Abnormal Grain Growth Research Articles

Laser Powder Bed Fusion Processing of Fe-Mn-Al-Ni Shape Memory Alloy—On the Effect of Elevated Platform Temperatures

In order to overcome constraints related to crack formation during additive processing (laser powder bed fusion, L-BPF) of Fe-Mn-Al-Ni, the potential of high-temperature L-PBF processing was investigated in the present study. The effect of the process parameters on crack formation, grain structure, and phase distribution in the as-built condition, as well as in the course of cyclic heat treatment was examined by microstructural analysis. Optimized processing parameters were applied to fabricate cylindrical samples featuring a crack-free and columnar grained microstructure. In the course of cyclic heat treatment, abnormal grain growth (AGG) sets in, eventually promoting the evolution of a bamboo like microstructure. Testing under tensile load revealed a well-defined stress plateau and reversible strains of up to 4%.

Formation of abnormal coarse grains and its effects on corrosion behaviors of solution treated ZK60 Mg alloy

Abnormal grain growth (AGG) took place during solution treatment of extruded ZK60 Mg alloy. The driving force of AGG was stored energy gradient, and the {0001} <11-20> grains acted as candidates. The grain size and topology advantages contributed to the propagation of AGG, while lots of stored energy was consumed by static recrystallization and grain growth, resulting in the ending of AGG. The extruded alloy had the worst corrosion resistance with a corrosion rate of 6.3 mm/y, and the welding zone was preferentially corroded. The corrosion rate dropped to 3.7 mm/y after solution due to the formation of abnormal grains.

In-situ observation of abnormal grain growth in a low-alloyed carbon steel using SEM-EBSD

Because abnormal grain growth (AGG) degrades mechanical properties of industrial polycrystalline materials such as steel, understanding and controlling AGG are important. In this study, AGG of Al-Nb-microalloyed low-carbon steel was investigated at 1100°C using in-situ scanning electron microscopy-electron backscatter diffraction (SEM-EBSD). Owing to the pinning particles of AlN and Nb(C,N), fine austenite grains formed initially, and AGG appeared owing to the dissolution of the pinning particles at high temperatures. Relatively large grains invaded the surrounding smaller grains with a size ratio of approximately 0.3, resulting in AGG. We developed the in-situ observation method to investigate the AGG of the carburization process using a diffusion couple of high- and low-carbon steels. The carbon diffusion into the low-carbon steel from the high-carbon steel enhanced the grain growth of the low-carbon steel. Although the detailed mechanism is still unclear, we clearly showed that carburization can reduce the pinning force. When focusing on the misorientation of the grain boundary, grain boundaries with misorientation angles of 50–59° remained during AGG, which is explained by their lower mobility owing to their lower grain boundary energy. These results suggest that the difference in grain-boundary mobility can induce duplex grain growth with a bimodal distribution, resulting in AGG.

Friction Stir Welding Parameters: Impact of Abnormal Grain Growth during Post-Weld Heat Treatment on Mechanical Properties of Al–Mg–Si Welded Joints

Friction stir welding (FSW) is an alternative method to join aluminum (Al) alloys in a solid-state condition. However, the coarsening or dissolution of precipitation hardening phases in the welding zone causes strength reduction or softening behavior in the welded area of age-hardened Al alloys. Therefore, this research aimed to improve the mechanical properties of an FSW Al–Mg–Si alloy via post-weld heat treatment (PWHT) and the possibility of controlling the abnormal grain growth (AGG) using different welding parameters. FSW was performed with different rotational and travel speeds, and T6 heat treatment was carried out on the FSW samples as the PWHT. The results showed a decrease in the strength of the FSW samples compared with that of the base material (BM) due to the dissolution of precipitation hardening particles in the heat-affected zone. However, the emergence of AGG in the microstructure after the T6-PWHT was identified as the potential event in the microstructure of the PWHT samples. It is found that the AGG of the microstructure in similar joints of Al6061(T6) was governed by the welding parameters. The results proved that PWHT was able to increase the tensile properties of the welded samples to values comparable to that of Al6061(T6)-BM. The increased mechanical properties of the FSW joints were attributed to a proper PWHT that resulted in a homogeneous distribution of the precipitation hardening phases in the welding zones.

Machine learning based a priori prediction on powder samples of sintering-driven abnormal grain growth

Reducing porosity in sintered products allows enhancement of several of its properties such as conductivity (electrical and thermal), strength, and translucency. One of the key challenges to porosity reduction and property improvement during sintering is abnormal grain growth (AGG). Abnormal grain growth occurs when a certain energetically favored particle becomes significantly larger than other particles in the matrix. This leads to an increase in porosity due to uneven particle size. There is no simple a priori test to determine whether a given powder sample will exhibit abnormal grain growth, when subjected to sintering. Here we show that a machine learning based approach predicts abnormal grain growth in powdered samples prior to actual sintering. This approach has the potential to allow for pre-selection of appropriate powder samples with an accuracy of 82%, to minimize the risk of abnormal grain growth in practical sintering processes.

Influence of Spark Plasma Sintering Conditions on the Optical and Mechanical Properties of Spinel.

The influence of spark plasma sintering (SPS) conditions on the optical and mechanical properties of MgAl₂O₄ spinel was investigated for application to infrared windows as parts of military systems. The thermal conditions of SPS, including the temperature and heating rate, have a significant impact on optical and mechanical properties. This study shows that the formation and growth of abnormal grains cause mechanical degradation with an increasing SPS temperature. In-line transmittance (Tin) was affected by the heating rate due to changes in oxygen vacancy and carbon contamination in SPSed samples. The fabricated spinel exhibited excellent flexural strength of 401 MPa and an average mid-infrared transmittance of 84.8% in the range of 3-5 μm.

Microstructure and mechanical properties in dissimilar friction stir welded AA2024/7075 joints at high heat input: effect of post-weld heat treatment

In this research, dissimilar AA2024-T351 to AA7075-T651 joints manufactured at high heat input by friction stir welding has been studied to analyze the microstructure and mechanical properties by employing post-weld heat treatment (PWHT, solution and subsequent various aging treatments). The results revealed that abnormal grain growth (AGG) occurs in the nugget zone (NZ) of the joints after PWHT compared to large numbers of fine equiaxed grains forming in the NZ of the as-welded joint. The degree of grain growth of AA2024 side at the interface is obviously higher than that of AA7075 side. Stable {111}1¯1¯2A1∗ and {111}112¯A2∗ simple shear texture components are presented in the bottom interface zone, which are not altered during PWHT. The joint performances subjected to five kinds of PWHT are not improved by PWHT due to the weak lack of penetration phenomenon between the two BMs in the bottom zone of the joints, which results in the fracture locations of all the six joints occurring in the NZ.

Research on Abnormal Grain Growth in the Conventional Sintering of (K0.5Na0.5)NbO3 Nano Powders

About 30nm grain sized (K0.5Na0.5) NbO3 powders were used as the raw materials, and the solid phase method was used to prepare potassium sodium niobate ceramics at 750~ 950°C under normal pressure sintering. The abnormal grain growth (AGG) at process of powder sintering was investigated. The consequences demonstrate that when the sintering temperature is greater than 850°C, AGG can be clearly observed. As the sintering temperature increases, the number of areas where abnormal grain growth occurs increases as well as the size. When the sintering temperature is 950°C, the area of abnormal grain growth has a maximum of approximately 1mm × 1mm, and it has good (111) orientation. The difference in the sintering driving force caused by the size distribution in the potassium sodium niobate nano powder is the root cause of abnormal grain growth.

Hot deformation behavior and microstructure evolution of carbon nanotube/7055Al composite

In this study, carbon nanotube (CNT) reinforced 7055Al composite was fabricated by high energy ball milling combined with powder metallurgy. The as hot-pressed composite had an inhomogeneous microstructure composed of coarse grains without CNTs and fine grains with uniformly dispersed CNTs. Processing maps were established and the hot deformation behavior as well as the microstructure evolution during deformation was investigated. It was indicated that the power dissipation efficiency of the composite was relatively lower at higher deformation temperature or lower strain rate. Abnormal grain growth (AGG) and cracking occurred when the composite was deformed at high temperature with low strain rate. A few of CNTs were embedded into coarse grains as AGG occurred, and the micro-cracks formed at the boundaries between the coarse and the fine grained zones. The cracking mechanism was considered as the stress concentration caused by the dragging effect of CNTs, and the reduced critical stress required for pore nucleation due to AGG at the boundaries between the coarse and the fine grained zones.

Abnormal Grain Growth: A Potential Way to Construct Gradient Structure

Abnormal grain growth (AGG) has been commonly observed but the mechanism still remains unclear. In this study, the formation of abnormally large grains was investigated in compressed nickel powders. By periodical sintering in the large volume press, AGG occurred and resulted in a gradient coarse-grained structure. Through the improvement of the cell assembly by introducing tungsten tablets with a higher shear modulus, the optimized grained gradient structure can be obtained, which arises from tuning the AGG associated with the suppress of deformation. This work would be a potential way to design heterogeneous gradient nanostructured materials with excellent strength and ductility.

Formation mechanism of sol-gel synthesized Li7−3xAlxLa3Zr2O12 and the influence of abnormal grain growth on ionic conductivity

The development of solid-state batteries requires the optimization of ceramic electrolyte such as garnet type cubic Li7La3Zr2O12(LLZO). Exploration of powders formation mechanism and the sintering behavior of ceramics are crucial to acquire high performance LLZO electrolytes. This study adopts quantitative analysis based on X-ray adiabatic principle to investigate the formation mechanism of Li7−3xAlxLa3Zr2O12 (x = 0, 0.1, 0.15, 0.2, 0.25, LALZO) powders synthesized by sol-gel method. The whole calcination procedure could be divided into three stages as follows, La2Zr2O7(LZO) formation (<750 °C), transformation from LZO to LLZO (750 °C–850 °C) and LLZO decomposition (>850 °C). The phase, morphology and conductivity of LALZO ceramics demonstrate abnormal grain growth (AGG) caused by over-sintering leads to random orientation and conductivity deterioration. The optimal sintering condition could not only make grains bound tightly but also avoid AGG. Ceramics consist of LALZO (x = 0.25) exhibit better total ionic conductivity of 3.08 × 10−4 S cm−1 (Ea = 0.27 eV) with density of 92.5% after sintering at 1100 °C for 15 h. This research provides guidance for quickly acquiring pure LLZO powders and excellent performance ceramics.

Effect of Annealing Prior to Cold Rolling on the Microstructure Evolution and Energy Losses of Low-Si, Ultra-Low-C Hot-Rolled Electrical Steel

The microstructure of a low Si, ultra-low-C, hot-rolled electrical steel strip is modified by annealing at T &lt; To, the α→γ transformation temperature. This heat treatment causes the abnormal anisotropic growth of surface grains which consumes the original hot-rolled microstructure. The growth of the surface grains first takes place parallel to the rolling direction and then in a columnar form parallel to the normal direction until grains growing in opposite directions from the surfaces impinge at the center of the strip. It is shown that cold rolling and a short annealing treatment at temperatures between 700 and 800 °C leads to microstructures which result in iron energy losses that can be as much as 30% lower than those observed in the same material not subjected to the annealing prior to cold rolling. The magnitude of the reduction in energy losses depends on strip thickness and processing parameters. The major effect is observed in material annealed at 710 °C and the relative effect (with respect to material that is not annealed prior to cold rolling) decreases as the strip thickness decreases. It is shown that these effects can be attributed to the effect of the processing conditions on texture and grain size. The maximum reduction in energy losses is observed when the final microstructure consists of ferrite grains ~1.5 times larger than those obtained if the material is not annealed prior to cold rolling.

Influence of CaO and SiO2 additives on the sintering behavior of Cr,Ca:YAG ceramics prepared by solid-state reaction sintering

Yttrium aluminum garnet optical ceramics (Y3−xCaxAl4.95Cr0.05O12) doped with 0.1 at.% of chromium ions and 0.5, 0.8 and 1.2 at.% of calcium ions were prepared by reactive sintering in vacuum at 1750°C. The influence of SiO2 and CaO sintering aids on optical properties and microstructure of Cr,Ca:YAG ceramics was investigated. A decrease in CaO concentration led to a reduction of the Cr,Ca:YAG ceramics transparency from 81 to 0% at 1064 nm, moreover, samples with a lower concentration of CaO had abnormal grains and high-level porosity. The obtained features were attributed to the interactions between silica and calcium oxide during vacuum sintering of Cr,Ca:YAG ceramics. We propose and discuss two possible ways in which CaO and SiO2 influence the formation of Ca,Cr:YAG ceramics. The first one involves the appearance of a liquid phase thanks to the CaO–SiO2 interaction during the heating stage of ceramics sintering which results in abnormal growth of ceramics grains. The second mechanism consists on the mutual consumption of Si4+ and Ca2+ ions during the isothermal stage of ceramic sintering, which leads to a decrease in Cr4+ concentration. Both mechanisms have a negative effect on the optical and laser properties of the ceramics.

Statistical analysis of EBSD data to predict potential abnormal grain growth in 3.0 wt% Si grain-oriented electrical steel

The role of grain topology as a predictor on abnormal grain growth (AGG) of primary recrystallized silicon steel sheet has been investigated by EBSD technique. Goss oriented grains in the primary recrystallized silicon steel sheet were observed to have larger grain sizes than average matrix grains. The average grain size of grains with the smallest deviation angle (<1°) from the Goss orientation was around 2.6 times larger than the average grain size of the matrix. Also, as the grain orientation approaches the Goss orientation, it had the higher number of neighboring grains. In this study, eight grains were identified as AGG candidates that have at least three times larger grain sizes than the average grain size, and have orientations close to the Goss orientation, i.e., with <5° deviation angle from Goss. However, it was not possible to determine which is more likely to happen for the abnormal grain growth among these grains.

Dynamics of particle-assisted abnormal grain growth revealed through integrated three-dimensional microanalysis

Secondary-phase particles are routinely dispersed in metals and ceramics to prevent grain growth and take full advantage of the small grain size in the mechanical properties of polycrystals. Somewhat surprisingly, the preferential or abnormal growth of a few grains is observed in particle-containing systems at relatively high temperature, which will limit the lifetime of the material. The origins and mechanisms of particle-assisted abnormal grain growth are widely contested. Here, we employ integrated three-dimensional X-ray imaging to throw new light on the complex interactions between grain boundaries and particles in an Al–Cu alloy as a model system. We observe abnormal grain growth in the presence of a highly non-random distribution of particles. The incipient grain size is set by the local distribution of particles such that the larger grains come from particle-poor regions. Subsequently, grains with a size advantage may “run away” from the grain size distribution, in agreement with predictions from an analytical model that takes into account the competing capillary and particle pinning pressures.

Effect of annealing process on the abnormal grain growth behavior during carburizing of 20CrMnTi steel

Abnormal grain growth (AGG) as an important physical metallurgical behavior is to be avoided during carburizing due to its adverse effect on properties such as fatigue, impact toughness and the quenching distortion. In this work, two types of annealing process which produce different morphology of precipitated particles were conducted to investigate its effect on the subsequent austenite grain growth behavior during carburizing of 20CrMnTi steel. Calculation from Humphreys’ model exhibited a good agreement with experimental observations, which indicates the large sized particle from high temperature annealing led to the locally insufficient pinning force for the blockage of grain boundary migration and is therefore responsible for the AGG. The isothermal precipitation kinetics for TiC particles is able to be described by the diffusional growth and LSW coarsening models with reasonable accuracy provided with the transition time from growth to coarsening stage. Design of microalloying and pre-treatment processing to suppress AGG can be assisted with the combinatory application of Humphrey’s model and precipitation kinetics model.

Abnormal grain growth during annealing in the Al/Al2O3 composite produced by accumulative roll bonding

The mechanical and physical properties of composites are related to the matrix microstructure and reinforcements dispersion. Microstructural evolution during annealing can change the properties of the composites. Consequently, in this paper, the microstructural evolution of Al/Al2O3 composite during annealing and the interpretation of metallurgical phenomena were investigated. Accumulative roll bonding technique was used for manufacturing the Al/Al2O3 composite. The Al/Al2O3 composite specimens were further annealed at 375 °C for 15, 30 and 60 min without any protected atmosphere. The grain orientation (texture) and the grain size of the composites were investigated. Results showed that by increasing the annealing time to 60 min, abnormal grain growth started. Moreover, the frequency of low angle boundaries (1.5–5) increased, while the frequency of ∑3 Coincident Site Lattice boundaries decreased. In fact, grain boundaries with low Coincident Site Lattice were less prone to reinforcement pinning. Then low Coincident Site Lattice boundaries were more susceptible to formation and growth of abnormal grains.

Analysis of abnormal grain growth behavior during hot-press sintering of boron carbide

Dense boron carbide bearings with a central through hole were fabricated by hot-press sintering. When the sintering temperature was above the critical grain growth temperature, an increase in the applied pressure at 2100 °C caused a distinct abnormal grain size region. Two grain growth mechanisms were explored; normal grain growth (NGG) is controlled by a grain boundary diffusion mechanism whereas the abnormal grain growth (AGG) pattern abides by the ‘Bose–Einstein’ grain growth law.

Normal and Abnormal Grain Growth in Hydrogenation-Disproportionation-Desorption-Recombination Processed Nd–Fe–B Strip-Cast Alloys

Abnormal grain growth (AGG) is a common phenomenon in liquid-phase sintering of Nd–Fe–B materials. To identify the growth mechanism and suppress AGG, we investigated the grain growth in hydrogenati...

Abnormal growth of columnar grains and formation of Σ3 grain boundaries in non-oriented electrical steels

The remarkably abnormal growth behavior of columnar grains and the formation of Σ3 grain boundaries (GBs) were studied via plastic deformation and annealing based on an electron backscattering diffraction (EBSD) investigation. Driven by the stored energy and GB mobility, coarse austenite columnar grains were formed by the abnormal growth of fine equiaxed grains with higher grain orientation spread (GOS) values in the reducing atmosphere of hydrogen. Moreover, extraordinarily coarse columnar grains with high GOS values and non-{1 1 1} texture were formed by microstructure inheritance during γ → α transformation. In addition, slender ferritic columnar or island grains with Σ3 GBs and low GOS values were generally obtained, and the formation mechanism was strictly associated with the Kurdjumov-Sachs (K-S) relationship, which was induced during variant selection driven by the GB mobility and surface effect in the reducing atmosphere of hydrogen.