Recent developments of the high strength and high ductility nanostructured materials

This talk will summarize the recent work related to a kind of new nanomaterials produced by the SMAT (surface mechanical attrition treatment).The concept of surface nanocrystallization of materials will be presented.In terms of the grain refinement mechanism induced by plastic deformation,a novel surface mechanical attrition(SMA) technique was developed for synthesizing a nanostructured surface layer on metallic materials in order to upgrade the overall properties and performance.The grain refinement mechanism of the surface layer during the SMA treatment will be analyzed in terms of the nanostructure observations in several typical materials.Very high yield stress(5 times of the base material) on the surface layer of the material obtained by the SMAT has been observed.The effect of surface nanostructures on the mechanical behavior and on the failure mechanism of metallic material shows the possibility to develop a new strength gradient composite using co-rolling and nitriding.The role of residual stress induced during the treatment will be investigated and discussed.The developed materials are also porosity free materials which can be used as reference material for the local mechanical behavior investigation technique such as the nanoindentation.A general concept for obtaining high strength and high ductility nanostructured materials will be presented.The exceptional high strength and high ductility steels have developed.The simulation of the mechanisms for improving ductility of high strength nanostructured materials will be presented.The potential applications for the land transportation vehicles(car,bus,train) and wind energy have been investigated.Some examples of concept design for the integration of the advanced nanostructured steels will be presented.     

Fatigue crack growth in Ti-matrix composites with spatially varied interfaces

Spatially varied interfaces (SVIs) is a design concept for composite materials where the interface mechanical properties are varied along the length and circumference of the fiber/matrix interface. These engineered interfaces can be used to modify critical titanium matrix composite properties such as transverse tensile strength and fatigue crack growth resistance in ways that produce a balanced set of properties. The SVI approach may also be used to probe interface failure mechanisms for the purpose of understanding complex mechanical phenomena. Single lamina Ti-6Al-4V matrix composites containing strongly bonded SiC fibers were fabricated both in the as-received condition and with a weak longitudinal stripe along the sides of the fibers. The striped SVI composites exhibited an increase in the overall fatigue crack growth life of the specimens compared to the unmodified specimens. This improvement was caused by an increased extent of debonding and crack bridging in SVI composites. This article is based on a presentation made in the symposium “Fatigue and Creep of Composite Materials” presented at the TMS Fall Meeting in Indianapolis, Indiana, September 14–18, 1997, under the auspices of the TMS/ASM Composite Materials Committee.     

Modeling Compression Behavior of Structured Geomaterials

The influence of the natural (or artificially induced) structure of a geomaterial on its compression behavior is investigated. An approach for modeling this influence for various structured geomaterials is proposed by using the disturbed state concept. An isotropic compression model is formulated on three basic assumptions. A special version of the proposed model is also described for situations where the compression is one-dimensional. The proposed compression model is used to simulate the behavior of a variety of structured geomaterials such as clays, sands, calcareous soils, clay-shale, soft rock, unsaturated soils, and soils artificially treated by adding chemical agents or mechanical reinforcement, and the model is evaluated on the basis of these simulations. A general discussion on the influence of the structure of geomaterials on their mechanical properties is also presented.     

Development of a high-performance green Fe–Al2O3 composites using Bayan Obo minerals: Enhancement effects of CeO2

With the deepening understanding for the concept of sustainable development, the utilization of minerals is no longer limited to the traditional way. In this study, an environment friendly method for preparing Fe–Al₂O₃ composites by using natural minerals was investigated. Additionally, the effects of CeO₂ on the properties of composites were studied. The mechanical properties of Fe–Al₂O₃ composites prepared by natural minerals are affected by the brittleness of glass phase. The strength and toughness of the glass phase in the composite are improved successfully by using rare earth oxides, indicating that the natural rare earths in Bayan Obo minerals have an enhanced influence on the properties of composite materials. The results show that the properties of glass phase can be significantly improved by addition of CeO₂. At the optimal addition of 3 wt% CeO₂, the composite achieves the density of 4.21 g/cm³, flexural strength of 401 MPa, Vickers hardness of 13.07 GPa and fracture toughness of 6.58 MPa⋅m^1/2. The composite has excellent mechanical properties, which can be used in engineering as a cheap structural material. This study aims at reducing waste emissions, improving energy efficiencies and avoiding waste of rare earth resources during the preparation of composite materials.     

Effects of Section Size, Surface Cooling Conditions, and Crucible Material on Microstructure and As-Cast Properties of Investment Cast Co-Cr Biomedical Alloy

ASTM F75, a biomedical grade cobalt alloy, is commonly used for orthopedic implants manufactured using investment casting methods. The quality of the cast implants is paramount. Defects such as porosity, inclusions, or misruns are not tolerated and the mechanical properties have to conform to predetermined standards. Generally, castings are subjected to post-casting thermo-mechanical treatments to homogenize structure and improve mechanical properties. However, casting parameters have a significant effect on the as-cast properties of materials, and so if solidification and secondary phase precipitation can be controlled at this stage to limit defects and optimize properties, then the need for such expensive additional treatments may be reduced. The initial objective of this work was to determine the effect of section size and cooling rate on the microstructural characteristics and resultant mechanical properties of the investigated alloy. This was done by conducting a series of instrumented control experiments involving solidification of the alloy in typical investment shell molds, characterization of the as-cast micro/macrostructure, and a program of mechanical testing. In the process of conducting these experiments, unexpected microstructural features were observed that could not be found in the current literature. Through a series of tests conducted to ascertain the origin of this alternative microstructure, it was found that the crucible material used for melting the alloy was highly influential in defining the as-cast properties.     

含Cu高熵合金的组织结构和力学性能分析

高熵合金（HEAs）是一种由5种或5种以上元素以接近等原子比的方式混合而成的一种新型合金。HEAs的概念为开发具有独特性能的先进材料提供了新的途径，这是传统的基于单一主导元素的微合金化方法无法实现的。由于Cu元素与HEAs中其他元素的混合焓均为正值，因而更容易偏聚形成富Cu的面心立方（fcc）结构。本文主要总结了合金成分、制备方法对含Cu HEAs组织结构的影响规律以及含Cu HEAs的热稳定性。例如Al的添加会使CoCrCuFeNi合金从fcc单相转变为fcc+bcc的双相结构，而Ni含量的增加则会将AlCoCrCuNi的多相组织转变为单相fcc结构。与传统铸造工艺相比，选区激光熔化和喷溅急冷等具有极高的冷却速度，限制了元素的扩散，因而制备而成的AlCoCuFeNi和AlCoCrCuFeNi合金均是bcc结构。组织结构的改变会进一步影响含Cu HEAs力学性能，因而本文也探讨了合金成分、制备工艺和服役温度与力学性能的关系。例如，V的添加可以提高合金的强度，以先进制备方法如选区激光熔化或激光粉末熔融得到的合金具有优于铸造合金的力学性能。     

成分起伏对钢液形核影响的热力学研究

 金属凝固过程中加入形核剂是改善金属材料组织和性能的常用方法。笔者以形核热力学为基础,讨论了外来成分起伏对钢液形核的影响,并通过实验进行了验证。研究结果表明,当金属液中加入的形核剂熔化后,改变了其周围钢液微区化学成分,造成成分起伏。当成分起伏的“方向性”和“程度”满足形核的热力学条件时,相变驱动力增加,可以促使钢液形核。根据热力学可以选择合适的形核剂。     

Validation of a Model of Plastic Deformation of Niobium Microalloyed Steels in the Two‐Phase Temperature Region

This article describes a thermo‐mechanical‐microstructural model for deformation of niobium microalloyed steel in the two‐phase range of temperatures. Results of physical and numerical modeling are presented. The physical simulation experiments include plastometric and dilatometric tests, as well as industrial rolling trial. Plastometric tests were performed to describe the flow stress for the wide range of temperatures, including ferritic, two‐phase, and austenitic states. Two‐stage deformation tests were performed to identify the microstructure evolution model. Dilatometric tests were used to identify the model of phase transformation. A model of the kinetics of the precipitation was adapted from the literature. The coefficients in all the models were identified using inverse analysis. Developed models were implemented in the finite element code. In order to improve the accuracy of the flow stress predictions in the two‐phase phase temperature range, internal variable dislocation density model was included, as well. The proposed combination of models correctly predicted microstructure changes and mechanical properties in the two‐phase range, during the transformations of the thermo‐mechanical treatment. Industrial trials were performed for the final validation of the models.     

Heavy Wall Thermo ‐ mechanically Rolled Plates for Offshore Construction Use

The growing energy consumption of the world made the exploitation of many offshore oil & gas fields feasible. In order to have the benefit of high strength and toughness together with good weldability, thermo‐mechanical rolling has been developed. In the construction of offshore structures plate thicknesses in excess of 50 mm are common which could up to now only be manufactured with restricted mechanical properties and a loss of productivity. The necessary fine grain structure is obtained by a minimum of four times total deformation in the thermo‐mechanical rolling process with accelerated cooling. For a steel mill this means that a change to a higher slab thickness was obligatory. In the plate mill the main drives were replaced in order to increase the torque and as a consequence get better deformation in the centre of the slabs. The accelerated cooling unit was revamped to a higher water pressure to obtain higher cooling rates and additionally enlarged to reach lower cooling stop temperatures. The results obtained with this new concept are presented in terms of strength, toughness and weldability for a 75 mm plate. This together with fracture mechanical data on the plate and on the weld demonstrates the effectiveness of the new technical concept and set a new benchmark in steel properties.