Nanostructured Metallic Materials Processing and Mechanical Properties Sung Whang Historical Use of Metals • • • • • • Gold Copper Bronze (Cu–Sn) Brass (Cu-Zn) Iron Steel ~ 35,000 years ago ~ 4,000 BC ~ 1,200 BC 2,000 – 1,000 BC ~ 1,200 BC ~ 500 BC Development of strong Metallic Materials • i) Alloying with two or more elements by melting or other atomic mixing technique. • Ii) Strengthening metals by reducing grain size or other second phase or precipitates, etc,. • Iii) Permanent or plastic deformation increase hardening and strength – called strain hardening or work hardening. In the microscopic terms, it means “Increase dislocation density” from 103 /mm2 up to max 109 /mm2. Polycrystalline Metals Yield Stress vs. Ave grain diameter in Brass Hall-Petch relation Hall-Petch Relation • σy = σo + k d-1/2 • Where σy is yield strength, σo yield strength for a very large grain or reference point, k material proportional constant, and d average grain diameter. Dislocations Dislocations in Deformed Metals Strain Hardening Classification of Metals by Grain size • Nanostructured metals: • A nanostructured material is made of grains or other microstructural entities that have average size 100 nm or less in length at least in one dimension. Other entities include precipitates, second phase, a third phase, etc. • Ultrafine grained metals (UFG) A materials that is made of grains of 100 nm – 1,000 nm diameter. But, practically often grain refined metals contain a mixture of both nano-grains and ultrafine grains. An alloy of such a mixture possess excellent strength and ductility. • Coarse grained metals (CG) or Conventional metals • grain size larger than 1,000 nm (1 micron) – few mm in diameter. Processing • ( a ) Severe Plastic Deformation: Starting materials have a bulk form. • ( b ) Mechanical Attrition: Starting materials have powders or fine pieces • ( C ) Electrodeposition: Anode – metal bar • ( d ) Amorphous Metals Route: Devitrify amorphous metal into fine crystalline precipitates (1) Severe Plastic Deformation High Pressure Torsion Equal Channel Angular Pressing Severe plastic deformation & Grain refinement • 1). SPD at/below o.4 Tm produces dislocation density as high as 1012 / mm2 under hydrostatic pressure. • 2). A portion of high density dislocations transform into grain boundaries under the proper heat treatment. Thus, this creates very fine grains of 100 nm – 500 nm in diameter. Pure Ti after ECAP Commercially pure Ti – 4 passes Grain size 300 – 400 nm Pure Ti + Thermomechanical Treat grain size 100 nm (2) Mechanical Attrition or Mechanical Alloying Pure metal or two metals or more Layered particle in mechanical alloying Mechanism of Mechanical alloying • (a) Large impact on particles by milling balls under protective atmosphere leads to large strain and very high dislocation density in the matrix. The grain size decreases as the milling progresses. • (b) For two different metals, the attrition leads to atomic mixing of two metals. • (c) The resulted grain size becomes much smaller for higher melting-temperature metals. Consolidation of milled powders • 1) Cold pressing high temperature sintering Issue: grain growth • 2) Cold pressing hot extrusion or hot isostatic pressing • 3) In-situ consolidation of low melting metals into spheres of a few mm diameter by cryomilling in liquid- N (77 K) or liq-Ar (87 K) . In-situ consolidation room temperature, inert gas atmosphere True stress-true strain for copper grain size 23 nm Mechanical Properties of Nanostructured Metals • Strength : The yield strength of nanostructured metals increases with decreasing grain size from 100 nm down to 20 nm, obeying Hall-Petch rule; But the yield stress decreases decreasing grain size below 20 nm. Instead of hardening, the material shows softening, i.e., the violation of Hall-Petch rule. • Ductility vs. grain size: The ductility is decreasing with decreasing grain size. Superplasticity • Definition: The ability of a polycrystalline metal to exhibit very high tensile elongation, say 200 %. • Superplasticity of metals facilitates sheet metal forming for manufacturing. • Superplasticity occurring temperature has a strong dependence on grain size. Summary on Mechanical Properties • 1) With decreasing grain size in nanostructured metals, the yield strength increases up to grain size of ~ 20 nm. Below 20 nm grains, the yield strength decreases with decreasing grain size, exhibiting a softening behavior. Thus, the deformation mechanism for nanostructured metals with above 20 nm grains is governed by dislocation pile-up mechanism while for the nano-grained material with grains less than 20 nm, the deformation is governed by dominant grain boundary sliding. Practical material can be produced from grain sizes from 20 nm to 1,000 nm. These metals contain nano-grains and ultrafine grains as well. 2) Nano- and ultrafine grained metals trade off ductility with higher strength. But, the increase in specific strength would be more than off-set any reduction in ductility. 3) When the grain size becomes ultrafine size or less, the alloy exhibits superplastic behavior at lower temperatures where the grain growth is minimal. 4) The fatigue testing shows that ultrafine grained metals exhibit superior performance in the high stress range compared to counterpart coarse grained metals (of same compositions). This is not the indication that nanograined metals with grains less than 20 nm would have the same fatigue behavior.