MCEN90014 · Materials Engineering
Diffusionless Transformations & TTT Diagrams
Diffusionless transformations are the Week 6 payoff of MCEN90014 Materials Engineering at the University of Melbourne, where cooling rate — not just composition — decides a steel's structure and hardness. It sits on the subject's Process→Structure→Property spine: austenite sheared into martensite, and the time–temperature–transformation (TTT) and continuous-cooling (CCT) diagrams that predict pearlite, bainite or martensite. Read one diagram correctly and the heat-treatment part of the final exam becomes routine.
What this chapter covers
- 01Distinguish diffusion-controlled from diffusionless (athermal, no composition change) transformations
- 02Describe martensite as FCC austenite shearing to a body-centred-tetragonal (BCT) cell with trapped carbon
- 03Explain why martensite is hard but brittle, and why it is tempered
- 04Use the M_start and M_finish lines: martensite amount depends on temperature, not time
- 05Compute the volume change of an FCC→BCC/BCT transformation from lattice parameters and atoms per cell
- 06Read an isothermal TTT diagram: start/finish C-curves, the nose, and the product fields
- 07Trace a quench–hold–quench path to the final microstructure (naming the final-quench martensite)
- 08Contrast TTT with the continuous-cooling (CCT) diagram and define the critical cooling rate in °C/s
- 09Rank pearlite, bainite and martensite by hardness from their microstructure
Volume change of the austenite→ferrite (FCC→BCC) transformation
- +2Volume per atom, austenite (FCC, n = 4): V_γ = a_γ³/4 = (0.360)³/4 = 0.046656/4 = 0.011664 nm³.
- +2Volume per atom, ferrite (BCC, n = 2): V_α = a_α³/2 = (0.287)³/2 = 0.023640/2 = 0.011820 nm³. Comparing per atom is essential because the two cells hold different numbers of atoms.
- +1Fractional change: ΔV/V = (V_α − V_γ)/V_γ = (0.011820 − 0.011664)/0.011664 = 0.000156/0.011664 = +0.0134.
- +1Interpret: ΔV/V = +1.34%, positive, so the lattice EXPANDS on FCC→BCC (BCC is less densely packed, APF 0.68 vs 0.74).
Key terms
- Diffusionless transformation
- A change of crystal structure by short-range, coordinated atom shear rather than long-range diffusion, so there is NO change in composition. It is essentially instantaneous (athermal), unlike diffusion-controlled pearlite or bainite that need time.
- Martensite (α′)
- The product of quenching austenite fast enough to outrun diffusion: FCC → body-centred-tetragonal (BCT), with carbon trapped interstitially (c > a). It is the hardest steel microstructure and brittle, so it is normally tempered.
- M_start / M_finish
- The temperatures at which martensite first forms (M_s) and is essentially complete (M_f, often M_90%) on cooling, drawn as horizontal lines. The fraction transformed depends on how far below M_s you cool, not on time; both fall as carbon or alloy content rises.
- TTT diagram
- Time–Temperature–Transformation (isothermal-transformation) diagram: temperature vs log time for a fully austenitised steel, with C-shaped start and finish curves. Read for holds at a fixed temperature.
- Transformation nose
- The point of fastest transformation on a TTT/CCT diagram (a balance between small driving force just below the eutectoid and slow diffusion far below it). Missing the nose on cooling gives martensite.
- Critical cooling rate (CCR)
- The slowest continuous cooling rate that yields a fully martensitic structure — the cooling curve just tangent to the CCT nose, in °C/s. Cool faster and you get 100% martensite; cool slower and some pearlite or bainite forms first.
- Pearlite
- A diffusion product of eutectoid austenite: alternating lamellae of ferrite and cementite (Fe₃C). Coarse pearlite forms just below 727 °C (small undercooling); fine pearlite forms nearer the nose and is harder.
- Bainite
- A partly diffusionless product formed below the nose: fine cementite needles or plates in a ferrite matrix. Upper bainite (higher temperature) has near-parallel needles; lower bainite (lower temperature) is finer and harder.
Diffusionless Transformations & TTT Diagrams FAQ
What is the difference between a TTT and a CCT diagram?
Both plot temperature against log time with C-shaped transformation curves, but a TTT (isothermal) diagram assumes you cooled instantly to a temperature and then HELD it, so you read start and finish times at a fixed level. A CCT (continuous-cooling) diagram overlays real cooling curves — the way actual parts cool — and its curves sit slightly lower and to the right. Use TTT for quench-and-hold heat treatments and CCT for constant-cooling-rate questions such as water, oil, air or furnace cooling.
Why is martensite so hard, and why do we temper it?
On a fast quench the FCC austenite shears to a body-centred-tetragonal cell without any diffusion, so the carbon stays trapped interstitially and heavily strains the lattice. That trapped carbon and the sheared, distorted structure block dislocation motion, making as-quenched martensite the hardest steel microstructure — and, for the same reason, brittle. Tempering (reheating below the eutectoid, then cooling) lets the carbon precipitate as fine cementite, giving tempered martensite: a little softer but far tougher, which is how most hardened parts are actually used.
Can AI help me with diffusionless transformations and TTT diagrams in MCEN90014?
Yes — Sia is an AI tutor that can explain the concepts step by step: it can walk you through tracing a quench–hold–quench path on a TTT diagram, show why any leftover austenite becomes martensite on the final quench, or check that your volume-change calculation divides by the right atoms-per-cell. It is a study aid for understanding the method, not a source of ready-made answers, and it will not sit an assessment or guarantee a grade. Always follow the University of Melbourne's academic-integrity and generative-AI rules for MCEN90014 and confirm what is permitted on Canvas.
Studying with AI? Sia — free AI mechanical engineering tutor works through MCEN90014 step by step.
Exam move
Anchor everything to one drawing: sketch a TTT diagram with its nose, the coarse- and fine-pearlite and bainite fields, and the three horizontal martensite lines (M_s, M_50%, M_f), then practise tracing a few quench–hold–quench paths to a named final microstructure — always finishing at room temperature so you remember that any untransformed austenite becomes martensite on the final quench. Because a formula sheet is provided in the final exam, spend your practice time on reading diagrams and choosing the right relation rather than memorising equations; the marks reward naming every phase and stating the direction (harder/softer, expands/contracts, martensite/pearlite) as much as the final number. Re-derive a volume change per atom (a³/n) and a critical cooling rate in °C/s by hand until the atoms-per-cell factor and the “faster-than-CCR gives martensite” direction are automatic, since a dropped factor or a reversed direction is the most common lost mark. The final exam is 10 questions of 10 marks each (100 marks, all compulsory) worth 50% of the subject with an exam hurdle, so treat every question as equally weighted, spend about a tenth of the exam on each, and confirm the exam duration on the timetable in Canvas.
