BME 320: Lecture #4, Part 3 Review (Metals: Dislocations & Deformation, Strengthening Metallic Biomaterials)

Dislocations are created in crystalline materials by . . .

the application of stress

Discounting stress-induced dislocations, dislocations may already . . . but external stress . . .

exist in metals as imperfections; can lead to their multiplication

Dislocations move when . . .

the material is strained and a critical shear stress threshold is reached

Movement of dislocations corresponds to . . . and constitutes the basis of . . . and . . .

rearrangement of atomic structure; plastic deformation; strengthening

. . . corresponds to plastic deformation of a material

dislocation movement across a single grain (crystal)

Restricting dislocations can make materials . . ., while easing dislocations can make materials . . .

stronger; easier to deform

Dislocations cannot move easily across grains when . . .

lattice organization differs significantly between neighboring grains (i.e. high angle of grain misalignment)

The greater the difference in lattice orientation between neighboring grains, the more likely it is that . . ., leading to . . .

dislocations will accumulate at the grain boundary; strengthening of the material

dislocation sliding

The movement of a dislocation through a crystal lattice under an applied stress, producing permanent (plastic) deformation

dislocation pile-up

The accumulation of multiple dislocations at an obstacle, such as a grain boundary, precipitate, or other lattice defect, that impedes further dislocation motion

Describe how grain-boundary misorientation (grain angle) influences material strengthening through its effects on dislocation motion and dislocation pile-up.

As grain-boundary misorientation (grain angle) increases, the crystallographic mismatch between adjacent grains becomes greater, making it more difficult for dislocations to move across the grain boundary. High-angle grain boundaries therefore act as stronger barriers to dislocation sliding than low-angle grain boundaries. When dislocations are unable to pass through a boundary, they accumulate and form dislocation pile-ups, which impede further plastic deformation and increase the material's strength. Thus, larger grain angles generally promote strengthening by restricting dislocation motion and enhancing dislocation pile-up effects.

What is the relationship between grain size and material strength?

Materials with smaller grains--and thus larger grain boundaries--tend to be stronger than those with large grains and smaller grain boundaries.

Explain how increasing grain boundaries can strengthen materials by making it more difficult for them to deform plastically.

Plastic deformation in metals occurs through the movement of dislocations along slip planes. Grain boundaries act as barriers to this motion because neighboring grains have different crystallographic orientations, making dislocation transmission more difficult. Increasing the number of grain boundaries increases the frequency with which dislocations are obstructed, promoting dislocation pile-ups and requiring greater applied stress for continued deformation. Consequently, fine-grained materials are typically stronger than coarse-grained materials.

Manipulating grain size can be used to . . .

improve mechanical properties

Hall-Petch relationship

Theorizes the relationship between grain size and mechanical strength, asserting that a material's yield strength increases as its grain size decreases because grain boundaries act as barriers to dislocation motion. Smaller grains create more grain boundaries, which impede dislocation sliding and promote dislocation pile-up, thereby increasing the stress required for plastic deformation.

Hall-Petch equation: . . .

sigmay (yield stress) = sigma0 (initial stress) + kd^-m, where k is a material-specific constant, d denotes grain size, and m represents a material-specific power

yield stress

The stress corresponding to the yield point at which the material begins to deform plastically

The Hall-Petch relationship posits there exists a(n) . . . relationship between change in yield strength and grain size to . . .

inverse; some power 'm'

Metallic biomaterials with smaller grains (microstructure) are . . . than those with larger grains.

stronger