Processing and mechanical properties of magnesium-lithium composites containing steel fibers
Deformation-processed metal-metal composites (DMMC) of Mg-Li alloys containing steel reinforcing fibers were prepared by infiltrating a preform of steel wool with the molten matrix. The Li content was varied to control the crystal structure of the matrix; Mg-4 wt pct Li is hexagonal close packed (hcp), while Mg-12 wt pct Li is body-centered cubic (bcc). The low carbon steel used as the reinforcing fiber is essentially bcc. The hcp/bcc and bcc/bcc composites were subsequently deformed by rolling and by extrusion/swaging and mechanically tested to relate the tensile strength of the composites to true deformation strain. The hcp/bcc composites had limited formability at temperatures up to 400 °C, while the bcc/bcc composites had excellent formability during sheet rolling at room temperature but limited formability during swaging at room temperature. The tensile strengths of the hcp/bcc composite rod and the bcc/bcc composite sheet and rod increased moderately with deformation, though less than predicted from rule-of-mixtures (ROM) calculations. This article presents the experimental data for these DMMC materials and comments on the possible effect of texture development in the matrix and fiber phases on the deformation characteristics of the composite material.
In situ strengthening of titanium with yttrium: texture analysis
In situ processing consists of heavily deforming a two-phase alloy of mutually immiscible elements to produce composite sheet or wire. In the well-studied Cu(fcc)-Nb(bcc) system, severe deformation by swaging and drawing reduces as-cast Nb filament phase thicknesses several hundred-fold after deformation. Cu-20 vol % Nb ultimate tensile strengths exceed 2000 MPa for material deformed to a true strain of η = 12, where η = ln (areaoriginal/areafinal).In an earlier study of in situ strengthening in immiscible hexagonal close-packed metals, Ti-50 vol % Y and Ti-20 vol % Y alloys were deformed by hot extrusion, hot and cold swaging. The composites were deformation processed to true strains as high as η = 7.6 to form a filamentary microstructure with filament thicknesses on the order of 0.1 μm. The deformation processing of these composites increased their ultimate tensile strengths from 318 to 945 MPa, but the specimens' original diameters were too small to allow deformation processing to the very high true strains achieved with the Cu-Nb composites.In this study, a larger casting of Ti-20 vol % Y was deformation processed to η = 12.8 in an attempt to achieve further refinement of the filament thickness. This composite formed the same filamentary microstructure up to η=7.27 observed in the earlier study of Ti-Y composites; however, at higher η values the filaments recrystallized into equiaxed grains, decreasing the ultimate tensile strength. X-ray texture analysis of the composite specimens showed a strong 〈10¯10〉 fibre texture in both the Ti and Y phases in the deformation processing range 2.25 ≤ η ≤ 7.27. This texturing is thought to constrain both the Ti and Y phases to deform by plane strain, which produces severe geometric restrictions on the ability of the plane straining filaments to achieve high η values without fracturing or recrystallizing.
Ductile two-phase alloys: Prediction of strengthening at high strains
When alloys containing two ductile phases are heavily deformed, composite-like microstructures develop and strengths well in excess of either of the phases in single phase form may be exhibited as a result of microstructure/dislocation density effects. In this paper a previously-published model for such strengthening is reviewed, and its application in a predictive capacity discussed. Flow stressvs fabrication strain data for the two components in single phase form and for one two-phase alloy are necessary for this purpose. The model may then be applied to predict strength for any other two-phase alloy as a function of composition, fabrication strain, and interphase spacing. The approach is illustrated using existing data for several alloy systems. For Ag-Fe and Cu-Nb alloys (with very limited mutual solubility) strengths can be predicted within 15 to 20 pct of the experimental values over the entire range of strains and volume fractions for which data are available. In systems where the potential for precipitation hardening exists (e.g., Cu-Fe) thermal history is important. When such hardening becomes a significant factor, the model cannot be used in its present form due to uncertainty over how to “add” the strengthening from this effect. Such hardening may, however, be useful in further improving the properties of these materials.