Question

DTO23/3 SECTION A Figure Ql illustrates four micrographs (A, B, C, and D) taken using a scanning electron microscope. For each micrograph: ) Draw a sketch outlining the main features. (ii) Identify the microstructure explaining your decision. (iüi) Explain in detail with the aid of illustrated sketches and graphs how these m (2 marks) I mark were formed. Where possible justify your answer for and against. (I mark) iv) Comment on the mechanical or environmental properties or an improvement strategy Each micrograph: 10 marks: total 40 marks Micrograph Q1A Micrograph Q1B Micrograph QID Mag. 2.000X

Answer 1

figure a represents fatigue fracture showing striations figure b represents ductile fracture showing void coalescence and shear figure c represents brittle fracture showing granular and bright appearance of grains figure d represents creep failure represents grain boundary sliding. fatigue fracture is caused by the crack initiation at the surface of the specimen gradually it grows in to the specimen forming striations/beachmarks. if the distance between those striations is high then it is high cycle fatigue and it is low it is called low cycle fatigue ductile fracture forms by void coalescence as the load continuosly applied.As the voids get together and finally collapsed whenever they reached their critical level.the microstructure appearance is dulland fibrous of grains brittle fracture occurs by cleavage when there is no sufficient plastic deformations mostly observed in hcp metals. the microstructural appearance is granlarand bright of grians creep failure occurs by sliding of the grian boundaries only when it happens if there are high angle boundaries because they tend to move very fast For faitigue failure, clean the surface clean and polish and remove any pores from the surface and induce compressive stresses on the surface. for ductile failure use of dispersoids increase the load carrying capacity and stop the migration of grains, for brittle materials remove any stress raisers that leads to the plain strain conditions for creep failure components with large grain size are preferable like single crystals