Direct chill casting of magnesium alloy under pulsed magnetic field

To refine the grain size and improve the hot workability, the billets of AZ80 magnesium alloy were cast by a new process of the pulsed magnetic field-direct chill casting. The effect of pulsed magnetic field on the grain size, segregation and mechanical properties of cast billet were investigated by experiments. The results indicate that the grain size of cast billet with pulsed magnetic field was greatly refined, and the homogeneity of microstructure was improved significantly. Meanwhile, the macrosegregation and microsegregation of main alloying elements in the cast billet with pulsed magnetic field were suppressed. Compared with the conventional cast billet, the yield strength, ultimate tensile strength and elongation of the cast billet with pulsed magnetic field were increased obviously.     

A multi-sphere based modelling method for maize grain assemblies

In this paper, the geometric shape, dimension parameters and density of maize grains were studied to establish a DEM analysis model of maize assemblies. According to the results, the shape of maize grains of the same variety can be divided into wide wedge, narrow wedge, ellipsoidal-conical and irregular shapes. Amongst these shapes, the quantity of wide wedge and narrow wedge shapes accounts for more than 90% of the total. When modelling maize assemblies, wedge shaped maize can be chosen to establish a geometric model with the upper base defined as the main dimension and other dimensions calculated based on the relationship with the main dimension. The geometric model of single maize grains was established using a Multi-Sphere (MS) model. This analysis model of maize grain is acceptable for scenarios with 14–16 sub-spheres. On the basis of this work, 2 varieties of maize grains are used as examples. The feasibility and validity of the modelling method of maize assemblies and single maize grains are preliminarily verified through comparison of experimental and simulated data to determine repose angles and bulk density.     

Role of tool pin profiles on wear characteristics of friction stir processed magnesium alloy ZK60/silicon carbide surface composites

Poor friction and wear resistance are the major drawbacks that restrict structural applications of ZK60 magnesium alloys. The surface properties of magnesium alloy can be enhanced by reinforcing particles in the surface using friction stir processing (FSP). Tool pin profile is the significant process parameters which influences the material flow, particle breakups and its distribution in the processed zone. In this study, an attempt was made in order to understand the major effects of tool pin profiles namely, cylindrical thread (CT), plain cylindrical (PC), plain tapered cylindrical (PTC) and square (SA) pin profiles on the microstructure characteristics and particle distribution of friction stir processed/silicon carbide particle surface composites. The surface composites fabricated by plain tapered cylindrical pin profile yield superior properties which is attributed to the higher shear force and balanced state of material flow and heat generation in the processed zone. The formation of smaller grains and hardness enhancement due to dispersion strengthening are the main causes to get better wear resistance of friction stir processed/silicon carbide particle magnesium alloy.     

稀土Y和Sm对AZ91D镁合金组织与性能的影响

采用控制变量法研究单一稀土Y和复合稀土Y,Sm元素对AZ91D镁合金微观组织与力学性能的影响,分析稀土元素对AZ91D合金的细化机理。结果表明：复合添加稀土Y和Sm对AZ91D合金的作用效果明显好于单一添加稀土Y对AZ91D合金的作用效果,添加Y和Sm后,生成了块状相Al₂Y相和针状相Al₂Sm相,可以作为α-Mg的有效异质形核点。当加入量为0.8%（质量分数,下同）Y+1.0% Sm时,α-Mg晶粒尺寸最为细小,分布最为均匀,其合金的硬度、抗拉强度及伸长率分别为67.42HV,153.37 MPa和3.62%,改善了铸态AZ91D合金的室温力学性能,但是超过这个最佳添加量后,合金的室温力学性能开始下降。     

Effect of Hf additions on phase transformation,microstructural stability,and hardness of nanocrystalline 304L stainless steels synthesized by mechanical alloying

304L stainless steels with Hf additions were nanostructured by mechanical alloying (MA) and annealed at temperatures up to 1100 °C. The results showed that face-centered cubic (fcc) phase in 304L transformed to body-centered cubic (bcc) phase during MA. The in-situ studies revealed that bcc-to-fcc phase transformation completed after 105 min annealing at 900 °C for 304L, whereas Hf addition increased the required time and temperature for the complete transformation. The grain size of 304L stainless steel was ~10 nm after MA and remained ~167 and ~293 nm after annealing at 900 and 1100 °C, respectively, with Hf addition in comparison to 960 nm average grain size of base 304L stainless steel after annealing at 900 °C. The hardness of 304L increased from ~200 HV to 408 HV after MA and remained 329 HV after annealing at 1100 °C with Hf addition as opposed to 195 HV hardness of 304L.     

热等静压烧结制备细晶粒Ce,Y:SrHfO3闪烁陶瓷

Ce:SrHfO₃陶瓷因具有高密度和高有效原子序数, 对高能射线具有很强的阻止能力。同时, Ce:SrHfO₃陶瓷还具有快衰减和高能量分辨率等优异的闪烁性能, 引起了研究人员的广泛关注。由于传统的烧结方法难以实现非立方结构Ce:SrHfO₃陶瓷的透明化, 本研究采用真空长时烧结和短时真空预烧结合热等静压烧结(Hot Isostatic Pressing, HIP)方法制备Ce,Y:SrHfO₃陶瓷。以金属氧化物和碳酸盐为原料, 1200 ℃下煅烧8 h可以获得平均粒径为152 nm的纯相Ce,Y:SrHfO₃粉体。1800 ℃真空烧结20 h获得平均晶粒尺寸为28.6 μm的不透明的Ce,Y:SrHfO₃陶瓷, 而两步烧结法可以制备光学透过率良好的Ce,Y:SrHfO₃陶瓷。本研究详细分析了陶瓷致密化过程中微结构的演变, 探究了预烧结温度对Ce,Y:SrHfO₃陶瓷密度、显微结构和光学透过率的影响。真空预烧(1500 ℃×2 h)结合HIP后处理(1800 ℃×3 h, 200 MPa Ar)所获得的Ce,Y:SrHfO₃陶瓷在800 nm处的最高直线透过率为21.6%, 平均晶粒尺寸仅为3.4 μm。在X射线激发下, Ce,Y:SrHfO₃陶瓷在400 nm处产生Ce³⁺ 5d-4f发射峰, 其XEL积分强度比商用锗酸铋(BGO)晶体高3.3倍, Ce,Y:SrHfO₃陶瓷在1 μs门宽下的光产额约为3700 ph/MeV。良好的光学和闪烁性能可以拓宽Ce,Y:SrHfO₃陶瓷在闪烁探测领域的应用。     

稀土元素Gd对Al-Si-Mg铸造合金微观组织和力学性能的影响

为了系统地研究稀土Gd对铸造Al-Si-Mg(A357)合金组织和性能的影响,采用OM,SEM,EPMA,XRD,DSC,TEM及拉伸实验等方法对不同Gd含量A357合金进行研究。结果表明:Gd的添加可以细化A357合金的晶粒并减小二次枝晶间距。此外,Gd可以有效地细化合金中的共晶硅,但是对片状共晶硅的形貌影响不大。晶粒和共晶硅的细化及二次枝晶间距的减小使添加Gd后的A357合金的力学性能有了显著的提高。其中,A357-0.5Gd(质量分数/%)合金热处理态抗拉强度为355MPa,相对于未添加Gd元素的A357合金提高了37MPa。当Gd质量分数为1.0%时,尽管组织得到进一步细化,但是大量粗大Al 2Si 2Gd第二相的形成导致了合金力学性能的下降。同时对Gd的细化机制进行探究,结合TEM分析结果可以推断,Gd变质处理后共晶硅上的孪晶密度并不足以引起共晶硅形貌的转变,使得Gd变质效果较弱。而Gd对共晶硅的细化作用可能与Gd增加成分过冷以及形成纳米相阻碍共晶硅生长有关。     

Improvement of mechanical properties of cast pure titanium by repeated heat treatment

ABSTRACT

Pure titanium components fabricated by casting have a coarse grain microstructure. To improve the mechanical strength of pure titanium components by refining the grain size, the cast samples were repeatedly heat-treated. During the heat treatment, the titanium samples were repeatedly heated above the alpha-to-beta (α–β) transition temperature and cooled to room temperature to undergo phase transformation. The heating cycle was performed 1, 3, 5, and 7 times. As the number of heating cycles increased, the grain size decreased. The tensile strength was 267.9?MPa in the as-cast sample and improved to 343.4?MPa after 7 heat-treatment cycles owing to the grain size refinement, while the elongation was maintained during the heat treatment.

This paper is part of a thematic issue on Titanium.     

序列折半划分问题的形式化推导

形式化推导是在程序正确性证明理论下所进行的程序开发,最终得到完全正确的算法程序。针对序列折半划分问题,现有的形式化推导方法将推导与证明交替进行,推导过程繁琐且大多无法直接获得可执行程序。为解决上述问题,提出了一种新的序列折半划分问题的形式化推导方法。该方法基于分划递推的核心思想,应用规约变换技术对问题规约进行变换并严格保证一致性,使得在推导过程中无需交替证明,进而导出递推关系式并得到高可靠性抽象算法程序Apla,最终通过转换工具自动生成可执行程序。实现了从程序规约到具体可执行程序的完整程序求精过程。以2个序列算法为例,验证了该方法的有效性和可行性,对相关问题的形式化推导具有指导意义。     

A New Class of Guided C2 Subdivision Surfaces Combining Good Shape with Nested Refinement

Converting quadrilateral meshes to smooth manifolds, guided subdivision offers a way to combine the good highlight line distribution of recent G‐spline constructions with the refinability of subdivision surfaces. This avoids the complex refinement of G‐spline constructions and the poor shape of standard subdivision. Guided subdivision can then be used both to generate the surface and hierarchically compute functions on the surface. Specifically, we present a C² subdivision algorithm of polynomial degree bi‐6 and a curvature bounded algorithm of degree bi‐5. We prove that the common eigenstructure of this class of subdivision algorithms is determined by their guide and demonstrate that their eigenspectrum (speed of contraction) can be adjusted without harming the shape. For practical implementation, a finite number of subdivision steps can be completed by a high‐quality cap. Near irregular points this allows leveraging standard polynomial tools both for rendering of the surface and for approximately integrating functions on the surface.