Precipitate Microstructure Research Articles

How calcium prevents precipitation hardening in Al–Mg–Si alloys

We have investigated the effect on precipitate microstructure and hardness upon adding small amounts of Ca to a base Al–Mg–Si alloy. The main investigative techniques were transmission and scanning electron microscopy. We found that large Ca-containing particles with composition CaAl2Si2 form during the production stages of the alloy. The particles leave less Si available in solid solution for the nucleation of hardening precipitates, leading to a coarser microstructure consisting of less coherent precipitates. The resulting hardness decrease is measurable for alloys containing more than 60atppm of Ca.

Influence of Finishing Rolling Variables on the Austenite Recrystallization and Grain Growth

The austenite recrystallization and grain growth during thermo-mechanical control processing (TMCP) of a pipeline steel grade is described and analysed in terms of precipitation state progress. The influences of finish rolling temperature and cooling rate on the link between microstructure-precipitation evolution and their consequent effect on the mechanical properties were examined. Two stage controlled rolling (roughing and finishing) was carried out on a laboratory rolling mill for a set of completed and interrupted schedules. Subsequent to rolling, two different cooling routes were used (air-cooling and accelerated water cooling (ACC) together with coiling simulation). From the combination of transmission electron microscopy (TEM) observations, detailed texture analysis and inductively coupled plasma-mass spectroscopy (ICPMS) precipitates quantification, consistent correlations between precipitation state and microstructure at every stage of TMCP can be recognized. The role of grain size and precipitation on final mechanical properties was discussed based on different strengthening mechanisms.

Fracture behavior of Alloy 625 with different precipitate microstructures

To study the effect of various ordered phases and their growth on room temperature tensile properties and fracture behavior of Alloy 625, specimens of this alloy were held isothermally at 813K, 973K and 1123K for 10h, 100h and 1200h, respectively. Specimens held at the aforementioned temperatures were subjected to transmission electron microscopic investigation to characterize Ni2(Cr,Mo), γ″ and δ ordered phases. Tensile testing of the heat-treated samples revealed the influence of these ordered phases on the tensile properties and fractures. Primary carbides, which were identified semi-qualitatively under electron-probe micro-analyzer (EPMA) have been found responsible for void nucleation resulting in large dimples in Alloy 625 with solid-solution matrix. Early stages of Ni2(Cr,Mo) and γ″ and their subsequent growth at 813K, was responsible for increase in strength without reducing ductility considerably. This behavior has been correlated with the reduction in size and depth of the dimples in the fractographs. Transmission electron micrographs showing three variants of γ″ in three orthogonal directions in the alloy matrix have revealed that the morphology of the ordered phase is lens shaped and the precipitate can grow up to ~150nm. With the growth of γ″ the alloy is seen to fracture in transgranular cleavage manner following ~23% of uniform strain. Influence of plate shaped, long and thick δ phase on the strength and ductility appears similar to that of γ″. δ—precipitation after 1200h of isothermal holding at 1123K is also seen to cause faceted appearance of the fracture surface, which is similar to that of γ″ after 1200h of isothermal holding at 973K. However, the facets are seen to be consisting of dimples, which are small and uniformly distributed over the surface.

Quench Sensitivity and Quench-Induced Precipitation in High Strength 7xxx Series Aluminum Alloys

This paper investigates the quench sensitivity of some selected 7xxx series Al alloys based on a Jominy End Quench method. The precipitate microstructures as a function of cooling rate during quenching are also characterized by using transmission electron microscopy (TEM). The results indicated that quench sensitivity and therefore the mechanical properties inhomogeneity in large plates or forgings can be predicted more accurately by the simultaneous combination of hardness and electrical conductivity measurements based on Jominy end quench. The hardness drop and conductivity increase in the novel alloy following a low cooling rate are much reduced compared to AA7050 and 7B04 because of a lower sensitivity to quench-induced precipitation on dispersoids. The novel alloy exhibited the least quench sensitivity, and the 7B04 Al alloy was the most quench sensitive. If the 90% of the maximum hardness is defined as the depth of quenching, the depth of 7B04 Al alloy, AA7050 through Jominy end quenching is about 20 and 55 mm respectively. Meanwhile, the depth of greater than 150 mm is achievable in the novel alloy, and hence it can be recommended to fabricate large section plates or forgings without compromising properties in the center of the part after a slow cool.

Precipitate Behavior and Microstructure Evolution of γ’ Phase in Ni-Al Alloys with the Internal Elastic Strain

The microstructure evolution and precipitation behaviors of ordered γ (Ni3Al) phase in Ni-Al alloys were studied using the phase field dynamic model. Under the interactions of internal elastic strain, the γ phase morphology changes from the separated cuboidal to connected rectangle shape with the decrease of the aging temperature or the increase of the Al concentration. It also shows that the ordering of precipitates is finished instantly when they precipitate from the matrix phase. The γ phase volume fraction and the phase transformation velocity are affected by the aging temperature and composition of the Ni-Al alloys.

Atomic Interchange Potentials Calculation of Ni-Al-Cr Alloy Based on Microscopic Phase Field Method

The effects of increasing atomic interchange potentials to the precipitation process and microstructure of Ni-Al-Cr alloy have been simulated based on the microscopic phase field theory. The first nearest neighbour atomic interchange potentials of Ni-Al-Cr alloys for L12 and D022 phase was calculated out according to the formula which were referenced on the relation equation between atomic interchange potentials and long range order parameters by Khachaturyan. The results indicated that Ni-Al (WNi-Al) and Ni-Cr (WNi-Cr) s first nearest neighbor atomic interaction potentials will increase linearly while the temperatures rose. Moreover WNi-Al increased but WNi-Cr decreased roughly linearly if Al atoms concentration rose, and conversely inversed. In addition, these atomic interchange potentials changing with temperature and concentration were in good agreement with earlier study.

Precipitation Behavior and Microstructure Evolution of Mg-Zn-Zr-Er Alloy Processed by Secondary Deformation and Heat Treatment

Preheated at 673K for 3h and 6h respectively, as-extruded Mg-9Zn-0.6Zr-2Er (wt.%) slabs with starting thickness of 8mm were hot-rolled at 673K to 2mm. The hot-rolled alloys were then solution treated at 673K for1.5h and aged at 473K for 10h. Microstructure evolution and second phase precipitation behavior during hot rolling and subsequent heat treatments were examined by optical microscopy (OM), scan electron microscopy (SEM) and micro hardness test. Mg-9Zn-0.6Zr-2Er alloy were fully recrystallized with fine equiaxed grains after preheating for 3 hours, prolonging heating time leads to a higher degree of dissolution of secondary phase into Mg matrix but a coarsening of the microstructure. During hot rolling process, the volume fraction of the DRX grains increased gradually with increasing reduction ratio, shearing bands became visble in the final pass. Thermal stable Mg-Zn-Er intermetallic compounds distributed along rolling direction at the first rolling passes and became more homogenous in the final pass. More nanosized MgZn2 precipitated during hot rolling in the alloy preheated for 6h than that for 3h, leading to an enhanced precipitation hardening effect in the former.

Interrupted ageing in steels: Hardness improvement and microstructural stabilization

Similarly to aluminium alloys, interrupted ageing in steels may increase hardness by 10%. By adding an intermediate stage between quenching and tempering, where quenched martensite is left to age at room temperature, carbon forms finer precipitate microstructures, which become more stable at room temperature. Using thermoelectric power to model carbon segregation to dislocations, it appears that room temperature ageing increases the number of effective nucleation sites for the subsequent tempering stage, as reflected in the microstructure and hardness.

An Atomistic-Based Hierarchical Multiscale Examination of Age Hardening in an Al-Cu Alloy

A large class of modern structural alloys derives its strength from precipitation hardening. Precipitates obstruct the motion of dislocations and thereby increase alloy strength. This paper examines the process using an atomistic-based hierarchical multiscale modeling framework. Atomistic modeling is employed to (1) compute solute-dislocation interaction energies for input into a semi-analytic solute hardening model and (2) evaluate precipitate strengths for use in dislocation line tension simulations. The precipitate microstructure in the dislocation line tension simulations is obtained from simple analytic precipitation kinetics relations. Fitting only the rate constants in the precipitation kinetics model, the macroscopic strength predictions of the hierarchical multiscale model are found to correspond reasonably well with experiments. By analyzing the potential sources of discrepancy between the model’s macroscopic predictions and experiments, this work illuminates the importance of specific atomic-scale processes and highlights important challenges that remain before truly predictive mechanism-based plasticity modeling can be realized.

Significant enhancement of the age-hardening response in Mg–10Sn–3Al–1Zn alloy by Na microalloying

A trace addition of Na to the Mg–9.8Sn–3.0Al–0.5Zn (wt.%) (TAZ1031) alloy substantially enhances the peak-aged hardness from 53 to 99 HV at 200 °C, and to 109 HV at 160 °C, and accelerates the precipitation kinetics. Na clusters act as heterogeneous nucleation sites for lath-shaped Mg2Sn precipitates, thereby refining the precipitate microstructure.

Calorimetric Analysis of Precipitation in Undercooled Ni-Si Alloy

In order to study the precipitaion of Ni3Si particle in undercooled Ni-Si alloy, calorimetric analyses were carried out using non-isothermal measurements by DSC. The scanning electron microscopy (SEM) and the transmission electron microscopy (TEM) measurements were used to describe qualitatively and quantitatively the precipitate microstructures. The non-isothermal DSC thermograms exhibited one reaction peaks and it indicated that the precipitation process is an exothermic reaction. The evolution for the precipitate was obtained in the as-solidified Ni-Si alloy subjected to DT=195K, meanwhile, the precipitate size was found increased with decreased heating rate in the TEM images. The largest precipitate size was about 120nm, and the precipitates still kept spherical shape. Model prediction for the precipitation of Ni3Si particle has been performed. Good agreement with experimental data has been achieved

Study of Microstructure, Microhardness and Phases of Precipitation during Ageing of Cu-Sn and Al-Ag Alloys

Ageing study of supersaturated solid solutions Cu-13 wt. % Sn and Al-20 wt. % Ag has shown that the two types of precipitation (continuous and discontinuous) would occur for various temperatures and plastic deformation rates (thickness reduction by cold rolling following rapid quenching in iced water). The precipitated phases take place generally on the grain boundaries (discontinuous precipitation DP) and inside the grains (continuous precipitation CP). A detailed study of phases' development conditions and the growth controlling mechanisms has been carried out, for several ageing temperatures with and without plastic deformation of the supersaturated solid solution using various experimental techniques (optical microscopy, X-ray diffraction and microhardness measurements).

The effect of experimental conditions on the microstructure of hematite particles precipitated by the forced hydrolysis of FeCl3 solutions

The effect of experimental conditions on the forced hydrolysis of FeCl3 solutions and specifically the effect of amidosulphonic acid on the microstructure of precipitates were investigated. Precipitates were analyzed by XRD, 57Fe Mössbauer, FT-IR and UV/Vis/NIR spectroscopies, as well as FE-SEM. The phase transformation β-FeOOH→α-Fe2O3 was accelerated with an increase in the hydrolysis temperature, whereas an addition of amidosulphonic acid at the start of the hydrolysis of Fe3+ ions suppressed the phase transformation β-FeOOH→α-Fe2O3 or decreased crystallinity at 120 and 160°C. The size and shape of hematite particles were strongly dependent on FeCl3 concentration and temperature and the presence of sulfonate/sulfate groups. Preferential adsorption of sulfonate/sulfate groups influenced the hematite crystal growth direction. The aggregation effect was also noticed in the formation of hematite particles.

Effects of Cu and Ag additions on age-hardening behavior during multi-step aging in AlMgSi alloys

Low Cu and Ag additions (≤0.10at%) were found to strongly affect the age-hardening behavior in AlMgSi alloys with Mg+Si>1.5at%. The hardness increased during aging at 170°C and the formation of β″ precipitates was kinetically accelerated. The activation energy of the formation of the β″ phase was calculated to 127, 105, 108 and 99KJmol−1 in the base, Cu-added, Ag-added and CuAg-added alloys, respectively using the Kissinger method. The negative effect of two-step aging caused by the formation of Cluster (1) during natural aging was not overcome by the addition of microalloying elements. However, it was suppressed by the formation of Cluster (2) through a pre-aging at 100°C. Quantitative analysis of the precipitate microstructure was performed using a transmission electron microscope equipped with a parallel electron energy loss spectrometer for the determination of specimen thickness. The formation of Cluster (2) was found to increase the number density of β″ precipitates, whereas the formation of Cluster (1) decreased the number density and increased the needle length. The effects of low Cu and Ag additions in combination with multi-step aging are discussed based on microstructure observations and hardness and resistivity measurements.

Quantitative Characterization of Precipitate Microstructures in Metallic Alloys Using Small-Angle Scattering

Quantitatively characterizing precipitate microstructures in metals by small-angle scattering poses specific challenges as compared to other areas of application of this technique. In terms of size and morphology evaluation, these include the presence of a significant size distribution, non-isotropic shapes, and interpretation complicated by a partial averaging due to a non-random texture. In terms of volume fraction evaluation, these include the imperfect knowledge of the chemical composition of very small objects. This paper, based on a presentation given at the “Neutron and X-Ray Studies of Advanced Materials V: Centennial” symposium of the 2012 TMS conference, reviews the strategies that can be applied in different characteristic cases to obtain a robust quantification of precipitate microstructures.

Phase-field microelastic modeling of microstructure evolutions in free-standing thin films

Phase-field microelastic modeling of microstructure evolutions in free-standing thin films is developed. Khachaturyan’s microelasticity theory is reformulated to account for the plane-stress condition of free-standing thin films. As examples, the model is applied to simulate two-phase (disordered face-centered cubic+ordered L12) microstructures formed by decomposition and ordering in free-standing thin films of different orientations. Significant effects of the film orientation on the precipitate microstructure morphology are observed, which are analyzed in terms of the anisotropy of the orientation-dependent effective elastic modulus tensor of the free-standing thin films. Distinctive microstructure features in free-standing thin films are discussed in comparison with that observed in bulk materials.

Hydrogen-induced decomposition of Zr-rich cores in an Mg−6Zn−0.6Zr−0.5Cu alloy

This study presents the first experimental evidence of a hydrogen-induced decomposition reaction in an Mg–6Zn–0.6Zr–0.5Cu alloy from combined transmission electron microscopy and atom-probe tomography characterization. The reaction takes place due to the presence of H in the Mg matrix, causes the decomposition of pre-existing, high-temperature Zr–Zn intermetallic rods into Zr-rich hydride and β′ (Zn3Mg2), and forms novel composite precipitates in the Zr-rich cores of the alloy during ageing at 180°C. The stoichiometry of the Zr–Zn rods was found to be Zn3(Zr1−x, Mgx)2, rather than Zn2Zr3, although both have a similar tetragonal crystal structure. The intrinsic link between the high-temperature Zr–Zn rods and the subsequent elongated composite precipitates, as depicted by the reaction, highlights the importance of engineering the Zr–Zn rod microstructures to control the final precipitate microstructure and effectively strengthen the Zr-rich cores, and hence the advanced Mg alloys.

Effect of room temperature storage time on precipitation in Al–Mg–Si(–Cu) alloys with different Mg/Si ratios

Abstract The effect of natural ageing time before artificial ageing has been investigated in four Al–Mg–Si(–Cu) alloys, with 0.4% Mg + 0.8% Si and 0.8% Mg + 0.4% Si, both with and without 0.14 at.% Cu. The precipitate microstructure was quantified by means of transmission electron microscopy. Varying the storage time before ageing for 170 min at 200°C, we observe an initial hardness increase after minutes, a decrease after several hours and another increase after weeks. The hardness decrease was most pronounced in the Mg-rich Cu-free alloy, caused by a reduced precipitate volume fraction. Adding Cu produces finer microstructures, higher hardness and reduces the negative effect of natural ageing regardless of the Mg/Si ratio of the alloy. With 1 week storage, an increase in the fraction of the Cu-containing precipitates L and Q′ was observed in the Cu-containing Si-rich and Mg-rich alloys respectively.

On the formation of an ultrafine-duplex structure facilitated by severe shear deformation in a Ti–20Mo β-type titanium alloy

Severe plastic deformation in the form of equal channel angular pressing (ECAP) has been adopted to introduce severe shear strain into a Ti–20 wt.% Mo β-type titanium alloy to elucidate the aging response of the severely deformed β matrix. Upon isothermal aging in the (α+β) phase field, selective heterogeneous α nucleation and growth resulted in a mixed precipitation microstructure. An ultrafine-duplex (α+β) structure composed of equiaxed α precipitates formed inside the shear bands (SBs) created during ECAP, whereas acicular α precipitates were favoured outside the SBs. This distinct precipitation structure has been correlated to the structural characteristics of the SBs: high disorder with dislocation cells characteristic of low-angle boundaries and enhanced atomic diffusivity. The highly disordered structure results in a weak variant selection and thereby promotes randomly orientated α precipitation without obeying the Burgers orientation relationship. Furthermore, the enhanced atomic diffusivity facilitates rapid growth of the α nuclei to form the ultrafine-duplex (α+β) structure.

Microscopic phase-field study for mechanisms of directional coarsening and the transformation of rafting types in Ni–Al–V ternary alloys

Directional coarsening (rafting) of precipitates and transformation on rafting type under stress are investigated by computer simulations for Ni–Al–V ternary alloys. The simulation technique is based on a microscopic phase-field model that characterizes simultaneously the spatiotemporal evolution of both precipitate microstructures and occupation probabilities. The initial configurations consisting of cubical γ′ and tetragonal θ are constructed according to experimental observations and microscopic phase-field simulation. For a given state of the lattice misfit and external load, the predicted rafted precipitates and the transformation from P-type rafting to N-type rafting with stress agree well with the experimental observations. This indicates that stress plays an important role in directional coarsening, which may promote or block the diffusion for atoms in different directions. The evolution of directional coarsening both in space and in time for solute atoms caused by the coupling of external load and misfit stress and precipitation is evaluated from morphologies and occupation probabilities, from which the diffusion of atoms is analyzed.