UNSW Sydney

TERM 1 2022 FINAL EXAMINATION

MATS6106: Mechanical Properties of Materials

Thursday, 5^th May 2022

14:00 – 16:00 (10 min reading + 1.5 h exam + 20 min Moodle submission)

1. TIME ALLOWED – 1.5 hours

2. READING TIME – 10 minutes

3. ONLINE MOODLE SUBMISSION TIME – 20 minutes

3. THIS EXAMINATION PAPER HAS 2 PAGES

4. TOTAL NUMBER OF QUESTIONS – 5

5. TOTAL MARKS AVAILABLE – 40

6. MARKS AVAILABLE FOR EACH QUESTION ARE SHOWN IN THE EXAMINATION PAPER

7. OPEN BOOK EXAMINATION

MATS 6106    Final Exam

The final exam paper will be released on Moodle at 2 pm on 5^th May. You will have 10 minutes downloading and reading time, and 1.5 h for answering the questions (2:00 – 3:40pm). You can either type your answers, save as a Word or pdf file and then submit it on Moodle, or write on paper by hand, then take a photo of the papers and upload them on Moodle. You will have 20 minutes after the end of the exam to submit your files (till 4:00pm). Note that late submissions will not be accepted – make sure you do not underestimate the time required for submission, and remember that demand on the submission server can be high at the end of the exam, so file upload may take several minutes.

If your internet connection fails at any time, you should take a photo/screenshot showing some evidence of the problem, clearing showing the current date and time. Send an email to Jianqiang Zhang ([email protected]) informing him of the problem as soon as possible.  You should also apply for special consideration (https://www.student.unsw.edu.au/special-consideration). You can email your files to Prof Zhang if the problem is specifically related to accessing Moodle.

Question 1 (7 marks)

A single crystal of copper was grown in the shape of a tensile bar with the [32] direction aligned with the tensile axis.

(a) Which of the {111}<110> slip system(s) would be most highly stressed when tension was applied along [32] and why is it? Show the slip-plane normal(s) and the slip direction(s) of this system(s) on the 100 standard stereographic projection shown below.

(b) What will be the ratio of the shear stress on this slip system to the tensile stress applied along [32]?

Question 2 (8 marks)

(a) On one set of axes, sketch the tensile stress-strain curves which might be obtained for a polycrystalline and a single crystal sample of the same material. Discuss any differences between the two stress-strain curves.

(b) For both mild steel and a fresh sodium chloride single crystal, the tensile stress-strain curves measured at a constant strain rate show initially a region of strain softening (decrease of tensile stress with strain) at the yield point. Explain in each case why this occurs and suggest how the effect might be eliminated.

Question 3 (9 marks)

a) A high-strength low alloy steel is strengthened by precipitates of TiC. If the volume percent of particles (f_V) is estimated to be 2%, what particle diameter (d) would be required to increase the strength by 150 MPa, assuming the particles are all spherical with the same diameter and distributed uniformly? For this low alloy steel, G=90 GPa, and the atomic diameter of iron is 0.248 nm. Assume the increased tensile yield strength is 3 times the Orowan shear stress, .

b) Calculate a limiting grain size of this high-strength low alloy steel due to the particle pinning effect of these TiC precipitates.

c) Discuss the particle pinning effect on mechanical property of the steel, referring to the significance of the particle dispersion parameter f_V/d.

Question 4 (8 marks)

For each of the following three materials, suggest two methods that you could use to increase the strength:

i. an aluminium 2024 alloy

ii. a medium carbon steel

iii. pure copper

Discuss the strengthening mechanisms involved in each of your suggested methods.

Question 5 (8 marks)

Account for the following observations:

i) the flow stress of a metal is generally found to be proportional to the square root of the dislocation density;

ii) the work hardening rate of an ordered alloy is generally higher than that of a disordered alloy of the same chemical composition;

iii) an over-aged alloy will work-harden more rapidly that a peak-aged alloy with the same chemical composition.

END OF EXAMINATION