, to avoid the arise of cracks in the early stages. The materials were pretreated by a thermomechanical treatment. Criteria for a selection were the mechanical properties and the microstructure. According to the results a steel with mass contents of 13 % Cr, 10 % Mn and O.35 % C (X 35 CrMn 13 10) should have the best cavitation resistance. In this steel a deformation of 400 °C produces a dislocation network that forces a transformation of austenite to very fine martensite plates during cavitation and gives a smooth abrasion. The cavitation resistance of steel X 35 CrMn 13 10 in a pretreated condition is in the range of published values of a Steinte 6B, but the production of the steel is more versatile and less expensive than that of a Stellite.     

Structure and formation of the metastable phasem-TiBe

The metastable ordered phasem-TiBe was discovered in the course of studying liquidquenched Ti-Be and Ti-Be-Zr alloys. m-TiBe forms, together with the α-Ti solid solution, on heating binary (and ternary) metallic glasses to 700°C and is also observed directly in as-quenched alloys lying outside the composition limits for glass formation. This phase has a B2 (CsCl-type) crystal structure. Its existence can be justified in terms of crystal chemical factors leading to the conclusion that TiBe most likely does not appear as an equilibrium phase in normally cooled alloys due to the greater stability of the Laves phase TiBe₂.     

Fatigue crack propagation of metastable beta titanium-vanadium alloys

The fatigue crack propagation behavior of three titanium-vanadium alloys (24, 28, and 32 wt pct V) which have tensile deformation modes ranging from coarse twinning to wavy and planar slip has been measured in laboratory air and correlated with their low cycle fatigue properties and microstructure. The fatigue crack growth rate of alloys with similar microstructures but different deformation modes, and of alloys with similar deformation modes but different microstructures have been compared. Increasing the deformation barrier mean free path and improving low cycle fatigue properties has been observed to reduce the fatigue crack growth rate at low and inter mediate ΔK levels. The fatigue crack growth data have been compared with that calculated from equations which use microstructure and low cycle fatigue parameters. The predictive capability of these equations which contain only measurable parameters has been found to be quite adequate.     

The stable and metastable Ti-Nb phase diagrams

The phase transformations which occur in the Ti-Nb binary alloy system have been discussed in two recent papers. The phase relationships were investigated by varying alloy composition and thermal history. In this paper, these results are summarized in complete and thermodynamically consistent calculations of the stable and metastable phase diagrams. The calculations of the metastable equilibria are relevant to the Ti-V and Ti-Mo systems, as well as to several other titanium and zirconium-based transition metal alloy systems. Formerly with the National Bureau of Standards, Gaithersburg, MD 20899.     

Steel powder with a metastable austenite structure

The effect of technological parameters of fabrication on wear resistance and phase transformations in the surface layer of chromium-nickel steel PK110Kr2N is studied. A statistical model is proposed for prediction of the content inhomogeneity of the steel. Enhanced abrasive wear resistance is attained by appropriate transformations in the surface layer.Scientific-Research Institute for Problems of Powder Technology and Coatings, Perm. Translated from Poroshkovaya Metallurgiya, Nos. 3–4, pp. 42–47, March–April, 1994.