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Chapter 8: Problem 45

Cite three metallurgical/processing techniques that are employed to enhance the creep resistance of metal alloys.

Short Answer

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Question: Explain three metallurgical or processing techniques used to improve the creep resistance of metal alloys. Answer: Three techniques used to improve the creep resistance of metal alloys are precipitation hardening, grain boundary strengthening, and solid solution strengthening. Precipitation hardening involves heat treatment, which forms precipitates within the metal, hindering dislocation motion and increasing mechanical strength. Grain boundary strengthening alters the material's microstructure to create smaller grains with more grain boundaries, resulting in greater resistance to dislocation movement. Solid solution strengthening adds alloying elements to a base metal, disrupting the atomic lattice and introducing lattice strain, which hinders dislocation motion and increases deformation resistance.

Step by step solution

01

Technique 1: Precipitation Hardening

Precipitation hardening, also known as age hardening, is a heat treatment process used to enhance the creep resistance of metal alloys. This technique involves heating an alloy to a specific temperature, holding it there for a given period, and then cooling it down. This process leads to the formation of precipitates within the metal, which hinder dislocation motion and increase the alloy's overall mechanical strength. Consequently, the creep resistance of the material is improved, making it less susceptible to deformation under constant stress at high temperatures.
02

Technique 2: Grain Boundary Strengthening

Grain boundary strengthening is another processing technique employed to enhance the creep resistance of metal alloys. The process involves altering the microstructure of the material to create smaller grains with more grain boundaries. This can be achieved by rapid cooling or adding specific alloying elements. A higher number of grain boundaries result in a greater resistance to dislocation movement and improve the creep resistance. This is because grain boundaries serve as barriers to dislocation motion, making it more difficult for the material to deform over time, especially at elevated temperatures.
03

Technique 3: Solid Solution Strengthening

Solid solution strengthening is a metallurgical technique in which one or more alloying elements are added to a base metal to form a solid solution. The process enhances the creep resistance of the resulting metal alloy by altering its overall chemical composition and microstructure. The added elements disrupt the regular atomic lattice and introduce lattice strain, hindering dislocation motion, and increasing resistance to deformation. This results in a more creep-resistant material, capable of maintaining its shape and mechanical properties under constant stress at high temperatures.

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Most popular questions from this chapter

What is the maximum carbon content possible for a plain carbon steel that must have an impact energy of at least \(200 \mathrm{~J}\) at \(-50^{\circ} \mathrm{C} ?\)

The following tabulated data were gathered from a series of Charpy impact tests on a commercial low-carbon steel alloy. $$ \begin{array}{|cc|} \hline \text { Temperature }\left({ }^{\circ} \boldsymbol{C}\right) & \text { Impact Energy (J) } \\ \hline 50 & 76 \\ \hline 40 & 76 \\ \hline 30 & 71 \\ \hline 20 & 58 \\ \hline 10 & 38 \\ \hline 0 & 23 \\ \hline-10 & 14 \\ \hline-20 & 9 \\ \hline-30 & 5 \\ \hline-40 & 1.5 \\ \hline \end{array} $$ (a) Plot the data as impact energy versus temperature. (b) Determine a ductile-to-brittle transition temperature as the temperature corresponding to the average of the maximum and minimum impact energies. (c) Determine a ductile-to-brittle transition temperature as the temperature at which the impact energy is \(20 \mathrm{~J}\).

A cylindrical component constructed from an S-590 alloy (Figure 8.31) is to be exposed to a tensile load of \(20,000 \mathrm{~N}\). What minimum diameter is required for it to have a rupture lifetime of at least \(100 \mathrm{~h}\) at \(925^{\circ} \mathrm{C} ?\)

The following creep data were taken on an aluminum alloy at \(480^{\circ} \mathrm{C}\left(900^{\circ} \mathrm{F}\right)\) and a constant stress of \(2.75 \mathrm{MPa}\) (400 psi). Plot the data as strain versus time, then determine the steady-state or minimum creep rate. Note: The initial and instantaneous strain is not included. $$ \begin{array}{cccc} \hline \text { Time } \text { (min) } & \text { Strain } & \text { Time } \text { (min) } & \text { Strain } \\ \hline 0 & 0.00 & 18 & 0.82 \\ \hline 2 & 0.22 & 20 & 0.88 \\ \hline 4 & 0.34 & 22 & 0.95 \\ \hline 6 & 0.41 & 24 & 1.03 \\ \hline 8 & 0.48 & 26 & 1.12 \\ \hline 10 & 0.55 & 28 & 1.22 \\ \hline 12 & 0.62 & 30 & 1.36 \\ \hline 14 & 0.68 & 32 & 1.53 \\ \hline 16 & 0.75 & 34 & 1.77 \\ \hline \end{array} $$

The fatigue data for a steel alloy are given as follows: $$ \begin{array}{|c|c} \hline \text { Stress Amplitude [MPa (ksi)] } & \text { Cycles to Failure } \\\ \hline 470(68.0) & 10^{4} \\ \hline 440(63.4) & 3 \times 10^{4} \\ \hline 390(56.2) & 10^{5} \\ \hline 350(51.0) & 3 \times 10^{5} \\ \hline 310(45.3) & 10^{6} \\ \hline 290(42.2) & 3 \times 10^{6} \\ \hline 290(42.2) & 10^{7} \\ \hline 290(42.2) & 10^{8} \\ \hline \end{array} $$ (a) Make an \(S-N\) plot (stress amplitude versus logarithm of cycles to failure) using these data. (b) What is the fatigue limit for this alloy? (c) Determine fatigue lifetimes at stress amplitudes of \(415 \mathrm{MPa}(60,000 \mathrm{psi})\) and \(275 \mathrm{MPa}(40,000 \mathrm{psi})\). (d) Estimate fatigue strengths at \(2 \times 10^{4}\) and \(6 \times\) \(10^{5}\) cycles.
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