Microstructure and mechanical properties of AZ91 magnesium alloy subject to deep cryogenic treatments

AZ91 magnesium alloy was subjected to a deep cryogenic treatment. X-ray diffraction (XRD), scanning electronic microscopy (SEM), and transmission electronic microscopy (TEM) methods were utilized to characterize the composition and microstructure of the treated samples. The results show that after two cryogenic treatments, the quantity of the precipitate hardening β phase increases, and the sizes of the precipitates are refined from 8–10 μm to 2–4 μm. This is expected to be due to the decreased solubility of aluminum in the matrix at low temperature and the significant plastic deformation owing to internal differences in thermal contraction between phases and grains. The polycrystalline matrix is also noticeably refined, with the sizes of the subsequent nanocrystalline grains in the range of 50–100 nm. High density dislocations are observed to pile up at the grain boundaries, inducing the dynamic recrystallization of the microstructure, leading to the generation of a nanocrystalline grain structure. After two deep cryogenic treatments, the tensile strength and elongation are found to be substantially increased, rising from 243 MPa and 4.4% of as-cast state to 299 MPa and 5.1%.     

Al-6.6Zn-1.7Mg-0.26Cu合金挤压材焊接接头的组织与性能

采用光学显微镜、透射电子显微镜、维氏硬度计和拉伸试验机,研究了Al-6.6Zn-1.7Mg-0.26Cu合金挤压材熔化极惰性气体保护焊接接头的显微组织和力学性能。结果表明：焊缝中心区为枝晶,靠近母材侧的焊缝熔合区为柱状晶,母材为等轴晶,但靠近焊缝熔合区的母材晶粒发生了长大。焊接接头的硬度以焊缝为中心呈对称分布,从母材到焊缝中心,硬度先下降后上升再下降。焊缝中心区的硬度最低,为86～105（HV）。焊接接头的抗拉强度为309 MPa,屈服强度为237 MPa,伸长率为4.75%,挤压材的焊接强度系数为0.76。     

Mantle superplasticity and its self-made demise

The unusual capability of solid crystalline materials to deform plastically, known as superplasticity, has been found in metals and even in ceramics. Such superplastic behaviour has been speculated for decades to take place in geological materials, ranging from surface ice sheets to the Earth's lower mantle. In materials science, superplasticity is confirmed when the material deforms with large tensile strain without failure; however, no experimental studies have yet shown this characteristic in geomaterials. Here we show that polycrystalline forsterite + periclase (9:1) and forsterite + enstatite + diopside (7:2.5:0.5), which are good analogues for Earth's mantle, undergo homogeneous elongation of up to 500 per cent under subsolidus conditions. Such superplastic deformation is accompanied by strain hardening, which is well explained by the grain size sensitivity of superplasticity and grain growth under grain switching conditions (that is, grain boundary sliding); grain boundary sliding is the main deformation mechanism for superplasticity. We apply the observed strain-grain size-viscosity relationship to portions of the mantle where superplasticity has been presumed to take place, such as localized shear zones in the upper mantle and within subducting slabs penetrating into the transition zone and lower mantle after a phase transformation. Calculations show that superplastic flow in the mantle is inevitably accompanied by significant grain growth that can bring fine grained (≤1?μm) rocks to coarse-grained (1-10?mm) aggregates, resulting in increasing mantle viscosity and finally termination of superplastic flow.     

A TiCx reinforced Fe (Al) matrix composite using in-situ reaction

A new Fe matrix composite reinforced by the in-situ generated TiCx grains was prepared using the element Fe and Ti3AlC2 powders as the starting materials.Several hot-pressing temperatures were tried for exploring the phase transformation behavior from Ti3AlC2 to TiC x.Microstructures of the hot-pressed product were observed and analyzed.A tensile test was carried out for the new composite material,and the fracture face was analyzed.The results showed that the starting Ti3AlC2 was wholly decomposed and transformed into submicron TiC x grains at the hotpressing temperature above 1100℃.Most of the Al ions escaped from Ti3AlC2 were aggregated at the grain boundary of Fe grains,but a small amount of Al ions could be absorbed by Fe and formed Fe(Al) alloy around the surface of Fe grains.The prepared material exhibited a higher tensile strength of about 660 MPa and a uniform deformation of about 7%.     

添加或未添加 Cu?Nb 的超低碳高强度钢焊接金属的显微组织和力学性能

Two types of ultralow carbon steel weld metals (with and without added Cu?Nb) were prepared using gas metal arc welding (GMAW) to investigate the correlation between the microstructure and mechanical properties of weld metals. The results of microstructure characterization showed that the weld metal without Cu?Nb was mainly composed of acicular ferrite (AF), lath bainite (LB), and granular bainite (GB). In contrast, adding Cu?Nb to the weld metal caused an evident transformation of martensite and grain coarsening. Both weld metals had a high tensile strength (more than 950 MPa) and more than 17% elongation; however, their values of toughness deviated greatly, with a difference of approximately 40 J at ?50°C. Analysis of the morphologies of the fracture surfaces and secondary cracks further revealed the correlation between the microstructure and mechanical properties. The effects of adding Cu and Nb on the microstructure and mechanical properties of the weld metal are discussed; the indication is that adding Cu?Nb increases the hardenability and grain size of the weld metal and thus deteriorates the toughness.     

Effect of initial orientation on microstructure and mechanical properties of AZ31 Mg alloy sheets via accumulated extrusion bonding

In this study, effects of initial orientation on microstructure evolution and mechanical properties of AZ31 Mg alloy sheets via accumulated extrusion bonding(AEB) was systematically studied. The samples with RD and TD parallel to extrusion direction(ED) were labeled as RED and TED, respectively. RD and TD pieces alternately stacked was named as RTED. The results revealed that under three-dimensional compressive stress, {10-12} tensile twinning dominated the first stage deformation in container. ...     

10Cr21Mn16NiN高锰氮奥氏体不锈钢组织与性能研究

研究了固溶处理温度对热轧态10Cr21Mn16NiN高锰氮奥氏体不锈钢微观组织、力学性能和腐蚀性能的影响,并进一步揭示了该材料的低温韧脆转变行为。结果表明,随着固溶温度的升高,屈服强度和抗拉强度逐渐降低,而延伸率和耐腐蚀性能逐渐增大。这是因为高温固溶促进了热轧阶段形成的有害相重新溶解,从而消除析出相对性能带来的不利影响。10Cr21Mn16NiN钢在低温冲击载荷下表现出明显的韧脆转变行为,韧脆转变温度在-110℃附近,高于-110℃可以获得强度与韧性的良好配合。     

Columnar grains-covered small grains Cu–Sn alloy prepared by two-phase zone continuous casting

A new theory of two-phase zone continuous casting(TZCC) has been established in order to improve mechanical properties,corrosion resistance and conductivity properties of metals with wide solid-liquid two-phase zone.A Cu-Sn alloy with continuous columnar grains-covered non-columnar small grains of same phase microstructure containing many self-closed grain boundaries were produced by the self-developed TZCC process.Compared with water-cooled mold continuous casting Cu-Sn alloy,the tensile strength and ductility of the TZCC alloy are greatly improved,the corrosion resistance is improved up to fifteenfold,and the conductivity is improved by 12.2%.The excellent high strength may be due to the effective blockage of dislocation motion by numerous self-closed grain boundaries,which suppress the propagation of grain boundary corrosion,and the extremely low electrical resistivity and high ductility may be attributed to continuous columnar grains.     

Microstructure and mechanical properties of the micrograined hypoeutectic Zn–Mg alloy

A biodegradable Zn alloy, Zn–1.6Mg, with the potential medical applications as a promising coating material for steel components was studied in this work. The alloy was prepared by three different procedures: gravity casting, hot extrusion, and a combination of rapid solidification and hot extrusion. The samples prepared were characterized by light microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction analysis. Vickers hardness, tensile, and compressive tests were performed to determine the samples’ mechanical properties. Structural examination reveals that the average grain sizes of samples prepared by gravity casting, hot extrusion, and rapid solidification followed by hot extrusion are 35.0, 9.7, and 2.1 μm, respectively. The micrograined sample with the finest grain size exhibits the highest hardness (Hv = 122 MPa), compressive yield strength (382 MPa), tensile yield strength (332 MPa), ultimate tensile strength (370 MPa), and elongation (9%). This sample also demonstrates the lowest work hardening in tension and temporary softening in compression among the prepared samples. The mechanical behavior of the samples is discussed in relation to the structural characteristics, Hall–Petch relationship, and deformation mechanisms in fine-grained hexagonal-close-packed metals.     

等温自由锻温度对7085铝合金组织与性能的影响

通过金相组织观察、扫描电镜分析和室温拉伸力学性能及剥落腐蚀实验,分析探讨等温自由锻温度对7085铝合金显微组织、力学性能和剥落腐蚀的影响.研究结果表明:在370℃和400℃等温自由锻时,合金发生严重再结晶,强度较低,伸长率稍高,剥蚀抗力较差;在420℃锻造时,合金出现大量细小且分布均匀的亚晶粒,抗拉强度、屈服强度、伸长率和剥蚀抗力均较好,分别达到533.2 MPa,495 MPa,13.3％和EA.在450℃锻造时,该合金的晶粒开始长大,强度下降,伸长率稍有升高,剥蚀抗力较差.综合考虑显微组织、强度、塑性和剥落腐蚀等因素,确定420℃为合金等温自由锻最佳锻造温度.     

Analysis of surface orange peel of aluminum alloy automobile sheet by using of EBSD and X ray diffraction

The formation cause of orange peel of aluminum-alloy automotive sheet after tensile deformation was analysed by using X-ray diffraction and electron back-scattered diffraction(EBSD).The test results showed that formation cause of surface orange peel after tensile deformation related to product texture and nonuniform deformation during the tensile process.The grain size has significant effect on deformation uniform and texture formation.Coarse grains were easy to produce nonuniform deformation and texture,which would produce surface orange peel after tensile deformation.     

高挤压比下挤压工艺对AZ31B镁合金组织与力学性能的影响

研究了高挤压比条件下挤压温度、速度对AZ31B镁合金微观组织、力学性能的影响。采用光学显微镜观察了显微组织,拉伸试验测试了力学性能,并配合扫描电镜观察了拉伸试样的断口形貌。结果表明,高挤压比条件下,动态再结晶较为充分,少量晶粒长大,混晶组织消失。低温、高速挤压有助于晶粒细化,并使晶粒尺寸分布均匀,因而可获得高的抗拉强度、屈服强度以及良好的塑性。350 ℃,2 m/min条件下挤压,试样抗拉强度与延伸率最高,为336.5 MPa与 23%。低温、高速下的挤压试样的拉伸断口韧窝较深且细密,呈现明显的韧性断裂特征,而高温、低速的断口为混合断裂。     

Zr对Al-10Zn-2.5Mg-1.6Cu合金板材组织与性能的影响

针对T6态Al-10Zn-2.5Mg-1.6Cu铝合金板材，研究了添加质量分数分别为0，0.046%，0.098%，0.151%，0.185%的Zr，铝板中合金相粒子和晶粒的组态以及板材的力学性能.结果表明，Zr含量增加对铝板中微米级T相和Al₇Cu₂Fe相粒子无明显影响，合金中未形成含Zr的结晶相，但纳米级Al₃Zr弥散相粒子数量逐渐增多.添加质量分数为0.046% Zr可细化T6态铝板等轴状Cube取向晶粒；Zr质量分数超过0.098%能抑制铝板再结晶形核，板材晶粒为纤维状，取向以Brass，S，R和Copper为主，且其体积分数随Zr含量增加而逐渐增大.Zr含量增加，T6态铝板的强度逐渐增大，而延伸率先增大后略有减小.Zr质量分数为0.151%的T6态Al-10Zn-2.5Mg-1.6Cu铝合金板材性能最佳，其抗拉强度、屈服强度和延伸率分别为706，645MPa和10.3%.     

轧制处理对生物医用Zn-Mg合金组织和性能的影响

采用金相显微镜、X射线衍射仪、扫描电子显微镜、万能材料试验机和电化学工作站等设备,分析和研究了轧制变形对生物医用Zn-Mg合金的显微组织及力学性能的影响。结果表明：铸态、轧制态的合金均由Zn和Mg₂Zn₁₁两相组成;在轧制变形过程中,物相未发生改变,抗拉强度逐渐提高,伸长率先提高后降低,耐腐蚀性能逐渐下降;随着轧制变形量的增加,晶粒沿轧制方向的变形程度逐渐增大,直至出现纤维状组织。在相同的退火条件下,轧制变形量越大的Zn-Mg合金,再结晶晶粒尺寸越细小、均匀。     

铁素体珠光体球铁强度和延伸率的计算细观力学分析

本文基于计算细观力学方法,用有限元网格模拟球铁细观结构,将搞拉强度判据和延伸率判据引入轴对称弹塑性大应变有限元程序,计算了在球墨体积百分数为12.566％、球化良好、分布均匀条件下。不同细观结构球铁的抗拉强度和延伸率。结果表明:(1)铁素体含量是影响铁素体球光体球铁抗拉强度的主要因素,其关系可用线性或双线性公式表示:(2)当球铁中铁素体含量在50％以上时,延伸率随铁素体含量增加而迅速增大;(3)当铁素体含量在50％以下时,延伸率主要由球光体组织性能控制,并受到铁素体分布形态的较大影响,以铁素体呈破碎状分布为优;(4)在一定尺寸范围内,球墨直径增大,将使铁素体球铁抗拉强度和延伸率提高;对球光体球铁,则使延伸率略为下降(小于1％),而抗拉强度提高。     

Microstructure and mechanical properties of ZE10 magnesium alloy prepared by equal channel angular pressing

ZE10 magnesium alloy was subjected to equal-channel angular pressing (ECAP) up to 12 passes in a die with an angle of 120° between the two channels at 250-300℃. An inhomogeneous microstructure of bimodal grains including fine grains of 1-2 μm as well as coarse grains of about 20μm was obtained after the initial 1-4 ECAP passes. The grain size became increasingly homogeneous with further ECAP processing and the grains were significantly refined to 1-2 ktm after 8 passes and further refined to 0.5-1 μm after 12 passes. The alloy's yield strength changed slightly but the ductility improved greatly initially up to 4-6 passes corresponding to the bimodal grain microstructure. And after the subsequent pressing of more than 8 passes, the tensile strength including yield strength improved while the elongation decreased gradually.     

Preparation of super-aligned carbon nanotube-reinforced nickel-matrix laminar composites with excellent mechanical properties

A homogeneous and compact super-aligned carbon nanotube(SACNT)-reinforced nickel-matrix composite was successfully prepared by electrodeposition. The mechanical properties of the laminar SACNT/Ni composites were substantially improved compared with those of pure nickel. With increasing content of SACNTs, the tensile strength of the composite increased and the elongation decreased because of the high-strength SACNTs bearing part of an applied load and the fine-grained strengthening mechanism. The nanohardness of the SACNT/Ni composites was improved from 3.92 GPa(pure nickel) to 4.62 GPa(Ni-4 vol%SACNTs). The uniform distribution of SACNTs in the composites and strong interfacial bonding between the SACNTs and the nickel matrix resulted in an improvement of the mechanical properties of the SACNT/Ni composites. The introduced SACNTs refined the nickel grains, increased the amount of crystal twins, and changed the preferred orientation of grain growth.     

Effect of heat treatment on microstructures and properties of a novel Al-Zn-Mg-Cu alloy for oil drilling

In view of the special requirements for strength, heat resistance and corrosion resistance of Al-Zn-Mg-Cu alloy for oil drilling, the Al-6.2 Zn-2.5 Mg-1.6 Cu alloy was prepared by increasing Cu content on basis of Russian Series 1953 alloy. The effect of heat treatment on the microstructures and properties of the alloy was characterized by optical microscope(OM), scanning electron microscope(SEM) and transmission electron microscope(TEM), and investigated by tensile test at room temperature, thermal exposure test and corrosion test. The results show that the strength after T6 aging treatment exhibit a decrease trend as an increase of the solution temperature from465 °C to 480 °C. After the solution treated by the rate of 470 °C/1 h, second phases dissolve into the matrix very well and the strength property reaches optimum. The alloy has better comprehensive properties treated by a solution treatment of 470 °C/1 h and then followed by an aging treatment of 120 °C/24 h + 170 °C/1 h + 120 °C/24 h. Under the aging state, the precipitated phases inside the grains are suitable in size, while on the grain boundary distribute discontinuously and the precipitate-free zone is obvious. Besides, the alloy still maintain high tensile properties. The yield strength, tensile strength and elongation are 650 MPa, 686 MPa,12.0%, respectively. The yield strength retention after heat exposure is 92%. The alloy has good corrosion resistance and the exfoliation corrosion degree. The average corrosion rate in the H_2S and CO_2 environment is 0.0024 mm/a, which is far less than the required 0.12 mm/a. It is insensitive to H_2S and CO_2 environments.     

Grain boundary characteristics and tensile properties of Ti14 alloy after semi-solid deformation

The microstructure and room-temperature tensile properties of Ti14, a new α+Ti₂Cu alloy, were investigated after conventional forging at 950℃ and semi-solid forging at 1000 and 1050℃, respectively. Results show that coarse grains and grain boundaries are obtained in the semi-solid alloys. The coarse grain boundaries are attributed to Ti₂Cu phase precipitations occurred on the grain boundaries during the solidification. It is found that more Ti₂Cu phase precipitates on the grain boundaries at a higher semi-solid forging temperature, which forms precipitated zones and coarsens the grain boundaries. Tensile tests exhibit high strength and low ductility for the semi-solid forged alloys, especially after forging at 1000℃. Fracture analysis reveals the evidence of ductile failure mechanisms for the conventional forged alloy and cleavage fracture mechanisms for the alloy after semi-solid forging at 1050℃.     

Microstructure and properties of in-situ synthesized Cu-1 wt%TiC alloy followed by ECAP and post-annealing

A dispersion-strengthened copper alloy with 1 wt% TiC for commercial electrical-contact wires was prepared by in-situ reaction casting, grain-ultrafining by equal-channel angular pressing (ECAP) and subsequent annealing with aim to obtain excellent comprehensive performance. The results showed that fine TiC particles were in-situ synthesized in the as-cast Cu matrix and aggregated in clusters, and thus mechanical properties of the as-cast alloy deemed insufficient. Continued ECAP at 473 K significantly refined the grains of the as-cast alloy and improved the distribution of TiC particles. Due to multiple strengthening mechanisms, the ECAP-processed alloys maintained good conductivity with obviously enhanced tensile strength and hardness values. After post-ECAP annealing, the elongation and conductivity of the fine-grained copper alloy increased with the adequate tensile strength. The novel combined process endows the alloy appropriate performance to serve current high-frequency electrification railway systems.