Unrecrystallized Regions Research Articles

Improved yield strength-ductility synergy of a high Mn TWIP steel by grain refinement and partial recrystallization

The industrial application of high Mn twinning-induced plasticity (TWIP) steels is limited by their low yield strength (YS). In this work, a combination of grain refinement and partial recrystallization was applied to increase the YS of a high Mn TWIP steel to 1215 MPa, along with a total elongation (TE) of 16 %. The high YS of the partially recrystallized steel mainly originates from the combination effect of high dislocation density up to 8.3 × 1014 m−2, fine grain size of about 1 μm in recrystallized regions, and dense mechanical twins with a small spacing of about 20 nm in unrecrystallized regions. This work provides a potential way to improve the YS-ductility synergy and broaden the application of TWIP steels.

Imaging and Segmenting Grains and Subgrains Using Backscattered Electron Techniques.

We present two new methods of processing data from backscattered electron signals in a scanning electron microscope to image grains and subgrains. The first combines data from multiple backscattered electron images acquired at different specimen geometries to (1) better reveal grain boundaries in recrystallized microstructures and (2) distinguish between recrystallized and unrecrystallized regions in partially recrystallized microstructures. The second utilizes spherical harmonic transform indexing of electron backscatter diffraction patterns to produce high angular resolution orientation data that enable the characterization of subgrains. Subgrains are produced during high-temperature plastic deformation and have boundary misorientation angles ranging from a few degrees down to a few hundredths of a degree. We also present an algorithm to automatically segment grains from combined backscattered electron image data or grains and subgrains from high angular resolution electron backscatter diffraction data. Together, these new techniques enable rapid measurements of individual grains and subgrains from large populations.

Enhanced strength-ductility synergy in a Mo and W co-doping NiCoCr multi-principal element alloy at ambient and cryogenic temperatures

A novel designed Ni48Co33Cr9Mo4W6 multi-principal element alloy (MPEA) with single face-centered cubic (FCC) solid-solution phase was fabricated by casting, homogenization, cold rolling, and various post deformation annealing (PDA) heat treatments. The single-phase FCC structured samples with homogeneous grains exhibited average grain sizes ranging from ∼1.4 to ∼47.8 μm. In addition, a single-phase FCC structured heterogeneous sample consisting of partially un-recrystallized region (∼47.1 vol%) and fully recrystallized region (∼52.9 vol%) was also achieved. Among all the samples with homogeneous grains, the sample processed using PDA treatment at 900 °C for 3 min (named as PDA900-3min) showed an average grain size of ∼1.4 μm, displaying the best mechanical properties. In detail, it exhibited a yield strength of ∼811 MPa and a total elongation of ∼40.2 % at ambient temperature (298 K), and these values were increased to ∼1085 MPa and ∼42.7 % at LN2 (77 K) temperature. Due to heterogeneous deformation induced (HDI) strengthening, the hetero-structured sample (named as PDA900-90s) present markedly higher strength and slightly lower ductility in comparison with the PDA900-3min sample at 298 K and 77 K. These two samples demonstrated synergetic improvement of strength and ductility through Mo/W co-doping and PDA heat treatments both at ambient and LN2 temperatures, attributing to solid solution strengthening and grain boundary strengthening, as well as heterogeneous deformation induced (HDI) strengthening. Additionally, Mo/W co-doping, reduces the stacking fault energy (SFE) of the alloy, facilitates the generation of stacking faults (SFs) and twins, which is prone to maintaining the higher work hardening rate. Therefore, our work suggests that Mo and W co-doping is an effective strategy for achieving enhanced strength-ductility synergy in FCC structured MPEAs.

Microstructures and mechanical properties of Ti-55511 alloy subjected to rolling in the α+β dual-phase region

In this study, dual-phase region rolling was conducted on a Ti-55511 alloy with different initial rolling reductions in the single-phase region. The mechanical properties and microstructural characteristics of the alloy with different deformation conditions were investigated. Microstructure observations show the formation of a heterogeneous structure with a gradient size distribution of the α phase at the recrystallized grain boundaries, induced by the non-uniform stress distribution. The α phase and the β matrix exhibit the Burgers orientation relationship in the recrystallized region with the gradient structure. However, in the unrecrystallized region of the partially recrystallized specimens and in the grain interiors of the fully recrystallized specimens, the α phase and the β phase exhibit Pitsch-Schrader orientation relationship (P–S OR). A large number of the nano-sized α" martensite bands were observed after the dual-phase region rolling. These bands originated from the α/β interfaces and grew towards the interiors of the α phase. As the deformation increased, these nano-bands underwent a transformation into the {10 1‾ 1}α deformation twins, resulting in the fragmentation of the α phase. Mechanical testing results show that the mechanical properties of the alloy can be improved by the dislocation strengthening, the generation of profuse α phase and other microstructures including the heterogeneous distribution of the α phase, the nano-sized α" martensite and the {10 1‾ 1}α deformation twins.

Exceptional combination of mechanical and corrosion properties in partially recrystallized CoCrFeNiMo medium-entropy alloys

Corrosion-resistant materials with good mechanical properties are an endless goal to develop reliable structural materials. Multiple strengthening mechanisms stemming from diverse microstructural factors have been developed, while the effect of microstructural factors on corrosion properties in high-entropy alloys has not been clearly defined yet. Herein, the corrosion behaviors of CoCrFeNiMo medium-entropy alloys with different annealing times are investigated. The influences of microstructures are summarized as follows: alloyed Mo accelerates repassivation, Mo-rich precipitates form micro-galvanic couples, and unrecrystallized microstructures induce pitting corrosion. Regarding mechanical properties, the alloyed Mo, Mo-rich precipitates, and unrecrystallized region with profuse dislocations lead significant solid solution strengthening, precipitation strengthening, and dislocation strengthening, respectively. Particularly, the sample annealed for 60 min possesses dissolved Mo in the matrix, fine precipitates, wide recrystallized regions, and low dislocation density, resulting in a wide passivation range of 1.512 V and high yield strength × fracture strain of 220.3 MPa simultaneously. The sample annealed for 60 min reveals a synergistic combination of diverse microstructural factors affecting corrosion and mechanical properties.

The effects of Ca on the microstructure, mechanical and corrosion properties of extruded Mg–2Zn–0.5Mn alloy

In this study, the microstructure, mechanical properties and corrosion behavior of as-extruded Mg–2Zn–0.5Mn–xCa (x=0, 0.2, 0.4) alloys were investigated. The results showed that alloying with Ca refined the grains of the extruded rods and increased the unrecrystallized region, which leaded to an increase in ultimate tensile strength and a decrease in elongation. With the addition of Ca, the overall texture intensity of the alloys increase, and it is found that the texture of the alloys is mainly determined by the coarse deformed grains. The corrosion experiment of the alloys in hanks balanced salt solution (HBSS) shows that the corrosion product film of the alloys becomes denser with the addition of Ca. At the same time, the addition of Ca element introduces Ca2Mg6Zn3 phase, which reduces local corrosion, the corrosion resistance is improved.

The microstructural features and strengthening mechanisms of Mg-Al-Zn-Ca-(RE) extruded alloys having high-volume fractions of second-phase particles

The microstructures and tensile properties of Mg-4∼6Al-2∼4Zn-2∼3Ca-0∼1RE (in wt.%) extruded alloys have been investigated. C15 and C36 Laves phases, Ca2Mg6Zn3, Al2REZn2, and Al11RE3 phases were formed as networks during solidification in different alloys. After extrusion, coarse dendritic were significantly refined due to dynamic recrystallization, second phases were fragmented into pieces arranged as stripes, and typical basal fiber texture {0002} <10-10> with the extrusive direction paralleled to (0001) were formed in all studied alloys. Laves phase particles were prone to be crushed into nanoparticles, which appeared more easily to hinder the recrystallized grain growth and could bring about more than 30 vol % ultra-fine grains after extrusion. During extrusion, twinning-induced dynamic recrystallization occurred at first, and both discontinuous and continuous dynamic recrystallization dominated the microstructural evolution. The volume fraction of second phases had critical roles in recrystallization process, and a nearly complete recrystallization degree could be obtained in alloys containing more than 9 vol% second phases. The strengthening mechanisms were evaluated based on classic strengthening models, which agreed well with the experimental results. It was found that grain boundary strengthening, second phase strengthening, and texture strengthening contributed around 60%, 30%, and 10%, respectively, to yield strengthening, except for Mg-4Al-4Zn-2Ca alloy because of the high percentage of unrecrystallized region. Moreover, the overestimate of second phase strengthening happened in alloys having more than 9 vol% second phases, which revealed that too many second phases had limited influence on microstructures and the resulting mechanical properties.

The effect of Gd on the microstructure evolution and mechanical properties of Mg-4Zn-0.6Ca alloy

In this work, the effect of Gd on the microstructure evolution and mechanical properties of Mg–4Zn–0.6Ca–xGd (x = 0, 0.3, 0.6 and 1, wt%) alloys were investigated systematically. The results show that Mg-4Zn-0.6Ca alloy was composed of α-Mg, Mg7Zn3 and Ca2Mg6Zn3 phase. The W-phase (Mg3Zn3Gd2) was synthesized after addition of Gd. The dynamic recrystallization was restrained and the average grain size decreased after 0.6 wt% Gd addition, causing the increase of unrecrystallized region. In addition, the volume fraction of second phase increased with increasing Gd content up to 1 wt%, and the effect of particles stimulated nucleation (PSN) was enhanced, resulting in a decrease in unrecrystallized region. The addition of Gd promoted the occurrence of the overall texture split and TD-split texture component, causing decreased basal texture intensity. An optimal mechanical property of Mg–4Zn–0.6Ca–1Gd alloy, with the yield and ultimate tensile strength of 240 MPa and 297 MPa, respectively, was achieved. The values were increased by 82 MPa and 37 MPa, respectively, compared with that of Mg–4Zn–0.6Ca alloy. In initial deformation stage, strain-hardening rate of alloys decreased with increase of Gd content. Mg-4Zn-0.6Ca-0.6Gd alloy possessed the lowest strain-hardening rate because of fine grain size. However, strain-hardening rate was increased when Gd content was increased to 1 wt%, which may be related to decreased texture intensity and presence of many second phases. This work reveals the effect of co-addition of minor Ca and Gd on the microstructure evolution and mechanical properties of Mg-Zn alloys, which provides theoretical basis for the development of high strength Mg alloy.

Microstructure, texture, and mechanical properties of the extruded and multi-directionally forged Mg–xGd alloys

The microstructural and textural evolutions as well as room-temperature mechanical properties of three binary Mg–Gd alloys with 1, 2, and 3 wt% of Gd were studied in the as-extruded condition and after two, four, and six passes of multi-directional forging (MDF). Microstructural observations of the extruded samples were indicative of partial dynamic recrystallization resulting in bimodal grain structures comprised of elongated un-recrystallized patches and fine recrystallized grains. After six passes of the MDF process, the fraction of the un-recrystallized regions decreased because of dynamic recrystallization, and thus, relatively uniform microstructures were achieved in all alloys. The greater fraction of the un-recrystallized patches and the finer grains in the recrystallized regions of the Mg–3Gd alloy confirmed the more pronounced retardation of recrystallization via solute drag mechanism at the higher Gd concentrations. Evaluation of crystallographic texture revealed that by applying the MDF process, the conventional fiber texture of the extruded condition was replaced with a new randomized texture, in which the basal planes had a tendency to rotate at angles of lower than 90° to the transverse direction. Mechanical properties were evaluated using shear punch testing (SPT) and the results showed that with increasing the number of MDF passes in each alloy, the achieved finer grain structure was responsible for improving the shear strength. In addition, increasing the Gd content of the alloys resulted in finer grain size, more solute atoms, and higher fraction of Mg 5 Gd second phase particles, all of which led to the enhancement in shear strength.

Microstructures and mechanical properties of a Mg–9Gd−3Y−0.6Zn−0.4Zr (wt.%) alloy modified by Y-rich misch metal

Microstructural evolutions and mechanical properties of a Y-rich misch metal modified Mg–9Gd−3Y−0.6Zn−0.4Zr (wt.%) alloy were investigated. In the as-cast sample, the intermetallic phases are Mg5RE, Mg3RE and 14H-type long-period stacking ordered (LPSO) phase. After solution, all Mg3RE and most Mg5RE were dissolved while the 14H-LPSO plates were coarsened. The as-extruded alloy has a bimodal structure of ultra-fine dynamically recrystallized (DRXed) grains and coarse un-recrystallized grains, and is with a typical non-fiber texture. Disintegrated Mg5RE and 14H-LPSO particles aggregate in extrusion stringers while fine dynamically precipitated Mg5RE particles distribute at grain boundaries. After peak-aging, ultra-thin basal γ’’ and prismatic β’ precipitated in DRXed grain and un-recrystallized regions, respectively. The yield strength of the as-extruded and peak-aged alloys reaches to 385 MPa and 481 MPa, respectively, at room temperature, and to 320 MPa and 350 MPa, respectively, at 250 °C. Grain boundary strengthening from both grain boundaries and sub-grain boundaries in DRXed and un-recrystallized regions, respectively, and precipitation strengthening were revealed as the dominant strengthening mechanisms.

Microstructure study of cold rolled Al0.32CoCrFeMnNi high-entropy alloy: Interactions between recrystallization and precipitation

The equimolar CoCrFeMnNi alloy is widely studied for its excellent mechanical properties at cryogenic temperatures. Interestingly, it presents a unique combination of strength and ductility mainly due to the occurrence of twinning at these low temperatures. Motivated by the desire to improve its room temperature strength, this study focuses on the optimization of the microstructure through alloying and thermomechanical treatments. More specifically, the effect of alloying with aluminium is studied with the aim of triggering precipitation strengthening. The composition Al0.32CoCrFeMnNi is chosen on the basis of thermodynamic data and submitted to various processing routes including cold rolling and annealing steps. The annealing temperature is shown to be the key parameter determining the governing microstructure transformations. At annealing temperatures below 900 °C, precipitation is the main phenomenon taking place. Above 900 °C, recrystallization is governing the evolution of the microstructure. At 900 °C, both mechanisms occur simultaneously. Recrystallization occurs first on grain boundaries and shear bands while precipitation is activated both in recrystallized and unrecrystallized regions with distinctive morphologies. It is shown to impede further recrystallization. Thermodynamic calculations are performed to explain the precipitation sequence observed in both regions. By increasing the rolling level from 60% to 80%, recrystallization kinetics is accelerated and complete recrystallization reached. A wide range of microstructure is achieved through the variety of thermomechanical treatments explored in this study. These various microstructures in turn translate into a wide range of hardness levels and tensile properties.

Effect of forced-air cooling on the microstructure and age-hardening response of extruded Mg-Gd-Y-Zn-Zr alloy full with LPSO lamella

The homogenized Mg-8.2Gd-3.8Y-1.0Zn-0.4 Zr (wt.%) alloy full of plate-shaped long period stacking ordered (LPSO) phases was hot extruded in the atmosphere and cooled by the forced-air, then the effect of forced-air cooling on the microstructure and age-hardening response of the alloy was investigated in this work. The results show that in comparison with the extruded sample cooling in the atmosphere, the forced-air cooling restricts dynamic recrystallization (DRX) and brings about finer dynamic recrystallized (DRXed) grain size, stronger basal texture and higher dislocation density. Furthermore, the forced-air cooling promotes the dynamic precipitation in the DRXed regions and facilitates formation of plate-shaped LPSO phases and γ′ phases with smaller interspacing in the unrecrystallized (unDRXed) regions, then slightly restricts the precipitation of β′ phases during aging. After peak-ageing treatment, the extruded sample with forced-air cooling shows superior tensile properties with a tensile yield strength of 439 MPa, an ultimate tensile strength of 493 MPa, and elongation to failure of 18.6 %.

Unique high-temperature deformation dominated by grain boundary sliding in heterogeneous necklace structure formed by dynamic recrystallization in HfNbTaTiZr BCC refractory high entropy alloy

Microstructural evolution of dynamically recrystallized (DRX) grains and grain boundary sliding (GBS) in the heterogeneous necklace structure of HfNbTaTiZr refractory high entropy alloy (RHEA) was studied systematically during high temperature deformation. Uniaxial compression testing was carried out to different strains at 1000 °C and a strain rate 10−3 s−1. Significant bulging of grain boundaries led initially to the formation of DRX grains. The fraction of DRX grains increased with strain, and typical necklace structures of fine (d ≤ 1.5 µm) DRX grains formed at strain ε ≥ 0.3. The DRX grains showed very limited grain growth, and heterogeneous microstructures composed of coarse unrecrystallized regions surrounded by the characteristic DRX necklace structure were formed at larger strains. Interrupted testing with marker grids revealed that DRX grains deformed by a GBS mechanism. The DRX necklace regions connected mesoscopically and also displayed diamond network morphologies, with unique “Y-shaped”, “T-shaped” and “X-shaped” junctions. The formation of different types of junctions were rationalized on the basis of GBS accommodated by local dislocation slip in unrecrystallized regions. The unrecrystallized regions showed preferred <001>/<111> micro-texture, consistent with conventional dislocation slip in the BCC crystals. On the other hand, the newly formed DRX grains initially had similar orientations to those of the parent grains, but they displayed a random texture with increasing strain, as expected from GBS. The randomized texture of DRX grains and the stability of DRX grains size represented GBS in the DRX necklace regions.

A high-strength low-rare-earth-alloyed magnesium alloy via traditional hot-extrusion

A high-strength low-rare-earth-alloyed Mg−3.5Sm−2Yb−0.6Zn−0.4Zr alloy was processed by an ingot metallurgy process with hot-extrusion followed by artificial aging. The peak-aged sample exhibited a higher yield strength of 449 MPa than most of extruded Mg−Gd based alloy containing high RE content, and even is comparable to that of the T8-treated 2024 Al alloy. The studied alloy exhibited a typical bimodal microstructure, consisting of dynamically recrystallized (DRXed) grains with random textures and coarse unrecrystallized regions with strong basal fiber texture. In addition, the fragmented Mg5RE and Mg41RE5 phases distributed at extrusion stringers. Abundant of fine Mg3RE particles precipitated dynamically in DXRed regions, which restrains DRXed grains growth effectively by grain boundary pinning. Within the unDXRed regions, uniformly dense Mg3RE and Mg12RE nano-precipitates were observed. Also there were many basal <a> dislocations and a few non-basal <c + a> dislocations in unDXRed regions. Subsequent artificial ageing significantly enhanced the alloy's strength by introducing basal precipitates, although decreased the ductility slightly. Finally, the ultrahigh yield strength was revealed to be attributed to the combined effects of a strong basal fiber texture, a bimodal microstructure, and numerous precipitated particles.

Microstructural Characterization of Aluminum 6061 Splats Cold Spray Deposited on Aluminum 6061-T6 Substrate

The microstructural evolution at the interface of Al6061 splats cold sprayed on Al6061 substrate at a supercritical impact velocity was investigated by SEM and S/TEM. Metallurgical bonding was observed in the peripheral (equatorial) region of the splat where a localized outward material flow promoted by continuous dynamic recrystallization (CDRX) occurred to displace the oxide from the interface. No recrystallization or metallurgical bonding was observed at the polar interface where oxide remained in fragments. CDRX produced elongated grains less than 100 nm thick at the equatorial interface which transformed to slightly coarsened (100 to 200 nm) ultrafine grains in the splat side of the interface. Rapid precipitation on the boundaries of recrystallized grains and in the unrecrystallized regions of the splat–substrate interface was discussed in relation to the sequence of structural evolution at the splat–substrate interface during the impact and subsequent cooling. Further work is needed to more fully understand the kinetics of dynamic recovery/recrystallization and precipitation in the presence of excess vacancies that can have impact on the dislocation mobility, mass transport, and thermodynamic stability.

Unique deformation behavior and microstructure evolution in high temperature processing of HfNbTaTiZr refractory high entropy alloy

Hot-deformation behaviors of an equiatomic HfNbTaTiZr refractory high entropy alloy having BCC structure were investigated. Uniaxial compression was carried out at different temperatures ranging from 1000 °C to 1200 °C and various strain rates from 10−4 s−1 to 10−2 s−1 where BCC single phase was stable. Stress-strain curves indicated distinctive sharp drops at yielding followed by continuous decreases of flow stress. Interrupted compression tests indicated that the sharp drop of the stress was explained from a viewpoint of dislocation unlocking possibly by solute atom(s) atmosphere or short-range ordering. Flow stress analysis was carried out by using Arrhenius type power law relationship. The apparent activation energy (Q) for hot-deformation estimated from the flow stress data was 258–232 kJ mol−1 over the entire range of strain. The flow stress analysis indicated high strain rate sensitivity (m) of the flow stress (m ∼ 0.33). The relatively high m value and dynamically recrystallized (DRX) microstructures composed of coarse unrecrystallized regions and fine DRX grains with necklace morphologies indicated a possibility of grain boundary sliding (GBS) in the fine-grained DRX regions. The fine DRX grain sizes indicated inhibited grain growth after DRX, which might be due to sluggish diffusion in the HfNbTaTiZr alloy. Preferred orientations of <001> and <111> parallel to the compression axis in the unrecrystallized coarse regions indicated normal dislocation slips in the BCC crystal, while weak and near-random texture observed in the fine-grained DRX regions supported the occurrence of GBS.

Avrami Kinetic-Based Constitutive Relationship for Armco-Type Pure Iron in Hot Deformation

The work presents a full mathematical description of the stress-strain compression curves in a wide range of strain rates and deformation temperatures for Armco-type pure iron. The constructed models are based on a dislocation structure evolution equation (in the case of dynamic recovery (DRV)) and Avrami kinetic-based model (in the case of dynamic recrystallization (DRX)). The fractional softening model is modified as: X = ( σ 2 − σ r 2 ) / ( σ d s 2 − σ r 2 ) considering the strain hardening of un-recrystallized regions. The Avrami kinetic equation is modified and used to describe the DRX process considering the strain rate and temperature. The relations between the Avrami constant k ∗ , time exponent n ∗ , strain rate ε ˙ , temperature T and Z parameter are discussed. The yield stress σ y , saturation stress σ r s , steady stress σ d s and critical strain ε c are expressed as the functions of the Z parameter. A constitutive model is constructed based on the strain-hardening model, fractional softening model and modified Avrami kinetic equation. The DRV and DRX characters of Armco-type pure iron are clearly presented in these flow stress curves determined by the model.

Unique Deformation Behavior and Microstructure Evolution in High Temperature Processing of HfNbTaTiZr Refractory High Entropy Alloy

Hot-deformation behaviors of an equiatomic HfNbTaTiZr refractory high entropy alloy having BCC structure were investigated. Uniaxial compression was carried out at different temperatures ranging from 1000°C to 1200°C and various strain rates from 10-4 s-1 to 10-2 s-1. Stress-strain curves indicated distinctive sharp drops at yielding followed by continuous decreases of flow stress. The sharp drop of the stress was explained from a viewpoint of dislocation pinning possibly by any solute atom(s). Flow stress analysis was carried out by using Arrhenius type power law relationship. The apparent activation energy (Q) for hot-deformation estimated from the flow stress data was 124 ~ 113 kJmol-1 over the entire range of strain. The flow stress analysis indicated high strain rate sensitivity (m) of the flow stress (m ~ 0.35). The relatively high m value and dynamically recrystallized (DRX) microstructures composed of coarse unrecrystallized regions and fine DRX grains with necklace morphologies indicated a possibility of grain boundary sliding (GBS) in the fine-grained DRX regions. The fine DRX grain sizes indicated inhibited grain growth after DRX, possibly because of the sluggish diffusion in the HfNbTaTiZr alloy. Preferred orientations of and parallel to the compression axis in the unrecrystallized coarse regions indicated normal dislocation slips in the BCC crystal, while weak and near-random texture observed in the fine-grained DRX regions supported the occurrence of GBS.

Microstructures and mechanical properties of a hot-extruded Mg−8Gd−3Yb−1.2Zn−0.5Zr (wt%) alloy

Microstructures and mechanical properties of a Mg−8Gd−3Yb−1.2Zn−0.5Zr (wt%) alloy have been investigated. The dominant intermetallic phases in the as-cast sample are Mg5RE (RE = Gd,Yb) phase, 14H-type long-period stacking ordered (LPSO) phase, and Mg2Zn2RE (W) phase and ordered Mg12RE phase. Furthermore, the ordered Mg12RE phase generally coexists with the W phase following an orientation relationship as [01¯1]w//[2¯30]Mg12RE, and (1¯11)w//(001)Mg12RE. After extrusion, the microstructure is consisted of un-recrystallized regions along with a small part of fine dynamically recrystallized (DRXed) regions. Simultaneously, the coarse Mg5RE, W and Mg12RE particles were disintegrated and mainly distribute at extrusion stringers while the fine LPSO plates mainly distribute in un-recrystallized regions. Moreover, amounts of nanoscale Mg5RE particles were dynamically precipitated in DXRed regions. Then, the as-extruded Mg−8Gd−3Yb−1.2Zn−0.5Zr alloy exhibits clearly higher strength than the classic rare-earth-containing magnesium alloys with comparative or even much higher rare earth content at both room temperature and high temperatures. The dominant strengthening mechanism was finally revealed as precipitation/dispersion strengthening.

Microstructures and mechanical properties of a hot-extruded Mg−8Ho−0.6Zn−0.5Zr alloy

The microstructures and mechanical properties of a Mg−8Ho−0.6Zn−0.5Zr alloy were thoroughly investigated. The results reveal five kinds of intermetallic phases, namely the 14H long-period stacking ordered (LPSO) phase, the 18R LPSO phase, Mg24Ho5, Mg0.5Ho1.5, and Mg3Ho. Ordinarily, the 18R LPSO phase locates at a lamellae grain boundary with the misorientation angle of 136.9° and is simultaneously coherent with two α-Mg grains. The corresponding orientation relationships are as (001)18R//(0001)α1 and [010]18R//〈112¯0〉α1, and (001)18R about 43.1° from (0001)α2 and [010]18R//〈112¯0〉α2. After hot-extrusion, the disintegrated intermetallic phases composed of Mg24Ho5 and Mg3Ho mainly distribute at extrusion stringers while the LPSO phases mainly locate in un-recrystallized regions. In addition, amounts of fine spherical intermetallic particles were formed when many parallelly distributed LPSO plates precipitated in a part of the recrystallized grains. The as-extruded Mg−8Ho−0.6Zn−0.5Zr alloy exhibits excellent combination of high strength and high ductility. The underlying strengthening mechanisms were revealed as fine-grain strengthening and dispersion strengthening.