On the strength of heavily cold worked in situ composites

The strengths of heavily deformed two phase materials are much in excess of those expected on the bases of the strain hardening behavior of the phases comprising such a composite. In this paper a model is developed to explain the strengths of this class of materials and the predictions of it are compared to results obtained in several systems. The basis for the model is that additional (geometrically necessary) dislocations are generated during deformation of two, as compared to single, phase materials as a result of the inherently greater strain incompatibility between adjacent grains in a two phase material. Thus the model predicts that, other factors being the same, the greater the disparity between the flow curves of the composite constituents, the greater the excess strength generated. The strain hardening behaviour of the individual phases influences also the strengths obtained in a composite. Composites comprised of materials for which dynamic recovery processes are particularly effective do not display large incremental strengths as these processes eliminate both geometrically necessary and statistical dislocations. Conversely, composites containing materials which do not dynamically recover (e.g. iron which work hardens linearly) display rather impressive excess strengths as a result of the complementary interaction between statistical and geometrical dislocations. The agreement between the model developed and experimental results is good. The two adjustable parameters of the model (one concerning the partitioning of the geometrical dislocations between the phases and the other a measure of the inherent strain incompatibility between them) have physically plausible numerical values. © 1985

Microstructural strengthening in cold worked in situ Cu-14.8 Vol. % Fe composites

Cu-14.8 vol. % Fe in situ fabricated composites have tensile strengths well above those predicted by the ROM. This increased strength correlates well with fiber spacing but not with prior composite deformation strain. Such microstructural scale strengthening also apparently explains data obtained previously on similar composites. This dependence of strength on scale serves to emphasize the hybrid nature of these fine composites as a mixture of conventional composites and microstructurally strengthened materials. It is worth emphasizing that while fiber spacing has been emphasized as the parameter to describe strengthening in the present work, it is better to think of this parameter in more general terms as representing the influence of the overall microstructural scale. Additionally, fiber size and shape undoubtedly play a concurrent role with that of fiber scale and their importance should rise as the volume fraction of the stronger fiber phase is increased. © 1981

Fabricability of and microstructural development in cold worked metal matrix composites

In some cases, in situ composites may be fabricated by mechanical working even when they contain phases which are inherently mechanically incompatible in bulk form. An extreme case is provided by composites containing low volume fractions of brittle chromium. The initial microstructure plays an important role in the fabricability of these composites and several examples of this are provided by our work. During wire drawing, interphase spacing is reduced in an axially symmetric manner irrespective of whether or not a lamellar or ribbon-like morphology is developed. The reduction in interphase spacing with rolling strain is similar to that observed during drawing in spite of the externally imposed plane strain conditions accompanying deformation by rolling. © 1984

The processing and properties of heavily cold worked directionally solidified Ni-W eutectic alloys

Certain two phase metallic alloys display impressive strengths following extensive deformation processing. Provided an appropriate phase morphology and/or texture is developed initially, somewhat surprising combinations of metals (e.g., copper-chromium) can be so processed. Thus this scheme offers the possibility for developing high strength metal matrix composites at a comparatively low price. In the work described, we consider another material combination-the Ni-W directionally solidified eutectic-as a candidate for this interesting class of material. This alloy can be cold worked to true deformation strains of four. The tensile strengths of alloys so deformed are impressive (2470 MPa), but so are those of the cold worked nickel-tungsten solid solution which is a component of the eutectic. Based on the work-hardening behavior of tungsten and on a recently advanced model which qualitatively explains the strengths of heavily cold worked two phase metals, it is argued that further deformation processing of these alloys would lead to substantially higher strengths. Estimates on the fracture toughness of the cold worked eutectic are made from tensile properties. Estimated toughnesses are remarkably high and point to the possibility that this process can produce high strength-high toughness metallic materials to a degree not possible via conventional processing. © 1986 The Metallurgical of Society of AIME