**Objective:** To study the time and temperature variations in the hardness of Al-4% Cu alloy on isothermal aging.

Introduction

Materials can be hardened by inhibiting the motion of crystal defects called dislocations. In pure metals, the presence of defects (such as vacancies, interstitials, dislocations and grain boundaries) can enhance the strength. In single phase alloys, additional resistance to deformation may arise from the presence of foreign atoms. In two-phase alloys, additional stress is needed to enable the dislocation to intersect the second-phase particles. A finely dispersed precipitate may, therefore, strengthen the material. This phenomenon is termed precipitation hardening.

Procedures

1. Stamp the five 2024 Al specimens with an identifying mark.

2. Measure the hardness of all of the specimens using Rockwell B.

3. Place all five in a heat-treatment crucible and into a furnace for solution treatment at 500°C (930°F) for 30 minutes.

4. Natural Aging - Remove one specimen and drop into a pail of water. Once the specimen is cool, measure the Rockwell B hardness at intervals of approximately 30 min, 90 min, 1 day, and 1 week for this specimen. It will be necessary for one member to come back to the lab during the week when the lab is in session to measure the hardness.

5. Artificial Aging - Remove the remaining four specimens and drop them into a pail of water. Once the specimens are cool remove them from the quenching bucket. Measure the Rockwell B hardness. Next, transfer the specimens to a furnace set at 190°C (370°F).

Remove one sample after 3 min, 10 min, 60 min and 90 min. Quench into water and measure the Rockwell B hardness. After one week, again measure the Rockwell B hardness of the 3- and 10-minute artificially aged specimens.

Write Up

1. Prepare a memo report.

2. Plot the hardness vs. time of aging for each aging temperature, using Excel.

3. How does aging temperature affect the time and hardness?

4. What happens to the 190°C (370°F) 3 and 10 minute specimens after one week? Why?

5. If you were going to use 2024 Al in an application at a temperature of 190°C (370°F), what problems could be encountered?

6. Discuss errors in this experiment and their sources.

**Attachment:-** assign.rar

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