DRXed Grain Size Research Articles

Effect of rotational-die ECAP parameters on microstructure and mechanical properties of Mg97Y2Zn alloys

In this work, the Mg97Y2Zn (at%) alloys have been processed by the rotary-die (RD) ECAP technique under different temperature or pass conditions to tailor the microstructure and mechanical properties. The results reveal that the DRX ratio (VDRX) increases with the increase of ECAP pass or temperature, weakening the texture intensity. The refinement of DRXed grains saturates after 9 passes of ECAP at 350 °C, while the size of DRXed grains (dDRX) monotonically decreases with the reduction of temperature from 400 °C to 300 °C in the case of the same pass number (9P). The combination of fine dDRX, large VDRX, and directionally arranged LPSO phases along the extrusion direction endows the 350 °C–9P alloy with the optimum strength and ductility synergy, exhibiting the tensile yield strength (TYS) of 267 MPa, ultimate tensile strength (UTS) of 318 MPa, and elongation of 10.3%. However, the larger VDRX, weaker texture, less low-angle grain boundaries combine to improve the elongation (14.7%) of the 400 °C–9P alloy at the expense of the TYS (231 MPa) and UTS (303 MPa). This paper provides useful information to optimize the RD-ECAP parameters for the fabrication of high-performance Mg97Y2Zn alloys.

Effect of Cu micro-alloying on the microstructure, mechanical and corrosion properties of Mg-Gd-Y-Zn based alloy applied as plugging tools

Dissolvable functional magnesium alloys are attracting strong attention due to the prominent advantages of high strength and suitable dissolution rate. Here, the effect of Cu micro-alloying (0, 0.4, 0.8 wt%) on the microstructure, mechanical and corrosion properties of Mg-9.5Gd-2.7Y-0.9Zn alloys are investigated. With the increase of Cu contents, the block 14 H long-period stacking ordered (LPSO) phase gradually increases, which promotes the nucleation of dynamic recrystallized (DRXed) grains. However, the migration of the sub-structure is inhibited caused by the plate LPSO phase and the growth of DRXed grains is suppressed. Eventually, the grain size of DRXed grains decreased. The alloy with 0.8 wt% Cu obtains higher yield strength compared with Cu-free alloys, which is mainly ascribed to multiple strengthening contributions containing second phase, dislocation, grain boundary and solid solution. Due to the potential difference between the LPSO phase and the Mg matrix in the investigated alloys, the LPSO phase acts as cathodic to accelerate corrosion. The 0.8Cu alloy exhibits the highest corrosion rate, which is the synergistic effect of the second phase (its larger volta potential compared with 0Cu and its higher fraction of LPSO phase), dislocation density, grain size and crystal orientation. Ultimately, the Mg-9.5Gd-2.7Y-0.9Zn-0.8Cu alloy exhibits an ultimate tensile strength of 380 MPa, tensile yield strength of 331 MPa, and decent corrosion rate (16.8 mg·cm−2·h−1 at 93 °C in 3 wt% KCl solution). This work strengthens understanding the Cu role in designing high-strength dissolvable Mg-RE alloys.

Microstructure and strengthening mechanism of hot-extruded ultralight Mg-Li-Al-Sn alloys with high strength

1 The extruded Mg-7Li-2Al-1.5Sn alloy possesses good mechanical properties with the yield strength, ultimate tensile strength and elongation of 250 MPa, 324 MPa and 11.9% at room temperature, respectively. 2 Dynamic precipitation and dynamic recrystallization occur during hot deformation. Nanoscale Sn-rich precipitates with a size of ∼14 nm are formed and the average size of DRXed grains in the extruded Mg-Li-Al-Sn alloys is ∼2 μm. 3 The strengthening mechanism of the extruded alloy mainly consists of grain boundary, precipitation and dislocation strengthening. Grain boundary strengthening is the most dominant mechanism in the test alloy. Duplex-structured Mg-7Li-2Al-1.5Sn alloys with high strength were fabricated and their strengthening mechanism was investigated. The Mg-7Li-2Al-1.5Sn alloys were prepared by casting and extruded at the temperature of 533 K with an extrusion ratio of 25:1. The microstructure and mechanical properties of Mg-7Li-2Al-1.5Sn alloys were systematically investigated by OM, XRD, SEM, TEM, and tensile tests. The results show that Mg-7Li-2Al-1.5Sn alloys are mainly composed of α-Mg, β-Li, LiMgAl 2 , Mg 2 Sn and Li 2 MgSn phases. The yield strength (YS), ultimate tensile strength (UTS) and elongation (EL) of the extruded alloy at room temperature reach 250 MPa, 324 MPa and 11.9%, respectively. A lot of Sn-rich precipitates (Mg 2 Sn and Li 2 MgSn) are precipitated during extrusion with an average size of ∼14 nm, which is beneficial to the grain refinement. Dynamic recrystallization occurs during hot deformation and the nano-precipitates effectively refine the dynamic recrystallized (DRXed) grains. Besides, the residual dislocations existed in DRXed and un-DRXed grains result in the dislocation strengthening in the extruded alloy. Mg-7Li-2Al-1.5Sn alloys possess excellent high-temperature mechanical properties with the YS, UTS and EL of 200 MPa, 237 MPa and 26.7% at 423 K, respectively. Sn-rich precipitates with good thermal stability can effectively prevent grain growth, which is good for the improvement of the high-temperature performance of Mg-Li-Al-Sn alloy.

Unveiling the microstructure evolution based on deformation mechanisms and dynamic recrystallization in as-extruded AZ31 Mg alloys during uniaxial compression

A series of uniaxial compression experiments were carried out on the as-extruded AZ31 Mg alloy with<10–10>-<11–20>//ED (extrusion direction) double fiber texture at 200–350 ℃. The microstructure evolution during the work hardening and dynamic softening stages were systematically investigated based on deformation mechanisms and dynamic recrystallization (DRX), respectively. Experimental and simulation results demonstrated that in the work hardening stage, the dominant deformation mechanisms were the basal<a>slip and {10−12} twinning at 250 ℃, while the basal<a>slip and prismatic<a>slip 350 ℃. The appearance of profuse {10−12} twins at 250 ℃ not only effectively refined the microstructure but also quickly promoted the formation of strong<0001>//CD (compression direction) texture. Moreover, the activation of twin variants was mainly dependent on their own Schmid factor that was higher in<10–10>//CD grains, resulting in the earlier disappearance of the<10–10>//ED texture component than the<11–20>//ED texture component. In contrast, owing to the limited of {10−12} twins, the samples at 350 ℃ exhibited a weak fiber texture with diffusely distributed of basal plane along the CD. In the dynamic softening stage, high temperature increased both the grain size and proportion of DRXed grains, resulting in a more homogeneous microstructure. More importantly, the continuous DRX (CDRX) transformed into discontinuous DRX (DDRX) as the temperature increased from 250 °C to 350 °C. The similar orientation between the new CDRXed grains and parent grains at 250 ℃ preserved the strong<0001>//CD texture, while at 350 ℃ the basal texture was significantly weakened due to the random orientation DDRXed grains and higher activation of pyramidal<c + a>slip. These findings are beneficial for the optimization of the temperature range of the MDF process.

A comprehensive analysis on microstructure evolution of Mg-5.65Zn-0.66Zr alloy during hot deformation

The microstructure evolution of Mg-5.65Zn-0.66Zr (wt.%) alloy was studied based on the hot compression tests. The results indicated that the flow stress increased rapidly to a peak point at the initial stage, and then it gradually decreased. Moreover, high temperature and low strain rate resulted in the decreasing of flow stress. All samples exhibited a necklace grain structure because of the occurrence of partial dynamic recrystallization (DRX). High temperature increased both the size and fraction of DRXed grains, while high strain rate showed an opposite tendency. At the conditions of 350 °C/0.001 s−1 and 350 °C/0.1 s−1, the twins were not exhibited and DRX played a dominant role. Importantly, the obvious split of basal texture was observed. The pyramidal <c+a> slip with high value of Schmid factor was active in large deformed grains, which corresponded to the peak split point in (0001) pole figure. A mechanism about the grain rotation was proposed to explain the relationship between the pyramidal slip and the split of basal texture. Finally, it was found that large number of {10–12} extension twins were formed during the initial stage at condition of 300 °C/1 s−1, and the number of twins decreased with the increase of strain. The twins greatly contributed to the fast formation of basal texture and grain rotation. Moreover, the non-basal slips were active in twining region, which could facilitate the nucleation of DRX.

A high-performance Mg-4.9Gd-3.2Y-1.1Zn-0.5Zr alloy via multidirectional forging after analyzing its compression behavior

A high-performance Mg-4.9Gd-3.2Y-1.1Zn-0.5 Zr alloy has been fabricated by multidirectional forging (MDF) after analyzing its compression behavior. The as-homogenized alloy exhibits a high activation energy Q of deformation (∼285 kJ/mol). The size of DRXed grains after compression tends to decrease as the Z-H parameter (Z) increases, showing a grain size exponent m of ∼4.0. Lamellar LPSO phases, kinking deformation, and bimodal microstructure are detected at the relatively low compression temperature of 350 and 400 °C, while sufficient DRX can be achieved at 500 °C, accompanied by the dissolution of lamellar LPSO. According to the processing maps, MDF was successfully conducted under an appropriate condition. After peak-aged at 200 °C for 78 h, the MDFed billet exhibits a tensile yield strength (TYS) of 331 and 305 MPa at room temperature and 200 °C, respectively. The high strength mainly results from the combination of fine grains, low Schmid factor for basal slip, sufficient β' ageing precipitates, and directionally arranged interdendritic LPSO phases, etc. This paper provides a feasible way for the fabrication of high-performance, low-RE-content, and large-scale Mg components for industrial production.

Enhanced mechanical properties of Mg-Zn-Y-Zr alloy by low-speed indirect extrusion

Effects of extrusion temperature on microstructure and mechanical properties of low Zn and rare earth (RE) containing Mg-2.5Zn-0.7Y-0.4Zr alloy were investigated. It was subjected to indirect extrusion at 300°C and 350°C at low extrusion speed (0.3mm/s). The microstructures of as-extruded alloys exhibited both a bimodal grain structure consisting of refined dynamic recrystallized (DRXed) grains and elongated strips of non-recrystallized (un-DRXed) grains. Additionally, the fragmented Mg3Zn2Y3 (W) phase particles distributed along the extrusion direction and promoted dynamic recrystallization. Meanwhile, the amount of nanoscale MgZn2 and W phases were dynamically precipitated in Mg matrix during the extrusion process. The average size and volume fraction of DRXed grains decreased with the extrusion temperature decreasing from 350°C to 300°C, while the volume fraction of precipitates increased. The alloy extruded at 300°C exhibited superior mechanical properties with the ultimate tensile strength (UTS) of 347.1MPa, yield strength (YS) of 329.3MPa and elongation (EL) of 12.8%. The enhancement of yield strength was mainly attributed to fine DRXed grains and dense nanoscale precipitates. The strengthening mechanism and microstructure evolution were also discussed.

Microstructure and mechanical properties of Mg-4Zn-xGd (x=0, 0.5, 1, 2) alloys

Abstract The present work mainly focuses on the effect of minor content of Gd element on the microstructure and mechanical properties of Mg-Zn alloy. The Mg-4Zn-xGd (x=0, 0.5, 1, 2) alloys were fabricated and subjected to extrusion at 280°C with the ratio of 25:1. Results revealed that the addition of Gd could inhibit the dynamic recrystallization (DRX) of Mg-4Zn alloy, and both the size and volume fraction of DRXed grains demonstrated a decreasing tendency as the Gd content increased from 0.5 to 2 wt.%. It is the weakening effect on DRX behavior that lead to the smaller amount of fine precipitate and the stronger texture intensity. As compared with the Mg-4Zn alloy, the yield strength and ultimate tensile strength of Mg-4Zn-xGd alloys were improved obviously by the addition of minor content of Gd, however, at the expense of elongation. Excellent tensile properties with the yield strength of ∼369.8 MPa and ultimate tensile strength of ∼400.3 MPa were obtained when the Gd content was 2 wt.%.

Microstructure and Tensile Properties of Mg–5Zn Alloy Containing Ca

The present study aimed at investigating the effect of Ca (0–3 wt%) on the microstructure and tensile properties of as-cast and as-extruded Mg–Zn alloy, in which tensile tests, X-ray diffraction, SEM, EDX and texture analysis were used to evaluate these features. Based on the results obtained, it is shown that the average grain size decreased from 305 to 84 μm by adding 3 wt% Ca to the as-cast Mg–Zn alloy. XRD and SEM results reveal that the dominant second phase in the presence of Ca is Ca2Mg6Zn3 which is formed continuously between dendrite arms spacing. It has been observed that the addition of 0.1 wt% Ca, significantly enhanced both UTS and elongation of as-cast alloy. It is found that the size of DRXed grains decreases with the increase of Ca (4 μm with 3 wt% Ca addition) after hot-extrusion. Further, it has been observed that Ultimate tensile strength (UTS) and elongation values of extruded alloys containing Ca are enhanced and the best result is obtained for Mg–5Zn–0.1Ca sample. Ultimate tensile strength (UTS) of 351 MPa and elongation percentage of 24% are obtained for Mg–5Zn–0.1Ca extruded alloy.

Effect of strain rate on plastic deformation bonding behavior of Ni-based superalloys

Plastic deformation bonding (PDB) has emerged as a promising solid state bonding technique with limited risk of phase transformations and residual thermal stresses in the joint. In this study, the PDB behavior of IN718 superalloy was systematically investigated by performing a series of isothermal compression tests at various processing conditions. It was revealed that, with increasing PDB strain rate at 1000 °C, different extents of dynamic recrystallization (DRX) occur in the bonding area of IN718 joints. The extent of DRX, average size of DRXed grains, and a newly proposed “interfacial bonding ratio (ΨBonding)” parameter (to quantify the bond quality) were initially reduced with increase in the strain rate up to 0.1 s−1 and later increased at further higher strain rates. Electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) based interfacial microstructure analyses indicated that the quality of the bonded joints is closely related with the development of fine DRXed grains at the bonding interface with the increasing strain, which promotes adiabatic temperature rise. It was revealed that the initial bulging and subsequent migration of the original interfacial grain boundary (IGB) were the main mechanisms promoting DRX in the well bonded IN718 superalloy joints. Moreover, the mechanical properties of the bonded joints were not only controlled by the recrystallized microstructure but also depended upon the Bonding parameter of the joints.

Effect of extrusion ratio on microstructure and mechanical properties of Mg−2.5Sn−1.5Zn−0.8Ca−0.1Y alloy

To examine the influence of extrusion ratio on microstructure and mechanical properties, a new as-extruded Mg−2.5Sn−1.5Zn−0.8Ca-0.1Y (mass fraction, %) alloy was invented and tested. Results show that incomplete dynamic recrystallization (DRX) appeared during hot extrusion ; the number of un-DRXed grains and the average size of DRXed grains became smaller with the extruding ratio rising from 5 to 10, and the basal texture became worse, together with coarse MgSnCa phase being crushed into particles and fine Mg2Sn phase precipitated from α-Mg matrix. The yield strength, tensile strength and elongation ratio under extruding ratio10 were 218.4 MPa, 288.6 MPa and 18.61% respectively, which have increased by 36.0%, 17.7% and 13.5% correspondingly by contrast with that of under extruding ratio 5.

Microstructure and mechanical properties of as-extruded Mg-Sn-Zn-Ca alloy with different extrusion ratios

Abstract The effect of extrusion ratio on microstructure and mechanical properties of as-extruded Mg-6Sn-2Zn-1Ca (TZX621) (mass fraction, %) alloy was investigated. It is found that incomplete dynamic recrystallization (DRX) took place in as-extruded TZX621 alloy. As the extrusion ratio was increased from 6 to 16, both fraction of un-DRXed grains and average size of DRXed grains in as-extruded TZX621 alloy decreased and the basal texture was weakened. Coarse CaMgSn phase was broken into particles and fine Mg2Sn phase precipitated from α-Mg matrix during hot extrusion. Yield strength, ultimate tensile strength and elongation of as-extruded TZX621 alloy with extrusion ratio of 16 reached 226.9 MPa, 295.6 MPa and 18.1%, which were improved by 36.0%, 17.7% and 13.5%, respectively, compared to those of as-extruded TZX621 alloy with extrusion ratio of 6.

Effect of Zn and Ca Addition on Microstructure and Strength at Room Temperature of As-Cast and As-Extruded Mg-Sn Alloys.

In this study, the effect of Zn and Ca addition on microstructure and strength at room temperature of Mg-Sn alloys was investigated by comparison of Mg-6Sn, Mg-6Sn-2Zn, and Mg-6Sn-2Zn-1Ca alloys in as-cast and as-extruded states. In the as-cast samples, α-Mg and Mg2Sn phases were the main phases of Mg-6Sn and Mg-6Sn-2Zn alloys, while the CaMgSn phase was formed in Mg-6Sn-2Zn-1Ca alloy due to the addition of the Ca element. Mg2Sn phase dissolved into the matrix during homogenization while CaMgSn phase remained. Incomplete dynamic recrystallization (DRX) took place in these alloys during hot extrusion. Fine Mg2Sn precipitates were observed in α-Mg matrix of as-extruded samples. Zn showed little influence on microstructure, whereas Ca reduced the volume fraction of un-DRXed grains and increased the size of DRXed grains. As-extruded Mg-Sn alloys exhibited typical fiber texture. The strength at room temperature of Mg-Sn alloys improved significantly after hot extrusion. The addition of Zn element was beneficial to the strength at room temperature of the Mg-6Sn alloy, while the further addition of Ca element was harmful to the strength. Among these alloys, the Mg-6Sn-2Zn alloy exhibited the best strength at room temperature in both as-cast and as-extruded states.

High strength Mg-9Al serial alloy processed by slow extrusion

The AZ91-0.5Ca alloy was extruded at 250°C with slow extrusion speed (0.03, 0.3, 3mm/s) in the current study. The results showed that both the volume fraction and average grain size of DRXed grains increased with the increase of extrusion speed. Furthermore, numerous fine Mg17Al12 phase were observed in the DRXed regions of the extruded alloys, the amount of which decreased by increasing extrusion speed. The decrease of extrusion speed led to the improvement of yield strength and ultimate tensile strength, however, at the expense of elongation. As the extrusion speed decreased to 0.03mm/s, the yield strength, ultimate tensile strength and elongation of present alloy were ~347.8MPa, ~402.2MPa and ~9.1%, respectively, which were superior to the present commercial Mg-Al serial alloys. Detailed reasons are given and analyzed.

Effect of ultra-slow extrusion speed on the microstructure and mechanical properties of Mg-4Zn-0.5Ca alloy

The Mg-4Zn-0.5Ca alloys extruded at 280°C by different ultra-slow extrusion speed (0.01, 0.05, 0.1 and 0.5mms−1) were investigated in present study. The results show that both the volume fraction and average size of DRXed grains increase with the increase of extrusion speed, which is different from that depicted by present Z parameter. To fit the extrusion conditions with low temperature and ultra-slow extrusion speed, a more accurate estimate/measurement of the actual temperature in the expression of the Z parameter is needed in the further study. Even though the amount of precipitates decrease with the increasing extrusion speed, the increasing amount of DRX regions lead to the decreased intensity of basal plane texture. Superior mechanical properties with the yield strength of 324.5MPa, ultimate tensile strength of 371.1MPa and elongation to failure of 7.9% are achieved in present Mg-4Zn-0.5Ca alloy extruded at the speed of 0.01mms−1. As the extrusion speed increased from 0.01 to 0.5mms−1, the strength decreases which is accompanied with the increase of elongation.

Effect of billet diameter on hot extrusion behavior of Mg–Al–Zn alloys and its influence on microstructure and mechanical properties

The effect of the initial billet size on the microstructural evolution and tensile properties of extruded AZ61 and AZ91 Mg alloys was investigated using homogenized billets with diameters of 50 and 80 mm. All the extruded alloys exhibit a bimodal structure consisting of fine dynamically recrystallized (DRXed) grains with numerous precipitates and relatively coarse DRXed grains with few precipitates. Although the size and hardness of the fine DRXed grains do not vary with the billet size, using a smaller billet results in a significant reduction in the size and area fraction of coarse DRXed grains because of an increase in the quantity of precipitates. This increase in both the fraction of fine grains and the number of Mg17Al12 precipitates improves the tensile strength of the extruded alloys. This size effect can be attributed mainly to the increase in the strain rate with decreasing billet diameter, which promotes dynamic recrystallization and precipitation during extrusion. The effect of billet size is more pronounced in the AZ91 alloy, which has a high concentration of alloy elements, than in the AZ61 alloy.

High-speed indirect extrusion of Mg–Sn–Al–Zn alloy and its influence on microstructure and mechanical properties

This investigation into the effect of indirectly extruding Mg–7Sn–1Al–1Zn (TAZ711) alloy at high exit speeds on the microstructure and tensile properties found no evidence of surface cracking, not even at the maximum speed tested of 27 m/min. This high-speed extrudability is attributed to the relatively high incipient melting temperature of the alloy (535 °C), which results from the formation of a thermally stable Mg2Sn phase. All extruded samples exhibited a completely recrystallized (DRXed) structure consisting of coarse DRXed grains with few particles in combination with relatively fine DRXed grains containing numerous fine precipitates. With an increase in extrusion speed, the total amount of Mg2Sn precipitates decreased and the size and quantity of coarse DRXed grains increased due to a rise in temperature during extrusion. This has the effect of reducing the strength of the extruded alloy through a reduction in precipitation and grain-boundary strengthening, yet the elongation of the extruded alloy remains essentially the same due to a loss of ductility caused by the increase in DRXed grain size.

Effect of homogenization temperature on the microstructure and mechanical properties of extruded Mg–7Sn–1Al–1Zn alloy

The effect of homogenization temperature (HT) on the microstructure and tensile properties of extruded Mg–7Sn–1Al–1Zn (TAZ711) alloy was investigated by conducting homogenization heat-treatment at 400, 450, and 500°C for 24h, followed by indirect extrusion. The results obtained through this revealed that second phases are nearly fully dissolved by homogenization at 500°C, but undissolved Mg2Sn particles remain after homogenization at 450 and 400°C. Furthermore, the increase in large undissolved particles with decreasing HT helped to promote dynamic recrystallization (DRX) during extrusion by particle stimulated nucleation (PSN), thus resulting in an increase in the area fraction of DRXed grains in the extruded alloy. A decrease in the amount of Sn supersaturated in the matrix, however, suppressed the formation of fine Mg2Sn precipitates during extrusion, which in turn led to an increase in the size of DRXed grains due to a reduction in grain boundary pinning by precipitates. Ultimately, this meant that although the yield and ultimate tensile strengths gradually improved with increasing HT due mainly to a decrease in the DRXed grain size, an increase in texture intensity, and an increased number of fine Mg2Sn precipitates, the elongation deteriorated due to crack initiation at twins formed within coarse non-DRXed grains.

Dynamic recrystallization behavior during hot deformation and mechanical properties of 0.2 μm SiCp reinforced Mg matrix composite

In this paper, 0.2μm SiCp/AZ91 composite was fabricated by stir casting technology. The influence of fine (<1μm) particles on the dynamic recrystallization (DRX) behavior of AZ91 matrix was investigated by researching the microstructure evolution of 0.2μm 1vol% SiCp/AZ91 composite during the hot extrusion process. Results showed that the DRXed ratio of AZ91 matrix increased as well as the average size of DRXed grains reduced due to the addition of fine particles. The SiCp could not only pin dislocation, but also was helpful to generate dislocations owing to the deformation mismatch between matrix and particles, thus causing the increase of dislocation density around SiCp, which was benefit to stimulate DRX nucleation. Meanwhile, the growth of DRXed grains was suppressed in the following deformation process due to the pinning effect of fine particles on grain boundaries. Thus, the fine particles had significant effect on stimulating DRXed nucleation and refining grain size. The yield strength and ultimate tensile strength of the as-deformed 0.2μm 1vol% SiCp/AZ91 composite are 275 and 335MPa, respectively, which are mainly attributed to the grain refinement, basal plane texture and uniform particle distribution induced by hot deformation.