Phase transitions and their attendant atomic rearrangement processes in polycrystalline substances are most important for tailoring the physical properties of modern engineering materials. Multiphase alloys, such as c-TiAl based intermetallics possess distinguished mechanical properties depending on their thermo-mechanical treatment history and thus their microstructure. Not surprisingly, both fundamental and industry-related research has been undertaken to find optimal process parameters. As these metallurgical investigations are often obtained off-situ, little is known about the kinetics of the phase transition, nor the path of atomic rearrangements. Here we report on novel, in-situ time resolved 2D diffraction and microscopy measurements which were conducted at elevated temperatures. The transition from a 2 -Ti 3 Al to c-TiAl has been followed in both reciprocal and real space and is found to appear homogeneously over the bulk and to occur through an oriented rearrangement of atoms. At higher temperatures, the transition reverses and starts to grow slowly and stepwise from the grain boundaries into irregular shapes which impinge leading to grain refinement.Usually, a phase transition can be monitored off the thermal equilibrium via dedicated experimental techniques such as Differential Scanning Calorimetry whilst metallurgical investigations such as microscopy and conventional diffraction based structure and phase analysis are performed off-situ on quenched and subsequently prepared samples. In this case, models then have to be used to reconstruct the conditions in the high temperature state. Little is known on the atomic arrangements at elevated temperatures, which can be assessed by neutron and X-ray diffraction. [1][2][3] The development of high-energy synchrotron X-ray diffraction brought a breakthrough in this field [4] especially as sophisticated mapping methods employing 2-dimensional (2D) detectors have evolved. [5,6] So far, those experiments were undertaken with accurately calibrated 2D detectors possessing read-out or scanning overhead times in the range of minutes. Some attention has been given to the development of time resolving setups, but mostly, 2D detectors have only been used in order to record a large solid angle to obtain good grain statistics for classical powder diffraction experiments. [7,8] In these types of experiments it is common to spin the sample during acquisition for a better powder average, [9] removing all the features of inter-and intra-grain relationships presented in this study. Complementary Laser Scanning Confocal Microscopy has been developed for highly time resolved high-temperature observations in real space. [10,11] Intermetallic titanium aluminides exhibit increasing technical importance for high-temperature applications in the automotive and aerospace industries. [12] We have investigated the phase evolution of the composition Ti-45Al-7.5Nb (concentration in atomic %) which were previously tempered for 5 min at 1320°C and 1335°C and subsequently oil quenched in orde...
A new experimental technique defined as concentric solidification has been developed to improve in-situ observations of solidification and high temperature phase transformations using laser scanning confocal microscopy (LSCM). The technique consists of applying a radial thermal gradient across a 10 mm diameter sample such that the maximum temperature is focused in the centre of the specimen. Careful control over the sample thickness, heating rate and peak temperature results in the formation of a liquid pool in the centre of the specimen. Surface tension balance between solid, liquid, gas and crucible result in minimal meniscus formation on the liquid pool, leading to a greatly enhanced in-situ observations. Examples of the range of observations possible as well as unique observations of segregation related phenomena are presented.
An experimental study has been conducted into the role of cooling rate on the kinetics of the peritectic phase transformation in a Fe-C alloy. The interfacial growth velocities of the peritectic phase transformation were measured in situ for cooling rates of 100, 50, and 10 K/min. In-situ observations were obtained using high-temperature laser scanning confocal microscopy (HTLSCM) in a concentric solidification configuration. The experimentally measured interface velocities of the liquid/austenite (L/␥) and austenite/delta-ferrite (␥/␦) interphase boundaries were observed to increase with higher cooling rates. A unique finding of this study was that as the cooling rate increased, there was a transition point where the L/␥ interface propagated at a higher velocity than the ␥/␦ interface, contrary to the findings of previous researchers. Phase field modeling was conducted using a commercial multicomponent, multiphase package. Good correlation was obtained between model predictions and experimental observations in absolute values of interface velocities and the effect of cooling rate. Analysis of the simulated microsegregation in front of the L/␥ and ␥/␦ interfaces as a function of cooling rate revealed the importance of solute pileup. This microsegregation plays a pivotal role in the propagation of interfaces; thus, earlier modeling work in which complete diffusion in the liquid phase was assumed cannot fully describe the rate of propagation of the L/␥ and ␦/␥ interfaces during the course of the peritectic transformation.
been observ~~~il~~situ utlhsing laser scanniJlg cont:oc,,1 1,111Cl"OS(:01?.Y (LSCM)." Well developed subboundaries with interfaCial' energies much smaller than that of delta-fen'ite grain boundaries fOfmed following transformation from austenite to delta-fen'ite on heating. Transfonnatioll stresses associated with the austenite to delta-fenite phase transt'onnation generate dislocations that subsequently recover into sub-boundaries by a process of polygonisation. Experimental evidence in suppol1 of this proposal was found in a ferritic stainless steel. Thermal cycling through the high temperature delta-ferrite/austenite/delta-fel1'ite phase b'ansformation leads to the development of a strong recovery substructure, wh.ich in turn. modifies the low temperature austenite decomposition product from WidmansUilten/bainite to polygonal fen'ite, with a commensurate chauge in hardness.
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