DRXed Grains Research Articles

Dynamic recrystallization of commercially pure titanium during cryogenic compression

In this study, a systematic investigation on the twin-induced dynamic recrystallization (DRX) of commercially pure titanium (CP-Ti) was carried out under uniaxial compression at room (RT) and cryogenic temperature (CT, −150 °C). The compression tests were intentionally interrupted at 0.02, 0.05 and 0.1 strain levels at both deformation temperatures and a detailed post-mortem analysis was performed via electron backscattered diffraction (EBSD) to examine the progressive evolution of microstructure. The results revealed that two major types of twins i.e. {10−12} extension twin (ETW) and {11−22} compression twin (CTW) were effectively activated at both deformation temperatures. During RT deformation, increased strain levels resulted in the higher evolution of ETWs and CTWs, where numerous twin lamellas, lateral twin thickening and twin-twin interactions (ETW-ETW, ETW-CTW) were observed. On the other hand, CT deformation led to significant grain refinement with the average grain size reduced by 94% (3.7 μm for CT-10 sample as compared to 63 μm of initial material). The recrystallization kinetics were understood with respect to three region of interests (ROIs). The DRX mechanism was identified as tDRX, where intersectional {10–12} ETWs and {11–22} CTWs network provided the preferential sites for the nucleation of DRXed grains due to high strain energy from the dislocation pile-ups at the TBs.

Systematic investigation of the microstructure evolution and dynamic recrystallization mechanisms of as-cast Fe–TiB2 composites under different hot deformation parameters

Fe–TiB2 composite shows great potential in lightweight materials for automotive parts due to its lightweight, high strength and high specific modulus etc. However, challenges remain with regard to establishing a fine hot working process of Fe–TiB2 composites. Here, a series of isothermal hot compression tests was conducted to systematically investigate the microstructure evolution and the dynamic recrystallization (DRX) mechanisms of as-cast Fe–TiB2 composites at different sets of hot deformation parameters. The results indicate the true stress drops more significantly after reaching its peak at low temperatures compared to high temperatures, since low temperatures favors the DRX through particle stimulated nucleation (PSN) in Fe–TiB2 composites, as featured by layers of DRXed grains surrounding TiB2 particles. Continuous dynamic recrystallization (CDRX) process gradually dominates as strain rate increase from 0.01 s−1 to 1 s−1 in Fe–TiB2 composites hot compressed to the true strain of 0.69 at 900 °C, as evidenced by the presence of DRXed grains with larger size and uniform distribution in matrix. DRX through PSN is the primary DRX mechanism at low temperature and low strain rate, while CDRX dominates the DRX mechanism at high temperature and high strain rate. Both DRX behaviors deepen with the increase of the strain. Different hot deformation parameters favor different DRX mechanisms. The increase in temperature and strain rate leads to a transition of the dominant recrystallization mechanism from the DRX through PSN to CDRX which is attributed to the competition for stored energy between the DRX through PSN and CDRX process.

Texture weakening and grain refinement behavior of the extruded Mg-6.03Zn-0.55Zr alloy during hot plane strain compression

A series of hot plane strain compression (PSC) experiments of the extruded Mg-6.03Zn-0.55Zr alloy were conducted on the Gleeble-3500 machine with strain rates of 0.01 s−1 to 10 s−1 and temperatures ranging from 300 °C to 500 °C to reveal the microstructure evolution during the secondary process. The effects of grain refinement and texture modification which were easily affected by the deformation mechanism and dynamic recrystallization (DRX) were systematically studied. Experimental results showed that the main deformation mechanism during PSC was basal <a> slip (74.3 %–94.9 %) and assisted by prismatic <a> slip (4.8 %–21.1 %) and pyramidal <c+a> slip (0.3 %–4.6 %). Since the pyramidal <c+a> slip and prismatic <a> slip were more preferred to activate owing to the significant decrease of critical resolved shear stress (CRSS) at a higher temperature, the activation of non-basal slip promoted the c-axes of the grains deviated from compression direction which weakened the basal fiber texture. Moreover, the activation of non-basal slip could promote DRX behavior to some extent. Further analysis of the DRX mechanism showed that the continuous DRX (CDRX) by continuous rotation of low angle grain boundaries was dominant at lower temperatures and higher strain rates, while discontinuous DRX (DDRX) which nucleated at the grain boundaries played a more and more important role as the temperature increased and strain rate decreased. After characterizing the orientations of the DRXed grains, the CDRXed grains owned a similar grain orientation with the deformed grain while the DDRXed grains exhibited a fairly dispersed distribution, indicating that the weakening effect of the basal texture was greater with a higher DDRX fraction.

Interaction of dynamic precipitation and dynamic recrystallization of a Mg-4Sn-3Al-1Zn alloy during hot compression

The intrinsic correlation between dynamic precipitation and Dynamic Recrystallization (DRX) in a rare earth (RE)-free engineering Mg-4Sn-3Al-1Zn (TAZ431, wt.%) magnesium alloy has been systematically investigated. Hot compression tests were performed on the TAZ431 alloy, in comparison to Mg-3Al-1Zn (AZ31, wt.%) as a reference alloy, with varying temperatures, strain rates, and strain levels (T = 300 ‐ 460 °C; ε̇ = 0.001 - 1 s‐1; ε = 0 to -0.9). Correlative microstructural characterizations including Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Electron Backscatter Diffraction (EBSD), were employed to explore the behaviors of precipitation, DRX and intragranular deformation as subjected to hot compression. The results indicate a significant interaction between Mg2Sn dynamic precipitation and the formation of DRXed grains. The TAZ431 alloy with Sn alloying exhibits profuse precipitation and larger DRX fraction within the microstructure, resulting in the increase of modulus and hardness. Nucleation of DRXed grains in the TAZ431 alloy was stimulated by relatively large (over 1 micrometer) undissolved Mg2Sn intermetallic particles. Meanwhile, the precipitation of nano-sized Mg2Sn occurred within DRXed grains and was facilitated by DRX process, consequently hindering the further growth of DRXed grains. The findings here may have implications for alloy design and thermal-mechanical processing of magnesium alloy structural components.

Superplastic behavior of a fine-grained Mg−Gd−Y−Ag alloy processed by equal channel angular pressing

An extruded Mg−6Gd−3Y−1.5Ag (wt%) alloy was processed by 6 passes of equal channel angular pressing (ECAP) at 553 K using route Bc to refine the microstructure. Electron back-scattered diffraction (EBSD) analysis showed a fully recrystallized microstructure for the extruded alloy with a mean grain size of 8.6 µm. The microstructure of the ECAP-processed alloy was uniformly refined through dynamic recrystallization (DRX). This microstructure contained fine grains with an average size of 1.3 µm, a high fraction of high angle grain boundaries (HAGBs), and nano-sized Mg5Gd-type particles at the boundaries of the DRXed grains, detected by transmission electron microscopy (TEM). High-temperature shear punch testing (SPT) was used to evaluate the superplastic behavior of both the extruded and ECAP-processed alloys by measuring the strain rate sensitivity (SRS) index (m-value). While the highest m-value for the extruded alloy was measured to be 0.24 at 673 K, the ECAP-processed alloy exhibited much higher m-values of 0.41 and 0.52 at 598 and 623 K, respectively, delineating the occurrence of superplastic flow. Based on the calculated average activation energy of 118 kJ mol−1 and m-values close to 0.5, the deformation mechanism for superplastic flow at the temperatures of 598 and 623 K for the ECAP-processed alloys was recognized to be grain boundary sliding (GBS) assisted by grain boundary diffusion.

Optimization in strength-ductility of Mg-Gd-Y-Zn-Zr alloy processed by upsetting-extrusion combined with intermediate thermo-mechanical treatment (ITMT)

In this work, the effects of intermediate annealing and intermediate annealing subsequent aging treatment on the second phase precipitation, dynamic recrystallization (DRX) behavior, and mechanical properties of Mg-9Gd-4Y–2Zn-0.5Zr (wt.%) alloy were comparatively studied. The results showed that the one-pass upsetting-extrusion (UE) deformation intermediate annealing treatment sample (UAE) exhibited the highest dynamic recrystallized (DRXed) grains number fraction and the largest average DRXed grain size, which was mainly attributed to the precipitation of a large number of β-Mg5RE phases (>1 μm) at the grain boundaries (GBs) and within unDRXed grains after annealing treatment. These β-Mg5RE phases promoted DRX through particle-stimulated nucleation (PSN) mechanism during subsequent extrusion process. However, the lack of the pinning effect of the fine second phase precipitations led to the growth of DRXed grains. The DRXed grains of the deformation intermediate annealing subsequent peak-aging treatment sample (UAAE) were significantly smaller than the UAE sample. This is mainly due to the fact that the densely distributed prismatic β′ phases precipitated by the peak-aging treatment were rapidly transformed into nano-scale β-Mg5RE phases and pinned at the DRXed grain boundaries to inhibit the growth of the DRXed grains during the subsequent extrusion process. The ultimate tensile strength (UTS) and failure elongation (EL) of the UAAE sample were 274 MPa and 14.3%, respectively, 4.2% and 85.7% higher than those of the UE sample, showing excellent strength-ductility synergy. The randomly oriented fine DRXed grains, coarse unDRXed grains with high-density dislocations and substructures, and nano-scale β-Mg5RE particles were co-contribute to the strength-ductility synergy.

In-Situ Study on the Tensile Deformation and Fracture Mechanism of a Bimodal-Structured Mg-Gd-Y Alloy.

The as-extruded (EX) Mg-Gd-Y alloy studied here exhibited a bimodal structure, composed of fine dynamic recrystallized (DRXed) grains with random orientations and longitudinal coarse hot-worked grains. The slip analysis showed the DRXed grains exhibited mainly basal slips, while the hot-worked grains exhibited mainly prismatic slips during the tensile deformation. The distribution of geometrically necessary dislocations (GNDs) showed that there was strain partitioning between the fine and coarse grain regions. The hetero-deformation induced (HDI) hardening occurred between the two domains. It improves the strength and strain hardening capability of the alloy, leading to good strength-ductility synergy. Microcracks tended to nucleate at the DRXed grain boundaries, as well as at the interface between the two domains. The calculation of geometric compatibility parameter (m') indicated that strain incompatibility between the adjacent grains induced the crack nucleation. The toughening effect of the fine DRXed grains hindered the crack propagation. However, the major crack formed at the interface between the two domains propagated unstably, due to the high stress concentration and the large crack size, causing the final failure.

Punching shear failure behavior of fine-grained ZK60 Mg alloy processed by a novel forward shear normal extrusion process at room and elevated temperatures

The shear punch test (SPT) of ZK60 Mg alloy hot deformed by a novel sever plastic deformation technique called the forward shear normal extrusion (FSNE) was carried out at room and elevated temperatures, and the influence of FSNE process temperature on shear deformation behavior was clarified. FSNE of Mg alloy was carried out at temperatures of 200 °C, 250 °C, 300 °C and 350 °C and then SPT behavior of deformed samples was studied at temperature range of 25–250 °C. The results demonstrated that the deformation behavior and mechanical properties during SPT were significantly affected by the initial microstructure owning to different temperatures of the FSNE process. The microstructures in 200 °C, 250 °C and 300 °C FSNE processed specimens were composed of coarse deformed grains and ultra-fine DRXed grains with a near fully DRXed structure in the highest temperature (350 °C). Basal plane intensity was the maximum value in the 200 °C FSNE processed specimen, whiles with increasing FSNE process temperature, the random crystallography orientation was gradually dominated. Mechanical properties results showed that the 350 °C processed specimen presented the superior RT shear properties with the ultimate shear strength (USS) of ∼ 159 MPa and peak shear strain (PSS) of 81% as well as the highest thermal stability. Shear failure surface was mainly composed of rollover, burnished, fractured and burr areas. The percentage of burnished area in punching-out specimens was affected by initial microstructure and strength and for the specimens having high strength, the percentage of this area was low. In each SPT process temperature, higher peak strain postponed rapture and resulted in higher burr area.

Dependence of interfacial bonding behavior on strain rate in CoCrFeMnNi high-entropy alloy joints produced by vacuum hot-compression bonding

Vacuum hot-compression bonding (VHCB) is a highly competitive solid-state bonding technology. To expand the application prospects of VHCB in high-entropy alloys (HEAs), the interfacial bonding behavior of CoCrFeMnNi HEA VHCB joints was investigated, and its intense dependence on the strain rate was clarified. The microstructural characterization results suggested that excellent interfacial bonding was subject to interfacial grain boundary (IGB) migration and dynamically recrystallized (DRXed) grains, and exhibited different bonding behaviors at various strain rates. Interfacial residual nanoscale IOPs facilitated the nucleation of DRXed grains through particle-stimulated nucleation effect; however, their pinning effect on IGB migration also negatively affected interfacial bonding. In addition, this study focused on the prominent contribution of twin boundaries to IGB migration and revealed the evolution of interfacial microtexture. The tensile test results indicated that the ratio of joint elongation to base material elongation (REL) exhibited similar evolution characteristics to the interfacial DRX fraction (XDRX). A new interfacial bonding quality prediction model was developed by describing the quantitative relationship between REL and XDRX, and the accuracy of the model prediction was verified by comparing it with the tested values.

Effect of dynamic recrystallization, LPSO phase, and grain boundary segregation on heterogeneous bimodal microstructure in Mg-9.8Gd-3.5Y-2.0Zn-0.4Zr alloy

The formation conditions and influence mechanisms of the heterogeneous bimodal microstructure of Mg-9.8Gd-3.5Y-2.0Zn-0.4Zr alloy were systematically studied by thermal compression experiments in the range of 0.001–1.0 s−1 and 350–500 °C. Combined with the microstructural characterization, the inherent relationships between the grain structure and the deformation parameters are deeply analyzed. The results indicate that heterogeneous bimodal microstructures composed of fine dynamically recrystallized (DRXed) grains with random orientation and coarsely deformed grains with basal orientation can only be formed under deformation conditions of 40.51 < lnZ < 46.33. This mainly stems from the dominant continuous dynamic recrystallization (CDRX) and particle stimulated nucleation (PSN) mechanisms under this deformation condition. However, dynamic recovery (DRV) and discontinuous dynamic recrystallization (DDRX) dominate isothermal compression process with lnZ > 47.41 and lnZ < 39.74, which eventually lead to unimodal distributions of deformed grains and DRXed grains, respectively. Furthermore, results reveal that deformation parameters have a strong influence on the long-period stacking order (LPSO) phase and grain boundary (GB) segregation. The lamellar LPSO phases and grain boundary co-segregation of Zn, Y, and Gd atoms jointly promote CDRX and inhibit DDRX by preventing GB migration under the condition of 40.51 < lnZ < 46.33, which is ultimately beneficial to the development of heterogeneous bimodal Mg-Gd-Y-Zn-Zr alloy.

Effect of Y and Gd solutes on grain refinement of the as-extruded Mg-Gd(-Y)-Zn-Mn alloys

Mg alloys exhibit high Hall-Petch coefficients, so obtaining refined grains in wrought alloys is important for their high performance. This study explored the roles of Y and Gd solutes on grain refinement of the Mg-(1.25–2.5)Gd-(0–1.25)Y-0.5Zn-0.4Mn(at%) extruded alloys, and the mechanical properties were tested. Microstructural analyses show that incomplete dynamic recrystallization (DRX) occurred after hot extrusion of Mg-Gd(-Y)-Zn-Mn alloys and DRX behaviors varied with Y and Gd contents. The addition of Y or an increase in Y/Gd atomic ratio (by partially substituting Gd with Y) resulted in a reduction of the average DRXed grain size, while adding Gd (or Y) or decreasing the Y/Gd atomic ratio facilitated DRX nucleation. Mechanical properties demonstrate that the joint alloying of two RE elements, Y and Gd, enhanced the experimented alloys’ strength surpassing the effect of using Gd alone. The Mg-2.5Gd-0.75Y-0.5Zn-0.4Mn(at%) extruded alloy displayed the optimal mechanical characteristics having a 402 MPa ultimate tensile strength, a 345 MPa yield strength, and 5.2% moderate ductility.

Texture tailoring and microstructure refinement induced by {11−21} and {10−12} twinning in an extruded Mg-Gd alloy

An extruded Mg-17Gd (wt%) alloy with < 0001 > fiber texture was compressed along different directions at room temperature. Electron backscatter diffraction characterization was used to investigate the texture tailoring, twinning and dynamic recrystallization (DRX) behaviors. The results indicate that {10−12} and {11−21} twinning activated. The [0001] pole tilted to compression direction with the strain increasing and twinning dominated the texture transition. Activation of {10−12} twinning can be governed by Schmid law, while it is more appropriate to explain the activation of {11−21} twinning by the basal slip transfer condition in the twinned grains according to the slip trace analysis. In addition, operation of {11−21} twinning can facilitate the nucleation of {10−12} twin in the same grain by changing Schmid factor (SF) in the vicinities of {11−21} twin. Grain refinement occurred in the specimens with large strain and the average grain size decrease from ∼27 µm to ∼12 µm. The mechanism of grain refinement can be summarized as follows: the mobility of twin boundaries will be restricted because of the twin-twin and dislocation-twin interaction and converted to random high angle grain boundaries (HAGBs). The random HAGBs offer the nucleation sites and the strain accumulation within twins offer the driving force which facilitate the formation of DRXed grains at random HAGBs. Additionally, the SF difference within parent grain may result in the formation of HAGBs, and these HAGBs can connected the random HAGBs that result from the dislocation-twin interaction which result in the formation of new grains.

Machine learning-assisted constitutive modeling of a novel powder metallurgy superalloy

The flow behavior and microstructure evolution during thermomechanical processing (TMP) of powder metallurgy superalloys are very important for their service behavior. In this contribution, a machine learning-assisted physical model was proposed to predict the flow stress, dynamic recrystallization (DRX) kinetic, and grain size evolution of a novel FGH4113A superalloy during the TMP. The work hardening and dynamic softening stages were modeled separately to predict the flow stress. The novel aspect of the proposed model is that the strong coupled effects of deformation temperature, strain rate, true stain, and primary γ' precipitates on the DRX kinetic were quantitatively described by the genetic algorithm-based artificial neural network (GA-ANN). Besides, the grain size modeling considered the pinning effect by primary γ' precipitates and the deformation-induced DRXed grain growth. Comparing the experimental and predicted flow curves, DRX fraction, and average grain size reveals that the proposed model has a preferable prediction accuracy than the conventional model. Then the specific DRX kinetic and grain size evolution characteristics were revealed. The promoting effect of strain rate on the DRX degree was gradually increased with the deformation temperature. Finally, the 3D processing map based on the developed model was established to guide TMP's optimized design and microstructure control. Three domains were divided and discussed: grain refinement, constant, and growth. The optimal processing window is defined as 1000–1050 °C/0.001–0.006 s−1 and 1075–1100 °C/0.008–0.01 s−1.

Strong yet strain-hardenable equiatomic CoCrFeMnNi high-entropy alloys by dynamic heterostructuring

Heterostructuring gives solutions to achieve better mechanical performance of metallic materials. For instance, partial static recrystallization by short-term annealing induces bimodal grain size distribution. Distinct from the well-studied ‘static heterostructuring’ by cold rolling followed by annealing, this study introduces ‘dynamic heterostructuring’ by hot rolling. Dynamic recrystallization (DRX) during the hot rolling as a softening mechanism leaves room for dislocation accumulation. With the expectations of enhanced strain hardening of the as-rolled materials with the aid of the DRX, we monitored tensile responses of the as-hot-rolled equiatomic CoCrFeMnNi high-entropy alloys (HEAs). Among the hot rolling temperatures from 800 ℃ to 900 ℃ with a thickness reduction ratio of 78.6% for the HEAs, the hot rolling at 850 °C results in a bimodal grain size distribution, i.e., partial DRX. It comprises relatively fine DRXed grains and coarse unrecrystallized grains: the latter is harder than the DRXed ones with relatively low dislocation density. The microstructural heterogeneity results in uniform elongation of ∼13% due to the accumulation of dislocations at the grain boundaries inside the soft DRXed grains. This endows the as-hot-rolled single-phase HEAs with strain hardenability without the aid of phase transformation, deformation twinning, or precipitates. In addition, the hot-rolled HEAs with high dislocation density have doubled yield stresses of those of conventionally cold-rolled and annealed counterparts. Considering the industrial benefits of the simplified thermo-mechanical control processes, this work extends the academic and practical value of DRX by dynamic heterostructuring through annealing-free hot rolling.

Optimizing LPSO phase to achieve superior heat resistance of Mg–Gd–Y–Zn–Zr alloys by regulating the Gd/Y ratios

The microstructural features of long period stacking ordered (LPSO) phase in Mg–(11-x)Gd–xY–1Zn–0.3Zr (wt.%) alloys were optimized by adjusting Gd/Y ratios to10:1, 8:3, 6:5, 4:7 and 2:9, respectively. Superior high-temperature strength was successfully obtained in the extruded 6Gd–5Y alloy with ultimate tensile strength (UTS) values of 338 MPa and 286 MPa at 250 °C and 300 °C, respectively. In the as-cast alloys, with the decrease of Gd/Y ratio, the type of intermetallic compound gradually changed from (Mg, Zn)3(Gd, Y) (W) to LPSO phase. After homogenization, the W phase was completely converted to LPSO phase except for the 10Gd–1Y alloy. Importantly, the block-shaped LPSO phases with the smallest size and the abundant lamellar LPSO phases were formed in the 6Gd–5Y alloy, which significantly influenced the microstructures of the extruded alloy. Firstly, the small sized block-shaped LPSO phase before extrusion provided favorable conditions for the formation of thin plate-shaped LPSO phase at DRXed regions, which effectively hindered the grain boundary migration at high temperature due to superior thermal stability. Secondly, the profuse lamellar LPSO phases prior to extrusion contributed to the highest density of low angle grain boundaries (LAGBs) and residual dislocations in the extruded 6Gd–5Y alloy, which obstructed dislocation slip at elevated temperatures. In addition, the abundant lamellar LPSO phase also caused the fine DRXed grain size and strong basal texture to enhance the high-temperature strength of the extruded 6Gd–5Y alloy.

Optimization in strength-ductility of Mg-RE-Zn alloy based on different repetitive upsetting extrusion deformation paths

This study investigated the effects of different repetitive upsetting extrusion (RUE) deformation paths on the microstructures and mechanical properties of Mg-9.32Gd-3.72Y-1.68Zn-0.72Zr (wt%) alloy. Bimodal microstructures were observed in all RUE deformed samples. Increasing the RUE ratio under the same cumulative strain enhanced the dynamic recrystallization (DRX) process, resulting in higher aspect ratios of deformed grains and dynamic precipitation of β phases. The bulk long period stacking ordered (LPSO) phases promoted the DRX process through a particle-stimulated nucleation mechanism, while the β phases hindered the growth of DRXed grains by a pinning effect during deformation. As the RUE ratios increased, the LPSO phase gradually aligned towards the extrusion direction. The R1P3 sample demonstrated the best balance between strength and ductility under small RUE ratio and multi-pass deformation, with ultimate tensile strength, tensile yield strength (TYS), and elongation reaching 382.5 ± 3.1 MPa, 302.3 ± 4.2 MPa, and 8.7 ± 0.5%, respectively. The different contributions of LPSO phases strengthening and heterogeneous deformation-induced (HDI) strengthening led to variations in TYS in different samples. Adjusting the uniformity of the deformed grain size in the bimodal microstructure based on HDI strengthening can further enhance the synergistic improvement of the alloy's strength and ductility.

Development of ultrahigh-strength low-alloy Mg-0.7Al-0.3Ca-0.4Mn (wt.%) alloy with excellent ductility via controlling extrusion temperature

Mg-0.7Al-0.3Ca-0.4Mn (AXM070304, wt.%) alloy was subjected to extrusion with a ram speed of 1 mm/s and an extrusion ratio of 25:1. The extrusion temperature varies from 170 °C to 400 °C. The influence of extrusion temperature on microstructure and mechanical properties of the dilute AXM070304 alloy was systematically explored. With raising the extrusion temperature, the average size of the dynamic recrystallized (DRXed) grains was increased, while the intensity of the basal fiber texture was decreased. When the extrusion temperature was escalated from 170 °C to 400 °C, the tensile yield strength (YS) of the low-alloy AXM070304 extrusion alloy was decreased from 437 to 246 MPa, while the elongation to failure (EL) was increased from 2.9 to 16.9%. The alloy extruded at 200 °C obtains an YS of 412 MPa, an ultimate tensile strength (UTS) of 418 MPa and an EL of 14.8%, exhibiting excellent ductility and ultrahigh strength. The ultrahigh YS was mainly due to the strengthening from ultra-fine recrystallized grains (0.61 μm) with grain boundary segregation of solute Al and Ca atoms. The growth of DRXed grains was inhibited mainly by the co-segregation of Al atoms and Ca atoms, nanosized Al2Ca and β-Mn particle phases at the grain boundaries. Plastic instability occurred under tensile loading in the ultrafine-grained AXM070304 alloy, which may be due to the fact that the grain boundary segregation of Al atoms and Ca atoms requires a high energy barrier for dislocation emission. Once the tensile stress reaches the peak value, the mobile dislocation density increases suddenly.

Systematic analysis of distinct flow characteristics and underlying microstructural evolution mechanisms of a novel fine-grained P/M nickel-based superalloy during isothermal compression

Isothermal forging (IF) is an effective method for forming difficult-to-deform materials like P/M superalloys. Understanding the isothermal compression microstructural evolution mechanism of a novel P/M superalloy provides the basis for its optimized IF planning. In this study, the isothermal compression tests of a novel fine-grained P/M nickel-based superalloy were carried out at 1000–1150 °C with strain rates of 0.001–0.01 s−1. The results indicated that the alloy exhibits three distinct flow characteristics: continuous softening after reaching the peak stress, near-steady superplastic flow, and discontinuous hardening, corresponding to different strain rate sensitivity exponent (m) values. Varied microstructural evolution mechanisms, including grain boundary sliding (GBS), dynamic recrystallization (DRX), and grain growth, are dominated in different m-value domains. Meanwhile, different roles of primary γ' play in microstructural evolution were clarified. A moderate fraction of primary γ' with 8.5%–14.2% can well coordinate the GBS and hinder excessive grain growth at a high m value domain (m > 0.4). When 0.2 < m < 0.4, the role of the primary γ' is changed to promote dislocation accumulation, accelerating the nucleation of DRXed grains. As the primary γ' is dissolved at 1150 °C, obvious grain growth was observed after compression. Work hardening effect by overgrown grains competed with DRX softening results in the discontinuous rising stress.

Introducing lamellar LPSO phase to regulate room and high-temperature mechanical properties of Mg-Gd-Y-Zn-Zr alloys by altering cooling rate

The mechanical properties of the extruded and peak-aged Mg-12.8Gd-4.7Y-2.6Zn-0.9Zr (wt.%) (GWZ1352) alloys at room and high temperatures were systematically investigated with respect to their microstructures. The microstructural features were tailored through altering the cooling rates after homogenization by water-quenching, air-cooling and furnace-cooling, respectively. As the cooling rate reduced, the number of lamellar long period stacking ordered (LPSO) phase increased substantially and its spacing decreased gradually. Additionally, solute segregation at grain boundaries occurred after furnace-cooling. After extrusion, the quenching (QE) alloy exhibited nearly full dynamic recrystallized (DRXed) grains, whereas the air-cooling (AE) and furnace-cooling (FE) alloys displayed a typical bimodal structure consisting of fine DRXed and coarse un-DRXed grains. The nucleation and development of the DRXed grains were hampered by the nano-spaced lamellar LPSO phase and the solute segregation of grain boundary. The excellent high temperature mechanical properties of the FE alloy were mostly due to the abundant lamellar LPSO phase. After aging treatment, plentiful fine β′ phase was precipitated in the quenching and aged (QEA) alloy. In contrast, the β′ phase with a larger size and lower density, accompanied with a wide precipitation-free zone (PFZ) at grain boundaries was formed in the air-cooling and aged (AEA) and furnace-cooling and aged (FEA) alloys, which severely deteriorated their mechanical properties both at room and high temperatures.

Strength-ductility balance of AZ31 magnesium alloy via accumulated extrusion bonding combined with two-stage artificial cooling

AZ31 Mg alloy with heterogeneous bimodal grain structure (smaller grain size of 5–20 µm and coarser grain size of 100–200 µm) was subjected to accumulated extrusion bonding (AEB) at 250 ℃ combined with two-stage artificial cooling in this work, viz. local water cooling and artificial cooling. The microstructure developed consecutively as a result of discontinuous dynamic recrystallization (DDRX) for the AEBed samples. {10–12} tensile twinning also played an important role for the AEB with local water cooling at the initial extrusion stage in the container. Local water cooling could further reduce the DRXed grain size to ~ 2.1 µm comparing that without water cooling. And the grain growth rate was reduced by artificial cooling out of extrusion die. Under the combination of two-stage cooling, the fine DRXed grains at sizing band were almost retained with average grain size of ~ 2.3 µm after the sheet out of extrusion die, and the unDRXed grains with high residual dislocation density accumulation were also reserved. The tensile tests results indicated that a good strength-ductility balance with a high ultimate tensile strength (319 MPa vs. 412 MPa) and fracture elongation (19.9 % vs. 30.3%) were obtained. The strength enhancement was mainly owing to the grain refinement and local residual plastic strain reserved by the artificial cooling. The excellent ductility originated from fine DRXed microstructure and ED-tilt double peak texture.