Formation Of Lamellar Microstructure Research Articles

Phase field simulation of the lamellar precipitation in the TiC-ZrC system

TiC-ZrC system with a high hardness is a promising material being widely used in processing and manufacturing industries. The effect of elastic strain on the formation of lamellar microstructure was investigated in the present work through the methodology combing our thermodynamic data and two-dimensional Cahn-Hilliard/elastic strain energy model. It is shown that elastic induced lattice distortion and stress field at the boundaries affect the precipitation process. The simulation results agree well with the previous experimental results in terms of the variation of the periodicity for the elemental distribution and the morphology of regularity for lamellar microstructure. It is demonstrated that phase field method coupled with thermodynamic database can make precipitation models to access those proven thermodynamic models, which can be applied to simulation of various material systems.

Quantitative study of surface relief produced by formation of lamellar microstructure in a γ-TiAl based alloy

Surface relief produced during lamellar transformation in a Ti48Al2Cr2Nb (at.%) alloy was quantitatively studied. Three-dimensional representations and quantitative data exhibit there are three kinds of surface reliefs, single-tilt, regular-terrace and irregular-strip, corresponding to narrow-width, moderate-width and large-width lamellae, respectively. Observation on fine structure reveals that kinks and ledges of the three kinds of surface reliefs are different. All the surface reliefs are related to diffusional terrace-ledge-kink growth mechanism and only single-tilt surface relief combines displacive character. The γ lamellae grow forward by nucleation of growth islands one upon the other and increase length and width by kink migration. The kink density and size control the lateral growth and further determine the lamellar width.

Phase Field Simulation of the Lamellar Microstructure Formation in TiAl Alloys

The details of the lamellar microstructure in TiAl intermetallic alloys, such as the lath thickness and interfaces type governs the strength, ductility, creep properties and the long term microstructure stability of the alloy. The lamellar microstructure coarsening may induce property degradation of materials when the working temperature is high especially for the aero-engine turbine blades. At the same time, the reliability of the structure will decreases dramatically during long term working. In order to customize highly stable microstructure in high temperature, the phenomenon of lamellar formation during the solid-solid α→α2+γ phase transformation in fully lamellar TiAl alloys was investigated by phase field simulations. The lamellar structure morphology obtained with simulation is coincides with the experimental results. It is found that the independent nuclei and twin-related nuclei co-exist in the nucleation stage for the random noise nucleation. During growth stage, the independent nuclei grow slowly or disappear for the interfacial energy and elastic energy minimization. While most twin-related nuclei survived. During the following coarsening stage, big nuclei swallow small nuclei for interfacial energy minimization. The statistical character of twin area fraction changes complicated during these processes and it will be analyzed in detail. These findings could shed light on the understanding of the lamellar formation and coarsening mechanisms during phase transformation in TiAl alloys.

Effects of short time electric pulse heat treatment on microstructures and mechanical properties of hot-rolled Ti–6Al–4V alloy

Effects of electric pulse heat (EPH) treatment on microstructures and mechanical properties of hot-rolled Ti–6Al–4V alloy are investigated. It is interesting to find that, the original bimodal microstructure of hot-rolled Ti–6Al–4V alloy is transformed to the typical lamellar microstructure within several minutes, and the minutes spent for accomplishing microstructure transformation become less with the increased heat treatment temperature. Before the formation of lamellar microstructure, recovery and recrystallization happen to the original bimodal microstructure; after the formation of lamellar microstructure, the average grain size shows an increasing tendency with the increased temperature and maintaining time. Quasi-static compression test results show that, the critical failure strain of the recrystallized microstructure is improved compared with the hot-rolled Ti–6Al–4V alloy, once the recrystallized microstructure is transformed to lamellar microstructure, the critical failure strain shows a decreasing tendency. Dynamic compression test results show that, the susceptibility to adiabatic shear bands (ASBs) of the hot-rolled Ti–6Al–4V alloy is obviously declined after EPH treatment, particularly the initially transformed lamellar microstructure exhibits the lowest susceptibility to ASBs, owing to the deformation zones around the main ASB and the bifurcation of the main ASB. The alloy EPH-treated at 800 °C for 20 min with the smallest grain size of 200 μm shows the best dynamic ductility.

Effect of electropulsing on anisotropy behaviour of cold-rolled commercially pure titanium sheet

The specimens cut from the cold-rolled pure titanium sheet at 0°, 45° and 90° to the rolling direction were treated by high density electropulsing (maximum current density J=(7.22–7.96)×10 3 A/mm 2, pulse period t p=110 μs). The mechanical properties and microstructures of the cold-rolled, electropulsed and conventional annealed commercially pure titanium sheet were examined by using uniaxial tension test machine and optical microscope (OM), respectively. The results show that the deformation behavior of the electropulsed pure titanium sheet is significantly different from that of conventional annealed pure titanium sheet. The difference of the mechanical properties between the 0°, 45° and 90° direction specimens is almost diminished. It is mainly due to the increase in dislocation mobility and formation of lamellar microstructure after the electropulsing.

A numerical model for the description of the lamellar and massive phase transformations in TiAl alloys

A phenomenological modelling approach has been developed to describe the massive transformation and the formation of lamellar microstructures during cooling in binary γ titanium aluminides. The modelling approach is based on a combination of nucleation and growth laws which take into account the specific mechanisms of each phase transformation. Nucleation of massive and lamellar γ is described with classical nucleation theory, accounting for the fact that nuclei are formed predominantly at α/α grain boundaries. Growth of the massive γ grains is based on theory for interface-controlled reactions. A modified Zener model is used to calculate the thickening rate of the γ lamellar precipitates. The model incorporates the effect of particle impingement and coverage of the nucleation sites by the growing phase. The driving pressures of the phase transformations are obtained from Thermo-Calc based on the actual temperature and matrix composition. CCT diagrams and lamellar spacings calculated with the model are in good agreement with experimental data obtained from dedicated heat treatment experiments and from the literature. The model permitted investigating the influence of cooling rate, alloy chemistry and average α grain size upon the amount of massive γ and the average thickness and spacing of the lamellae. In particular it indicates that the Al depletion of the α phase during lamellar precipitation seems to play an important role in the suppression of the massive transformation at moderate cooling rate and in the large lamellar spacings observed at low cooling rate.

A Numerical Model for the Description of Massive and Lamellar Microstructure Formation in Gamma-TiAl

A phenomenological modeling approach has been developed to describe the massive transformation and the formation of lamellar microstructures during cooling in gamma titanium aluminides. The modeling approach is based on a combination of nucleation and growth laws which take into account the specific mechanisms of each phase transformation. Nucleation of both massive and lamellar γ is described with classical nucleation theory, accounting for the fact that nuclei are formed predominantly at α/α grain boundaries. Growth of the massive γ grains is calculated with a mathematical expression for interface-controlled reactions. A modified Zener model is used to calculate the thickening rate of the γ lamellar precipitates. The model incorporates the effect of particle impingement and rapid coverage of the nucleation sites due to growth. The driving pressures of the phase transformations are obtained form Thermo-Calc based on actual temperature and matrix composition. The model permitted investigating the influence of alloy chemistry, cooling rate and average α grain size upon the amount of massive γ and the average thickness and spacing of the lamellae. Calculated CCT diagrams were compared with experimental data collected from the literature and showed good agreement.

A structural aspect of α(α2) → lamellar α2 + γ transformation in γ-TiAl

A structural mechanism for fine lamellar microstructure formation (α(α2) → α2 + γ fine lamellae) in γ-TiAl-based as-atomized powders is discussed. The powder particles show a high density of basal stacking faults, formed by dissociation (on a basal plane) of a single screw dislocation jogged at multiple locations inside an α–α2 grain. dislocations emitted from the prior α(α2)–α(α2) grain boundaries which repeatedly cross-slip from the basal plane into prismatic plane, give rise to a staircase-like stacking-fault nucleation mechanism. The basal stacking faults eventually escape from the pinning jogs and grow to form γ lamellae. It is inferred that the thickness and distribution of γ lamellae can be modified by controlling the jog heights and the number of dislocations emitted from the grain boundary.

Grain coarsening in Ti – 45Al based titanium aluminides at supertransus temperature and subsequent lamellar structure formation

The phenomenon of grain growth in the α phase field and the effect of annealing time/grain size on the formation of lamellar microstructure upon subsequent cooling was studied in Ti - 45Al - 2Nb - 0.4Mn and Ti- 45Al - 2Nb alloys. The grain size of both alloys increased with annealing time and the grain growth exponent of Ti - 45Al - 2Nb- 0.4Mn and Ti - 45Al- 2Nb at 1350°C was observed to be 0.52 and 0.38, respectively. The difference in the grain growth exponent values has been attributed to the initial grain size and the grain boundary morphology. Interlamellar spacing in the lamellar microstructure, which formed upon furnace cooling from the α phase field, was also observed to increase with increase in annealing time, i.e. interlamellar spacing increased with increase in grain size.

Formation of lamellar microstructures in Al-rich TiAl alloys between 900 and 1100°C

The formation of lamellar microstructures in a Ti–62 at.% Al alloy at temperatures of 900–1100°C was investigated by transmission electron microscopy (TEM). The lamellar structure consists of γ-TiAl+r-Al 2Ti in the equilibrium state. h-Al 2Ti and Al 5Ti 3 are observed in the as-cast alloy, but Al 5Ti 3 transforms into γ-TiAl above 900°C. The phase h-Al 2Ti transforms into r-Al 2Ti by two alternative processes, i.e. by continuous or discontinuous transformation. While the first process is comparably sluggish, areas generated by the second process enlarge quickly, leading to the formation of a fine lamellar microstructure of γ-TiAl and r-Al 2Ti. This fine lamellar microstructure coarsens through lamellar fault or/and grain boundary migration during long time annealing. In the equilibrium state, the fully coarsened lamellar microstructure contains regular misfit dislocations, i.e. two types of [110] edge dislocations in the (001) planes at the interlamellar boundaries.