Influence of magnesium on grain refinement and ductility in a dilute Al–Sc alloy

Experiments were conducted to determine the influence of magnesium additions on grain refinement and tensile ductility in an Al–0.2% Sc alloy processed by equal-channel angular pressing (ECAP). The experiments show ECAP reduces the average grain size to within the range of ∼0.70 to ∼0.20 μm for alloys containing from 0 to 3% Mg but the as-pressed grain size increases to ∼0.3 μm in an alloy with 5% Mg because it is then necessary to use additional annealing treatments during the pressing process. The ultrafine grains introduced by ECAP are stable to high temperatures in the alloys containing from 0 to 3% Mg: in all alloys, the average grain size is <5 μm after annealing for 1 h at temperatures up to ∼750 K. High superplastic ductilities were achieved in the alloy containing 3% Mg but alloys containing 0.5% and 1% Mg exhibited the enhanced ductilities generally associated with conventional Al–Mg alloys. The results suggest the addition of ∼3% Mg is optimum for achieving superplastic elongations at rapid strain rates in the Al–0.2% Sc alloy.     

Effect of second phase particles on grain refinement during equal-channel angular pressing of an Al-Mg-Mn alloy

The process of grain refinement under severe plastic deformation was examined in an Al-5.4% Mg-0.5% Mn-0.1% Zr alloy, which was subjected to equal-channel angular pressing (ECAP) in the strain interval from 1 to 12 at a temperature of ∼300 °C. It was shown that the size and distribution of the second phase particles precipitated under homogenization annealing strongly affect grain refinement. Extensive grain refinement under ECAP was provided by a dispersion of Al₆Mn particles with an average size of ∼25 nm that precipitated during the homogenization annealing at an intermediate temperature. The fully recrystallized structure with an average crystallite size of ∼0.55 μm evolves through continuous dynamic recrystallization. In contrast, homogenization annealing at a high temperature leads to the formation of coarse Al₆Mn particles with a plate-like shape. Under further ECAP, the formation of coarse recrystallized grains takes place in this material due to the discontinuous growth of recrystallized grains during the inter-pass annealing between the ECAP passes. The role of second phases in grain refinement is discussed in terms of pinning and driving forces for recrystallization.     

Fatigue of a fine-grained high-strength Al-6Mg-Sc alloy produced by equal-channel angular pressing

Room-temperature mechanical properties of a fine-grained high-strength Al-6Mg-Sc alloy 1570 produced by equal-channel angular pressing (ECAP) have been studied. The alloy was deformed to a true degree of deformation ? ～ 8 at a temperature of 325°C. The average size and the volume fraction of new small grains after ECAP were 1.2 μm and 0.7, respectively. It is shown that the formation of a submicrocrystalline structure makes it possible to increase the yield strength and endurance limit of the alloy 1570 with respect to the microcrystalline and coarse-grained states with small loss in plasticity. The influence of the microstructure which is formed in the process of ECAP on the behavior of the material upon high-cycle fatigue is discussed.     

Structure and properties of aluminum alloy 1421 after equal-channel angular pressing and isothermal rolling

Sheets from the aluminum alloy 1421 with an ultrafine-grained (UFG) structure and a weak crystallographic texture were prepared by the method of equal-channel angular pressing (ECAP) through a die with channels of a rectangular cross section and by subsequent isothermal rolling. Both operations were carried out at a temperature of 325°C. It is shown that severe plastic deformation (SPD) leads to the formation of a completely recrystallized uniform microstructure with an average grain size of 1.6 µm in the alloy. At room temperature the alloy 1421 demonstrates high static strength (σ_u = 545 MPa, σ_0.2 = 370 MPa) in the absence of a significant anisotropy. At temperatures of hot deformation, the alloy showed ultrahigh elongations under superplasticity (SP) conditions. At a temperature of 450°C and initial deformation rate of 1.4 × 10^?2 s^?1 the maximum elongation at fracture was ～2700%. At static annealing at a temperature of SP deformation, the UFG structure formed in the process of SPD remains stable. The SP deformation is accompanied by an insignificant grain growth and pore formation.     

Low-temperature superplasticity of ZK40 magnesium alloy processed using equal channel angular pressing

Microstructure evolution and superplastic behaviors of ZK40 magnesium alloy were investigated in the temperature range of 473～623 K. Transmission electron microscopy (TEM) was used to study the microstructure changes, twinning occurred significantly after being processed by equal channel angular pressing (ECAP) for one pass through the die, the mean grain size was 5.6μm. Finer grains can be obtained after further processing through ECAP, the average grain size of the alloy processed by ECAP for three passes was as low as 0.8 μ_m; this alloy exhibited low temperature superplasticity at 473～523 K, elongations obtained at the same initial strain rate of 1×10~(-3) s~(-1) were 260％ at 473 K and 612％ at 523 K, respectively. Corresponding values for the ZK40 alloy processed by ECAP for only one pass were 124％ at 473 K and 212％ at 523 K, respectively; poor superplastic behavior of this material was related to the long-range stresses associated with the non-equilibrium grain boundaries within the coarse grains. The incompatibility between fine and coarse grains was thought to be unfavorable to the improvement of superplasticity.     

Annealing behavior of Al-Mg-Mn alloy processed by ECAP at elevated temperature

An Al-Mg-Mn alloy was subjected to equal channel angular pressing(ECAP) at 350 ℃ for 6 passes. Static annealing was conducted on the deformed alloy at various temperatures from 400 to 450 ℃ for 1h respectively. The microstructural evolutions of both the deformed and the annealed materials were studied by electron back scattering diffraction pattern(EBSD) analysis. A fine-grained structure with (sub)grain size of about 2 μm is obtained after 6 ECAP passes, and the fraction of high-angle boundaries is 48.08%. As the annealing temperature increases, the average misorientations of the grain boundaries and the fraction of high-angle boundaries increases gradually. No grain growth takes place in the 400 ℃ annealed sample, while after annealing at 450 ℃ a coarse-grained structure replaces the initial fine-grained structure produced by ECAP. The aspect ratios remain almost constant and the (sub)grains keep equiaxed in the range of the present experiment. As the annealing temperature increases, the strength decreases obviously, which attributes to the relaxation of the internal stresses and the grain growth, while the elongation increases slightly.     

Influence of equal-channel angular pressing on the structure and mechanical properties of low-carbon steel 10G2FT

Results are presented of the investigation of mechanical properties, microstructure, and phase composition of low-carbon steel 10G2FT (Fe-1.12Mn-0.08V-0.07Ti-0.1C) before and after equal-channel angular pressing (ECAP). It has been established that the ECAP of steel 10G2FT at T = 200°C in the case of the ferritic-pearlitic state and at T = 400°C in the case of the martensitic state leads to the formation of a predominantly submicrocrystalline structure with an average size of structural elements of approximately 0.3 μm, causes an increase in the strength properties, a decrease in the plasticity, and the localization of plastic flow. It has been experimentally shown that the initially martensitic structure after ECAP causes higher strength properties in comparison with the ferritic-pearlitic structure.     

Microstructure evolution and mechanical behavior of AZ31 Mg alloy processed by equal-channel angular pressing

AZ31 Mg alloy bar was subjected to 8-pass equal-channel angular pressing(ECAP) at 623 K. Microstructure evolution was observed by optical microscopy(OM) on cross section and X-ray diffraction analysis. The room temperature mechanical properties of the ECAP processed specimens were also investigated. A fine-grained structure with an average sub-grain size of 9 μm is obtained after 7 ECAP passes. XRD analysis indicates that after ECAP, in placing of planes and become the dominant directions that are favourable for grain refinement. ECAP processed AZ31 Mg alloy exhibits significant improvement in elongation but decrease in strength. The elongation of the specimen increases continuously up to 2 passes and then remains stable at further passes. This improvement can be related to the evolution of crystallographic texture and the scattered orientation of the basal plane (0001).     

Ultrafine-grained pure Ti recycled by equal channel angular pressing with high strength and good ductility

Machining chips of commercially pure Ti (ASTM grade 2) were consolidated into full density by equal channel angular pressing (ECAP) with an average grain size as low as 0.8 μm, yield strength up to 650 MPa, and ductility of ～16%. Effect of recycling condition on the microstructure and mechanical properties were investigated in terms of ECAP temperature, number of passes and chemical composition. Using electron backscatter diffraction it is evident that continuous dynamic recrystallization (however, which is purely a phenomenological terminology) plays a significant role in grain (with misorientation ≥15°) formation, whilst benefitting from high stacking fault energy, this continuous conversion of subgrain (<15°) into grain can be essentially considered as an extended recovery with a substantial presence of low angle grain boundaries in the recycled Ti. The Hall–Petch relationship is adapted to explain the strengthening of the recycled Ti. Additionally, using scanning electron microscopy fractography, the ductility was analyzed by a modified Griffith criterion. Last, superior energy efficiency of ECAP reduces environmental impact when comparing to conventional melting/casting. ECAP develops an innovative solid-state process for improving the recycling value of waste Ti.     

Experimental and numerical investigation on pure aluminum by ECAP

The equal channel angular pressing(ECAP) experiments were carried out with industrial pure aluminum and an in-house mould. The comparison of material grain size before and after ECAP was performed by applying the technique of electron back scattered diffraction(EBSD). The results show that the grains in the material after ECAP are refined and the yield stress and ultimate strength are increased. In order to investigate the deformation mechanism during ECAP and the reason for driving grain size refinement, three-dimensional numerical simulations of the ECAP process were carried out. Based on the Lode parameter analysis, the deformation of the material sample is found very complicated, not just pure shear during extrusion through the angular channel. The simulation confirms that a strong strain gradient in the sample material is imposed by the ECAP.     

Developing superplasticity in a magnesium alloy through a combination of extrusion and ECAP

A new processing procedure was applied to a cast Mg-9% Al alloy. This procedure involves the sequential application of extrusion and equal-channel angular pressing and is designated EX-ECAP. Experiments show that the Mg-9% Al alloy has an initial grain size of ~50 μm after casting but this is reduced to ~12 μm after extrusion and it is further reduced to ~0.7 μm when the extruded alloy is subjected to ECAP for 2 passes at 473 K. Although the cast alloy exhibits extremely limited ductility and the extruded alloy is only moderately ductile, it is demonstrated that processing by EX-ECAP produces excellent superplastic ductilities including the occurrence of both low temperature superplasticity and high strain rate superplasticity. The EX-ECAP process is less effective when the ECAP step is conducted at 573 K because, although the pressing is then very easy, there is significant grain growth at this higher temperature.     

等径角挤压2A12铝合金超细晶组织结构研究

使用Φ=90°和ψ=30°的挤压模具在室温下对2A12铝合金进行了8次等径角挤压,成功制备了晶粒尺寸为200nm左右、具有大角度晶界的块体超细晶材料,并且采用HitachiS-800透射电镜,研究了挤压过程中2A12铝合金的组织结构及其变化。结果表明挤压一次后,晶粒内位错密度急剧增加,形成位错纠缠,晶粒细化效果最明显;挤压前分布在α-Al基体上的针状第二相Al2Cu和颗粒状Al2Cu Mg,在剪切力的作用下,针状Al2Cu变成颗粒状,弥散分布在α-Al基体上,Al2CuMg颗粒因晶粒细化进入了晶界位置,而且在以后的挤压中,这些化合物颗粒大小基本保持不变。继续挤压,位错逐渐由晶内移动到晶界上,在晶界上形成胞状组织,最后逐渐变成了清晰的小角度晶界或大角度晶界,从而实现组织的超细化。     

工业纯钛在120°模具中的多道次ECAP室温变形组织与性能

在室温,采用通道夹角为120°的变形模具对工业纯钛(Commemial Pure Titanium,CP-Ti)以Bc方式实施四道次ECAP(EqualChannel Angular Pressing)挤压变形,成功获得表面光滑无裂纹的变形试样.文中主要研究了工业纯钛在室温下进行ECAP多道次变形的组织结构演变,并测试了变形试样的力学性能.微观结构显示工业纯钛在室温下进行多道次ECAP变形时,只在前两道次产生了大量的变形孪晶,且随道次增加变形孪晶逐渐消失.最终获得的试样晶粒平均尺寸由最初的约28μm细化到约250 nm,试样断裂强度和显微硬度分别提高到773和2486 MPa,而试样仍保持较好的延伸率(可达16.8%).     

Microstructure,Texture Evolution,and Mechanical Properties of ECAP-Processed ZAT522 Magnesium Alloy

In this work,the high-strength Mg-5Zn-2Al-2Sn(ZAT522,in wt%) Mg alloys was obtained at 220℃ and 130℃ by a two-step equal channel angular pressing(ECAP).For each stage,two passes were used.The results showed a remarkable grain refinement after the first stage of ECAP(A2 samples),leading to a fine-grained structure with average size of 1.40 μm.The additional stage(A4 samples) caused further grain refinement to 1.18 μm,and an ultra-fine grain structure(700 nm)appeared in the precipitate-rich region.The grain refinement mechanism for both samples was discussed in detail.To this end,the original extrusion fiber texture evolved into a new strong texture characterized by the base planes tilted toward the ECAP shear plane,with a higher Schmid factor value of 0.34.Compared with the as-extruded alloy,the yield strength of the A2 samples increased from 180 to 245 MPa,which was mainly attributed to the combined effects of grain boundary strengthening and precipitation strengthening.In the case of A4 samples,the dislocation strengthening resulted in a net increase in yield strength to 335 MPa,while the ductility was significantly reduced.     

Using ECAP to achieve grain refinement, precipitate fragmentation and high strain rate superplasticity in a spray-cast aluminum alloy

An aluminum 7034 alloy, produced by spray casting and with an initial grain size of ˜2.1 μm, was processed by equal-channel angular pressing (ECAP) at 473 K to produce an ultrafine grain size of ˜0.3 μm. It is shown that the rod-like MgZn₂ precipitates present in the as-received alloy are broken into very small spherical particles during ECAP and these particles become distributed reasonably uniformly throughout the material. The presence of these fine MgZn₂ particles, combined with a distribution of fine Al₃Zr precipitates, is very effective in restricting grain growth so that submicrometer grains are retained at elevated temperatures up to at least ˜670 K. Tensile testing of the pressed material revealed high elongations to failure, including elongations of >1000% when testing at a temperature of 673 K at initial strain rates at and above 10⁻² s⁻¹. These results confirm the occurrence of high strain rate superplasticity in the spray-cast alloy.     

Microstructural development in equal-channel angular pressing using a 60° die

Billets of pure aluminum and an Al–1%Mg–0.2%Sc alloy were successfully processed using equal-channel angular pressing (ECAP) with a die having an internal channel angle of 60°. Careful inspection of the microstructures after ECAP revealed excellent agreement, at both the macroscopic and the microscopic levels, with the theoretical predictions for shearing using a 60° die. The grain sizes introduced with the 60° die were slightly smaller than with a conventional 90° die; thus, the values with these two dies were ∼1.1 and ∼1.2 μm in pure Al and ∼0.30 and ∼0.36 μm in the Al–Mg–Sc alloy, respectively. Tensile testing of the pure aluminum at room temperature revealed similar strengthening after processing using either a 60° or a 90° die. In tests conducted at 673 K, the Al–Mg–Sc alloy processed with the 60° die exhibited significantly higher elongations to failure due primarily to the larger strain imposed with this die. It is shown using orientation imaging microscopy that superplastic flow in the Al–Mg–Sc alloy produces an essentially random texture and a distribution of boundary misorientations that approximates to the theoretical distribution for an array of randomly oriented grains.     

等通道转角挤压变形Mg-1.5Mn-0.3Ce镁合金的组织、织构与力学性能

采用等通道转角挤压(ECAP)工艺以Bc路径在623K温度下对Mg-1.5Mn-0.3Ce镁合金进行变形,观察显微组织与织构,测试了力学性能。显微组织分析表明,镁合金经ECAP变形晶粒尺寸明显得到细化,经6道次ECAP变形后晶粒尺寸由原轧制态的约26.1μm细化至约1.2μm,且细小的第二相粒子Mg12Ce弥散分布于晶内及晶界处;同时经ECAP变形后,原始轧制织构随变形道次的增加不断减小,并开始转变为ECAP织构,织构强度不断增强;力学性能结果表明,由于晶粒细化作用大于织构软化作用,前3道次ECAP变形镁合金强度随道次的增加不断提高,与Hall?Petch关系相符,在第3道次时其抗拉强度和屈服强度达到最大值,分别为272.2和263.7MPa;在4道次之后形成较强的非基面织构,镁合金强度下降,与Hall?Petch呈相悖关系。断口分析表明,轧制态与ECAP变形镁合金的断裂方式都是沿晶断裂,由于6道次变形镁合金晶粒细化,存在更多的韧窝并获得16.8%最大室温伸长率。     

Structural state and superplasticity of an aluminum-lithium alloy subjected to equal-channel-angular pressing

The structural-phase state of rods processed by different regimes of equal-channel angular pressing (ECAP) has been studied. A microcrystalline structure has been shown to be formed during the pressing process. A dislocation substructure and subgrains are formed in the grains. In the rods subjected to ten ECAP passes at 370°C, a uniform structural state is formed in the entire bulk material. The mechanical behavior of rods processed by ECAP has been investigated. Temperature and strain-rate conditions that are necessary to obtain the maximum deformation to fracture have been determined for the specimens of each structural state. It was established that the maximum deformation to fracture corresponds to the rods subjected to ten ECAP passes at 370°C. It corresponds to the deformation rate equal to 10^?2 s^?1 at 370°C. The maximum degree of deformation was ～1900%. The plasticity of the specimens with the smallest grain size proved to be low.     

Controlling Corrosion Resistance of a Biodegradable Mg–Y–Zn Alloy with LPSO Phases via Multi-pass ECAP Process

Mg-RE(rear earth) alloys with long period stacking(LPSO) structures have great potential in biomedical applications. The present work focused on the microstructure and corrosion behaviors of Mg 98.5 Y_1 Zn_(0.5) alloys with 18 R LPSO structure after equal channel angular pressing(ECAP). The results showed that the ECAP process changed the grain size and the distribution of LPSO particles thus controlled the total corrosion rates of Mg 98.5 Y_1 Zn_(0.5) alloys. During the ECAP process from 0 p to 12 p, the grain size reduced from 160–180 μm(as-cast) to 6–8 μm(12 p). The LPSO structures became kinked(4 p), then started to be broken into smaller pieces(8 p), and at last comminuted to fine particles and redistributed uniformly inside the matrix(12 p). The improvement in the corrosion resistance for ECAP samples was obtained from 0 p to 8 p, with the corrosion rate reduced from 3.24 mm/year(0 p) to 2.35 mm/year(8 p) in simulated body fluid, and the 12 p ECAP alloy exhibited the highest corrosion rate of 4.54 mm/year.     

等通道转角挤压制备超细晶Mg15Al双相合金组织与性能

对高铝双相合金Mg15Al在553K以Bc路线进行了不同道次的等通道挤压(ECAP),获得了超细晶高铝镁合金。通过OM,SEM,TEM分析了ECAP前后合金的微观组织结构及断口形貌,并测试了不同挤压道次后合金的硬度和室温拉伸性能,分析了ECAP细化晶粒机理及其性能改善原因。结果表明,随挤压道次增加,累计形变增强,网状硬脆相β-Mg17Al12破碎,合金晶粒显著细化,但对单相区和两相混合区细化效果不同。在α、β两相共存区内,4道次ECAP后形成100nm～200nm的细晶粒;在α单相区,4道次ECAP后晶粒为1μm以下,且在初晶α-Mg内析出弥散细小的β相,起到细晶强化和弥散强化作用。8道次ECAP后,晶粒略有长大。ECAP使合金的硬度、抗拉强度和延伸率同时得到提高,尤其是4道次ECAP后,硬度提高了32.04%,抗拉强度σb从150MPa提高到269.3MPa,延伸率δ由0.05%提高到7.4%;8道次ECAP后,硬度、抗拉强度略有下降,延伸率略有上升。SEM断口观察显示ECAP使合金拉伸断口形貌由铸态的解理断裂特征转变为延性韧窝断裂特征。