Strengthening Mechanisms Mohamed Hassan Khalili m.khalili@aui.ma Strengthening Mechanisms Outline + Types of strengthening mechanisms + Solid Solution + Dislocations + Grain size + Precipitate Types of strengthening mechanisms Materials Science – Dr. Mohamed Hassan Khalili 4 Types of strengthening mechanisms Strengthening mechanisms + Solute Impurities due to defect and diffusion + Dislocations High density of dislocations increases the strength + Grain size Decreasing the grain size in a polycrystalline + Precipitation Formation of particular phases in the material Solid Solution Materials Science – Dr. Mohamed Hassan Khalili 6 Solid Solution Solubility in solids + Unlimited Solubility: Some combinations of materials only form one phase when mixed, no matter the ratios of each species, e.g., water and alcohol, copper and nickel. These systems have unlimited solubility. Such systems can also form solid solutions, which are different from mixtures + Limited Solubility: In limited solubility systems, one species can form a single-phase solution with another below a certain composition. Any excess added afterwards precipitates out as a second phase, e.g., salt in water Solid Solution Solubility in solids Conditions for Unlimited Solid Solubility + The Hume-Rothery rules for unlimited solid solubility in alloys are: Size factor: The atoms/ions should be of a similar size, with less than a 15% difference in radii Crystal structure: The materials must have the same crystal structure Valence: The ions should have the same valence, otherwise they prefer to form compounds Electronegativity: The ions/atoms should have similar electronegativity, otherwise compound formation is preferred + These rules are necessary but not always sufficient to guarantee solid solution formation. Solid Solution Solid Solution Strengthening + In metallic materials, forming a solid solution often results in solid-solution strengthening, due to increased resistance to dislocation movement. + Degree of Solid-Solution Strengthening: It depends on 2 factors + First, a large difference in atomic sizes results in increased strength for the alloy + Second, the greater the amount of alloying element added, the greater the strengthening. The effects of several alloying elements on the yield strength of copper. Solid Solution Solid Solution Strengthening Effect of Solid-Solution Strengthening on Properties: + Yield strength, tensile strength & hardness of the alloy are better than those of pure metals + Nearly always, the alloy’s ductility is lower than that of the pure metal (zinc-copper is an exception). + Electrical conductivity in alloys is lower than in pure metals + Resistance to creep and strength at elevated temperatures is improved by solid-solution strengthening. The effect of adding zinc to copper on the properties of the alloy. Solid Solution Solid Solution Strengthening Isomorphous Phase Diagrams + A phase diagram shows the phases and their compositions at any combination of temperature & alloy composition Knowing which phase we have is crucial for material processing: • Casting: make sure that the metal is in a liquid phase. • Heat treatment: the alloy needs to maintain in solid phase. • Producing different phases produce different properties. Solid Solution Solid Solution Strengthening Isomorphous Phase Diagrams Example: You need to produce a Cu-Ni alloy having a minimum yield strength of 20,000 psi, a minimum tensile strength of 60,000 psi, and a minimum % elongation of 20%. You have in your inventory a Cu-20% Ni alloy and pure nickel. Knowing that copper is cheaper than nickel, design a method for producing castings having the required properties. Dislocations Materials Science – Dr. Mohamed Hassan Khalili 13 Dislocations Dislocation density and yield strength ππ¦π Theoretic ππ¦π Perfect single crystal Strain hardened polycrystalline material + The presence of dislocations make the material weaker than the theoretical strength Well annealed polycrystalline material + A higher density of dislocations makes the material stronger as they start resisting to slip mechanism. Dislocations density Dislocations π′ys > πys Strain hardening (cold working): + Strain hardening is the application of a stress higher than yield strength to a metal, deforming it and increasing strength by increasing the dislocations density. π Higher strength π′ys + Strain hardening leaves a permanent strain. + Strain hardening is effective in simultaneously shaping & strengthening metals at the expense of reducing ductility. πys Permanent strain Lower ductility (Lower elongation) π Dislocations Strain hardening (cold working): Properties vs percent of cold work ππ π0 Percent of cold work %CW = π0 − ππ × 100 π0 Dislocations Strain hardening (cold working): Properties vs percent of cold work Example: A 1-cm-thick copper plate is cold-reduced to 0.50 cm and later further reduced to 0.16 cm. Determine the total percent cold work and the yield strength of the 0.16 cm plate. Copper Dislocations Strain hardening (cold working): Properties vs percent of cold work Example: Design a manufacturing process to produce a 1-mm-thick copper plate having at least 448.2 MPa tensile strength, 413.7 MPa yield strength, and 5% elongation. Copper Dislocations Strain hardening (cold working): Some side-effects of strain hardening + Strain hardening causes grains to become elongated in the direction of applied stress From left to right, the above are grain structures of a low-carbon steel at 10%, 30%, and 90% cold work. → This results in anisotropic properties + Residual stresses from the applied stress remains within a structure after cold working These may be beneficial or undesirable and can be removed by a heat treatment called stressrelief anneal Dislocations Annealing: Annealing is a heat treatment used to remove some or all of the effects of cold working. + Low temperature annealing can eliminate residual stresses without affecting properties + High temperature annealing can reverse cold work + There are 3 stages of annealing: + Recovery, + Recrystallization, + Grain growth. Dislocations Annealing: Stages of annealing: Cold worked After recovery Recovery: + Heating causes the dislocations to move and form a polygonised subgrain structure. + The dislocation density and properties are unchanged, but residual stresses are removed. Dislocations Annealing: Stages of annealing: Cold worked After recovery After recrystallization Recrystallization: + Heating at higher temperatures forms new grains at the boundaries of the polygonised dislocation structure. + This reduces the number of dislocations, reducing strength and increasing ductility. + The recrystallization temperature is not fixed and depends on many variables. Dislocations Annealing: Stages of annealing: Cold worked After recovery After recrystallization After grain growth Grain growth: + Grain growth occurs at still higher temperatures, and causes smaller grains to merge into larger grains. + Grain growth occurs in most materials at high enough temperatures, even without previous recovery and recrystallization. Dislocations Annealing: Stages of annealing: Cold worked After recovery After recrystallization After grain growth Grain growth: + Grain growth occurs at still higher temperatures, and causes smaller grains to merge into larger grains. + Grain growth occurs in most materials at high enough temperatures, even without previous recovery and recrystallization. Grain Size Materials Science – Dr. Mohamed Hassan Khalili 25 Grain Size Unit cell orientations in grain A Polycrystalline material contains many grains separated by thin regions called grain boundaries. Unit cell orientations in grain B Grain Size + In a polycrystalline material grains are often randomly oriented, consequently some grain are more favorably oriented for slip during deformation. + It requires more stress to initiate slip in a polycrystalline metal in part due to the constraints at the grain boundaries. + Polycrystalline metals are thus strong then the same metal as a single crystal. + Decreasing the grain size, increases the strength. The expression used to describe this relationship is called the Petch-Hall equation. ππ¦ = π0 + πΎπ −1/2 Grain Size Solidification: + Solidification is casting materials from a liquid to a solid phase. + All metals and some ceramics and polymers go through solidification during processing. + Solidification starts when the material is below its melting temperature. Understanding the solidification process is important to control the grain size distribution Grain Size Solidification: Nucleation: + In solidification, nucleation is the formation of the first nanocrystallites from molten material. + The driving force for solidification is the free energy per unit volume 4 3 2 οG = ο° r οGv + 4ο° r ο³ sl 3 Grain Size Solidification: Nucleation: + In solidification, nucleation is the formation of the first nanocrystallites from molten material. + The driving force for solidification is the free energy per unit volume 4 3 2 οG = ο° r οGv + 4ο° r ο³ sl 3 Grain Size Solidification: The manner of the growth of solid nuclei depends on how heat is removed During solidification, 2 types of heat must be removed: latent heat of fusion and specific heat of the liquid Growth mechanisms: + Planar Growth: In a well-inoculated liquid at heterogeneous nucleation can occur equilibrium, Solidification occurs via planar growth in this case: the material solidifies layer by layer in front of a solidification front, which removes heat from the liquidsolid interface via conduction Grain Size Solidification: The manner of the growth of solid nuclei depends on how heat is removed During solidification, 2 types of heat must be removed: latent heat of fusion and specific heat of the liquid Growth mechanisms: + Dendritic Growth: In poorly inoculated liquids, the liquid has to be undercooled for solidification A small solid protuberance/dendrite at the interface is encouraged to grow since the liquid ahead of the solidification front is undercooled. Dendritic growth proceeds until the liquid is not undercooled any more, after which planar growth takes over Dendritic growth normally represents only a small fraction of the total growth in pure metals. Grain Size Solidification: Effect on the material properties: Dendrites in an aluminium alloy Dendrites formation effect on properties Precipitation Materials Science – Dr. Mohamed Hassan Khalili 34 Precipitation Dispersion Strengthening Limited Solubility When we exceed the limit of solubility a second phase forms (precipitate) Precipitation Dispersion Strengthening Solidification Eutectic phase diagram of Pb-Sn alloy + In dispersion-strengthened alloys, small, strong & hard particles of one phase (precipitate) are introduced into weaker but ductile second phase (matrix). + Dispersion-strengthened alloys may be produced by a eutectic reaction (e.g., cast iron, aluminum alloys), in which a liquid solidifies into 2 solid phases. Single phase 10% Dispersion strengthening (two phases) Precipitation Dispersion Strengthening Eutectic phase diagram of Pb-Sn alloy + In dispersion-strengthened alloys, small, strong & hard particles of one phase (precipitate) are introduced into weaker but ductile second phase (matrix). + Dispersion-strengthened alloys may be produced by a eutectic reaction (e.g., cast iron, aluminum alloys), in which a liquid solidifies into 2 solid phases. More complex microstructure at 30% Sn Precipitation Dispersion Strengthening Lead-Tin (Pb-Sn) alloys + In dispersion-strengthened alloys, small, strong & hard particles of one phase (precipitate) are introduced into weaker but ductile second phase (matrix). + Dispersion-strengthened alloys may be produced by a eutectic reaction (e.g., cast iron, aluminum alloys), in which a liquid solidifies into 2 solid phases. Precipitation Dispersion Strengthening To increase strength and toughness, the following guidelines should be followed: + (a) Matrix should be ductile, and dispersed phase should be hard and strong + Dispersed phase should be discontinuous, while matrix should be continuous + (b) Dispersed phase particles should be small & numerous to better prevent slip + (c) Dispersed particles should be round, i.e., less likely to initiate crack propagation + (d)Higher concentrations of dispersed phase increase strength Precipitation Dispersion Strengthening Example: Determine: (a) the solubility of tin in solid lead at 100°C, (b) the maximum solubility of lead in solid tin, (c) the amount of β that forms if a Pb-10% Sn alloy is cooled to 0°C, (d) the masses of tin contained in the α and β phases, 183 Assume that the total mass of the Pb-10% Sn alloy is 100 grams. 6% 2% 99.5% End of chapter Dr. Mohamed Hassan Khalili 41
0
advertisement
Download
advertisement
Add this document to collection(s)
You can add this document to your study collection(s)
Sign in Available only to authorized usersAdd this document to saved
You can add this document to your saved list
Sign in Available only to authorized users