The fracture toughness of niobium-based,in situ composites

The fracture resistance of Nb-Cr-Ti alloys orin situ composites of three different compositions, Cr₂Nb, and a Nb-10Siin situ composite was studied at ambient temperature. The crack-tip deformation and fracture behaviors were characterized using near-tip measurement techniques and fractographic analyses. The relevant fracture and toughening mechanisms were identified and related to the microstructure. Despite fracture by a combination of cleavage and slip band decohesion, the Nb solid-solution alloy exhibited a resistance-curve behavior with a relatively high toughness and local ductility. The source of toughness was modeled and explained in terms of a cracking process that involved alternate slip band decohesion and cleavage. Thein situ composites, on the other hand, exhibited cleavage fracture but considerably lower toughness with little or no resistance-curve behaviors. The difference in the fracture behavior appears to arise from two factors: (1) the presence of a high constraint in the Nb solid-solution matrix in thein situ composites, and (2) the lack of plastic flow associated with cleavage of the constrained Nb solid-solution matrix.     

Optical and mechanical properties of zinc sulphide diamond composites

Pure ZnS, an infrared transparent but mechanically weak material, was toughened with diamond particles. The optical properties of the pure material were preserved in the composite when the diamond particles were uniformly dispersed. Careful control of the hot pressing parameters was required in order to: (a) limit the phase transformation to a non-cubic phase of ZnS at high temperatures, and (b) maintain a small grain size. Measurement of the reflectivity of this composite in the region of the lattice vibration spectrum was used to compare the measured electrodynamic properties with those predicted by two differential theoretical models that assume different microstructural morphology. Good agreement was found with the Bruggemann model that assumes both components in the composite have the same type of interconnectedness. The mechanical properties of the 10% diamond composite showed a two fold increase in toughness without a change in the hardness with respect to pure ZnS. The bulk modulus and the shear modulus of the composite were measured and were found in agreement with a composite sphere model that assumes a good interfacial bonding between ZnS and diamond. The increase in toughness was explained in terms of the lowering of the local stress intensity factor (relative to the applied value) in the presence of diamond particles.     

Aspects of fracture in particulate reinforced metal matrix composites

The tensile deformation and fracture behaviour of the aluminium alloy 6061 reinforced with SiC has been investigated. In the T4 temper plastic deformation occurs throughout the gauge length and the extent of SiC particle cracking increases with increasing strain. In the T6 temper strain becomes localised and particle cracking is more concentrated close to the fracture. The elastic modulus decreases with increasing particle damage and this allows a damage parameter to be identified. The fraction of SiC particles which fracture is less than 5%, and over most of the strain range the damage controlling the tensile ductility can be recovered, indicating that other factors, in addition to particle cracking are important in influencing tensile ductility. It is suggested that macroscopic fracture is initiated by the SiC particle clusters that are present in these composites as a result of the processing. The matrix within the clusters is subjected to high levels of triaxial stress due to elastic misfit and the constraints exerted on the matrix by the surrounding particles. Final fracture is then produced by crack propagation through the matrix between the clusters.     

MoSi2复合材料断裂韧性的测量及评价

对压痕法和单边切口梁法测量MoSi2基复合材料的断裂韧性进行了研究和分析,提出了压痕法测量MoSi2基复合材料断裂韧性的计算修正式K1C=0.0336(l/α)-1/2(Eφ/H)2/5 Hα1/2/φ.结果表明单边切口梁法的测量结果随试样切口宽度的增加而增大,压痕法测量的结果随计算式和试验载荷的不同而不同.使用上述修正式将缩小压痕法与SENB法的测量偏差.     

Preparation of metallic glass based composites and measurement of the fracture toughness

采用铜模吸铸法制备了（Zr0.55Al0.1Ni0.05Cu0.30）100-xTix（x=0、2、4、6、8）板状哑铃型金属玻璃基复合材料试样。用X射线衍射（XRD）、岛津AG-10TA万能材料力学试验机和JSM-6700F场发射扫描电子显微镜（SEM）对试样的组织结构以及断裂韧性进行了测试。结果表明,当x=0、2、4时,试样为非晶-晶体复合材料,当x=6、8时,试样为晶体材料。表明通过调整Ti的含量可以制备出金属玻璃基复合材料。采用三点弯曲法测定了复合材料的断裂韧性,当x=0、2、4时,试样的断裂韧性KIC值分别为10.529、5.142和3.446MPa.m1/2。     

硫化锌沉淀分离-EDTA滴定法测定废水中硫化物

水样经盐酸酸化,将其中的HS-和S2-全部转化为H2S,加入过量的金属锌,反应产生的大量氢气作为载气与一并逸出的硫化氢通入热的氨性溶液及过量的锌标准吸收液中,以铬黑T为指示剂,用EDTA标准滴定溶液滴定形成ZnS沉淀后的剩余Zn2+,实现了采用吹气吸收-沉淀分离-EDTA滴定法测定实验室废水中硫化物含量的方法。讨论了水样的酸化效果,吸收液选择,气体分离等实验条件对测定的影响,避免了碘量法所产生的干扰。测定结果与标准方法相符合,相对标准偏差为3.02%~3.08%,加标回收率在96.0%~99.5%之间。     

Effects of thickness and precracking on the fracture toughness of particle-reinforced al-alloy composites

The effect of specimen thickness on the fracture toughness of a powder metallurgically processed 7093 Al/SiC/15p composite was evaluated in different microstructural conditions. The fracture toughness in the underaged condition increased significantly with a decrease in specimen thickness, even at thicknesses well below the value specified by ASTM-E 813 for a valid J _Ic test. The influence of thickness was considerably lower in the peak-aged (PA) condition. This relative insensitivity is believed to be due to the low strain to failure associated with severe flow localization in the PA condition. The effect of precracking on the fracture toughness of discontinuously reinforced aluminum (DRA) was also evaluated. The dependence of fracture toughness on specimen thickness and precracking is explained in terms of the hydrostatic stress, which has a strong influence on the plastic straining capability of the DRA material. The fracture toughness was modeled using a critical strain formulation, with the void growth strain dependent on hydrostatic stress through the Rice and Tracey model. The predicted toughnesses for the thick and thin specimens were in good agreement with the experimental data.     

The effects on fracture toughness of ductile-phase composition and morphology in Nb-Cr-Ti and Nb-Siin situ composites

Niobium-chromium alloys, both single and two phase, were alloyed with titanium in order to enhance fracture toughness and fatigue crack growth resistance. The selection of titanium as an alloying element and the relationship of electronic bonding to toughness are examined. The results indicated that toughness increased with a decreasing number of D +s electrons. Titanium was found to increase the toughness of solid-solution Nb-Cr alloys from ≈8 to 87 MPa√m, while for the twophase “insitu composites,” toughness was increased from ≈5 to 20 MPa√m, although this is less than expected. Fracture toughness of the composites correlated nonlinearly with the volume fraction of the phases. The evidence suggests that the toughness of the composites is decreased due to fracture of the intermetallic particles and constraint on matrix deformation imposed by the intermetallic. Fracture characteristics of the Nb-Cr-Ti materials are compared to those of Nb-Cr and Nb-Si materials.     

Effect of hot-rolling reduction on shape of sulfide inclusions and fracture toughness of AISI 4340 ultrahigh strength steel

Commercial AISI 4340 ultrahigh strength steels with hot-rolling reductions of 80 to 98 pct have been studied to determine the effect of the shape of sulfide inclusions on plane-strain fracture toughness(K _IC ) of the ultrahigh strength low alloy steels. The significant conclusions are as follows: decreasing the hot-rolling reduction from 98 to 80 pct for the steels modified the shape of sulfide inclusions from the stringer (average aspect ratio = 17.5) to the ellipse (average aspect ratio = 3.8). This improved theK _IC in the longitudinal testing orientation by about 20 MPa · m^1/2 at similar strength levels. This could be due to the fact that the ellipsed sulfide-inclusions separate from the matrix during plastic deformation, producing large voids. During testing these act to blunt and arrest cracks propagating across the specimen which would normally cause failure. The decrease in the hot-rolling reduction also developed theK _IC in the transverse testing orientation by about 17 MPa · m^1/2 at increased ductility and Charpy impact energy levels. This can be attributed to the fact that lamellate fracture, which occurs in a brittle manner along the interfaces of the sulfide-inclusion/matrix at the crack tip, is considerably suppressed by modifying the shape of the inclusions from the stringer to the ellipse.     

Microstructural analysis of fracture toughness variation in 2XXX-series aluminum alloy composites reinforced with SiC whiskers

The effects of local microstructure on fracture properties in powder-metallurgy (P/M)-processed 2124/SiC/15w and 2009/SiC/15w composites are analyzed in this study. Ductility and fracture toughness of the 2009/SiC/15w, in which dispersoid-forming elements such as manganese and iron were nearly absent, were greater than in the 2124/SiC/15w, while its tensile and yield strengths were somewhat less. Microstructural examination and fracture parameter analysis revealed that the improved fracture toughness of the 2009/SiC/15w compared to the 2124/SiC/15w was due to the increase in the critical microstructural distance,l* when manganese-containing particles are absent. 2009/SiC/15w was also heat-treated in T4P and overaged (OA) conditions. The OA 2009 composite showed lower fracture toughness than the 2009-T4P composite and the critical fracture strain of the OA condition was much lower, too. Detailed fractographic analyses indicated that interface precipitates facilitate premature SiC whisker failure in the OA condition. Formerly Research Assistant, Department of Materials Science and Engineering Pohang University of Science and Technology, Pohang, 790-784 Korea     

Relationships of fracture toughness and dislocation mobility in intermetallics

An analytical method has been developed and used to compute the Peierls-Nabarro (P-N) barrier energy, U _P-N, for relevant slip systems in several intermetallics, including NiAl, FeAl, Nb-Ti-Al (B2), Ni₃Al (L1₂), TiAl (L1₀), TiCr₂, NbCr₂ (C14, C15), Nb₅Si₃ (D8 _l ), Mo₅SiB₂ (D8 _l ), and Mo₅Si₃ (D8 _m ). The P-N barrier energy and a generalized fault energy, γ _F, are combined and used as a measure of dislocation mobility. Furthermore, a fracture model has been developed to describe the process of thermally activated dislocations moving away from the crack tip and to predict the corresponding fracture resistance. A ductility index defined in terms of the ratio of γ _s/(U _P-N+γ _F), where γ _s is the surface energy, is used to correlate with the fracture toughness, K _C, of individual intermetallics. The correlation indicates that fracture toughness increases with increasing values of γ _s/(U _P-N+γ _F), in accordance with the fracture model formulated based on thermally activated slip. The use of the fracture model for predicting the effects of slip behavior, temperature, and alloy additions on fracture resistance is demonstrated for selected intermetallics including NiAl, TiAl, Laves phase, and Nb₅Si₃.     

The effect of silicon and nickel additions on the sulfide spacing and fracture toughness of a 0.4 carbon low alloy steel

This paper discusses the effects of silicon and nickel additions on the mechanical properties of a 0.4 carbon low alloy steel. The four experimental steels used in the study were obtained by making additions of 1.5 wt pct nickel and 2 wt pct silicon, both separately and in combination, to a 0.4C/1.5Ni/1.0Cr/0.5Mn base steel. The base + Ni + Si steel, resulting from combined nickel and silicon additions, has a yield strength of 1682 MN/m² and impact and fracture toughnesses of 65 J and 115 MN/m^3/2, respectively. The other three steels have comparable strength levels but more typical impact and fracture toughnesses of about 30 J and 75 MN/m^3/2, respectively. The microstructures of the four steels are almost identical. The only significant observed difference among the four steels is that the sulfides in the base + Ni + Si steel are almost three times as large as those in the other three steels. As the four steels have similar sulfide volume fractions, there is a similar difference in sulfide spacings. The improved toughness of the base + Ni + Si steel is attributed to the increased sulfide spacing due to the influence of combined nickel and silicon additions on the average sulfide size.     

Leaching of zinc sulfide concentrate from the ganesh-himal deposit of nepal

The leaching of zinc concentrate obtained from the zinc ore located in the Ganesh-Himal region of Nepal has been carried out in the presence of ammonium persulfate (APS) as an oxidant. Leaching variables such as time, temperature, acid, and oxidant concentration, E _h , and particle size of sphalerite concentrate were studied to optimize the condition and to understand the mechanism of the reaction. The zinc extraction increased up to a temperature of 333 K and a further rise in the temperature to 343 K resulted in lowering of metal extraction. A decrease in particle size and increase in oxidant concentration enhanced the dissolution rate of zinc. The leaching data best fitted to the mixed controlled kinetic model. An activation energy of 43 kJ/mol was obtained for the dissolution of zinc in the temperature range 303 to 333 K with APS. The leaching mechanism was further established by characterizing the original concentrate and the leach residue by X-ray diffraction (XRD) phase identification and scanning electron microscopy (SEM) studies.     

Kinetics of zinc oxide formation from zinc sulfide by reaction with lime in the presence of water vapor

A novel reaction scheme for transforming certain metal sulfides to the corresponding oxides has been developed. In this process, steam oxidizes the sulfide into the oxide, and the hydrogen sulfide produced reacts with lime to form calcium sulfide and regenerate steam. There is no net consumption or generation of gaseous species. Thus, the overall reaction can be carried out in a closed system as far as the gas phase is concerned. This eliminates the possibility of emitting hydrogen sulfide out of the reactor. Only certain metal sulfides are thermodynamically amenable to this treatment. In this paper, the reaction of ZnS to ZnO by this scheme is described, together with a detailed formulation of the rate equation for the overall reaction based on the kinetics of the component gas-solid reactions. Although the present work was done with CaO, other suitable oxides may be used in its place. A further potential application of this process is to the selective oxidation of certain sulfide(s) from complex sulfide ores as a treatment prior to the separation of minerals.     

Oxidation of zinc sulfide in a fluidized bed

Kinetics of oxidation of ZnS particles in a batch-type fluidized bed were studied at temperatures between 800 and 910°C. A two-phase model was employed for the fluidized bed, and the partial pressure of oxygen and the gas-film mass transfer coefficient on the particle surface were separately evaluated in gas bubbles and in the emulsion phase. The calculated fractional reaction coincided well with the experimental results. The difference in O₂ partial pressure between gas bubbles and emulsion phase was found to be fairly large especially under the vigorous fluidizing condition. Furthermore, it was shown from the mathematical model that the reaction of ZnS particles in the gas bubbles is negligible because of the extremely low solid concentration and that the overall rate of reaction in the emulsion phase is virtually controlled by the rate of gas-film mass transfer at higher temperature. The resistance of interfacial reaction within the particle also becomes significant when the temperature is lowered. Y. Fukunaka and T. Monta are both former Graduate Students, Kyoto University, Kyoto, Japan.     

Correlation of microstructure and fracture toughness in two 4340 steels

A correlation is made of plane strain fracture toughness and microstructure in two steels corresponding to AISI 4340 composition. The steels were deoxidized with aluminum and titanium-aluminum additions, respectively. In the case of the aluminum killed steel, austenitizing at temperatures above 950 °C led to large austenite grain sizes, whereas in the titanium steel grain sizes were maintained below about 70 μm even after austenitizing at temperatures approaching 1200 °C. This allowed a comparison of variations in plane strain fracture toughness with austenitizing temperature between microstructures that underwent large increases in grain size and those that did not. The results are interpreted using a simple fracture model which indicated that particle spacing is of primary importance in controlling toughness. The overall observed phenomenology, however, is not explainable using simple models that essentially require that either critical stresses or critical strains be achieved over distances scaling with microstructure. This finding suggests that more detailed crack tip models than presently exist are required if the full effects of heat treatment are to be understood and explained. Formerly Graduate Student at Brown University