RX Grain Research Articles

Achieving superior strength-ductility synergy in a heterostructured magnesium alloy via low-temperature extrusion and low-temperature annealing

A low-alloyed Mg-1.2Zn-0.1Ca (wt.%) alloy was fabricated via low-temperature extrusion and annealing at 250 °C for different times (10, 30, and 90 min) to attain heterostructures with different fine-grained fractions, focusing on the effect of heterostructure on the mechanical properties. Partial dynamic recrystallization (RX) occurred during extrusion at 150 °C, and a lamellar structure consisting of fine RX grains and coarse unRX grains was obtained. The subsequent annealing promoted static RX in the as-extruded alloy, leading to an increased fine-grained fraction from 67% to 95%. Meanwhile, the co-segregation of Zn and Ca atoms impeded the migration of grain boundaries, thus achieving a fine grain size of 0.8–1.6 μm. The sample annealed for 10 min with a fine-grained fraction of 73% and an average RX grain size of 0.9 μm exhibited a superior combination of high yield strength (305 MPa) and good ductility (20%). In comparison, an excellent elongation of 30% was achieved in the alloy with a nearly fully-RXed microstructure and an average grain size of 1.6 μm after 90 min annealing, despite a lower yield strength of 228 MPa. In unRX grains, the hard orientation with 〈01–10〉 parallel to the extrusion direction and high-density dislocations made it more difficult to deform compared with the RX grains, thus producing hetero-deformation induced (HDI) strengthening. Besides fine grains and high-density dislocations, HDI strengthening is the key to achieving the superior mechanical properties of the low-alloyed Mg alloy.

An In Situ Resistance-Based Method for Tracking the Temporal Evolution of Recovery and Recrystallization in Ni-Base Single-Crystal Superalloy at Super-Solvus Temperatures

This study explores the influence of thermo-mechanical behavior and microstructure on recovery and recrystallization in single-crystal superalloys during casting and subsequent solution heat treatment, using miniature testing. Here, the temporal evolution of resistance was measured using in situ electrothermal mechanical testing (ETMT) to track the process of recovery and recrystallization (RX). It was found that recrystallization is dominant only when recovery is incomplete and is dependent on both the history dependence of the strain path as well as the magnitude of the accumulated plastic strain. A precursor to recrystallization is the occurrence of subgrains and deformation twins on the sample surface, where a characteristic butterfly-type morphology of γ′ precipitates is always observed in highly strained regions. The migration of RX grain boundaries is accompanied by the elimination of lattice curvature associated with the density of geometrically necessary dislocations. Homogenized samples provide the most reliable results, while interpreting resistivity changes with recovery and recrystallization is more challenging when inhomogeneity (microsegregation, local variation of mechanical properties) in as-cast material prevails.

In-situ observations and modeling of metadynamic recrystallization in 300M steel

The metadynamic recrystallization (MDRX) behaviors and microstructure evolution of 300 M steel are studied by hot compression experiments and high temperature laser scanning confocal microscopy (HTLSCM) experiments. The effects of insulation temperature, pre-strain rate and holding time on the MDRX behaviors are discussed in detail. Based on the statistical results of the microstructure evolution, the MDRX kinetics model and recrystallization (RX) grain size model of 300 M steel are established, respectively. It can be confirmed from the in-situ observations that strain-induced boundary migration mechanism and curvature-driven grain boundary migration mechanism are the main MDRX mechanisms of 300 M steel. The MDRX develops rapidly under the high temperature or low pre-strain rate. The insulation temperature has a more important effect on the MDRX rate. The RX grain size increases with the increased insulation temperature, pre-strain rate and holding time. Compared to the pre-strain rate, the effects of insulation temperature on the maximum MDRX fraction and RX grain size are negligible. Comparisons between the experimental and predicted results verify that the proposed MDRX kinetics models and RX grain size model can quantificationally reveal the MDRX behaviors of 300 M steel well.

In-situ observations and modeling of static recrystallization in 300 M steel

The study of static recrystallization (SRX) has been widely concerned for its importance in regulating the microstructure and mechanical properties of deformed materials. In the paper, the effects of SRX on the flow behaviors of 300 M steel are investigated by thermal compression experiments. The SRX mechanism of 300 M steel is investigated by high temperature laser scanning confocal microscopy (HTLSCM) setup. The variations in the flow behaviors and microstructure under different experimental conditions are discussed in detail. Also, the relationships between microstructure evolution and mechanical properties are analyzed. The SRX mechanism is detected by the analysis of microstructure. According to the statistical results of microstructure, the SRX kinetics models and the average recrystallization (RX) grain size model are proposed. It can be found that the flow stress increases with the increased strain rate and reduced deformation temperature. The yield stress is affected by both the softening mechanisms of static recovery and SRX. The main SRX mechanism of 300 M steel is strain-induced boundary migration (SIBM). Compared to the pre-strain rate, the insulation temperature has a more significant effect on the SRX rate, maximum SRX volume fraction and average RX grain size. And the effects of pre-strain rate on the RX grain size are negligible at low pre-strain rate. The comparison between the calculated and experimental results is consistent, which implies that the established models can describe the SRX process accurately. Besides, compared to the softening volume fraction calculated based on the stress-strain curves, the SRX volume fraction calculated based on the statistic results of microstructure can reflect the SRX process more precisely, which indicates the effectiveness of the research method used in the paper.

Microstructural and bulk properties evolution of cold-sprayed copper coatings after low temperature annealing

The current study details an experimental analysis that elucidates the relationship between the processing history, microstructural evolution, and bulk properties of cold-sprayed pure FCC copper coatings. Particular attention is paid to determining the effect of low temperature ex situ and in situ annealing (350 °C; 1 h) on the evolution of microstructure (i.e. grains and subgrains) and porosity, as well as the combined influence of these defects on the bulk properties (microhardness, and electrical conductivity) and fracture mechanics of cold-sprayed copper.SEM, TEM, in situ TEM, and fractography analysis together revealed that the inherently brittle nature of the cold-sprayed metals cannot be explained solely by their porosity or microstructure; their fracture mechanics is most likely governed by the coupling interaction between porosity and the nanocrystalline dynamically/static recrystallized (RX) grains adjacent to the pores. Under tensile loading, pores (0.5–20 µm) in the as-sprayed copper can easily act as crack tips which then propagate along the adjacent RX grains (∼100 nm), resulting in a brittle cleavage fracture. Although low-temperature annealing was not sufficient to remove the porosity, it was capable of promoting recovery and grain growth in these RX grains. The ductility of the region ahead of the crack tips was therefore enhanced to promote dimpling and potentially crack blunting. Consequently, this low-temperature annealing improved the tensile ductility of the cold-sprayed copper by 2 orders of magnitude (from ∼0.2% to ∼25% strain).

A model for simulation of recrystallization microstructure in single-crystal superalloy

In the present investigation, a coupled crystal plasticity finite-element (CPFE) and cellular automaton (CA) model was developed to predict the microstructure of recrystallization in single-crystal (SX) Ni-based superalloy. The quasi-static compressive tests of [001] orientated SX DD6 superalloy were conducted on Gleeble3500 tester to calibrate the CPFE model based on crystal slip kinematics. The simulated stress–strain curve agrees well with the experimental results. Quantitative deformation amount was introduced in the deformed samples of simulation and experiment, and these samples were subsequently subjected to the standard solution heat treatment (SSHT). Results of CA simulation show that the recrystallization (RX) nucleation tends to occur at the third stage of SSHT process due to the high critical temperature of RX nucleation for the samples deformed at room temperature. The inhomogeneous RX grains gradually coarsen and compete to reach more stable status by reducing the system energy. Simulated RX grain density decreases from 7.500 to 1.875 mm−1, agreeing well with the value of 1.920 mm−1 from electron backscattered diffraction (EBSD) detection of the experimental sample.

Experimental and Numerical Studies on Recrystallization Behavior of Single-Crystal Ni-Base Superalloy.

The recrystallization (RX) behavior of superalloy during standard solution heat treatment (SSHT) varies significantly with deformation temperature. Single-crystal (SX) samples of Ni-base superalloy were compressed to 5% plastic deformation at room temperature (RT) and 980 °C, and the deformed samples were then subjected to SSHT process which consists of 1290 °C/1 h, 1300 °C/2 h, and 1315 °C/4 h, air cooling. RT-deformed samples showed almost no RX grains until the annealing temperature was elevated to 1315 °C, while 980 °C-deformed samples showed a large number of RX grains in the initial stage of SSHT. It is inferred that the strengthening effect of γ’ phases and the stacking faults in them increase the driving force of RX for 980 °C-deformed samples. The RX grains nucleate and grow in dendritic arms preferentially when the microstructural inhomogeneity is not completely eliminated by SSHT. A model coupling crystal plasticity finite element method (CPFEM) and cellular automaton (CA) method was proposed to simulate the RX evolution during SSHT. One ({111} <110>) and three ({111} <110>, {100} <110>, {111} <112>) slip modes were assumed to be activated at RT and 980 °C in CPFEM calculations, respectively. The simulation takes the inhomogeneous as-cast dendritic microstructure into consideration. The simulated RX morphology and density conform well to experimental results.

Influence of gibbosity on recrystallization behavior of single crystal blade casting

Recrystallization (RX) defects formed in the gibbosities induced by platinum chaplet pins were studied by experiments and simulations for single crystal (SX) blade castings. Recrystallization occurs in 39 of the 80 gibbosities and can penetrate more than 250 μm into the SX blade. The RX occurrence ratios for the gibbosities in the upper, middle and lower positions of the blade are 83%, 50%, and 8% respectively, indicating a strong sensitivity of RX to the location height of the gibbosity. The microstructure is SX without the presence of micro misoriented grains in the cross-section of gibbosity of as-cast sample. Secondary carbide particles are found at the interface between the RX and SX grains. A composite image of backscattered electron (BE) and electron backscatter diffraction (EBSD) shows that the migration of the RX interface is hindered by coarse eutectic particles. A thermal elastoplastic model that considers the anisotropic mechanical properties of SX superalloys and is coupled with a finite-element method (FEM) was utilized to predict the deformation behavior during directional solidification. The FEM results reveal that the equivalent plastic strain (PEEQ) tends to concentrate in the gibbosities, and this gives a reasonable and reliable explanation for RX formation.

Effect of Recrystallization on Crack Initiation and Propagation during Creep in a Directionally Solidified Ni-Based Superalloy

Crack initiation and propagation during creep test in a recrystallization (RX) grain containing directionally solidified (DS) nickel-base superalloy was studied by employing electronic backscattered diffraction (EBSD) technique. Creep samples were locally deformed and then heat treated to get local RX at the center of the gauge length. Creep test was carried out at 980 °C/235 MPa. It was found that during creep test, precipitate free zones (PFZ) were formed along the RX grain boundary and the cracks were initiated and propagated along the interface (a) between PFZ and RX grains, (b) between PFZ and the matrix or (c) within PFZ. It indicated that the crack initiation and the propagation were crystal orientation dependent.

Influence of surface recrystallization on the low cycle fatigue behaviour of a single crystal superalloy

ABSTRACTThis paper investigated the effect of surface recrystallization (RX) on the low cycle fatigue (LCF) behaviour of a single crystal (SX) superalloy. LCF tests on both raw and recrystallized samples showed that fatigue life was significantly reduced by surface RX. Fractography indicated that fatigue cracks initiated from the casting defects in RX layer and multiple crack initiations were commonly observed. Moreover, RX grains exhibited predominantly transgranular cracking, in contrast to the intergranular fracture reported in literature. The fatigue crack propagation behaviour was discussed in light of fracture mechanics and crack growth life model. The fatigue cycles required to penetrate RX layer were estimated to be about one magnitude lower than that in forming an equivalent crack in SX specimens. It is suggested that the earlier crack initiation and promoted crack propagation in RX layer, as well as the trend of multiple initiations, are responsible for the fatigue degradation by RX.

Effect of carbon content on the recrystallization of a single crystal nickel-based superalloy

Carbon has recently been reintroduced to single crystal (SX) superalloys because of its beneficial effects, such as reducing the occurrence of casting defects and enhancing the tolerance of low angle grain boundaries. Meanwhile, the addition of carbon leads to changes in the carbide morphology and volume fraction and may subsequently affect the recrystallization (RX) behavior. The purpose of this paper was to identify the effect of the carbon content on RX in an SX nickel-based superalloy. The SX specimens were prepared with an identical crystal orientation but different carbon contents. It was found that with the increase of the carbon content from 0.012wt% to 0.17wt%, the volume fraction of carbides increased linearly, while the average size of the carbides was nearly unchanged. After shot peening and heat treatment, a layer of RX grains was generated on the surface. As the carbon content was increased, the RX depth increased at first and later decreased. These changes were interpreted as the combined effect of the carbides on the stored energy during deformation and the pinning force during RX grain growth.

Influence of the characteristic of recrystallization grain boundary on the formation of creep cracks in a directionally solidified Ni-base superalloy

Creep tests of a directionally solidified (DS) Ni-base superalloy specimens containing local recrystallization (RX) were carried out at 980 °C/235 MPa and the influence of the characteristic of RX grain boundary on the formation of creep cracks was investigated. The RX grain boundary morphology, misorientation and dislocation pattern at high, low angle and twin boundaries during creep were examined. It was shown that only a few high angle grain boundaries (GBs), but not all of them cracked during creep and even after ruptured. This is associated with the dislocation mobility at GBs or the morphology of GBs. The possibility of crack formation was discussed based on the transmission behavior of dislocations at different GBs.

Intermediate temperature creep of directionally solidified Ni-based superalloy containing local recrystallization

Intermediate temperature creep of directionally solidified Ni-based superalloys containing local recrystallization (RX) was studied. A sharp reduction of the creep rupture life was observed with increasing the transverse RX area fraction on the cross section of the specimen. Creep curves of specimens containing local RX showed that the steady state creep stage was much shorter than that of the “good” specimens. The creep rates were also slightly higher when there was local RX. The creep and failure mechanism was discussed based on the dislocation morphology in the vicinity of the RX grain boundaries and the formation and propagation of cracks in the RX region according to the microstructure observation of the interrupted creep specimens.