Understanding fracture toughness in gamma TiAl

The ambient-temperature ductility and fracture toughness of TiAl-base intermetallic alloys have been improved in recent years by both alloy additions and microstructural control. Two-phase TiAl alloys have emerged as a new class of lightweight, high-temperature materials with potential importance for aerospace applications. This overview summarizes recent advances in the basic understanding of the fracture processes and toughening mechanisms in TiAl-base alloys and the relationships between microstructures and mechanical properties.     

Aging and fracture of human cortical bone and tooth dentin

Mineralized tissues, such as bone and tooth dentin, serve as structural materials in the human body and, as such, have evolved to resist fracture. In assessing their quantitative fracture resistance or toughness, it is important to distinguish between intrinsic toughening mechanisms, which function ahead of the crack tip, such as plasticity in metals, and extrinsic mechanisms, which function primarily behind the tip, such as crack bridging in ceramics. Bone and dentin derive their resistance to fracture principally from extrinsic toughening mechanisms, which have their origins in the hierarchical microstructure of these mineralized tissues. Experimentally, quantification of these toughening mechanisms requires a crack-growth resistance approach, which can be achieved by measuring the crack-driving force (e.g., the stress intensity) as a function of crack extension (“R-curve approach”). Here this methodology is used to study the effect of aging on the fracture properties of human cortical bone and human dentin in order to discern the microstructural origins of toughness in these materials.     

加工道次对FSP制备高熵合金增强铝基复合材料组织和性能的影响

采用搅拌摩擦加工制备了以AlCoCrFeNi_2.1高熵合金为增强相的6061铝合金复合材料(AlCoCrFeNi_2.1/6061Al),重点研究了加工道次对复合材料组织均匀性、界面结合以及力学性能的影响. 结果表明,随搅拌摩擦加工道次的增加,AlCoCrFeNi_2.1/6061Al复合材料组织均匀性及力学性能均得到明显改善. 复合材料中基体与增强相界面结合良好,界面处扩散层厚度随加工道次增加而增大. 相较于不添加增强相的6道次搅拌摩擦加工铝合金,AlCoCrFeNi_2.1增强相颗粒的引入可进一步细化晶粒并提高抗拉强度,且随着加工道次增加,复合材料抗拉强度及断后伸长率均显著升高. 2,4道次下的断口存在明显的颗粒聚集区,而6道次下断口表面颗粒分布均匀且呈现大量韧窝,为典型的韧性断裂. 该现象主要归因于载荷传递效应、弥散强化和细晶强化3大强化机制.     

Microstructural model of intergranular fracture during tensile tests

A microstructural model of intergranular fracture in textured materials is presented. In this model, the material is represented by a two-dimensional microstructure with non-regular polygonal grains which represents material's texture and grain shape measured in experiments or calculated from Monte Carlo simulations. The grain boundary character, grain boundary energy, and fracture stress are assigned to each grain boundary according the grain boundary character distribution. Intergranular fracture susceptibility is analyzed by defining the probability of finding a continuous path along the grain boundaries which are intrinsically susceptible to fracture. In this analysis the orientations of the grain boundary with respect to the applied or residual tensile stress axis is considered. The probability of intergranular fracture for each grain boundary depends on the intergranular fracture resistance, the interface orientation relative to the stress axis, and a value of the tensile stress acting on the grain boundary. The crack arrest distance and the fracture toughness are calculated in terms of the frequency of low-energy grain boundaries, fracture stress of low-energy grain boundary, angle distribution of grain boundary interfaces, and anisotropy of grain shape. The results indicate that the fracture toughness increases and the crack arrest distance decreases dramatically with increasing the frequency of the low-energy grain boundaries. Lowering the grain boundary energy can improve the fracture toughness and decrease the crack arrest distance. The angle distribution of grain boundary interfaces and the grain shape factor are also very effective in controlling the fracture toughness. High fracture toughness of polycrystalline materials is related to the presence of a high frequency of low-energy boundaries which are resistant to fracture. The best fracture toughness for brittle materials can be achieved by controlling the frequencies of the low-energy grain boundaries, the grain boundary character, and the boundary inclination.     

Fracture toughness of cemented carbides obtained by electrical resistance sintering

The unique combination of hardness, toughness and wear resistance exhibited by WC-Co cemented carbides (hardmetals) has made them a preeminent material choice for extremely demanding applications, such as metal cutting/forming tools or mining bits, in which improved and consistent performance together with high reliability are required. The high fracture toughness values exhibited by hardmetals are mainly due to ductile ligament bridging and crack deflection (intrinsic to carbides). In this work two WC-Co grades obtained by using the electric resistance sintering technique are studied. The relationships between the process parameters (cobalt volume fraction, sintering current and time, die materials, etc.), the microstructural characteristics (porosity, cobalt volume fraction, carbide grain size, binder thickness and carbide contiguity) and mechanical properties (Vickers hardness and fracture toughness) are established and discussed. Also the presence of microstructural anisotropy and residual stresses is studied. The sintering process at 7 kA, 600 ms and 100 MPa, in an alumina die, followed by a treatment of residual stress relief (800 °C, 2 h in high vacuum), allows to obtain WC-Co pellets with the best balance between an homogeneous microstructure and mechanical behaviour.     

Mechanical Behavior of a Magnesium Alloy Nanocomposite Under Conditions of Static Tension and Dynamic Fatigue

In this paper, the intrinsic influence of nano-alumina particulate (Al₂O_3p) reinforcements on microstructure, microhardness, tensile properties, tensile fracture, cyclic stress-controlled fatigue, and final fracture behavior of a magnesium alloy is presented and discussed. The unreinforced magnesium alloy (AZ31) and the reinforced composite counterpart (AZ31/1.5 vol.% Al₂O₃) were manufactured by solidification processing followed by hot extrusion. The elastic modulus, yield strength, and tensile strength of the nanoparticle-reinforced magnesium alloy were noticeably higher than the unreinforced counterpart. The ductility, quantified by elongation-to-failure, of the composite was observably lower than the unreinforced monolithic counterpart (AZ31). The nanoparticle-reinforced composite revealed improved cyclic fatigue resistance over the entire range of maximum stress at both the tested load ratios. Under conditions of fully reversed loading (R = ?1) both materials showed observable degradation in behavior quantified in terms of cyclic fatigue life. The conjoint influence of reinforcement, processing, intrinsic microstructural features and loading condition on final fracture behavior is presented and discussed.     

Review of research progress on aluminium–magnesium dissimilar friction stir welding

The paper critically assesses the research progress towards aluminium–magnesium dissimilar friction stir welding (FSW). First, the theoretical requirements are explored through the understanding of joining mechanism and heat generation in aluminium–magnesium FSW. Next, the observed trends in microstructural characterisation and mechanical properties are analysed. Finally, the effects of welding parameters and how it influences process variables and materials responses are discussed in detail, and several suggestions are made based on these discussions.     

陶瓷基金属间化合物复合材料的研究进展

介绍了近年来国内外研究陶瓷基金属间化合物复合材料的进展情况。总结归纳了Ni3Al、FeAl和Fe3Al金属间化合物增强陶瓷基复合材料的性能特点,这些复合材料在抗酸腐蚀、耐磨性、高温强度等方面较普通WC/Co复合材料有明显的优势。并重点介绍了以液相烧结、无压熔渗、机械合金化和热压等方法制备陶瓷基金属间化合物复合材料的工艺特点。对于陶瓷基金属间化合物复合材料而言,在选择良好基体的基础上,更重要的是选择合适的金属间化合物增强相。     

Spark plasma sintering for multi-scale surface engineering of materials

Recently, significant progress has been made in understanding the effect of multi-scale microstructural features, including nano-, micro-, and macro-features, on the properties of materials. Controlling the length scale of micro-structural features provides tremendous opportunities for enhancing the properties of materials, including extraordinary strength and hardness, unprecedented damage from tribological contacts, and improvements in a number of functional properties of the materials. Spark plasma sintering (SPS) process which combines the effects of uniaxial pressure and pulsed direct current is becoming increasingly important for the processing of bulk shapes of amorphous and nanostructured materials. These materials can also be good candidates for high-performance coatings. This article presents a review of our ongoing efforts to use SPS to produce engineered coatings of amorphous and nanostructured materials for various applications, including structural, tribological, and biomedical applications.     

Phase assemblage effects on the fracture and fatigue characteristics of magnesia-partially stabilized zirconia

A detailed investigation on the relationships between phase assemblage and fracture and fatigue characteristics of Mg-PSZ has been conducted. In doing so, three completely different microstructural conditions were first attained through different thermal treatments and then their flexural strength, fracture toughness and crack growth resistance and fatigue crack growth (FCG) behaviour were evaluated. The obtained results are discussed considering the interplay between microstructural features and dominant crack-microstructure interaction and its influence on the operation of given toughening and mechanical fatigue mechanisms for each phase assemblage studied. FCG resistance, under both sustained and cyclic loading, is found to be closely related to the corresponding fracture toughness of each phase assemblage. However, real mechanical fatigue effects are estimated to be, once they are rationalized with respect to particular environmental-assisted cracking behaviours, an exclusive function of crack path type. Finally, different cyclic fatigue mechanisms for Mg-PSZ are pinpointed depending upon the prevalent transgranular or intergranular FCG morphology.     

Optimizing the corrosion fatigue properties of Co-Cr-Mo Hip joints

Because of their affordability and their adaptability to different designs, cast Co-Cr-Mo alloys are the materials most used for hip joint endoprostheses. These alloys combine excellent biocompatibility with a high corrosion resistance. Most hip joint endoprostheses are manufactured by conventional casting. The microstructural defects caused by this casting method lead to premature fractures. Aseptic loosening of endoprostheses also contributes to fracture. This article shows that using unidirectional solidification prolongs the mean value of fatigue life by at least six times over conventional casting; comparing the lowest values, the fatigue life is more than ten times higher. The comparison is made for two different kinds of solidified tension-compression specimens without any heat treatment to study only the influence of the solidification process. It should also be noted, however, that heat treatment adapted to microstructural parameters can elevate fatigue life.     

Processing maps: A status report

In the last two decades, processing maps have been developed on a wide variety of materials including metals and alloys, metal matrix composites, and aluminides, and applied to optimizing hot workability of materials and for process design in bulk metal working. Processing maps consist of a superimposition of efficiency of power dissipation and the instability maps, the former revealing the “safe” domain for processing and the latter setting the limits for avoiding undesirable microstructures. The dynamic materials model, which forms the basis for processing maps, is discussed in relation to other materials models. The application of dynamical systems principles to understanding of deterministic chaos in the system will help in achieving a greater degree of microstructural control during processing. The patterns in the hot working behavior as revealed by the processing maps of several classes of alloys relevant to technology are reviewed briefly. Processing maps have also been applied to analyze several industrial problems including process optimization, product property control, and defect avoidance, and a few examples are listed. With the processing maps reaching a matured stage as an effective tool for optimizing materials workability, expert systems and artificial neural network models are being developed to aid and prompt novice engineers to design and optimize metal processing without the immediate availability of a domain expert, and the directions of research in this area are outlined.     

Technology for obtaining samples of layered composite materials with metallic matrix

Layered composite materials significantly improved the mechanical process of fracturing, which means better fracture strength, while preserving surface properties such as hardness, resistance to wear and resistance to high temperatures. The properties are significantly influenced by the interphase mass transfer at the surface matrix-fiber reinforcement. We developed a mathematical model to determine the molar flux at the interface in stationary and in a nonstationary regime. The technological parameters are: hydraulic pressure, reinforced material, alloy type, fiber diameter, mass ratio between the reinforcement and the composite masses and mould preheating temperature. A mould patented in Romania was mounted on a hydraulic press to obtain the samples. We studied the material structure, matrix and fiber element distribution, metallic matrix element distribution and matrix and fiber element content variation. The results recorded revealed a 75% to 120% increase of the fracture strength, which means an improvement of the mechanical process of fracturing. We concluded that the reinforcement material, mass ratio and fiber diameter have a significant influences on the fracture strength.     

A Study of the Tensile Deformation and Fracture Behavior of Commercially Pure Titanium and Titanium Alloy: Influence of Orientation and Microstructure

In this paper, the tensile deformation and fracture behavior of commercially pure titanium and the titanium alloy (Ti-6Al-4V) are presented and briefly discussed. Samples of both commercially pure titanium and the Ti-6Al-4V alloy were prepared from the as-provided plate stock along both the longitudinal and transverse orientations. The specimens were then deformed to failure in uniaxial tension. The intrinsic influence of material composition and test specimen orientation on microstructure, tensile properties, and resultant fracture behavior of the two materials is presented. The conjoint influence of intrinsic microstructural features, nature of loading, and specimen orientation on tensile properties of commercially pure titanium and the Ti-6Al-4V alloy is highlighted. The fracture behavior of the two materials is discussed taking into consideration the nature of loading, specimen orientation, and the role and contribution of intrinsic microstructural effects.     

J-R fracture characteristics of ferritic steels for RPVs and RCS piping of nuclear power plants

J-R fracture resistance tests have been performed on 3 heats of SA508-Gr.3 nuclear reactor pressure vessel (RPV) steel as well as 2 heats of SA516-Gr.70 and a heat of SA508-Gr.1a steels for nuclear reactor coolant system (RCS) piping. For the latter two steels, dynamic in addition to static J-R fracture resistances were investigated. From the test results of the SA508-Gr.3 steels, the J-R fracture resistance was superior in the following order: Si-killing steel, modified VCD steel and VCD steel. Microstructural analyses were carried out to correlate J-R fracture resistances with microstructural characteristics. According to the test results for SA508-Gr.1a and SA516-Gr.70 steels, all of the tested steels showed steep drops in fracture resistance at certain temperature and loading rate combinations. One heat of SA516-Gr.70 steel was very sensitive to dynamic strain aging and its fracture resistance was significantly low. It was concluded that microstructural and chemical factors affect the J-R fracture and DSA characteristics of SA516-Gr.70 steels. This article is based on a presentation made in the “Symposium on Nuclear Materials and Fuel 2000”, held at the Korea Atomic Energy Research Institute (KAERI), Taejon, Korea, August 24–25 under the auspices of the Ministry of Science and Technology (MOST).     

Effects of dislocation dynamics and microstructure on crack growth mechanisms in two-phase titanium aluminide alloys

The fracture behaviour of two-phase titanium aluminide alloys was characterized by fracture toughness tests performed in a wide temperature range on chevronnotched three point bending bars. Temperature and rate dependent deformation processes were characterized by temperature and strain rate cycling tests. The alloy investigated had compositions and microstructures which are currently being considered for engineering applications. The paper considers the effects of microstructure and crack tip plasticity on the crack growth resistance. The temperature dependence of the fracture toughness was rationalized in terms of micro-processes which determine the glide resistance of the dislocations in the plastic zone of crack tips. The implications of such observations for the engineering application of the materials are addressed briefly.     

Fractography of critical and subcritical cracks in hard materials

In testing hard and brittle materials like hard metals and ceramics for fracture resistance, stable and unstable crack propagation can be observed. The question arises if critical and subcritical cracks proceed in the same way, and, as a corollary, if results from subcritical fracture can be used to describe catastrophic failure. Results from earlier work are re-evaluated to demonstrate the equivalence or difference of stable and unstable crack growth in hard materials. It is shown and statistically assured by χ² tests for the length distributions of the fracture facets that fracture at room temperature produces identical surface geometry in tungsten carbide–10 wt% cobalt alloys as well as in an alumina–3 wt% SiO₂ ceramic while significantly different fracture surfaces are obtained for the ceramic at high-temperature testing. Some general features of fracture mechanisms are discussed. Assessment of fracture surfaces (measurement of distributions of the lengths and the angles of inclination of profiles along the fracture surface) was carried out using an instrumented stereometer. Recent progress in quantitative analysis of rough surfaces is reviewed which appears to be very useful for a large number of applications in the field of hard materials.     

Main features of designing with brittle materials

Brittle material behavior and mode of failure are contrasted with those characteristics of ductile materials. The stochastic nature of brittle fracture, which results from the random occurrence of fracture-initiating microstructural imperfections, necessitates a probabilistic fracture mechanics approach to design with brittle materials. It is also clearly shown which main properties of brittle materials have to be optimized to improve the reliability of mechanically loaded components made of brittle materials. Important features of designing with brittle materials are elucidated and illustrated by an exemplary design calculation of a ceramic disc spring. It is shown how even environmentally induced subcritical crack growth, characteristic of ceramic materials, can be adequately accounted for in the assessment of reliability.     

Mechanisms governing deformation and damage during elevated-temperature fatigue of an aluminum-magnesium-silicon alloy

The cyclic deformation and fracture characteristics of aluminum alloy 6061 are presented and discussed. The specimens were cyclically deformed using fully reversed tension-compression loading under total strain-amplitude control, over a range of temperatures. The alloy showed evidence of softening to failure at all test temperatures. The degree of softening during fully reversed deformation increased with test temperature. The presence of shearable matrix precipitates results in a microstructure that offers a local decrease in resistance to dislocation movement, causing a progressive loss of strengthening contributions to the matrix. At elevated temperatures, localized oxidation and embrittlement at the grain boundaries are exacerbated by the applied cyclic stress and play an important role in accelerating crack initiation and subsequent crack propagation. The fracture behavior of the alloy is discussed in light of competing influences of intrinsic microstructural effects, deformation characteristics arising from a combination of mechanical and microstructural contributions, cyclic plastic strain amplitude and concomitant response stress, and the test temperature.     

Ordered intermetallic alloys,part I: Nickel and iron aluminides

This article summarizes recent progress in research and development on nickel and iron aluminide intermetallic alloys. Ordered intermetallics possess attractive properties for structural applications at elevated temperatures in hostile environments; however, brittle failure and poor fracture resistance limit their use as engineering materials. In recent years, efforts to understand this brittle fracture behavior have identified both intrinsic and extrinsic factors governing brittle fracture. Parallel work on alloy design using physical metallurgy principles has led to the development of aluminide alloys with improved mechanical and metallurgical properties for structural use.