Heterogeneous Nucleation Of Precipitates Research Articles

Metastable phases of Ag–Si: amorphous Si and Ag-nodule mediated bonding

Metastable phases such as supersaturated solid solutions, supercooling, and amorphous phases are well-known in metallurgy. They are often composed in non-equilibrium states and can be transformed into a stable phase by overcoming an energy barrier with driving forces. Particularly, it has been widely used for material strengthening and heterogeneous nucleation of precipitates in solids is mainly induced by heat treatments for supersaturated solid solutions. However, little is known about the metastable phases of the Ag–Si alloy, although it is a well-known simple binary eutectic alloy. Here, we show that the metastable phases composed of amorphous Si and supersaturated Ag solid solution are induced by the eutectic reaction under rapid cooling of Ag–Si. Furthermore, the solute Si in the Ag matrix reacts with oxygen to precipitate Ag by-products, which grow as nodules. The Ag nodules have high crystallinity and robust interfacial structures, and the nodule growth leads to the formation of cross-links between the Ag–Si particles. We also demonstrate the Ag nodule-mediated bonding where the rapidly cooled Ag–Si ribbon is directly used as a bonding medium, indicating the possibility of using it as a high-temperature bonding material with low-temperature processes.

Unusual Precipitation at Grain Boundaries in Non-Equiatomic CoCrFeMnNi High Entropy Alloys

In Cr-rich CoCrFeMnNi alloys, the precipitation of the σ phase at grain boundaries during recrystallization is so fast that ultrafine-grained structure is formed due to the pinning effect of the precipitates. The average grain size of the fcc parent phase is found to be consistent with modified Zener-Smith model. If conventional alloys come to equilibrium, volume fraction of precipitates should approach a saturation value. However, it is interesting to note that the volume fraction of the σ phase in Cr-rich CoCrFeMnNi alloys is inversely proportional to the average grain size of the fcc parent phase. For instance, in Co20Cr25Fe20Ni15Mn20 alloys, the volume fraction changes from 6.5% to 1.2% with increasing average gran size from 14 μm to 210 μm even after annealing at 1273 K for 100 h. It is well known that heterogeneous nucleation of precipitates at grain boundary is energetically favorable and fast diffusion through grain boundary can assist the precipitation. However, they cannot account for the grain size dependence of the volume fraction after reaching equilibrium. Based on stereology, the reciprocal of grain size is proportional to grain boundary area. Thus, chemical fluctuation at grain boundaries (e.g. segregation) is considered to be related to the unusual precipitation at the grain boundaries.

κ-Carbide assisted nucleation of B2: A novel pathway to develop high specific strength steels

Controlling morphology of B2 phase as nano-sized particles that are homogenously distributed within austenite matrix has been remained as a challenging task particularly for hot rolled lightweight steels, where the nucleation sites for B2 precipitation are less abundant than in cold rolled ones. Utilizing the phenomena occurring in age-hardenable aluminum alloys, i.e., heterogeneous nucleation of precipitates from pre-existing GP zones, here we solved this challenge by using κ-carbide nanoparticles to promote the formation and to engineer the morphology of B2. This strategy has been applied to a hot rolled Fe-21Mn-10Al-1C-5Ni model steel resulting in the formation of a large fraction of B2 nanoparticles (sizes of 20-500 nm) uniformly distributed within austenite grains. Besides their contribution to precipitation strengthening, these B2 nanoparticles lead to a dramatic improvement of work hardening capability of the steel. As a result, novel precipitation hardened hot rolled lightweight steel with excellent combination of ultrahigh specific strength and ductility has been fabricated. Such properties of the present steel are not only better than those of other hot rolled lightweight steels strengthened by B2 and/or κ-carbide but also comparable to those of the steels subjected to additional cold rolling, and it is expected to expand the applications of B2-strengthened steels in response to an increasing demand for lightweight constructions.

Microstructure and High Temperature Tensile Properties of Mg–10Gd–5Y–0.5Zr Alloy after Thermo-Mechanical Processing

The microstructure, high-temperature tensile properties and fracture behavior of the Mg-10Gd-5Y-0.5Zr alloy after thermo-mechanical processing (pre-tension between solution and aging treatment) were investigated. The pre-deformed alloy shows the accelerated aging kinetics compared to the un-deformed alloy. Microstructure of pre-deformed samples showed not only the homogeneous nucleation of the precipitate but also heterogeneous nucleation of precipitates on the dislocation and twin boundaries. Tensile results show that the pre-deformation enhanced the strength of the alloy, while it deteriorates the ductility of the alloy. The ultimate tensile strength (UTS) of the T6 treated un-deformed and pre-deformed alloy at room temperature are 331 MPa and 366 MPa, respectively. Tensile strength of the T6 treated alloy in both un-deformed and deformed conditions was enhanced by raising the test temperature and then reduced by further raising the test temperature. The higher strength of the pre-deformed alloy could be related to the higher density of the precipitates, which grow on the twin boundaries and can hinder the dislocation movement and strengthen the alloy. The results shows that thermo-mechanical processing can significantly improve the room- and high-temperature mechanical properties and enhance the formation of precipitates in Mg-10Gd-5Y-0.5Zr alloy, which can lead to wider application of the alloy in industries such as aerospace or powertrains that need better room- and high-temperature mechanical properties.

The Microstructure Evolution and Deformation Behavior of AZ80 During Gradient Increment Cyclic Loading

Cyclic loading–unloading uniaxial tension experiments were conducted at temperatures ranging between 293 K and 623 K and a strain rate of 10−3 s−1 to study the cyclic accumulated plastic deformation (CAP) behavior of extruded AZ80. The 673 K/4-h heat treatment to the as-extruded AZ80 led to a noticeable decrease in yield strength which was associated with both dissolution of the β-Mg17Al12 phase and growth of the matrix grain size. The critical number of cycles needed to soften the material (Nc) decreased from 5 to 4 when the cyclic strain amplitude (ea) increased from 3.3 to 5.0 pct for the as-extruded AZ80. The average cyclic hardening rate (Θ) increased from 11 to 23 MPa/cycle after heat treatment, and this was attributed to the more pronounced twinning process in the coarse-grained microstructure. During the 293 K to 473 K CAP deformation, the increasing accumulated cyclic tension strain may have accelerated the propagation of secondary twinning leading to the Luders-like post-yield softening. Twinning was prevalent at low temperature (293 K to 473 K) in the ea = 3.0 pct CAP deformation for the heat-treated alloy, and twin-assisted precipitation occurred during the 523 K CAP deformation, which implied that the high diffusivity in the twin boundary accelerated the heterogeneous nucleation of precipitates. The preferred cracking locations changed from twin boundaries to grain boundaries when the CAP deformation temperature increased from 473 K to 523 K. As for the 623 K CAP deformation, cavities initiated at the grain boundaries, and the volume fraction of the cracks/cavities increased from 0.01 to 0.05 with increasing temperature.

Modelling the strain-induced precipitation kinetics of vanadium carbonitride during hot working of precipitation-hardened Ferritic–Pearlitic steels

A model for describing the strain-induced precipitation kinetics of vanadium carbonitride in the hot deformed austenite of precipitation-hardened Ferritic–Pearlitic (AFP – Ausscheidungshärtende Ferritisch–Perlitische) steels is presented. The model considers heterogeneous nucleation of precipitates on dislocation nodes and predicts the volume fraction, size and composition of the precipitates as well as the composition of the remaining austenite during the nucleation, growth and coarsening stages of the precipitation process. The effect of static recovery on the precipitation kinetics is also incorporated in the model. A fairly good agreement between the model predictions and experimental observations is demonstrated.

Modelling the kinetics of strain induced precipitation in Nb microalloyed steels

Strain induced precipitation is a key phenomenon that controls the microstructure evolution during the finish rolling stages of microalloyed steels. Extensive research has shown that the precipitation of Nb(CN) delays the onset of recrystallisation. This paper presents a model to describe the precipitation kinetics during isothermal holding following high temperature deformation in Nb-containing steels. The model is based on the assumption that heterogeneous nucleation of precipitates on dislocations and enhanced coarsening due to pipe diffusion are responsible behind the accelerated kinetics observed in strain induced precipitation. Results show a very good agreement between reported experimental observations and predictions of the present model for precipitate size and volume fraction evolution.

Ａｌ‐１％Ｍｇ２Ｓｉ‐０．４％Ｓｉ合金の時効挙動に及ぼすＭｎ添加の影響

Aging phenomena of Al-1%Mg2Si-0.4%Si alloys containing Mn up to 0.8% were studied by means of electrical resistivity measurement, transmission electron microscopy and tensile test. With an increase of Mncontents, the aging rate once decreases and then increases at 150 and 200°C aging. At the peak aging condition at 200°C, mean spacing of precipitates once decreases and then increases with increasing Mn content. In the 0.8% Mn additional alloy aged at 250°C, the aging rate decreases as compared with that of Mn free alloy. This alloy shows absolutly low age-hardenability because extremely coarse precipitates distribute randomly in the matrix. The interface between insoluble particles and matrix act as sinks for excess vacancies, so the heterogeneous nucleation of precipitates and a decrease of aging rate occur due to reduce the excess vacancies in the matrix.

Heterogeneous Nucleation of Precipitates in the Grain Boundary Regions of an AlZnMg Alloy

Heterogeneous Nucleation of Precipitates in the Grain Boundary Regions of an AlZnMg Alloy

Heterogeneous nucleation of precipitates in an Al–12 at% Zn alloy at dislocation loops

The heterogeneous nucleation of precipitates at dislocation loops DL was studied by TEM in an Al-12 at% Zn alloy at RT and above all at temperatures situated about 15 K to both sides of the upper limit temperature T for rapid homogeneous nucleation of precipitates. When quenching the specimens from sufficiently high temperatures into water of RT, the present DL are effective sites for heterogeneous nucleation of precipitates at RT and at Ta = T + 15 K, but not when ageing the samples at Ta = T − 15 K. The reasons for this striking difference are discussed and the results are compared with those obtained by other authors. Die heterogene Keimbildung von Ausscheidungen an Versetzungsschleifen DL wird in einer AlZn(12 at%)-Legierung mittels TEM während der Alterung bei RT und bei Temperaturen zu beiden Seiten der oberen Grenztemperatur der schnellen homogenen Keimbildung T(etwa 15 K davon entfernt) untersucht. Wenn die Proben von ausreichend hohen Temperaturen in Wasser von RT abgeschreckt werden, dann sind die vorhandenen DL während der Alterung bei RT und bei Ta = T + 15 K effektive Keimlagen für die heterogene Keimbildung, aber nicht bei Lagerung bei Ta = T − 15 K. Die Gründe für diesen auffallenden Unterschied werden diskutiert und die Ergebnisse mit denen anderer Autoren verglichen.