21-Mat-A5 Phase Transformations and Thermal Treatment · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, May 2014 — 10-Met-A5, Mechanical Behaviour and Fracture of Materials. Three hours, closed book, any non-communicating calculator permitted. Eight questions of 20 marks each; the rubric states that five questions constitute a complete paper and that only the first five appearing in the answer book are marked. All eight are answered here, because this set is a study resource rather than an exam script. Several questions ask explicitly for essay-format answers, and the marking scheme rewards clarity and organisation, so the discursive answers below are written as structured prose rather than as note form.
The printed exam header reads 10-Met-A5, Mechanical Behaviour and Fracture of Materials. The paper examines strengthening and deformation, creep and fatigue testing, fracture mechanics, toughening of engineering materials, deformation processing selection, and environmental degradation; it has no classical phase-transformation or heat-treatment (TTT/CCT diagram, hardenability, tempering-curve) questions. The answers below are written to the printed subject.
Reference texts. The answers below are keyed to the works normally recommended for this syllabus code:
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Conditions. SCC requires the simultaneous presence of a sustained tensile stress (residual or applied, and can be well below $\sigma_y$), a specific corrodent for the specific alloy/temper in question, and a susceptible microstructure; remove any one of the three and cracking stops. Classic pairings include austenitic stainless steel in hot chloride-bearing water, admiralty brass in ammoniacal environments ("season cracking"), and high-strength aluminium alloys (7xxx-T6) in humid or chloride-containing air. Example: a cold-worked brass fitting stored with residual tensile stress from forming, exposed to trace ammonia vapour, cracking within weeks with no externally applied load at all.
Appearance. Cracking is typically branched and can be intergranular or transgranular depending on the alloy/environment pairing, propagating by a combination of localised anodic dissolution at the crack tip (the crack advances along an active path while the crack walls behind it repassivate) and, in some systems, a hydrogen contribution. The fracture surface looks brittle — little to no macroscopic plastic deformation accompanies the crack even though the bulk alloy is perfectly ductile in air — and the component often shows extensive branching, secondary cracks, and a corrosion product or tarnish film on the crack faces.
Conditions. Atomic hydrogen enters the lattice — from a corrosion reaction's cathodic half-cell, from electroplating or acid pickling, from cathodic protection systems, from welding without adequate pre/post-heat, or from service in sour ($H_2S$) environments — and diffuses to regions of high triaxial stress (ahead of a notch or crack tip) or to internal trap sites (grain boundaries, inclusion interfaces, dislocations). High-strength steels ($\sigma_y \gtrsim 1000$ MPa, e.g. quenched-and-tempered fasteners or landing-gear steel) are especially susceptible because their high triaxial stress concentrations and limited crack-tip plasticity make them intolerant of even small hydrogen contents. Example: a high-strength bolt, acid-pickled and zinc-plated without an adequate hydrogen-relief bake, failing suddenly under static clamp load days after installation with no active corrosion visible at the fracture.
Appearance. Fracture is typically intergranular (along prior-austenite grain boundaries in a quenched-and-tempered steel) and shows classic delayed cracking — failure occurring after an incubation time under a static, sub-yield load, distinguishing it from an overload fracture. Internal hydrogen porosity/flaking can produce characteristic "fisheye" features on a fracture surface (a bright, coin-shaped region around an internal defect). Unlike SCC, hydrogen embrittlement can occur with no active corrodent present at the crack itself if the hydrogen was pre-charged earlier in processing, though in service the two mechanisms frequently overlap and can be difficult to separate by fractography alone.
Conditions. A cyclically stressed component operating in a corrosive (or even just moderately aggressive, e.g. humid or aqueous) environment shows crack initiation and growth rates well beyond what either the cyclic stress or the corrodent alone would produce, because each stress cycle mechanically ruptures the protective passive film at the crack tip, exposing fresh, highly reactive metal to the environment before repassivation can re-establish protection — a synergy the two damage modes do not show independently. Example: an offshore platform's welded tubular joint, cyclically loaded by wave action, in seawater; unlike the same joint's fatigue performance in air, corrosion fatigue shows no fatigue limit — the S–N curve continues to decline indefinitely with cycles rather than flattening — and the crack-growth rate is strongly frequency-dependent (lower frequency gives more time per cycle for the corrosion reaction, so more damage per cycle), a diagnostic signature that distinguishes it from purely mechanical fatigue.
Appearance. Multiple crack-initiation sites are common, frequently at corrosion pits rather than the single dominant slip-band initiation site typical of fatigue in air. The fracture surface is transgranular, still often showing fatigue striations/beach marks, but the surface is corroded or oxidised, sometimes obscuring the finer striation detail that a purely mechanical fatigue fracture would show clearly.
Conditions. In a flowing liquid, local pressure can drop below the liquid's vapour pressure (at a pump impeller vane, a ship propeller tip, downstream of a valve constriction, or on a hydraulic-turbine blade), nucleating vapour bubbles that are then swept into a higher-pressure region and collapse (implode) violently against or very near a solid surface. Each collapse generates a high-velocity liquid micro-jet and a local shock wave capable of transient pressures reaching the gigapascal range, repeatedly impacting a small area of the surface. Example: the suction-side surface of a centrifugal pump impeller handling water near its vapour pressure, or the tips of a ship's propeller blades.
Appearance. The mechanism is fundamentally mechanical (repeated micro-impact loading causing local plastic deformation, work hardening and eventually fatigue-like crack nucleation and material removal) rather than a chemical corrosion process, though a corrosive fluid can act synergistically to accelerate it ("cavitation corrosion"). The characteristic appearance is a rough, sponge-like or heavily pitted surface with numerous small, closely spaced craters, concentrated specifically in the region of the flow where the local pressure drop and subsequent bubble collapse occur — distinguishing it visually from the branched cracks of SCC, the delayed intergranular cracks of hydrogen embrittlement, or the striated crack faces of corrosion fatigue.
