Strain Hardening
Phenomenon where ductile metals become stronger and harder when they are deformed plastically is called strain hardening or work hardening. Increasing temperature lowers the rate of strain hardening. Hence materials are strain hardened at low temperatures, thus also called cold working. During plastic deformation, dislocation density increases. And thus their interaction with each other resulting in increase in yield stress. Strain hardening (work hardening) is the reason for the elastic recovery. The reason for strain hardening is that the dislocation density increases with plastic deformation (cold work) due to multiplication. The average distance between dislocations then decreases and dislocations start blocking the motion…
Solid-Solution Strengthening
Adding another element that goes into interstitial or substitutional positions in a solution increases strength. The impurity atoms cause lattice strain (Figs. 7.17 and 7.18) which can "anchor" dislocations. This occurs when the strain caused by the alloying element compensates that of the dislocation, thus achieving a state of low potential energy. It costs strain energy for the dislocation to move away from this state (which is like a potential well). The scarcity of energy at low temperatures is why slip is hindered. Pure metals are almost always softer than their alloys.
Strengthening by Grain Size Reduction
This is based on the fact that it is difficult for a dislocation to pass into another grain, especially if it is very misaligned. Atomic disorder at the boundary causes discontinuity in slip planes. For high-angle grain boundaries, stress at end of slip plane may trigger new dislocations in adjacent grains. Small angle grain boundaries are not effective in blocking dislocations. The finer the grains, the larger the area of grain boundaries that impedes dislocation motion. Grain-size reduction usually improves toughness as well. Grain size can be controlled by the rate of solidification and by plastic deformation.
Mechanisms of Strengthening in Metals
General principles. Ability to deform plastically depends on ability of dislocations to move. Strengthening consists in hindering dislocation motion. We discuss the methods of grain-size reduction, solid-solution alloying and strain hardening. These are for single phase metals. We discuss others when treating alloys. Ordinarily, strengthening reduces ductility.
Dislocations and Strengthening Mechanisms
Basic Concept of dislocation Dislocations can be edge dislocations, screw dislocations and exist in combination of the two. Their motion (slip) occurs by sequential bond breaking and bond reforming . The number of dislocations per unit volume is the dislocation density, in a plane they are measured per unit area. Characteristics of Dislocations There is strain around a dislocation which influences how they interact with other dislocations,…
Stress and Temperature Effects
Both temperature and the level of the applied stress influence the creep characteristics. The results of creep rupture tests are most commonly presented as the logarithm of stress versus the logarithm of rupture lifetime. Creep becomes more pronounced at higher temperatures. There is essentially no creep at temperatures below 40% of the melting point Creep increases at higher applied stresses. The behavior can be characterized by the following expression, where K, n and Qc are constants for a given material: dε/dt = K σn exp(-Qc/RT)
Creep
Creep is the time-varying plastic deformation of a material stressed at high temperatures. Examples: turbine blades, steam generators. Keys are the time dependence of the strain and the high temperature. The Creep Curve Creep in metals is defined as time dependent plastic…
Crack Initiation and Propagation
Stages is fatigue failure: I. crack initiation at high stress points (stress raisers) II. propagation (incremental in each cycle) III. final failure by fracture Stage I - propagation • slow • along crystallographic planes of high shear stress • flat and featureless fatigue surface…
Fatigue
Fatigue is the catastrophic failure due to dynamic (fluctuating) stresses. It can happen in bridges, airplanes, machine components, etc. The characteristics are: • long period of cyclic strain • the most usual (90%) of metallic failures (happens also in ceramics and polymers) • is brittle-like even in ductile metals, with little plastic deformation • it…
Ductile brittle transition
Ductile to brittle transition occurs in materials when the temperature is dropped below a transition temperature. Alloying usually increases the ductile-brittle transition temperature, for ceramics, this type of transition occurs at much higher temperatures than for metals. The notched-bar impact test can be used to determine whether or not a material experiences a ductile-to-brittle transition as the temperature is decreased. In such a transition, at higher temperatures the impact energy is relatively large since the fracture is ductile. As the temperature is lowered,the impact energy drops over a narrow temperature range as the fracture becomes more brittle. The transition can also be observed from the fracture surfaces, which appear fibrous or…
