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Forces grain boundaries & phases to become visible under microscope
& gets rid of impurities left over from grinding and polishing |
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| Grains are crystals that have 1 specific direction |
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| iron composition, creates ductility within the steel |
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| layers of iron and iron carbide, creates hardness & strength within the steel |
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| the art of preparing a metal's surface for analysis via grinding, polishing, & etching to study a metal specimen's microstructure |
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| Describe the importance of Metallography |
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| Metallography is important because it permits one to study the microstructure and learn more about a specimen's properties as a result. |
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| Why are grinding and polishing required before observation of metallograph? |
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To remove scratches & impurities on specimen surface
Grinding levels & cleans the specimens surface, and removes more material than polishing.
Polishing removes the artifacts of grinding, while removing very little material. |
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| Describe how grain size effects the mechanical properties of materials |
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Definition
| In general, the smaller the grain size, the stronger the material will be at low temperatures because most grain boundaries have high dislocation density. |
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| How does aging time affect the hardness of the specimen? |
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Definition
For any specific aging temperature the hardness of the specimen increases with time until it reaches a maximum point.
Aging time and temperature determine the size and number of precipitates. If the aging time is too long or at too high of a temperature, some precipitates will grow larger while others will wither and disappear; and the hardness will be directly and adversely affected. In other words, a decrease in the number of precipitates corresponds to a decrease in hardness. This reduction in strength and hardness resultant from overexposing a specimen for too long of a time to age hardening is known as over aging. The key point to note about the trend is that as time increases, the specimens will become harder but at a certain point, the material will become softer. |
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| How does the aging temperature affect the hardness of the specimen? |
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Definition
The aging process fulfills the goal of hardening the specimen by the uniqueness of the structure that is formed during this process. The increasing hardness is dependent on the fact that the size of the individual particles in the alloy's microstructure do not control the specimen's hardness. Rather, the density of precipitates comprising the structure proportionally controls the alloy's hardness. Thus, the overarching goal of the aging stage is to prevent these particles from growing larger. In pursuit of this goal, the influence of time and temperature are introduced as hardness manipulating variables. Aging time and temperature determine the size and number of precipitates. If the aging time is too long or at too high of a temperature, some precipitates will grow larger while others will wither and disappear; and the hardness will be directly and adversely affected. In other words, a decrease in the number of precipitates corresponds to a decrease in hardness. This reduction in strength and hardness resultant from overexposing a specimen for too long of a time to age hardening is known as over aging. These time and temperature dependent characteristics introduce the unique hardness attributes of an age hardening specimen, that during this process maximum hardness is reached sooner at higher temperatures and the maximum achieved hardness generally decreases as the aging temperature is raised. In other words, aging performed at lower temperatures results in a higher maximum hardness, but it takes a longer amount of time.
In regards to this laboratory experiment, at a lower temperature of 230°C, the 6061 aluminum specimen took longer to reach the maximum hardness peak. However, it had the higher hardness compared to that of the 280°C specimen. The specimen heated at 280°C was able to reach the maximum hardness peak much faster, but the hardness peak was lower than that of the 230°C specimen. |
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| Reduction in strength & hardness resultant from overexposing a specimen for too long of a time to age hardening. Time & Temperature dependent characteristic. |
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| Artificial Aging/Precipitation Heat Treatment |
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| When the age-hardening process occurs at an elevated temperature (above room temperature) |
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| Age-hardening at room temperature. Takes longer than Artifical Aging. |
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the process of hardening alloy by circulating finely dispersed precipitates of a solute in a supersaturated matrix. The process is governed by 3 stages:
1) Solution Treatment (alpha): Heated to T b/w Solvus & Solidus temperatures. Held there until all solute dissolved into a single phase & a nearly homogeneous solution is the product.
2) Quenching (alpha+theta): Quickly lower temp. & lock in the microstructure it had at the higher temp. by lowering temp. to a point where the atoms have very little mobility. Occurs w/o phase change & below the homogenization temperature.
3) Aging (alpha+theta):
*Facilitate formation of coherent precipitates.
*Heating the supersaturated solution below the solvus temperature.
*Formation of extremely small, uniformly dispersed particles of a 2nd phase within the original phase matrix. |
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| Cold Working/Strain Hardening/Work Hardening |
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Definition
Cold work is the strengthening of a metal by plastic deformation.
Cold work is when the metal is plastically deformed at a temperature lower than the material's recrystallization temperature.
Cold work makes a material stronger and harder, but also more brittle and less tough. aka Work Hardening or Strain Hardening.
Cold work = percentage decrease in cross-sectional area by plastic deformation
Grains become directional |
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| Plastic deformation of a metal |
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| can be defined as "work" done to a metal |
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when plastic deformation occurs above the recrystallization temperature
Grain size does not change, remain uniform. |
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| Springback (during coldwork) |
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Definition
| When the deformed material elastically returns partially back to its original shape. |
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1) Recovery: During recovery the metal is heated to a point where internal stresses are released but recrystallization is not yet happening.
2) Recrystallization: Happens when the metal is heated to the recrystallization temperature and grains start to reform. Increases softness, ductility, and toughness.
3) Regrowth: the smaller grains eat up the larger grains so there are more grains.
*Recrystallization temperature goes DOWN with INCREASED CW, SMALLER grain size, and INCREASED annealing time.
*Too high of an annealing temperature can cause excessive grain growth or induce incipient melting. |
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| Describe the mechanism of Work Hardening |
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Definition
Work hardening occurs when the applied stress exceeds the original yield strength. This process generates new dislocations and the strengthening of the material results from the interactions between these dislocations during the plastic deformation process. As the dislocation density increases, they strengthen the specimen by storing a fraction of the applied energy in the form of residual stress as dislocation movement becomes increasingly difficult as the dislocations hinder each other. This results in an increase in hardness.
As shown above in Figure 1-1, when the original yield strength is exceeded the yield stress of the material is improved. Notice, however, that the plastic region of the curve becomes shorter for each subsequent cold working operation performed on the specimen. Thus, the material will reach its ultimate yield stress much faster during plastic deformation, causing the material to be more prone to fracture. |
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| the stress at the maximum on the stress-strain curve. It is the maximum stress the specimen can sustain without failure. |
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| The maximum strain the material can sustain without failure. |
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| The reduction in cross-sectional area at the moment of fracture and at the moment of necking. |
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| Reversible deformation. When the force is no longer applied the object returns to its original shape |
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| Necking is the decrease in diameter in a specimen near the rupture point. It occurs as the sample leaves the elastic deformation region and begins to deform plastically. |
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Irreversible deformation
The object will return slightly back to its original form after plastic deformation but will not fully reform to the original form. |
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| true stress on the material at the time of rupture |
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| What properties of the material are altered by the tensile test? |
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Definition
As the material deforms-->
intermingling of the dislocations-->
increase in slip resistance-->
material Harder and Stronger, Ductility decreases, brittleness increases |
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| Measure very small values of strain by enhancing measurements for these very small values |
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| can be used to find poisson's ratio, lateral and axial strains |
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| What does it mean by negative poisson's ratio? Are there such materials? |
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Definition
| A negative Poisson's Ratio decribes how as the specimen is pulled in say, the lateral y-direction, the thickness becomes thicker in the axial direction or x-direction. A good example is polymer foams. However in most cases when you pull on something like silly putty, thickness decreases as it is stretched out. |
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