Bimodal Grain Structure Research Articles

Effect of microstructure evolution on Lüders strain and tensile properties in an intercritical annealing medium-Mn steel

The influence of microstructural characteristics on Luders strain and mechanical properties was explored by means of altering thermo-mechanical circumstances in an intercritical annealing (IA) medium-Mn Fe-11Mn-0.09C-0.25Si (wt.%) steel. By IA of cold-rolled samples with severe plastic deformation, exclusively equiaxed dual phases were obtained because of active recovery and recrystallization. The equiaxed austenite ( $$\upgamma_{\text E}$$ ) with a larger size and inadequate chemical concentration was more readily transformed into martensite, and subsequent transformation-induced plasticity (TRIP) effect was triggered actively at relatively higher IA temperature, lessening localized deformation. In addition, grown-in dislocations were prone to multiply and migrate around a broad mean free path for coarser equiaxed ferrite ( $$\upalpha_{\text E}$$ ) due to weakening dynamic recovery; therefore, it was the ensuing increased mobility of dislocations instead of reserving plentiful initial dislocation density that facilitated the propagation velocity of Luders bands and the accumulation of work hardening. In contrast, the bimodal-grained microstructure with lath-like and equiaxed austenite ( $$\upgamma_{\text L} + \upgamma_{\text E}$$ ) satisfactorily contributed to a smaller yield point elongation (YPE) without compromise of comprehensive mechanical properties on the grounds that austenitic gradient stability gave rise to discontinuous but sustainable TRIP effect and incremental work hardening. Hence, Luders strain is closely related to the absence of work hardening in the region which yields locally. It follows that the decreased stability of retained austenite, favorable mobility of dislocations and the bimodal-grained structure all prominently make up for the insufficiency of work hardening, thereof resulting in a limited YPE.

Effects of initial grain size and orientation on the twin behavior in ZK60 Mg alloy

To investigate the microstructures evolution of the hot-extruded Mg-5.1 wt% Zn-0.18 wt% Zr alloy (denoted as ZK60) fabricated via powder metallurgy route and to reveal the deformation mechanism, the microstructures of the ZK60 alloy hot-extruded at 623 K (denoted as 623 K ZK60), the ZK60 alloy hot-extruded at 673 K (denoted as 673 K ZK60) and the ZK60 alloy hot-extruded at 623 K subsequent compressed by 2% was analyzed and compared. It was find that the microstructure and texture of the 623 K ZK60 alloy subsequent compressed by 2% at room temperature were similar to the initial microstructure and texture of the 673 K ZK60. In addition, the twinning process was delayed before the yield points (σ0.2) by initial grain size and strong pyramidal preferred orientation in both the tensile and compressive tests. Compared with the 673 K ZK60 alloy, the 623 K ZK60 alloy with bimodal-grained structure (BGS) showed stronger strength, better ductility and lower tensile/compressive yield asymmetries compared with the FG samples due to the twinning suppressed in the BGS before the yield points.

Tuning texture and precipitation using Y/Gd atomic ratio in iso-concentration Mg–Y–Gd–Ag–Zr extruded alloys

Three Mg–Y(–Gd)–Ag–Zr alloys with an iso-concentration of total alloying elements but different Y/Gd atomic ratios were hot extruded at 350 °C and their microstructures and mechanical properties were comparatively studied. Extruded Mg–2.4Y–0.4Ag–0.1Zr (at.%) base alloy shows a bimodal-grained structure composed of fine equiaxed recrystallised grains and coarse elongated unrecrystallised grains and has a 011¯0//ED (extrusion direction) texture. In contrast, when Y is partially replaced with Gd, extruded Mg–1.6Y–0.8Gd–0.4Ag–0.1Zr (at.%) and Mg–0.8Y–1.6Gd–0.4Ag–0.1Zr (at.%) alloys with the Y/Gd atomic ratio of 2 and 0.5, respectively, exhibit a near fully recrystallised structure and have a [0001]//ED texture. While basal γ″ precipitates dominate but few β′ precipitates form in Mg–2.4Y–0.4Ag–0.1Zr alloy, a large number density of both β′ and γ″ precipitates form in Mg–1.6Y–0.8Gd–0.4Ag–0.1Zr and Mg–0.8Y–1.6Gd–0.4Ag–0.1Zr alloys after post-extrusion ageing. Precipitation in the Gd-containing alloys is considerably enhanced by partial substituting Y with Gd. The peak-aged Mg–1.6Y–0.8Gd–0.4Ag–0.1Zr alloy keeps a relatively good balance between grain boundary strengthening, dispersion strengthening and precipitation hardening and thus obtains a good combination of strength and ductility, whose room temperature tensile yield strength, ultimate tensile strength and elongation are 377 MPa, 479 MPa and 8.0%, respectively.

Achieving ultra-high strength in Mg–Gd–Ag–Zr wrought alloy via bimodal-grained structure and enhanced precipitation

Mg–13.1Gd–1.6Ag–0.4 Zr (wt%) alloy was either iso-thermally extruded at 350 °C or differential-thermally extruded with respectively pre-heated billet at 500 °C and die at 350 °C. The iso-thermal extrusion leads to a near fully recrystallized structure and a [0001]//ED (extrusion direction) texture. In contrast, the differential-thermally extruded alloy develops a bimodal-grained structure composed of fine equiaxed recrystallized grains and coarse elongated unrecrystallized grains with a 011¯0//ED texture. The differential-thermally extruded alloy has a higher number density of precipitates after post-extrusion ageing than that of the iso-thermally extruded counterpart. Moreover, precipitation in the differential-thermally extruded alloy is further enhanced with cold rolling before ageing. Finally, the alloy obtains room temperature tensile yield strength of 421 MPa and ultimate tensile strength of 515 MPa via differential-thermal extrusion, cold rolling and ageing, mainly ascribed to the coupled strengthening from the bimodal-grained structure and enhanced precipitation. Strength of the alloy is noticeably higher than those of Mg–Gd(–Y)–Ag extruded alloys with similar compositions reported previously and is comparable to those of other high-strength Mg wrought alloys. The findings suggest that differential-thermal extrusion plus strain ageing is a suitable approach for achieving high strength in age-hardenable Mg alloys.

Abnormal twinning behaviors in ZK60 alloy by powder metallurgy process

A novel Mg-5.1 wt%Zn-0.18 wt%Zr (denoted as ZK60) alloy with a bimodal-grained structure (BGS) was fabricated via powder metallurgy (PM), which exhibits higher strength, ductility and tension/compression symmetry than fine grains prepared by other progress. Through detailed electron backscatter diffraction (EBSD) analysis, fine grains (DRXed) with weaker basal texture (0001) <11–20> are favorable for basal <a> slip and early deformation. When the fine grains are filled with dislocations, the coarse grains (unDRXed) could accommodate abundant newly formed dislocations and the strong (11–22) <11–23> pyramidal texture promote the pyramidal <c+a> slip, thereby suppressing {10–12} extension twinning (ET).

The Fabrication and Electrocaloric Effect of Bimodal-Grain Structure (Ba0.60Sr0.40)TiO3 Using the Induced Abnormal Grain Growth Method

Bimodal-grain structure (Ba0.6Sr0.4)TiO3 ceramics had been prepared using induced abnormal grain growth method (IAGG). Two grain size distributions, between 10 ∼ 20 μm and 100 ∼ 300 μm, were found during the sintering temperature of 1250 ∼ 1350°C. Our results suggested that the ultra-large grain exhibited the high dielectric and ferroelectric properties comparable to the single crystal. At the same time, the fine-grain caused a phase diffusion transition at the Curie temperature about 20°C. The direct measured electrocaloric effect showed that the best performance is achieved with ΔT of 0.6 K, and ΔS of 1.02 J kg−1K−1 near room temperature and electric field strength of 2 MV/m.

Bimodal grain-structure formation in a Co–Cr-based superalloy during ultrahigh-homologous-temperature annealing without severe plastic deformation

Bimodal grain structures are often produced in a metal by employing severe plastic deformation followed by low-homologous-temperature annealing (≤0.4Tm, where Tm is melting temperature). Here, a bimodal grain structure was achieved in a Co–20Cr–15W–10Ni–C superalloy via moderate cold rolling (strain ≈0.3) and ultrahigh-homologous-temperature annealing (≥0.8Tm). During annealing, carbide-assisted heterogeneous static-recrystallization played a key role in forming the bimodal grain structure. A 40% increase in combined strength and ductility was observed.

Towards heterogeneous AlxCoCrFeNi high entropy alloy via friction stir processing

A heterogeneous bimodal-grained AlxCoCrFeNi high entropy alloy was realized through friction stir processing (FSP) with the addition of Al powders. With the bimodal-grained structure and second phases, strength was increased and ductility was retained as compared with the friction stir processed base alloy. Face-centered cubic grains in the matrix were refined from ∼500 µm to ∼15 µm by FSP and subsequent recrystallization. With addition of Al powders, localized Al-rich regions facilitated the formation of body-centered cubic phase (Al, Ni-rich) and Al rich precipitates, which further refined grain size to ∼5 µm and enhanced local strength.

Texture and Mechanical Properties of Al–Mg Alloy with Unimodal and Bimodal Grain-Structures Formed by Accumulative Roll Bonding and Annealing

Texture and Mechanical Properties of Al–Mg Alloy with Unimodal and Bimodal Grain-Structures Formed by Accumulative Roll Bonding and Annealing

The role of bimodal-grained structure in strengthening tensile strength and decreasing yield asymmetry of Mg-Gd-Zn-Zr alloys

This paper studied the role of bimodal-grained structure in strengths and asymmetry behaviors of Mg-15Gd-1Zn-0.4Zr (wt%) alloys under the as-extruded, compressive-yielded and fractured conditions. The bimodal-grained structure consisting of fine recrystallized grains and coarse un-recrystallized grains formed when the alloys were extruded with relatively low temperature. Compared with the equiaxed structure, the bimodal-grained structure possessed higher tensile strength attributed to the fine recrystallized grains plus the strong and hard un-recrystallized texture with its basal planes nearly parallel to the loading direction. However, the bimodal-grained structure weakened the ultimate compressive strength, because the un-recrystallized texture is favorable for tensile twining in the compressive test. Eventually, the aged sample with the bimodal-grained structure exhibited ultra-high tensile yield strength and ultimate tensile strength of 465 MPa and 524 MPa, respectively. Especially, the inhomogeneous bimodal-grained structure with stronger texture resulted in extraordinary low compressive-tensile yield asymmetries of 0.99 and 0.98 compared with those (from 1.09 to 1.48) of the homogeneous equiaxed structures with relatively random textures, because the fine recrystallized grains in the bimodal-grained structure undertook major deformation by slips and suppressed twining before the yield points in the mechanical tests. The relationship between the microstructures and properties allows us to produce high tensile strength Mg components by applying low extrusion temperature and obtaining bimodal-grained structure.

Bimodal-grained Ti fabricated by high-energy ball milling and spark plasma sintering

Bimodal-grained Ti containing coarse and fine grains was fabricated by high-energy ball milling and spark plasma sintering (SPS). The microstructure and mechanical properties of the compacts sintered by Ti powders ball-milled for different time were studied. Experimental results indicated that when the ball-milling time increased, the microstructure of sintered Ti was firstly changed from coarse-grained to bimodal-grained structure, subsequently transformed to a homogeneous fine-grained structure. Compared with coarse-grained Ti and fine-grained Ti, bimodal-grained Ti exhibited balanced strength and ductility. The sample sintered from Ti powders ball-milled for 10 h consisting of 65.3% (volume fraction) fine-grained region (average grain size 1 μm) and 34.7% coarse-grained region (grain size > 5 μm) exhibited a compress strength of 1028 MPa as well as a plastic strain to failure of 22%.

The effects of multi-cyclic thermo-mechanical treatment on the grain refinement and tensile properties of a metastable austenitic steel

Abstract The effects of the multi-cyclic cold rolling and annealing process on the grain refinement and tensile properties of a metastable austenitic steel were investigated. The multi-cyclic thermo-mechanical process yielded the bimodal ultrafine-grained structure through both the reverse transformation and recrystallization during annealing. The kinetics and temperatures of the reverse transformation were related to the average size of coarse grains. Not only yield and tensile strengths but also ductility of the bimodal-grained structure increased with the number of thermo-mechanical cycles.

Preparation and Thermal Stability of a Mechanically Alloyed Oxide Dispersion Strengthened Ferritic Steels

Oxide dispersion strengthened ferritic steels (so-called nanostructured ferritic alloys, NFAs), which are candidate structural materials in next generation nuclear power plant, have attracted much attention during recent years. In this work, iron oxide as oxygen carrier and titanium, yttrium hydrides were together mechanically milled with Fe-14Cr-3W gas-atomized powder. The thermal stability and recrystallization behaviour of the as-milled ferritic powder were studied by means of metallography, SEM, TEM and microhardness test. After ball milling for 48h, complete solid solution of bcc-Fe was formed in the as-milled powder. The thermal analysis results show that dispersed oxides with an average diameter of 5nm precipitate from the supersaturated matrix at about 850 °C. During annealing at temperatures from 800 to 1000 °C, a large number of equiaxed grains as fine as few hundreds of microns were found embedding in the matrix; the recrystallized grains stay quite stable and show minor dependence on annealing temperature and time. After being heated to 1200 °C for extended time, abnormal grain growth took place and resulted in bimodal grained structure. The effect of secondary particles on the thermal stability and recrystallization behavior of the ferritic steel was also discussed.