Study on fabrication and mechanism in of porous metals by spark plasma sintering

Porous stainless steel with regular spherical particles were fabricated by spark plasma sintering (SPS) and compared with those obtained by conventional hot pressing (HP). The effects of the parameters during sintering processes on properties, such as porosity and compressive strength, were investigated. Microstructure studies showed that the regular spherical stainless steel particle could form porous ingots with regular pores in the way of close sphere package. Results indicated that the compressive properties of the SPS samples were superior to those of the HP samples. The study on relative diffusion coefficient in SPS and HP specimens indicated that SPS enhanced neck growth and accelerated the atom diffusion. Porous metals could be superfastly prepared by SPS under the conditions of higher heating rate, shorter holding time and lower sintering temperature.     

Effect of fluorine content on mechanical properties of sintered fluoridated hydroxyapatite

The effect of fluorine content, preparing method, and sintering temperature on both the bulk density and biaxial flexural strength of sintered fluoridated hydroxyapatite (FHA) was studied. Both uniaxially pressed un-milled (UPU) and cold isostatically pressed milled (IPM) FHA discs were sintered at temperatures between 1200∼1400 °C at an interval of 100 °C. It was found that the fluorine content had significant impact on the sintering behavior, densification, and mechanical properties of FHA discs. At a low fluorine content, the specimens decomposed between 1200–1400 °C, which resulted in a relatively low sintered density and biaxial flexural strength of the specimens. In comparison, the specimens with high fluorine content did not decompose even at the sintering temperature of 1400 °C. The sintered density and biaxial flexural strength of these specimens increased with the sintering temperature. The maximum sintered density and biaxial flexural strength of the UPU specimens were only 85% of the theoretical density and 45 MPa, respectively. In contrast, much higher sintered density (∼95% of the theoretical density) and mechanical strength (∼100 MPa) were attained for the specimens subjected to the cold isostatic pressing and ball milling. A close correlation between the sintered density and biaxial flexural strength of the specimens was revealed, where the biaxial flexural strength increased exponentially with the sintered density.     

Hot consolidation and mechanical properties of nanocrystalline equiatomic AlFeTiCrZnCu high entropy alloy after mechanical alloying

The present study is aimed to investigate the consolidation behaviour and mechanical properties of nanocrystalline equiatomic AlFeTiCrZnCu high entropy alloy after mechanical alloying. The consolidation was achieved by cold pressing with conventional sintering, vacuum hot pressing and hot isostatic pressing techniques. The microstructure and mechanical properties were evaluated. The hardness and compressive strength of nanocrystalline equiatomic AlFeTiCrZnCu high entropy alloy after vacuum hot pressing are 9.50 and 2.19 GPa and those after hot isostatic pressing are 10.04 and 2.83 GPa, respectively. The wear resistance is found to be higher than the commercially used materials such as Ni-hard faced alloy.     

Damage assessment in cellulose–cement composites using dynamic mechanical characteristics

This paper reports on an experimental investigation of test methods that can detect damage in cement composites, incorporating cellulose macro-nodule fibers, subjected to freezing/thawing and immersion in hot water. Conventional methods of ascertaining damage, such as porosity and flexural strength testing, were carried out, along with dynamic mechanical testing for determination of elastic modulus and damping capacity. Increase in porosity and reduction in flexural strength (as compared to normally cured specimens) were observed for low fiber volume composites subjected to freezing and thawing, whereas no significant changes were observed for specimens with higher fiber volumes. Determination of dynamic elastic modulus and specific damping capacity at regular intervals of exposure showed that the high volume of porous macro-nodules in the mixture prevented damage due to freezing and thawing by acting as stress release sites. No appreciable change in porosity and flexural strength was observed for specimens continuously exposed to hot water; however, reduction in relative modulus and increase in damping capacity was observed. The changes in these system properties suggest that a certain degree of damage might be occurring under sustained hot water exposure, which the determination of porosity and flexural strength could not capture. Using a stiffness-loss map, it is shown in this paper that the damping characteristic is more sensitive than the stiffness in detecting damage as a result of continuous exposure to hot and wet conditions.     

碳化硅多孔陶瓷气孔率和强度影响因素

研究了陶瓷粘结剂含量、碳化硅颗粒粒径以及烧结温度对高温气体过滤用碳化硅多孔陶瓷抗弯强度和气孔率的影响. 利用X射线衍射测试了多孔陶瓷烧结后的物相组成. 陶瓷粘结剂含量的增加使碳化硅多孔陶瓷的气孔率快速下降, 在陶瓷粘结剂含量15wt%时, 碳化硅多孔陶瓷可具有较高的气孔率(37.5%)和抗弯强度(27.63MPa). 随着碳化硅颗粒粒径从300?m减少到87um, 碳化硅多孔陶瓷的气孔率和抗弯强度可同时提高, 气孔率从35.5%增加到了42.4%, 而抗弯强度从19.92MPa增加到了25.18MPa. 碳化硅多孔陶瓷的烧结温度从1300℃增加到1400℃过程中, 其气孔率从38.7%迅速下降到35.4%, 而其抗弯强度一直在27MPa左右, 没有大幅变化, 所以该多孔陶瓷的烧结温度应该选在陶瓷粘结剂熔点(1300℃)附近, 不宜过高.     

高强隔热刚玉-镁铝尖晶石-莫来石多孔陶瓷材料的制备

以氧化铝、石英粉和电熔镁砂为主要原料,以纸浆废液为结合剂,通过原位反应烧结制备复相高强隔热陶瓷,研究MgO添加量对所制备多孔陶瓷的显气孔率、抗折强度、耐压强度和抗热震性能的影响。采用X射线衍射(XRD)、扫描电子显微镜(SEM)和电子万能试验机对材料的物相组成、显微结构和力学性能进行表征,并对多孔陶瓷的显气孔率和抗热震性能进行测试。结果表明:5%(质量分数)电熔镁砂与氧化铝、石英粉在1450℃下原位反应烧结3h可制备得到刚玉-镁铝尖晶石-莫来石多孔复相陶瓷,耐压强度达270.25MPa,抗折强度超过45MPa,同时显气孔率达26.46%,常温导热系数为1.469W·m^-1·K^-1,隔热性能良好,且3次热震后的残余抗折强度保持率超过27%,是极具应用前景的工业窑炉内衬材料。其中MgO含量变化会直接影响该多孔陶瓷三相组成、相形态、气孔孔径及分布,使得多孔陶瓷抗折强度、耐压强度和抗热震性能呈现非单调变化的规律。     

溶胶–凝胶法制备不同形态的半透明羟基磷灰石陶瓷

为探索制备不同形态半透明羟基磷灰石(T-HA)陶瓷的方法, 采用微米级HA粉体为原料, 甲壳素为粘结剂, 用溶胶–凝胶法制备出球状和纤维状的陶瓷初坯, 然后进行常压烧结得到纯HA陶瓷, 最后经过热等静压烧结得到T-HA陶瓷。溶胶–凝胶法赋形简单, 制备出的球状T-HA陶瓷的球形度良好, 纤维状T-HA陶瓷的纵横比高, 其致密度为99.1%, 平均晶粒尺寸为2.2 μm。其中球状半透明HA陶瓷的抗压强度为10.2 MPa, 高于常规烧结得到的球状致密和多孔HA陶瓷(分别为8.9和4.7 MPa)。仿生矿化和细胞培养的结果显示半透明HA陶瓷具有良好的生物相容性。     

Neue Kapseltechnik für Diamantverbundwerkstoffe mittels PVD‐Verfahren

Novel encapsulation technique for diamond composites using PVD‐process For machining of mineral materials diamond tools consisting of a steel body combined with diamond impregnated segments are used. Frequently, these segments are hot pressed. Other process routes are pressureless sintering of green compacts partly combined with hot isostatic pressing and hot isostatic pressing of encapsulated powder mixtures. The compaction effect of hot isostatic pressing require a low porosity of sintered components realized by using ultra‐fine metal powder or an impermeable capsule made of metal or glass. The Institute of Materials Engineering pursues a novel process route by physical vapor deposition of a coating on pressureless sintered composites. The thin coating acts as a capsule and guarantees the pressure transfer in the following hot isostatic pressing process. Although bronze powders with particle sizes up to 90 μm are used, the manufacturing of diamond composites with low porosities is possible. In comparison to conventional encapsulation‐techniques the main advantages of this novel process route are the use of comparatively coarse metal powders and a larger geometric flexibility.     

Effect of processing conditions on the characteristics of pores in hot isostatically pressed alumina

Effect of processing conditions on the characteristics of residual pores was studied with an optical microscope in hot isostatically pressed translucent alumina ceramics. Green bodies formed by isostatic pressing were sintered at 1300, 1400 and 1600°C and then hot isostatically pressed at a temperature 50°C below the respective sintering temperature for 1 h at 100 MPa. All specimens were fully dense within experimental accuracy (±0.1%), and the grain size increased with increasing sintering/hot isostatic pressing temperatures. A variety of pores were found in all specimens. The distribution of pores was uniform at various locations within the specimen. The pore population decreased with increasing pore size, but was finite in the size range exceeding 84 m. The pores in this range increased with increasing sintering/hot isostatic pressing temperature. Except for these large pores, the pore population was similar under all processing conditions.     

多孔铝基陶瓷型芯的制备及其性能

为了解决铝基陶瓷型芯难于烧结和不易脱芯的难题, 在样品中加入了一定量的SiO₂. 利用干压法制备了多孔氧化铝基陶瓷型芯样品, 研究了不同SiO₂的添加量、烧结温度和保温时间对样品性能的影响. 研究结果表明: 经1500℃烧结保温2 h和1600℃烧结降温2 h的样品, 其线收缩率变化不大. 经1700℃烧结保温2 h的样品, 由于SiO₂的挥发导致样品的气孔率升高, 抗弯强度明显降低. 经1500℃烧结保温2 h的样品综合性能最好, 当SiO₂添加量为10wt%时, 样品的线收缩率为1.1%, 抗弯强度为63 MPa, 气孔率为35.5%, 体积密度为2.29 g/cm³; 当保温时间≥4 h时, 其线收缩率、抗弯强度、气孔率和体积密度变化不大, 有望成为高温型芯的候选材料.     

氮气压力对多孔氮化硅陶瓷显微组织和力学性能的影响

以α-Si₃N₄为原料, Y₂O₃为烧结助剂, 在三种不同的氮气压力(0.12、0.32和0.52 MPa)下烧结制备了多孔氮化硅陶瓷。研究了氮气压力对氮化硅的烧结行为、显微组织和力学性能的影响, 分别通过SEM观察显微组织并统计晶粒的长径比, 通过XRD对物相进行分析, 并对烧结试样进行三点弯曲强度测试。随着氮气压力的提高, 多孔陶瓷的线收缩率降低、气孔率提高, 这是由于低熔点的液相中N含量随氮气压力的提升而增加, 导致了液相粘度提高, 抑制陶瓷致密化。随着氮气压力的提高, 组织中的棒状β-Si₃N₄生长良好, 晶粒长径比增大, 其原因是高的液相粘度抑制了β-Si₃N₄形核, 有利于β-Si₃N₄生长。由于β-Si₃N₄棒状晶的作用, 陶瓷弯曲强度随氮气压力的升高得到改善, 但是气孔率的升高降低陶瓷的强度。在0.52 MPa的氮气压力下烧结的多孔陶瓷气孔率达58%, 弯曲强度为140 MPa。     

Effect of hot isostatic pressing on mechanical properties and microstructure of 70–30 cupronickel castings

Abstract

The present work is part of an investigation into the use of hot isostatic pressing to recover 70–30 cupronickel castings. These alloys have particularly good corrosion resistance and, when strengthened with silicon and chromium, produce a material capable of use in very severe conditions of stress and massive corrosion. However, it is not possible to recover such castings by the application of repair welding, because of the possibility of reduced corrosion resistance in the vicinity of the weld. Hot isostatic pressing represents an alternative method of casting recovery. The results reported in the present work refer to the effect of hot isostatic pressing on mechanical properties, microstructure, and the level of segregation in the alloys. Hot isostatic pressing may be used to remove casting defects in the form of fine pores up to total porosity of 5%. However, in cases where porosity takes the form of very large defects, the mechanical properties of the recovered region are inferior to those of the originally sound material. This effect is probably associated with the presence of very finely distributed oxide particles in the originally defective parts of the casting. The optimum hot isostatic pressing temperature for the best overall combination of properties was 950°C.

MST/1732     

热等静压对ZL111—T6铝合金叶轮组织和性能的影响

通过对比试验研究了热等静压处理对ZL111-T6铝合金增压器叶轮显微组织、拉伸性能及疲劳性能的影响。结果表明：热等静压处理可有效消除ZL111-T6铝合金叶轮内部的铸造疏松和缩孔缺陷;热等静压使ZL111-T6铝合金的抗拉强度、屈服强度和伸长率都略有提高,同时使各项性能数据的离散度大幅度减小,提高了铸件产品性能的稳定性;相比未经热等静压处理的ZL111-T6铝合金叶轮,热等静压处理使其疲劳寿命提高了一倍以上。     

微量SiO2添加对Pr:Lu3Al5O12陶瓷光学及闪烁性能的影响

在制备透明陶瓷时, 广泛采用烧结助剂来提升陶瓷的光学质量。但烧结助剂的添加可能会恶化陶瓷的发光性能。本研究采用真空预烧结合热等静压烧结制备了0.25at%Pr:LuAG闪烁陶瓷, 研究了微量SiO₂烧结助剂对陶瓷光学及闪烁性能的影响。结果表明, 添加少于200 ppm的微量SiO₂(1 ppm表示添加量为1×10^-6 g/g)能有效促进热等静压过程中气孔的排出, 有效提升了Pr:LuAG陶瓷的光学性能。150 ppm SiO₂添加的Pr:LuAG陶瓷在400 nm处的直线透过率约为77%。同时研究了预烧温度及时间对Pr:LuAG陶瓷光学性能的影响。在实现完全闭气孔结构时, 进一步升高预烧温度或延长保温时间会降低热等静压过程中的致密化速率, 不利于气孔的排出, 从而降低了Pr:LuAG陶瓷的光学质量。此外, 添加微量SiO₂对Pr:LuAG陶瓷闪烁性能的影响较小。添加微量SiO₂结合热等静压烧结是制备Pr掺杂石榴石闪烁陶瓷的有效途径。     

热等静压烧结Nd-Fe-B永磁体

热等静压烧结通常用来促进合金致密化和细化组织。本文尝试对Nd32．54Dy0．5Fe65．36Al0．6B1.0预烧结体分别进行热等静压烧结和常规烧结，旨在研究热等静压烧结能否促进Nd-Fe-B合金的致密化和晶粒细化。首次发现；高温热等静压烧结时，预烧结样品内的部分富Nd液相外流，在样品内形成空洞，被高压Ar气填充，造成样品不能收缩，磁体的密度比常规烧结磁体的低，力学性能明显恶化；且液相外流引起晶粒转动，导致取向度降低，最终磁体的剩磁和磁能积降低。     

Hot Isostatic Pressing of MgAl2O4 Spinel Infrared Windows

Optically transparent MgAl₂O₂ spinels have been produced by hot isostatic pressing (HIP). The spinel samples were prepared by hot pressing and subjected to capsule-free HIPing. The hot-pressed samples were made at 1500°C with a pressure from 14 to 41 MPa for 2 to 4 hours. In the HIP process, the pressure ranged from 14 to 207 MPa at 1500°C. The effect of hot isostatic pressure on the bulk density, microstructure and optical properties of the spinel sample were investigated here. The bulk density of the sample increased with HIP pressure and the sample HIPed at 207 MPa resulted in a bulk density of 3·576 g/cm³, about 99·94% of the theoretical density. A bimodal grain size distribution exists in samples HIPed at pressures ≤ 138 MPa. The extent of the abnormal grain growth decreased with pressure. The transparent spinel with uniform and fine grain size of 2 μn was obtained at 207 MPa. The transmission at short wavelength increased significantly with HIPing pressure. The transmittance of the sample HIPed at 207 MPa at a wavelength of 0·7 μn was 72%.     

Effects of amount, location, and character of porosity on stiffness and strength of ceramic fiber composites via different processing

The porosity dependence of ceramic fiber composite Youngs modulus, and especially tensile strengths, is reviewed. Though limited, data shows markedly different porosity dependencies for composite matrices derived from: (1) chemical vapor infiltration (CVI), (2) preceramic polymer pyrolysis, or (3) hot pressing of powders. CVI results in initially limited, then accelerating, rates of property decreases as porosity increases, as for typical monolithic ceramics. In contrast, hot pressing and polymer pyrolysis result in the opposite behavior, i.e., high initial then diminishing rates of property decreases. This markedly differing behavior is attributed to pores being rounded and especially away from the fiber-matrix interfaces in CVI while in hot pressing and polymer pyrolysis fiber-matrix interface, cusp/lenticularshaped pores (more difficult to remove and dominant at lower porosity levels) are more detrimental to properties, similar to grain boundary pores in monolithic ceramics. Competition between such interfacial pores and those totally in the matrix in both their elimination and the effects of those pores remaining in the processed composite is consistent with data differences and scatter. Implications for properties achievable by the above 3 types of processing, as well as for sintering of composites and possible use of porous layers at the fiber-matrix interface to limit oxidative embrittlement are noted.     

A statistical approach to strength evaluation incorporating porosity effect for porous ceramics

ABSTRACT It has not been clear whether the conventional effective volume proposed for dense brittle materials can be applied satisfactorily to the strength evaluation of porous ceramics. In the present study, a modified effective volume was proposed by incorporating the porosity effect in the statistical evaluation of strength properties of porous ceramics. The modified effective volume was derived as the conventional effective volume multiplied by a function of porosity p. In this work, a power function of (1 + p)^a was adopted as the simplest porosity function. To clarify the applicability of the modified effective volume, bending tests were conducted using smooth and notched specimens of 3 wt% MgO partially stabilised zirconia with six different porosities. The porosity dependence appeared in the relation between the conventional effective volume and the mean strength of various zirconia ceramics with different porosities. The exponent a of the porosity function was determined from experimental data obtained by using identically shaped specimens with distinct porosities, and the modified effective volume was calculated for several types of specimens used in the experiments. It was revealed that the mean strength was almost uniquely correlated with the modified effective volume independent of porosity. The experimental correlation verified the applicability of the modified effective volume to strength evaluation of porous ceramics.     

Hot isostatic pressing of beta-alumina

The effect of hot isostatic pressing on the properties of sintered beta-alumina has been investigated. The mechanical strength is increased as compared with as-sintered material. The ionic resistivity is decreased by hot isostatic pressing but this has been shown to be related to the heat treatment rather than the application of pressure. The changes in mechanical and electrical properties are related to changes in ceramic microstructure.     

喷射成形多孔材料的致密化工艺综述

介绍了6种常用的喷射成形多孔材料致密化工艺,即热挤压、轧制、锻造、热等静压、楔形压制和陶粒压制工艺,重点介绍了各种工艺的原理、特点、优缺点以及针对某些缺点而采取的一些改进措施.发现热挤压、轧制、锻造、热等静压和陶粒压制工艺能单次使喷射成形多孔材料整体成形致密化,而楔形压制是一种通过局部小变形累积以实现整体成形致密化的工艺;楔形压制在致密化喷射成形大尺寸材料方面具有独到的优势.最后,探讨和展望了今后的发展方向.