Investigation On Ductilization By Composite Of Micron-size/Ultrafine-Grained Microstructure

Nanoscrystalline/ultrafine-grained materials perform the unprecedented mechanical strength attributed to the very small grain sizes, which is five to ten times than that of the conventional counterpart. But their ductility, particularly uniform elongation in tension, has been rather low and in most cases nowhere close to that of normal metals. This drawback could be an insurmountable hurdle in bringing nanoscrystalline/ultrafine-grained materials into industrial application. Previous studies indicated the appearance of earlier necking due to the lack of limitation to plastic instability results from the restrain of small grain size to dislocation-mediated mechanism in nanoscrystalline/ultrafine-grained materials with low strain hardening ability.So far, one of the attractive strategies to improve the tensile ductility of nanocrystalline/ultrafine-grained materials is introducing micron-size grains of some volume fraction to achieve, namely, bimodal grain size distribution. Plastic instabilities would be overcome by the extra strain hardening ability of micron-size grains, which increases the tensile ductility without much loss of strength simultaneously. Pure Cu and Al-Mg alloys with bimodal grain size distribution has succeeded in achieving the high strength accompanied with large tensile ductility, especially bimodal Cu performs an ultra-high strength (430 MPa) close to that of fully nanoscrystalline Cu and a tremendously large elongation (60%) approach that of coarse-grained Cu altogether. However, there still exist some problems as:first, micron-size grains are obtained from secondary recrystallization, which is sensitive to annealing parameters. Therefore, the repeatability of microstructure is low; secondly, only the volume fraction of micron-size grained aggregate was investigated in the reported works. Are there any other factors that influence the mechanical behavior, especially the uniform tensile strain of bimodal materials? Last, couterexamples such as a consistent decrease of strength and ductility with increasing volume fraction of micron-size grains was found in electrodeposited Ni by Chokshi et al., which is contrast to the results of bimodal Cu. Additionally, bimodal structure is classical mixed grain structure, which is often eliminated in metal forming. The reason that bimodal structure is beneficial for increasing the tensile ductility still needs to be clarified.Present investigation focuses on the above three aspects of bimodal materials. The controllable bimodal structure is firstly fabricated through microstructure controlling based on physical metallurgy principle together with Severe Plastic Deformation (SPD). Then investigating the key parameters that influence the tensile ductility of bimodal materials by means of microstructure observation, macro-and micro-mechanical properties testing. Last, finding the relation between the ductilization and key material properties through micromechanical modeling analysis. The primary innovation points and conclusions are:(1) Proposing a novel route to obtain a bimodal structure with controllable microstructural parameters. Taking eutectoid alloys for example, hypo-eutectoid alloys are firstly selected to SPD for refining the microstructure to nanometer or sub-micron scale. Then annealing makes the grains inside the pre-eutectoid phase grow up to micron scale, while those grains in the eutectoid phase maintain the initial size because of pinning effect of duplex grain boundaries. Therefore, the initial dual phase pre-eutectoid+eutectoid microsturcture is in-situ transformed to bimodal structure consists of nanometer/sub-micron grains+micron-size grains. It is easy to identify the volume fraction and spatial distribution of micron-size grains through controlling these microstructural parameters of pre-eutectoid phase.(2) Experimental procedures as follows are proceeded to realize the new route and fabricate the controllable bimodal structure.①By usage of level law, three alloy systems of Zn-A1, Fe-C and Cu-A1 with different hypo-eutectoid composition are selected as experimental materials and quantificational controlling of volume fraction of pre-eutectoid phase was achieved.②Phase transformation processing is made according to phase diagram and phase transformation dynamic data, the size controlling of pre-eutectoid phase is achieved.③Microstructure observation shows a fully nanostructure in all three kinds of alloys subjected to High Pressure Torsion (HPT) and Equal Channel Angular Pressing (ECAP), which make an incredible increase in both microhardness and tensile strength of the deformed samples.④Short time annealing ultimately achieve bimodal structures in three alloy systems, which eliminates the inconvenient of low controllability of microstructure. Especially with various microstructural parameters in Cu-Al alloys:0%,20%,40%for the volume fraction and 20μm, 100μm for the average size of uniform spatial distributed micron-size grains aggregates.(3) Experimental results indicate besides volume fraction, other two factors that determine the tensile ductility of bimodal materials are the average size (or interspace) and strain hardening coefficient of micron-size grained region. The relationship between microstructural parameters and mechanical properties are systematically investigated by means of microstructure observations, tests in tension and nanoindentation in Cu-Al bimodal alloys. Three key factors that influence the mechanical behavior of bimodal materials are concluded as:volume fraction, average size and strain hardening coefficient of micron-size grained region.(4) Building the dual-phase ductilization micromechanical modeling and finding the microstructural parametric window to reach the effective ductilization. By integrating three factors that influence the mechanical behavior of bimodal materials, a micromechanical model is proposed based on the propagation and multiplication of localized deformation bands. The model prediction agrees well with the present and reported results of some bimodal materials (Cu-Al alloys, Fe-C alloys, pure Cu and Al-Mg alloys). Besides, with defining a criterion for ductility improvement (uniform elongation valueεu≥5%), the model also gives the microstructural parametric window to reach the effective ductilization, which clarifies if the effective ductilization can be achieved through bimodal mechanism and provides guidance in designing dual-phase composite for improving ductility.