The increase in the stress needed for deformation as plastic deformation multiplies dislocations, which become entangled with each other. Strength rises but ductility falls. Also called strain hardening.
Japanese name: 加工硬化
Work hardening appears in 6 practice questions across 5 topics, including Metals and alloys, Heat treatment and metallic materials, Materials processing and joining.
Field: Metals and alloys
When a metal is plastically deformed, dislocations multiply and their density increases by several orders of magnitude; they attract and entangle one another through their stress fields and become harder to move. Further deformation therefore requires a larger stress. This is work hardening. It is why cold-rolled sheet and drawn wire are stronger than before working, and for pure metals that cannot be strengthened by heat treatment, 5000-series aluminum alloys, austenitic stainless steels and the like, work hardening is the main strengthening method. In exchange, ductility and toughness decrease.
For many metals, the relation between true stress and true strain in the plastic region can be approximated as σ = Kεⁿ, where n is called the strain-hardening exponent (work-hardening exponent) and K the strength coefficient. The larger n, the more readily the material strengthens as deformation proceeds and the less localized deformation spreads, so necking is delayed and uniform elongation is larger. Under constant volume, necking can be estimated to begin when the true strain equals n. Annealed FCC copper and austenitic stainless steels have large n and are well suited to stretch forming, where the sheet thins as it stretches (deep drawability depends mainly on the r-value).
When a work-hardened metal is heated, recovery occurs first, as dislocations rearrange and internal stresses fall; at higher temperatures, recrystallization produces new grains, lowering the strength greatly and restoring ductility. Alternating cold working and annealing allows large deformations without cracking. In hot working, done above the recrystallization temperature, recovery and recrystallization proceed during working, so almost no work hardening remains and large deformations can be applied with small forces, though it is inferior to cold working in surface oxidation and dimensional accuracy.
Answer: B. 5000 series (Al-Mg)
The 5000 series (Al-Mg) are non-heat-treatable alloys that get their strength from solid-solution strengthening by Mg and from work hardening by cold working. They have excellent corrosion resistance and weldability and are used for ships and tanks. The 2000 series (Al-Cu), typified by duralumin, are heat-treatable and strengthened by Al-Cu precipitates. The 6000 series (Al-Mg-Si) are also heat-treatable, using Mg-Si precipitates, and are widely used for extrusions. In addition, the 7000 series (Al-Zn-Mg) are the strongest heat-treatable alloys, while the 1000 series (pure aluminum) and 3000 series (Al-Mn) are non-heat-treatable. Learn to tell from the alloy system whether the strengthening mechanism is precipitation or work hardening.
Answer: B. Solution heat-treated and then artificially aged
In aluminum alloy designations, the four-digit number gives the alloy series (the 6000 series is Al-Mg-Si) and the temper designation after the hyphen gives the processing condition. T6 means solution heat-treated and quenched, then artificially aged by heating to about 150–200 °C, strengthened by fine Mg₂Si-type precipitates. Solution heat-treated and naturally aged at room temperature is T4, somewhat weaker than T6. Work hardening by cold working is designated H and is used for non-heat-treatable alloys such as the 5000 series. In addition, O means annealed (the softest condition) and F means as-fabricated. Because strength varies widely with temper even for the same alloy, drawings specify both.
Both strengthen by obstructing dislocation motion, but in work hardening the obstacles are the entanglements of dislocations multiplied by deformation, whereas in precipitation hardening they are fine precipitates formed by aging.
For drawing and rolling, it is generally expressed as the reduction in cross-sectional area (percent cold work). Note that it is calculated from the area, not the diameter.
Annealing above the recrystallization temperature produces new strain-free grains, and strength and ductility return to roughly their condition before working.
The 5000 series (Al-Mg) are non-heat-treatable alloys that get their strength from solid-solution strengthening by Mg and from work hardening by cold working.
Which of the following aluminum alloys cannot be strengthen…Work hardening by cold working is designated H and is used for non-heat-treatable alloys such as the 5000 series.
In the aluminum alloy designation “A6061-T6” (JIS; 6061-T6 …For lead the guide is 0.4 × (327 + 273) = 240 K, about −33 °C, so rolling at room temperature (25 °C) counts as hot working; recovery and recrystallization proceed during working and almost no work hardening remains.
Taking the recrystallization temperature as roughly 0.4 tim…In hot working, recovery and recrystallization take place during working, so almost no work hardening remains, and the option citing large residual work hardening is wrong.
When carbon steel parts of the same shape are made by casti…Because this softening invites further deformation, deformation concentrates in shear bands about 10 to a few tens of nm thick, and with no work hardening, a single shear band runs through the specimen in tension and it fractures with almost no elongation.
Which statement about plastic deformation of metallic glass…Recovery Recrystallization Dislocation Solid-solution strengthening Shrinkage cavity Metallic glass Creep Brass Columnar grains Stainless steel