Chap11pt3

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Published on April 17, 2008

Author: Renzo

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Slide1:  Chapter 11 (Part I) Thermal Processing of Metal Alloys Precipitation Hardening Slide2:  Part I: Thermal Processing of Metal Alloys Heat Treatment Precipitation Hardening Part II: Metal Alloys and Fabrication of Metals Outline of Chapter 11 Thermal Processing of Metal Alloys:  Thermal Processing of Metal Alloys Heat Treatment of Steels (11.8) Hardenability Influence of quenching medium, specimen size, and geometry Annealing Processes (11.7) Annealing of ferrous alloys Full annealing Normalizing Process annealing Stress relief Precipitation Hardening (11.9) Strengthening Mechanisms (Methods) in Metals:  Strengthening Mechanisms (Methods) in Metals Restricting dislocation motion harder & stronger material Create barriers to block dislocation movement: Grain boundary: grain size reduction Point defect: solid solution strengthening Dislocation: strain hardening (cold work) Second phase: precipitation hardening Precipitation Hardening (or Age Hardening):  Precipitation hardening is commonly used to process copper alloys and other nonferrous metals for commercial use. The examples are aluminum-copper, copper-beryllium, copper-tin, magnesium-aluminum, and some ferrous alloys. Precipitation Hardening (or Age Hardening) Requisite Features on Phase Diagrams for Precipitation Hardening:  Two requisite features must be displayed by the phase diagrams of alloy systems for precipitation hardening An appreciable maximum solubility of one component in the other, on the order of several percent A solubility limit that rapidly decreases in concentration of the major component with temperature reduction Requisite Features on Phase Diagrams for Precipitation Hardening Precipitation Hardening:  Precipitation Hardening Which of the above systems can be precipitation-hardened? The answer is (c)! An appreciable maximum solubility of one component in the other, on the order of several percent A solubility limit that rapidly decreases in concentration of the major component with temperature reduction Precipitation hardening is accomplished by two different heat treatments:  Precipitation hardening is accomplished by two different heat treatments Solution heat treatment During solution heat treatment all solute atoms are dissolved to form a single-phase solid solution Quenching or rapid cooling to room temperature to form a nonequilibrium supersaturated solid solution (to prevent diffusion and the accompanying formation of any second phase) Precipitation hardening is accomplished by two different heat treatments:  Precipitation hardening is accomplished by two different heat treatments Precipitation heat treatment (aging) The supersaturated solid solution is heated to an intermediate temperature within the two-phase region at this temperature diffusion rates become appreciable The precipitates of the second phase form as finely dispersed particles. (aging) Precipitation Hardening: Temperature vs. Time Plot:  Precipitation Hardening: Temperature vs. Time Plot FIG. 11.20 Schematic temperature-versus-time plot showing both solution and precipitation heat treatments for precipitation hardening. Slide11:  Microstructure of Copper-Beryllium before and after Precipitation Hardening (a) Solution heat-treated (b) Precipitation hardened (x 750) Slide12:  Microstructure of A 7150-T651 Aluminum Alloy after Precipitation Hardening Fig. 11.24 A transmission electron micrograph showing the microstructure of a 7150-T651 aluminum alloy (6.2Zn-2.3Cu-2.3Mg-0.12Zr-balanced Al) that has been precipitation hardened. The light matrix phase is an Al solid solution. Mechanism of Precipitation Hardening:  Formation of very small particles of a second, or precipitate, phase. During precipitation hardening, lattice strains are established at the precipitate-matrix interface. There is an increased resistance to dislocation motion by these lattice strains in the vicinity of the microscopically small precipitate particles. Mechanism of Precipitation Hardening A Microstructural Sketch of Copper-Beryllium after Precipitation Hardening:  A Microstructural Sketch of Copper-Beryllium after Precipitation Hardening Precipitates Grain Boundary Overaging in Precipitation Hardening:  Overaging With increasing time, the strength or hardness increases, reaches a maximum, and finally diminishes. This reduction in strength and hardness that occurs after long time periods is known as overaging. FIG. 11.21 Schematic diagram showing strength and hardness as a function of the logarithm of aging time at constant temperature during the precipitation heat treatment. Overaging in Precipitation Hardening Precipitate Effect on Tensile Strength:  2014 Al Alloy: • TS peaks with precipitation time • Increasing T accelerates process Adapted from Fig. 11.25 (a) and (b), Callister 6e. Precipitate Effect on Tensile Strength Slide17:  • %EL reaches minimum with precipitation time Adapted from Fig. 11.25 (a) and (b), Callister 6e. Precipitate Effect on Ductility Artificial Aging and Natural Aging:  Artificial aging Aging at some temperature higher than room temperature Natural aging Some solution-treated and quenched alloys age at room temperature Natural aging requires long times — often several days — to reach maximum strength. The peak strength is higher than that obtained in artificial aging, and no overaging occurs. Artificial Aging and Natural Aging Typical Precipitation Hardened Alloys:  Typical Precipitation Hardened Alloys Al 2014 Forged Aircraft Fittings, Al Structures 2024 High strength forgings, Rivets 7075 Aircraft Structures, Olympic Bikes Cu Beryllium Bronze: Surgical Instruments, Non sparking tools, Gears Mg AM 100A Sand Castings AZ80A Extruded products Ni Rene' 41 High Temperature Inconel 700 up to 1800F Fe A-286 High Strength Stainless 17-10P Slide20:  Step 1: Solution-treat at a temperature between 340 to 451ºC to form a single-phase a solid solution Step 2: Quench to room temperature fast enough to prevent the precipitate phase  from forming Step 3: Age at a temperature below 340ºC to from a fine dispersion of  phase Precipitation Hardening in the First Aerospace Alluminum Alloy: The Wright Flyer Crankcase:  Aluminum has had an essential part in aerospace history from its very inception. An aluminum copper alloy (with a Cu composition of 8 wt%) was used in the engine that powered the historic first flight of the Wright brothers in 1903. Precipitation Hardening in the First Aerospace Alluminum Alloy: The Wright Flyer Crankcase F. W. Gayle and M. Goodway, Science, vol. 266, 1994, pp. 1015 The Aluminum-Rich Side of the Aluminum-Copper Phase Diagram:  The Aluminum-Rich Side of the Aluminum-Copper Phase Diagram The Wright Flyer Crankcase: Two Different Size Precipitates:  Two different size of the precipitate particles (10-22 nm vs. 3 nm) were observed using transmission electron microscopy. The original alloy had undergone precipitation hardening (10-22 nm) as a result of being held in the casting mold for a period of time and at a temperature that was sufficient to cause precipitation hardening. Since when the alloy was developed in 1903 until about 1993 (almost 90 years), the alloy had continued to age naturally (3 nm). The Wright Flyer Crankcase: Two Different Size Precipitates The Wright Flyer Crankcase:  Two types aging involved in the precipitation hardening for this case: Artificial aging from the original casting practice Natural aging over the last 90 years The use of a precipitation-hardened alloy in the first aerospace application occurred 16 years before the theory of precipitation hardening was proposed. The Wright Flyer Crankcase

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