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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

The fatigue data for a brass alloy are given as follows: $$ \begin{array}{cc} \hline \text { Stress Amplitude (MPa) } & \text { Cycles to Failure } \\ \hline 170 & 3.7 \times 10^{4} \\ \hline 148 & 1.0 \times 10^{5} \\ \hline 130 & 3.0 \times 10^{5} \\ \hline 114 & 1.0 \times 10^{6} \\ \hline 92 & 1.0 \times 10^{7} \\ \hline 80 & 1.0 \times 10^{8} \\ \hline 74 & 1.0 \times 10^{9} \\ \hline \end{array} $$ (a) Make an \(S-N\) plot (stress amplitude versus logarithm of cycles to failure) using these data. (b) Determine the fatigue strength at \(4 \times 10^{6}\) cycles. (c) Determine the fatigue life for \(120 \mathrm{MPa}\).

A large plate is fabricated from a steel alloy that has a plane strain fracture toughness of \(82.4 \mathrm{MPa} \sqrt{\mathrm{m}}(75.0 \mathrm{ksi} \sqrt{\mathrm{in} .}) .\) If the plate is exposed to a tensile stress of \(345 \mathrm{MPa}(50,000\) psi) during service use, determine the minimum length of a surface crack that will lead to fracture. Assume a value of \(1.0\) for \(Y\).

A structural component in the form of a ( wide plate is to be fabricated from a steel alloy that has a plane-strain fracture toughness of \(98.9 \mathrm{MPa} \sqrt{\mathrm{m}}(90 \mathrm{ksi} \sqrt{\mathrm{in} .})\) and a yield strength of \(860 \mathrm{MPa}(125,000 \mathrm{psi})\). The flaw size resolution limit of the flaw detection apparatus is \(3.0 \mathrm{~mm}(0.12 \mathrm{in}\).). If the design stress is one-half the yield strength and the value of \(Y\) is \(1.0\), determine whether a critical flaw for this plate is subject to detection.

(a) Estimate the activation energy for creep (i.e., \(Q_{c}\) in Equation \(\left.8.25\right)\) for the S-590 alloy having the steady-state creep behavior shown in Figure 8.32. Use data taken at a stress level of 300 MPa (43,500 psi) and temperatures of \(650^{\circ} \mathrm{C}\) and \(730^{\circ} \mathrm{C}\). Assume that the stress exponent \(n\) is independent of temperature. (b) Estimate \(\dot{\epsilon}_{s}\) at \(600^{\circ} \mathrm{C}(873 \mathrm{~K})\) and \(300 \mathrm{MPa}\).

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. $$ \begin{array}{cccc} \hline \text { Time } \text { (min) } & \text { Strain } & \text { Time } \text { (min) } & \text { Strain } \\ \hline 0 & 0.00 & 18 & 0.82 \\ \hline 2 & 0.22 & 20 & 0.88 \\ \hline 4 & 0.34 & 22 & 0.95 \\ \hline 6 & 0.41 & 24 & 1.03 \\ \hline 8 & 0.48 & 26 & 1.12 \\ \hline 10 & 0.55 & 28 & 1.22 \\ \hline 12 & 0.62 & 30 & 1.36 \\ \hline 14 & 0.68 & 32 & 1.53 \\ \hline 16 & 0.75 & 34 & 1.77 \\ \hline \end{array} $$
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