Microstructure and Impression Creep Characteristics of Cast Mg-5Sn-xBi Magnesium Alloys

The microstructure and creep behavior of a cast Mg-5Sn alloy with 1, 2, and 3 wt pct Bi additions were studied by impression tests in the temperature range 423 K to 523 K (150 °C to 250 °C) under punching stresses in the range 125 to 475 MPa for dwell times up to 3600 seconds. The alloy containing 3 wt pct Bi showed the lowest creep rates and, thus, the highest creep resistance among all materials tested. This is attributed to the favorable formation of the more thermally stable Mg₃Bi₂ intermetallic compound, the reduction in the volume fraction of the less stable Mg₂Sn phase, and the dissolution of Bi in the remaining Mg₂Sn particles. These particles strengthen both the matrix and grain boundaries during creep deformation of the investigated system. The creep behavior of the Mg-5Sn alloy can be divided into the low- and high-stress regimes, with the respective average stress exponents of 5.5 and 10.5 and activation energies of 98.3 and 163.5 kJ mol⁻¹. This is in contrast to the creep behavior of the Bi-containing alloys, which can be expressed by a single linear relationship over the whole stress and temperature ranges studied, yielding stress exponents in the range 7 to 8 and activation energies of 101.0 to 107.0 kJ mol⁻¹. Based on the obtained stress exponents and activation energies, it is proposed that the dominant creep mechanism in Mg-5Sn is pipe-diffusion controlled dislocation viscous glide the low-stress regime and dislocation climb creep with back stress in the high-stress regime. For the Mg-5Sn-xBi alloys, however, the controlling creep mechanism is dislocation climb with an additional particle strengthening effect, which is characterized by the higher stress exponent of 7 to 8.     

Mg-6Al-1Zn-Y镁合金组织及力学性能的研究

在Mg-6Al-1Zn合金的基础上添加不同质量分数的Y（分别为0％，0．5％，0．9％，1．4％），制备了4种实验合金，研究了Y的添加对合金组织性能的影响。通过X射线衍射、金相显微镜、扫描电镜、电子探针等手段分析了Mg-6Al-1Zn合金添加Y后组织结构的变化。研究结果表明，添加了不同含量Y的合金中都出现了一种新相Al2Y相。随着Y含量的增加，Al2Y相数量增多而Mg17Al 12相则减少。Y能细化合金铸态及挤压态显微组织，其细化作用在添加了0．9％Y的镁合金中尤为明显。铸态合金室温拉伸试验表明：该合金具有最佳的综合力学性能。当Y含量添加至1．4％时，Al2Y相变得粗大且出现团聚现象而导致了拉伸性能的下降。经过挤压后，合金的力学性能大幅度上升。     

Influence of Extrusion on the Microstructure and Mechanical Behavior of Mg-9Li-3Al-xSr Alloys

Mg-9Li-3Al-xSr (LA93-xSr, x = 0, 1.5, 2.5, and 3.5 wt pct) alloys were cast and extruded at 533 K (260 °C) with an extrusion ratio of 28. The microstructure and mechanical response are reported and discussed paying particular attention to the influence of extrusion and Sr content on phase composition, strength, and ductility. The results of the current study show that LA93-xSr alloys contain both α-Mg (hcp) and β-Li (bcc) matrix phases. Moreover, the addition of Sr refines the grain size in the as-cast alloys and leads to the formation of the intermetallic compound (Al₄Sr). Our results show significant grain refinement during extrusion and almost no influence of Sr content on the grain size of the extruded alloys. The microstructure evolution during extrusion is governed by continuous dynamic recrystallization (CDRX) in the α-Mg phase, whereas discontinuous dynamic recrystallization (DDRX) occurs in the β-Li phase. The mechanical behavior of the extruded LA93-xSr alloy is discussed in terms of grain refinement and dislocation strengthening. The tensile strength of the extruded alloys first increases and then decreases, whereas the elongation decreases monotonically with increasing Sr; in contrast, hardness increases for all Sr compositions studied herein. Specifically, when Sr content is 2.5 wt pct, the extruded Mg-9Li-3Al-2.5Sr (LAJ932) alloy exhibits a favorable combination of strength and ductility with an ultimate tensile strength of 235 MPa, yield strength of 221 MPa, and an elongation of 19.4 pct.     

Microstructural features of rolled Mg-3Al-1Zn

The microstructures of hot- and cold-rolled Mg-3Al-1Zn (AZ31) are examined using scanning electron and optical microscopy. It is shown that the microstructures following multipass hot rolling and annealing are more uniform than those formed by heavy single pass rolling and annealing. The importance of twins in producing intragranular recrystallization is evident, although the most dominant nucleation site is grain boundaries. The cold-rolled structure after a rolling reduction of 15 pct is dominated by the presence of deformation twins. Twin trace analysis suggests that approximately two thirds of the twins are a form of “c-axis compression” twin. A number of “c-axis tension” twins were also observed and additional in-situ scanning electron microscopy experiments were performed to confirm earlier observations that suggest these twins can form after deformation, during unloading. This article is based on a presentation made in the symposium entitled “Phase Transformations and Deformation in Magnesium Alloys,” which occurred during the Spring TMS meeting, March 14–17, 2004, in Charlotte, NC, under the auspices of ASM-MSCTS Phase Transformations Committee.     

织构对先进锆合金蠕变性能各向异性的影响

研究了两种组织形貌相似的先进锆合金M5TM和N36锆合金核燃料包壳管材的单轴拉伸和内压蠕变性能.利用x射线衍射仪分析了它们的织构.试验发现两种先进锆合金包壳材料的蠕变性能表现出明显的各向异性.根据试验条件下的蠕变机理,结合弹性粘塑性自洽模型定性地分析了织构对锆合金管材蠕变各向异性的影响,解释了先进锫合金各向异性随应力指数变化的共性规律.揭示了织构与先进锆合金管材蠕变各向异性的定性关系.由于成分和织构因素的共同作用,在研究的试验条件下,N36合金的初始蠕变应变、稳态蠕变速率低于M5合金.织构是合金蠕变行为产生各向异性的主要原因,对于再结晶状态的先进锆合金包壳管,具有(0002)织构特征时,应力指数越高(即施加的应力水平越高),其蠕变的各向异性值越大.     

The Influence of Induction Sintering on Microstructure and Deformation Behavior of Ti-5Al-5Mo-5V-3Cr Alloy

The influence of the induction sintering process at different temperatures on the behavior of the powder metallurgy Ti-5Al-5Mo-5V-3Cr alloy was investigated. Material for the research was produced by elemental powder blending, followed by the uniaxial cold compacting process. Powder compacts were induction heated and sintered within the temperature range of 1000 °C to 1300 °C. The influences of process parameters on the material behavior during sintering and its properties were studied. The microstructure examination was performed with particular attention to the pore size and distribution as well as the homogenization of the microstructure. The sintering temperature of 1200 °C proved to be critical for the dissolution of most alloying powder particles. Hot compression tests were performed to determine the formability of the obtained material. Significant differences in flow stress behavior between samples sintered at temperatures below and above 1200 °C were observed. The mechanical properties of the material before and after deformation were compared. The evolution of the microstructure of sintered Ti-5Al-5Mo-5V-3Cr alloy after hot deformation was analyzed with an emphasis on its influence on the material properties. Based on the conducted research, it was found that the adequate homogenization of the chemical composition and microstructure was achieved at the temperature of 1250 °C, and a further increase did not reflect in a significant improvement.

     

Al-4.9Zn-1.6Mg-0.4Mn-0.1Zr-xEr合金时效行为研究

采用金相(OM)、透射电镜(TEM)及能谱分析(EDS)、硬度测试的方法研究了单级与双级时效热处理制度对Al-4.9Zn-1.6Mg-0.4Mn-0.1Zr-xEr合金板材硬度和显微组织的影响.结果表明,添加Er元素后合金中形成了纳米级的A13(Zr,Er)相,可以强烈地钉扎位错,细化再结晶晶粒.无论单级时效还是双级时效,Er元素的添加均可以提高合金的硬度,加快合金的时效硬化速率,使合金达到硬度峰值的时间缩短,但Er含量对提高合金硬度的影响不大.同时,晶界处出现的第二相颗粒析出带得到消除,合金中存在明显的沉淀无析出带.此外,Er元素可以促进Zn,Mg元素的固溶,而析出的A13(Zr,Er)颗粒又可以促进MgZn2相的析出.     

Creep Behavior of Nickel-Copper Solid Solution Alloys below 0. 55 Tm

In two recent creep studies of inhomogeneous nickelcopper solid solution alloys,i.e. cast weld metal with solidification-induced composition gradients^[1] and nickelcopper laminate composites with controlled composition gradients across the layers,^[2] the creep rates at an intermediate temperature (500 °C) were shown to decrease with an increase in homogenization. The creep behavior in inhomogeneous alloy systems reflects the composite effects of position-dependent creep properties as controlled by solid solution alloy content. To utilize composite modeling techniques in creep analyses of materials with composition gradients, creep data of homogeneous materials as a function of alloy content are required. Therefore, this study was undertaken to evaluate the creep behavior of nickel-copper solid solution alloys at intermediate temperatures and to provide a base set of data to evaluate the effect of gradients described above.^[1,2] I. D. CHOI, formerly Graduate Research Assistant, Colorado School of Mines.     

High-Temperature Creep and Oxidation Behavior of Mo-Si-B Alloys with High Ti Contents

Multiphase alloys in the Mo-Si-B system are potential high-temperature structural materials due to their good oxidation and creep resistance. Since they suffer from relatively high densities, the current study focuses on the influence of density-reducing Ti additions on creep and oxidation behavior at temperatures above 1273 K (1000 °C). Two alloys with compositions of Mo-12.5Si-8.5B-27.5Ti and Mo-9Si-8B-29Ti (in at. pct) were synthesized by arc melting and then homogenized by annealing in vacuum for 150 hours at 1873 K (1600 °C). Both alloys show similar creep behavior at stresses of 100 to 300 MPa and temperatures of 1473 K and 1573 K (1200 °C and 1300 °C), although they possess different intermetallic volume fractions. They exhibit superior creep resistance and lower density than a state-of-the-art Ni-base superalloy (single-crystalline CMSX-4) as well as other Mo-Si-B alloys. Solid solution strengthening due to Ti was confirmed by Vickers hardness measurements and is believed to be the reason for the significant increase in creep resistance compared to Mo-Si-B alloys without Ti, but with comparable microstructural length scales. The addition of Ti degrades oxidation resistance relative to a Mo-9Si-8B reference alloy due to the formation of a relatively porous duplex layer with titania matrix enabling easy inward diffusion of oxygen.     

Deformation Structure of Unidirectionally Compressed Ultrafine-Grained Mg-3Al-1Zn Alloy

Ultrafine-grained (UFG) Mg-3Al-1Zn (AZ31) alloys with gain sizes ranging from 0.46 to 3.22 μm were prepared by equal channel angular pressing (ECAP) and annealing. The deformation structure of UFG AZ31 alloy resulting from uniaxial compression was studied by optical and electron microscopy. The deformation was noted to proceed with the development of shear bands (SBs), which has not been reported in an UFG hcp metal. Characterization of these SBs was performed, and comparison with the SBs formed in UFG bcc and fcc metals was given. { 10[`1]2} \{ 10\bar{1}2\} tension twins inside SBs were found in all specimens compressed, irrespective of the grain size. Discussion on the limiting grain size of twinning in the UFG AZ31 alloy is also given.     

Refinement of Eutectic Si Phase in Al-5Si Alloys with Yb Additions

A series of Al-5 wt pct Si alloys with Yb additions (up to 6100 ppm) have been investigated using thermal analysis and multiscale microstructure characterization techniques. The addition of Yb was found to cause no modification effect to a fibrous morphology involving Si twinning; however, a refined plate-like eutectic structure was observed. The Al₂Si₂Yb phase was observed with Yb addition level of more than 1000 ppm. Within the eutectic Al and Si phases, the Al₂Si₂Yb phase was also found as a precipitation from the remained liquid. No Yb was detected in the α-Al matrix or plate-like Si particle, even with Yb addition up to 6100 ppm. The absence of Yb inside the eutectic Si particle may partly explain why no significant Si twinning was observed along {111}_Si planes in the eutectic Si particle. In addition, the formation of the thermodynamic stable YbP phases is also proposed to deteriorate the potency of AlP phase in Al alloys. This investigation highlights to distinguish the modification associated with the ever present P in Al alloys. We define modification as a transition from faceted to fibrous morphology, while a reduction of the Si size is termed refinement.     

高性能Mg-6Sn-3Al-1Zn合金的制备工艺研究

通过改善铸态镁合金制备工艺,得到一种组织均匀、性能优异的Mg-6Sn-3Al-1Zn合金,合金抗拉强度、屈服强度、延伸率分别达到219MPa、82MPa、16%,晶粒尺寸为133.35μm。     

High Cycle Fatigue of Cast Mg-3Nd-0.2Zn Magnesium Alloys

This paper investigates the high cycle fatigue properties of a recently developed high-strength cast magnesium alloy [Mg-3Nd-0.2Zn (all compositions in wt pct except when otherwise stated)] with varied Zr contents for grain refinement (NZ30K) and the influence of heat treatment conditions. The NZ30K alloy containing 0.45Zr and heat treated to the peak-aged T6 condition [14 hours at 473 K (200 °C)] shows the highest fatigue strength, about 100 MPa, which is about 25 pct higher than that of commercial AZ91D-T6 alloy. In the absence of casting flaws, the high cycle fatigue properties of the NZ30K alloy strongly depend on its grain size and heat treatment conditions. The dependency of fatigue strength on grain sizes follows the Hall–Petch relationship. The NZ30K alloy also shows a significant response to heat treatments. The fatigue strength increases in a near linear fashion with increasing yield strength of the material through heat treatment.     

Ca对挤压镁合金高温抗氧化性能的影响

采用SEM与XPS,对Mg-11Al-3Zn+xCa高温氧化膜的物相组成和化学成分进行定性和定量分析。结果表明:含Ca镁合金高温氧化时,表面形成了致密的CaO/MgO保护层,改善了Mg-11Al-3Zn疏松的蜂窝状氧化膜结构,消除了氧化膜开裂现象,显著地增强了镁合金高温抗氧化性能。     

A Comparative Study on the Static Recrystallization Behavior of Cold-Rolled Mg-3Al-1Zn Alloy Stimulated by Electropulse Treatment and Conventional Heat Treatment

Cold-rolled AZ31 Mg alloy strips, with a reduction of 33 pct, were subjected to electropulse treatment (EPT) and conventional heat treatment (HT) to evaluate the respective influences of electropulses and temperature on the recrystallization behavior of AZ31. The highest measured temperature during the EPT (543 K) was used in HT. The electron backscattered diffraction results demonstrated that the EPT-stimulated recrystallization was completed within 8 seconds, whereas for HT, recrystallization was still far from completion even after 240 seconds. It was found that both the nucleation and grain growth of these two processes were totally different. In the EPT samples, nucleation tended to occur preferentially near extension twin boundaries and grain boundaries by continuous recrystallization, whereas in the HT samples, nucleation occurred mainly by grain boundaries bulging via discontinuous recrystallization. As grain growth proceeded, the texture intensities of the EPT samples decreased gradually and finally evolved into an obvious transverse-direction-split texture. This is likely attributable to the impact of electropulses on the boundary energy and the contribution of nonbasal dislocations; however, the basal-type textures of the HT samples were notably strengthened, which is associated with a 30 deg〈0001〉 orientation with respect to the deformed texture.