Formation and transformation of metastable LPSO building blocks clusters in Mg-Gd-Y-Zn-Zr alloys by spinodal decomposition and heterogeneous nucleation

To study the formation and transformation mechanism of long-period stacked ordered(LPSO)structures,a systematic atomic scale analysis was conducted for the structural evolution of long-period stacked ordered(LPSO)structures in the Mg-Gd-Y-Zn-Zr alloy annealed at 300℃~500℃.Various types of metastable LPSO building block clusters were found to exist in alloy structures at different temperatures,which precipitate during the solidification and homogenization process.The stability of Zn/Y clusters is explained by the first principles of density functional theory.The LPSO structure is distinguished by the arrangement of its different Zn/Y enriched LPSO structural units,which comprises local fcc stacking sequences upon a tightly packed plane.The presence of solute atoms causes local lattice distortion,thereby enabling the rearrangement of Mg atoms in the different configurations in the local lattice,and local HCP-FCC transitions occur between Mg and Zn atoms occupying the nearest neighbor positions.This finding indicates that LPSO structures can generate necessary Schockley partial dislocations on specific slip surfaces,providing direct evidence of the transition from 18R to 14H.Growth of the LPSO,devoid of any defects and non-coherent interfaces,was observed separately from other precipitated phases.As a result,the precipitation sequence of LPSO in the solidification stage was as follows:Zn/Ycluster+Mg layers→various metastable LPSO building block clusters→18R/24R LPSO;whereas the precipitation sequence of LPSO during homogenization treatment was observed to be as follows:18R LPSO→various metastable LPSO building block clusters→14H LPSO.Of these,14H LPSO was found to be the most thermodynamically stable structure.

Formation of Nanometer Coherent Structures During Spinodal Decomposition and Ordering Coexistence Phase Transformation in Fe-24Al Alloys

The nanometer coherent structure evolution of spinodal decomposition and ordering coexistence phase transformation in Fe-24Al alloys is investigated by the microscopic phase field kinetic model.The results show that the concentration and long-range order parameters all continuously change towards to their equilibrium values during phase transformation.With the increase of elastic interaction energy,the anisotropy along [01] or [10] elastic soft direction is more obvious and the time reaching equilibrium state is also shortened.According to the results,the formation of nanometer coherent structures during phase transformation is composed of the initial decreasing stage of order degree stage,the incubation stage,the continuous increasing stage of concentration order parameter and long-range order parameter,and the later stable stage.The spinodal decomposition and ordering is interaction;the initial ordering stage is a necessary condition of the coexistence phase transformation.The nanometer coherent structures are not found to grow during the whole phase transformation.The simulation results are in accordance with the results in experiment obtained by the aging treatment in Fe-24Al alloys.

Microscopic Phase Field Study of the Spinodal Decomposition in Fe-Mo Alloys

The spinodal decomposition in Fe-Mo alloys is studied by microscopic phase field method based on atomic scale.The results show that critical temperature of spinodal decomposition increases with the increment of Mo content,up to a maximum value of 1340 K at the Mo content of 50%.The spinodal decomposition shows a maximum rate at the Mo content of 50% for different Fe-Mo alloys.Comparing with the interconnected microstructure without considering elastic strain energy,the microstructure arrayed alternatively along the elastic soft direction of [01] and [10] is obtained under the effects of elastic strain energy,which will changes into a special 'macrolattice' structure during subsequent coarsening stages.The coarsening of microstructure is restrained by elastic interaction generated by atomic-size difference,and a larger atomic-size difference induces a stronger restraining effect.

ON THE ORDERING TRANSFORMATIONS IN Ti_3AL-Nb ALLOY

Study was made of the behaviour of ordering transformation in Ti_3AI-Nb alloy,including the ordering at high temperatures,the transformation of high temperature β-phase during cooling,and the decomposition of metastable β-phase during aging.The results show that the ordered primary α_2 and high temperature β in alloy form at 1060℃.The transformation of high temperature β-phase proceed by β→α_2+ω type during cooling,and the decomposition of metastable β and ω type proceeded by(β+ω)_(metustabte)→(α_2+β)_(stable)during aging at 700℃.

Microstructures of spinodal phases in Cu-15Ni-8Sn alloy

The microstructures of spinodal phases in Cu-15Ni-8Sn alloy were studied by TEM. It was found that when the alloy is completely in a as-quenched state, spinodal decomposition is quick. Ordering appears after spinodal decomposition. The ordered phase with DO22 structure has three variants obtained from coarsening spinodal structure. The reason of ordering appeared after spinodal decomposition is that the content of solute atoms needed by ordering is higher than the average, which can be reached by the composition fluctuation of spinodal decomposition. It was speculated that the morphology of the ordered phase is needle-like.

Ordering-induced Elinvar effect over a wide temperature range in a spinodal decomposition titanium alloy

Temperature-independent elastic modulus is termed as Elinvar effect,which is available by tuning the continuous spin transition of ferromagnetic alloys via composition optimization and the first-order martensitic transformation of shape memory alloys via plastic deformation.However,these reversible mechanisms are restricted generally in a narrow temperature range of less than 300 K.Here reports,by tuning a spinodal decomposition in a Ti-Nb-based titanium alloy via aging treatment,both the Elinvar effect in a wide temperature range of about 500 K and a high strength-to-modulus ratio of about 1.5%can be obtained by a continuous and reversible crystal ordering mechanism.The results demonstrate that the alloy aged at 723 K for 4 h has a nanoscale plate-like modulatedβ+α"two-phase microstructure and its elastic modulus keeps almost constant from 100 to 600 K.Synchrotron and in-situ X-ray diffraction measurements reveal that the crystal ordering parameter of theα"phase increases linearly with temper-ature from 0.88 at 133 K to 0.97 at 523 K but its volume fraction keeps a constant of about 33.8%.This suggests that the continuous ordering of theα"phase toward the high modulusαphase induces a posi-tive modulus-temperature relation to balance the negative relation of the elastically stableβphase.The aged alloy exhibits a high yield strength of 1200 MPa,good ductility of 16%and a high elastic admissible strain of 1.5%.Our results provide a novel strategy to extend the Elinvar temperature range and enhance the strength by tuning the crystal ordering of decomposition alloys.

Structure and Migration Characteristic of Heterointerfaces During the Phase Transformation from L1_2 to DO_(22) Phase

Based on the microscopic phase-field model, the structure and migration characteristic of ordered domain interfaces formed between DO22 and L12 phase are investigated, and the atomistic mechanism of phase transformation from L12 （Ni3Al） to DO22 （Ni3V） in Ni75AlxV25-x alloys are explored, using the simulated microstructure evolution pictures and the occupation probability evolution of alloy elements at the interface. The results show that five kinds of heterointerfaces are formed between DO22 and L12 phase and four of them can migrate during the phase transformation from L12 to DO22 except the interface （002）D//（001）L. The structure of interface （100）D//（200）L and interface （100）D//（200）L·^1/2[001] remain the same before and after migration, while the interface （002）D//（002）L is formed after the migration of interface （002）D//（002）L·^1/2[100] and vice versa. These two kinds of interface appear alternatively. The jump and substitute of atoms selects the optimization way to induce the migration of interface during the phase transformation, and the number of atoms needing to jump during the migration is the least among all of the possible atom jump modes.

AN X RAY DIFFRACTION STUDY OF THE AGING TRANSFORMATION IN Al Li Cu Zr ALLOY

Theinitialstageofagingtransformationin Al 2 27% Li 2 68 % Cu 0 11% Zr 0 13% Ceal loy at100℃wasstudied by meansof x ray diffraction.It wasconcludedthatordering of Liatom happens whenthealloyissolution treated andquenched, and beforetheformationofδ’ phasethereexitscontinuous phasetransformation processes, namely ordering ,spinodal de composition and both coexistence. Gunier Preston zone was not found in the aging process ofthestudied alloy.

INVESTIGATION OF PHASE SEPARATION OF THE Fe-Ni-Co-Al-Cu-Ti PERMANENT MAGNET ALLOY

Microstructure formed in the Fe-Ni-Co-Al-Cu-Ti permanent magnetic alloy with different treatments was studied by means of TEM observation, XRD method and SAXS technology. The results indicated that spinodal decomposition and orderiing transformation coexisted in the alloy and spinodal decomposition was completed in a short time. The ratio of component of spinodal and ordered microstructure was dependent on the cooling rate. The variation of gyration radius RG of the rod-like precipitates could be accounted for by the different growth modes of the precipitates related to strain energy and interface energy.

PHASE SEPARATION PROCESSES AT EARLY STAGE OF AGEING IN ZA27 ALLOY

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Microstructure Evolution and Phase Transformation of Ti–6.5Al–2Zr–Mo–V Alloy During Thermohydrogen Treatment

Thermohydrogen treatment （THT） is an effective way to refine microstructure and improve the mechanical properties of the titanium alloys. In the current work, as-cast Ti-6.5Al-2Zr-Mo-V alloy was hydrogenated with different hydrogen contents and processed solution aging. Accordingly, the microstructure evolution and phase transformation were analyzed. Results show that during solution aging, eutectoid decomposition occurs and the product is a mixture of coarse primaryα, fine eutectoid product and undecomposed βH. The size of primary α is closely dependent on the hydrogen content, and large primary α can be obtained at medium hydrogen content. Further, the influence of hydrogen content on the growth of primary α phase was revealed. The primary α is much fine, and the eutectoid product is relatively homo- geneous with 0.984 wt% H. After THT, the ultimate strength is beyond 1,100 MPa that has increased by 23.15% compared with that in as-cast state.

Aging transformation in Cu-3.2Ni-0.75Si alloy

The aging processes in Cu 3.2Ni 0.75Si alloy were examined by means of transmission electron microscope , X ray diffractometer and high resolution transmission electron microscopy. It is concluded that before the formation of Ni 2Si phase there exist continuous phase transformation processes, namely ordering, spinodal decomposition. Thereafter, the solute rich zones form Ni 2Si phase which is coherent with the matrix.

Computer simulation of γ′ and θ phase precipitation of Ni-Al-V alloy using microscopic phase-field method

The precipitation processes of γ′ and θ phases in Ni75Al6.5V18.5 alloy were simulated at different temperatures and the precipitation sequence of two phases and morphological evolution were investigated. The simulation demonstrates that the two phases precipitate simultaneously at high temperature and γ′ phase precipitates earlier than θ phase at 1 000 K and 1 120 K. With the temperature decreasing, the velocity of precipitation quickens, the quantity of θ phase increases and the size reduces; but the volume fraction increases, the quantity of phase increases and the size reduces as well. The two phases nucleate and grow independently at high temperature and the θ phase precipitates from the boundaries of γ′ phase at 1 000 K and 1 120 K. We also find that there are many kinds of domain boundaries between the same and different phases. The results of average deviation of composition and average absolute long range order parameter show that the ordering and compositional clustering of γ′ phase happen simultaneously at high temperature, the congruent ordering occurs prior to spinodal decomposition at 1 000 K and 1 120 K and the ordering advances and quickens as the temperature decreases. Ordering and compositional clustering of θ phase occur simultaneously at each temperature and are quickened with temperature decreasing.

Theoretical study of coexistence of ordering and spinodal decomposition

According to the conventional theory of solid solutions (the nearest neighbor atomic interaction model),ordering and spinodal decomposition/clustering are mutually exclusive processes.However,it has been found that the coexistence of ordering and spinodal decomposition (COSD) occurs in a large number of alloys.This fact gave a strong challenge to the conventional theory.A statistical investigation revealed that the COSD was closely related to large atomic-siae factors.It was thus proposed that the COSD stemmed from the long-range elastic interactions due to atomic-si?E disparity.In order to verify this idea,the formulism of concentration waves was applied to calculating the elastic interactions.The results proved that long-range atomic elastic interactions promoted both ordering and spinodal decomposition.A possible application of the COSD reaction was proposed,i.e.using this reaction to fabricate high-performance "natural nano-alloys".

Microstructural Changes of Cu-Ni-Si Alloy during Aging

Age hardening in Cu-3.2Ni-0.75Si (wt pct) and Cu-1.0Ni-0.25Si (wt pct) alloys from 723 to 823 K is studied. After an incubation period strengthening appears which is due to precipitates in the Cu-l.ONi-0.25Si (wt pct) alloy. On other hand an immediate increase of the yield strength characterizes the aging of the alloy. This is followed by the regions of constant yield strength and further by a peak. The microstructure of the alloy was studied by, means of transmission electron microscope (TEM) and X-ray diffraction (XRD). Spinodal decomposition takes place followed by nucleation of the ordering coherent (Cu,Ni)3Si particles, further precipitation annealing coherent δ-Ni2Si nucleated within the (Cu,Ni)3Si particle. Any change of the yield strength can be described by an adequate change of the structure in the sample. The nature of the aging curves with a 'plateau' is discussed. The formulas of Ashby and Labusch can be used to explain the precipitation.

Effect of Cu on Discontinuous Precipitation of a Symmetrical AlZn Alloy

The effect of Cu additions on discontinuous precipitation of AlZn alloy were studied with optical microscopy, scanning electron microscopy and X-ray diffraction. It is found that the effect of addition of 2 at. pct Cu on cellular nucleation site is not remarkable, while the effect of aging temperature on cellular nucleation site of AlZn and AIZn-2Cu alloys is fairly obvious. The cell growth rate of discontinuous precipitation of AlZn alloy is remarkably accelerated with the addition of 2 at. pct Cu. The discontinuous precipitation microstructure of AlZn alloy is not apparently affected with the addition of 2 at. pct Cu and the cellular front is still fine microstructure of spinodal decomposition. The phase constituents consist of a fcc Al-rich phase, a hcp Zn-rich phase and a CuZn4 phase. The transformation rate of discontinuous precipitation of AlZn alloy is remarkably accelerated with the addition of 2 at. pct Cu.

The Strengthening of Cu-15Ni-8Sn Alloy

The microstructure, property and relation between them of Cu-15Ni-8Sn alloy are studied by means of TEM and the measurement of hardness. The results show that y ’ metastable phase strengthens alloy because of its ordering structure. The ordering structure includes two types of and L12 ordering. Their strengthening for the alloy is much stronger than that of spinodal decomposition.

Thermomechanical treatment of super high strength Cu-8.0Ni-1.8Si alloy

The microstructures and properties of Cu-8.0Ni-1.8Si alloy subjected to different heat treatments were examined by mechanical and electrical properties measurements,optical and transmission electron microscopes observation. The results show that the precipitation process during aging can be accelerated by the cold deforming before aging. As the Cu-8.0Ni-1.8Si alloy is subjected to solution treatment at 970 ℃ for 4 h,cold rolling to 60% reduction,and then aging at 450 ℃ for 60 min,its properties are σb=1 050 MPa,σ0.2=786 MPa,δ=3.2% and conductivity 27.9%(IACS). The strengthening mechanisms of the alloy include spinodal decomposition strengthening,ordering strengthening and precipitation strengthening. The precipitation of the alloy is nano-scale Ni2Si phase.

Microstructure and Mechanical Properties of Mg–RE–TM Cast Alloys Containing Long Period Stacking Ordered Phases: A Review

Casting magnesium alloys hold the greatest share of magnesium application products due to their short processing period, low cost and near net shape forming. Compared with conventional commercial magnesium alloys or other Mg–RE-based alloys, the novel Mg–RE–TM cast alloys with long period stacking ordered(LPSO) phases usually possess a higher strength and are promising candidates for aluminum alloy applications. Up to now, two ways: alloying design and casting process control(including subsequent heat treatments), have been predominantly employed to further improve the mechanical properties of these alloys. Alloying with other elements or ceramic particles could alter the solidifi cation pattern of alloys, change the morphology of LPSO phases and refi ne the microstructures. Diff erent casting techniques(conventional casting, rapidly solidifi cation, directional solidifi cation, etc.) introduce various microstructure characteristics, such as dendritic structure, nanocrystalline, metastable phase, anisotropy. Further heat treatments could activate the transformation of various LPSO structures and precipitation of diverse precipitates. All these evolutions exert great impacts on the mechanical properties of the LPSO-containing alloys. However, the underlying mechanisms still remain a subject of debate. Therefore, this review mainly provides the state of the art of the casting magnesium alloys research and the accompanying challenges and summarizes some topics that merit future investigation for developing high-performance Mg–RE–TM cast alloys.

Landau theory of martensitic transformation in Fe-Mn-Si based alloys

Considering the features of martensitic transformation in Fe-Mn-Si based alloys, the Landau theory is established by introducing the density of stacking faults as a new order parameter η and the corresponding free energy function. By using such an order parameter, the stacking fault mechanism of the nucleation and growth for the γ(fcc) →ε(hcp) martensitic transformation can be reasonably explained, and a further detailed mechanism is proposed. The stacking faults are generated and overlapped in an irregular form at the beginning and then becoming regular to create some transition structures till a stable phase forms at a certain temperature. The importance of the interface soliton is to complete the transformations into various structures of martensite but not the twinned one. The thermodynamics of fcc→hcp transformation and those between different transition structures are described by the free energy function established in the present note.