Mechanical alloying fracturing

Fig. 1.10 Mechanical alloying through repetitive cold welding and fracturing...

TEM observations were performed in the as-received and deformed samples in order to reveal the effects of microstructure on the fatigue response of the studied alloy. Fracture surfaces of the deformed fatigue test specimens were comprehensively examined in a scanning electron microscope (JEOL JSM6500F) equipped with field emission gun to determine the macroscopic fracture mode and characterize the fine-scale topography and microscopic mechanisms governing fatigue fracture. 

Available in mid-1978, the first version of the handbook will concentrate on selected structural alloys. The properties presently being compiled include electrical and magnetic, thermal, elastic, mechanical, and fracture. Many of the necessary data are not available now and will be provided by the other projects in this program. 

The difference in volume fraction of MC carbides and the nature of the residual phases are reflected in the mechanical and fracture test data. Yield strengths at all temperatures were nearly identical for both alloys in the standard STD A condition. This indicates that the intrinsic alloy strength, which is derived from precipitations of y and y" phases, was not influenced by carbides and other phases. Ductility, fracture toughness, and the FCGR were apparently strongly influenced by carbides and particularly by the Laves phases, which tended to precipitate in the grain boundaries. 

Depending on the behavior of materials under mechanical actions, they can be classified, vis-a-vis mechanosynthesis or mechanical alloying processes, into two major groups ductile and brittle materials. The former are essentially the materials showing plastic deformation, that is, metals and alloys, and the latter are basically the materials fracturing under mechanical action without noticeable plastic deformation, that is, ceramics. Most of the perovskites are considered in the latter group. 

The sensitivity tests are carried out on artificial defects (nickel-chromium specimens of NFA 09.520,see figure 3 of annex 1) and natural defects (one part in "light" alloy, one part in stellite grade 1 containing micropores, 2 specimens of fracture mechanical type CT20 in Z2 CN 12.10 (NFA 03.180). 

Two approaches have been taken to produce metal-matrix composites (qv) incorporation of fibers into a matrix by mechanical means and in situ preparation of a two-phase fibrous or lamellar material by controlled solidification or heat treatment. The principles of strengthening for alloys prepared by the former technique are well estabUshed (24), primarily because yielding and even fracture of these materials occurs while the reinforcing phase is elastically deformed. Under these conditions both strength and modulus increase linearly with volume fraction of reinforcement. However, the deformation of in situ, ie, eutectic, eutectoid, peritectic, or peritectoid, composites usually involves some plastic deformation of the reinforcing phase, and this presents many complexities in analysis and prediction of properties. 

Stacking faults thereby providing barriers to sHp. If carbides are allowed to precipitate to the point of becoming continuous along the grain boundaries, they often initiate fracture (see Fracture mechanics). A thorough discussion of the mechanical properties of cobalt alloys is given in References 29 and 30 (see also Refractories). 

The abrasion resistance of cobalt-base alloys generally depends on the hardness of the carbide phases and/or the metal matrix. For the complex mechanisms of soHd-particle and slurry erosion, however, generalizations cannot be made, although for the soHd-particle erosion, ductihty may be a factor. For hquid-droplet or cavitation erosion the performance of a material is largely dependent on abiUty to absorb the shock (stress) waves without microscopic fracture occurring. In cobalt-base wear alloys, it has been found that carbide volume fraction, hence, bulk hardness, has Httie effect on resistance to Hquid-droplet and cavitation erosion (32). Much more important are the properties of the matrix. 

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