Altering Material Properties Flashcards
The size of the grains, or average grain diameter, in a polycrystalline metal influences the mechanical properties. Adjacent grains normally have different crystallographic orientations and, of course, a common grain boundary. During plastic deformation, slip or dislocation motion must take place across this common boundary. The grain boundary acts as a barrier to dislocation motion for two reasons:
- Since the two grains are of different orientations, a dislocation passing into grain B will have to change its direction of motion; this becomes more difficult as the crystallographic misorientation increases.
- The atomic disorder within a grain boundary region will result in a discontinuity of slip planes from one grain into the other.
Why is a fine-grained material harder and stronger than a course grained material.
The former has a greater total grain boundary area to impede dislocation motion
What is the Hall Petch equation?
σy =σi+ (k / √d)
Where:
yield strength - σy
average grain diameter - d
The other factors are constants.
When is the Hall Petch Equation not valid?
For very large and very fine grain polycrystalline materials.
What is the advantage of grain size reduction?
Grain size reduction improves not only strength, but also the toughness of many alloys.
What is solid solution strengthening?
Alloying with impurity atoms that go into either substitutional or interstitial solid solution. Increasing the concentration of the impurity results in an attendant increase in tensile and yield strengths.
Why are alloys stronger than pure metals.
Alloys are stronger than pure metals because impurity atoms that go into solid solution ordinarily impose lattice strains on the surrounding host atoms. Lattice strain field interactions between dislocations and these impurity atoms result, and, consequently, dislocation movement is restricted.
What is strain hardening?
Strain hardening is the phenomenon whereby a ductile metal becomes harder and stronger as it is plastically deformed. Sometimes it is also called work hardening, or, because the temperature at which deformation takes place is “cold” relative to the absolute melting temperature of the metal, cold working. Most metals strain harden at room temperature.
Why does strain hardening decrease ductility?
The dislocation density in a metal increases with deformation or coldwork, due to dislocation multiplication or the formation of new dislocations, as noted previously. Consequently, the average distance of separation between dislocations decreases-the dislocations are positioned closer together. On the average, dislocation-dislocation strain interactions are repulsive. The net result is that the motion of a dislocation is hindered by the presence of other dislocations. As the dislocation density increases, this resistance to dislocation motion by other dislocations becomes more pronounced. Thus, the imposed stress necessary to deform a metal increases with increasing cold work.
How may the effects of strain hardening by removed?
An annealing heat treatment.
What is precipitation hardening?
The strength and hardness of some metal alloys may be enhanced by the formation of extremely small uniformly dispersed particles of a second phase within the original phase matrix; this must be accomplished by phase transformations that are induced by appropriate heat treatments. The process is called precipitation hardening because the small particles of the new phase are termed “precipitates”.
What are the two methods of precipitation hardening?
Solution heat treatment
Precipitation heat treatment
How does solution heat treatment work?
The treatment consists of heating the alloy to a temperature within the α phase field-say, T0-and waiting until all the β phase that may have been present is completely dissolved. At this point, the alloy consists only of an α phase of compositionC0. This procedure is followed by rapid cooling or quenching to temperature T1 which for many alloys is room temperature, to the extent that any diffusion and the accompanying formation of any of the β phase are prevented. Thus, a nonequilibrium situation exists in which only the α-phase solid solution supersaturated with B atoms is present at T1; in this state the alloy is relatively soft and weak. Furthermore, for most alloys diffusion rates at T1 are extremely slow, such that the single α phase is retained at this temperature for relatively long periods.
How does precipitation heat treatment work?
The supersaturated α solid solution is ordinarily heated to an intermediate temperature T2 (Figure 7.3a) within the α +β two-phase region, at which temperature diffusion rates become appreciable. Theβ precipitate phase begins to form as finely dispersed particles of composition Cβ which process is sometimes termed “aging.” After the appropriate aging time at T2 the alloy is cooled to room temperature; normally, this cooling rate is not an important consideration. Both solution and precipitation heat treatments are represented on the temperature-versus-time plot, Figure 7.3b. The character of theseβ particles, and subsequently the strength and hardness of the alloy, depend on both the precipitation temperature T2 and the aging time at this temperature. For some alloys, aging occurs spontaneously at room temperature over extended time periods. 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.
What is full annealing?
Very slow furnace cooling
