Temperature Transformation Diagram Research Articles

Effects of time, temperature, and steel composition on growth and dissolution of inclusions in liquid steels

The current knowledge of stability and growth/dissolution kinetics of inclusions in liquid steels is synthesised to understand their growth and dissolution behaviour. In particular, it is shown that the effects of time and temperature on the diffusion controlled growth and dissolution behaviour of inclusions can be represented by a set of time–temperature–transformation (TTT) diagrams. The TTT diagrams for the growth of oxide, nitride, and sulphide inclusions show the characteristic C shape, with each plot having an optimal temperature where the highest growth rate is achieved. The inclusion dissolution rate depends strongly on the initial inclusion radius and temperature. Apart from the stability of various inclusions in a given composition of steel, the diagrams provide important kinetic information for diffusion controlled growth and dissolution of oxide, nitride, and sulphide inclusions.

Mineral assemblages formed in Oxford Clay fired under different time–temperature conditions with reference to brick manufacture

SUMMARY Time–temperature–transformation (TTT) diagrams have been determined for high-lime (10.2%) and moderate-lime (5.2%) bulk raw materials from the Peterborough Member of the Oxford Clay Formation of King’s Dyke [TL 525 297] and Orton [TL 517 293] brickworks. Study of the chemistry and mineralogy of the brickclays provides the starting point for 79 firing experiments on each raw material, at temperatures between 800 and 1100°C, and for firing times at maximum temperature from 15 minutes to 336 hours. Quantified X-ray diffraction of each fired briquette enables individual TTT diagrams to be constructed for each phase. Finally, a summary diagram shows the distribution of mineral phase assemblages in time–temperature space. The original mineralogy (quartz, illite, kaolinite, chlorite, calcite, aragonite, K-feldspar, albite, pyrite, gypsum, anatase and apatite) changes on heating to a mixture of calc-silicates (anorthite, melilite and pyroxene) with quartz, hematite, anhydrite and glass in commercial bricks and also wollastonite and cordierite when longer firing times and temperatures are used. The mineralogical changes can be followed in quantified equations using the actual compositions and quantities of the phases. The calc-silicates are produced by lime (from calcite) reacting with the clay minerals or their breakdown products such as metakaolinite. The glass has granitic affinities. The data provide a case for considering shorter commercial firing times.

Time–temperature–transformation diagram and microstructures of bulk glass forming Pd40Cu30Ni10P20

Isothermal crystallization studies were performed on the bulk glass forming alloy Pd40Cu30Ni10P20 in the undercooled liquid region between the glass transition and liquidus temperature, resulting in a complete time–temperature–transformation (TTT) diagram for crystallization. The TTT diagram shows a typical “C” shape with the nose at 50 s and 680 K. Assuming steady state nucleation and a diffusion-controlled growth rate, the TTT diagram was successfully fit over the entire range of the measurement. The microstructure after isothermal crystallization shows a modulation in Cu and P for all degrees of undercooling. The primary solidified phase is Cu3Pd, which forms distinct dendrites at low undercooling. From additional constant cooling experiments, the critical cooling rate to bypass crystallization was determined to be 0.33 K/s.

Application of artificial neural network for prediction of time–temperature–transformation diagrams in titanium alloys

A model of artificial neural network for simulation of time–temperature–transformation (TTT) diagrams for titanium alloys was designed. Standard backpropagation multilayer feedforward network was created and trained using data from published literature. The influence of aluminium, vanadium, molybdenum and oxygen on transformation kinetics in titanium alloys was assessed on the base of the trained neural network. The results are in good agreement with what is expected from phase transformation theory. Using the model, TTT diagrams for some commercial alloys were predicted. A graphical user interface was created for the use of the model.

Experimental determination of a time–temperature-transformation diagram of the undercooled Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 alloy using the containerless electrostatic levitation processing technique

High temperature high vacuum electrostatic levitation was used to determine the complete time–temperature–transformation (TTT) diagram of the Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 bulk metallic glass forming alloy in the undercooled liquid state. This is the first report of experimental data on the crystallization kinetics of a metallic system covering the entire temperature range of the undercooled melt down to the glass transition temperature. The measured TTT diagram exhibits the expected ‘‘C’’ shape. Existing models that assume polymorphic crystallization cannot satisfactorily explain the experimentally obtained TTT diagram. This originates from the complex crystallization mechanisms that occur in this bulk glass-forming system, involving large composition fluctuations prior to crystallization as well as phase separation in the undercooled liquid state below 800 K.

Crystallization and melting behaviors of poly(aryletheretherketone) (PEEK) on origin of double melting peaks

AbstractThe crystallization and melting behaviors of poly(aryletheretherketone) (PEEK) films were investigated, using differential scanning calorimetry and metallurgy concepts. The shape of the time–temperature–transformation (TTT) diagram, established for PEEK, results from both nucleation and growth phenomena. The double melting behavior exhibited by isothermally crystallized PEEK samples are discussed through the TTT diagram and the influence of the thermal history in the molten state. The upper melting peak arises first and the lower melting peak is developed later. The location of such a second endotherm is shifted toward the higher temperature with increasing either the crystallization temperature or the annealing time while the location of the upper melting peak seems to be unchanged. The double melting behavior is related to a bimodal distribution in size and/or perfection of lamellae developed in a two‐step crystallization. With increasing temperature and/or annealing time in the molten state, the pattern of the endothermic curves is modified. The observed changes are discussed through two origins: the progressive disappearance of remnants of the former crystals and a thermal degradation leading to a cross‐linking of the polymer. © 1994 John Wiley & Sons, Inc.

Kinetics of the α‐to‐α′H Polymorphic Phase Transition of Ca2SiO4 Solid Solutions

The α‐to‐α′H transition of Ca2SiO4 solid solutions (C2S(ss)) is a nucleation and growth process. This process was shown on time–temperature–transformation (TTT) diagrams for C2S(ss) with different concentrations of foreign oxides (Na2O, Al2O3, and Fe2O3). The kinetic cutoff temperature and the activation energy for growth of the α′H phase increase steadily with increasing concentration of impurities. The activation energy for nucleation also increases above 950°C. The α′H phase, which exists in equilibrium with the α phase at 1280°C, is formed at a maximum rate at around 1100°C regardless of the chemical composition. The TTT diagrams enable us to predict, as a function of cooling rate, the phase constitution of C2S(ss) at ambient temperature.

Isothermal decomposition of supercooled austenite in steels

A brief rationalisation is made of the isothermal decomposition kinetics of supercooled austenite in steels. From the experimental evidence of the past several decades, it is concluded that each type of isothermal decomposition product, i.e. grain boundary allotriomorphs, Widmanstätten ferrite (and cementite), pearlite, upper bainite, lower bainite, lath martensite, and twinned martensite, has its own independent C-curve in time–temperature–transformation (TTT) diagrams. Special emphasis is placed on the isothermal transformation kinetics of martensite. It is demonstrated that martensite transformation follows C-curve kinetics in isothermal conditions; this is a general rule and holds for all steels. The reason why most experimental TTT diagrams fail to display separate C-curves for different products is briefly explained. A temperature–composition–product (TCP) diagram is constructed for Fe–C alloys (plain carbon steels) to depict the general pattern of the decomposition process and to display the conditions for the formation of the various decomposition products.MST/1623

Isothermal decomposition of supercooled austenite in steels

A brief rationalisation is made of the isothermal decomposition kinetics of supercooled austenite in steels. From the experimental evidence of the past several decades, it is concluded that each type of isothermal decomposition product, i.e. grain boundary allotriomorphs, Widmanstätten ferrite (and cementite), pearlite, upper bainite, lower bainite, lath martensite, and twinned martensite, has its own independent C-curve in time–temperature–transformation (TTT) diagrams. Special emphasis is placed on the isothermal transformation kinetics of martensite. It is demonstrated that martensite transformation follows C-curve kinetics in isothermal conditions; this is a general rule and holds for all steels. The reason why most experimental TTT diagrams fail to display separate C-curves for different products is briefly explained. A temperature–composition–product (TCP) diagram is constructed for Fe–C alloys (plain carbon steels) to depict the general pattern of the decomposition process and to display the conditions for the formation of the various decomposition products.MST/1623

Thermodynamic analysis of isothermal transformation diagrams

AbstractA thermodynamic method has been developed to allow the prediction of isothermal transformation diagrams, starting simply from a knowledge of the chemical composition of the steel concerned. The method has been extensively tested and has been shown to be capable of faithfully reproducing the critical ‘bay region’ of time–temperature–transformation (TTT) curves. The TTT diagram has been treated as being composed of two overlapping ‘C’ curves, one representing the diffusional polygonalferrite and pearlite transformations, and the other representing the displacive Widmanstatten ferrite and bainite reactions. It is possible to predict relative shifts in these component curves, as a function of alloying element content, thus making the technique potentially useful in theoretical steel design. While the analysis is formally based on Russell's theory of incubation periods, it is believed that a number of difficulties prevent a fundamental interpretation of the results.

Direct Evidence for Three Transformation Mechanisms for the β→α Phase Change in Uranium - Chromium Alloys

PURE uranium exists in three allotropic forms: α, which is orthorhombic and stable up to 667° C; β, which is tetragonal and stable between 667° and 775° C; and γ which is body-centred cubic and is stable between 775° C and the melting point 1,132° C. In the pure metal the phase changes occur with great speed, and investigation of the mechanisms of the changes is extremely difficult. The addition of small concentrations (∼ 0.5 atomic per cent) of certain solutes, of which chromium is one, retards the β → α change so effectively that β can be retained at moderate cooling rates to sub-critical temperatures and then allowed to proceed isothermally. White1 and others2,3, using either dilatometric or metallographic techniques, showed that the time–temperature–transformation (TTT) diagram associated with isothermal β→α transformation in various dilute uranium alloys consisted of two separate C-curves. Metallographic and crystallographic investigations4–7 have shown conclusively that the low-temperature C-curve (hereafter referred to as C3), which in U–0.5 per cent Cr alloy extends from room temperature to about 400° C, is associated with a martensitic mechanism. It was supposed that the upper C-curve, covering the range 400°–620° C in this alloy, was associated with transformation involving thermally activated atomic movement. However, metallographic observation showed that the micro-structure of the α formed at temperatures between about 525° C and 620° C was quite different from that formed between 525° C and 400° C, suggesting that two formation mechanisms were possible, one in each temperature range. In order to explore this possibility further Dixon and Burke8 redetermined the TTT diagram for a U–0.5 atomic per cent Cr alloy using an electrical resistivity method which was regarded as a more sensitive technique than those used in previous investigations. Evidence was found to suggest that the upper C-curve was composed of two overlapping C-curves, intersecting at about 520° C as required by the two-mechanism hypothesis.