R. Martínez-Sánchez

Influence of Size on the Microstructure and Mechanical Properties of an AISI 304L Stainless Steel—A Comparison between Bulk and Fibers

In this work, the mechanical properties and microstructural features of an AISI 304L stainless steel in two presentations, bulk and fibers, were systematically studied in order to establish the relationship among microstructure, mechanical properties, manufacturing process and effect on sample size. The microstructure was analyzed by XRD, SEM and TEM techniques. The strength, Young’s modulus and elongation of the samples were determined by tensile tests, while the hardness was measured by Vickers microhardness and nanoindentation tests. The materials have been observed to possess different mechanical and microstructural properties, which are compared and discussed.

Mechanical Response and Microstructure of Al-SiO2 Composites Prepared by Means of a Solid-State Route

This study deals with the production of some Al-SiO2 composites and the evaluation of milling intensity over the distribution of silica particles into the Al matrix. Samples for mechanical characterization were prepared from powders by compaction and sintering using a solid-state route complemented with mechanical milling. The mechanical response was modified as a direct function of the milling intensity, but an adverse effect was observed with prolonged milling times. Electron microscopy studies reveal a homogeneous dispersion of insoluble particles into the Al matrix, which is associated with the high grain refinement in the synthetized composites giving an important improvement on the composites strength. Also, the silica spheroidal structure is not altered nor destroyed (mechanically and/or chemically) during the composite synthesis.

Microstructural Characterization of NiCoAlFeCuCr High-Entropy Alloys

High-entropy alloys (HEA’s) contain at least five principal elements in equiatomic ratio or nearequiatomic ratio [1]. They usually form simple FCC and BCC structures, and even amorphous phases [2]. Excellent physical and chemical properties are reported for HEA ́s, like mechanical resistance, thermal stability and corrosion resistance to mention some [3]. These properties can be affected by the number, contents, and nature of the component elements. The effect of milling time in a multi-component quinary (NiCoAlFeCu) and hexanary (NiCoAlFeCuCr) systems was studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). Raw materials were commercial Nickel, Cobalt, Aluminum, Iron, Copper and Chromium, pure elemental powders supplied by Sigma-Aldrich. Equiatomic mixtures were mechanically alloyed to form quinary (NiCoAlFeCu) and hexanary (NiCoAlFeCuCr) systems. The milling process was carried out from 0 to 30 h in a high energy shaker ball mill (SPEX-8000M). An argon atmosphere was used to avoid oxidation and methanol (1 ml) was employed as the control agent process. The milling ball-to-powder weight ratio was set at ~5:1. In figure 1, it is observed a characteristic lamellar structure at 10 h of mechanical alloying. With increasing milling time, the layer spacing becomes finest, conducting to homogeneous microstructure and chemical distribution. The images in Fig. 2 show non-homogeneous microstructures formed at 10 h of milling (the lowest milling time). In the NiCoAlFeCu alloy were observed three well-defined phases: A is a Co-rich phase, B is a Ni-rich phase and C is an apparent hexanary solid solution. The NiCoAlFeCuCr alloy also presents the three same phases mentioned before, and just chrome zones. From the analyses done in several system (do not shown), was observed that Fe is a BCC solid solution former, Cu is a dominant element and favor the FCC solid formation which present structural characteristics similar to Cu solid solution. However, Cr apparently is Ni solid solution stabilizer. On the evidence of mechanically alloyed systems XRD spectra, elemental characteristic peaks disappear as the milling time increases while a solid solution peaks appear. TEM samples were prepared by powders deposition over a copper grid. No evidence of amorphous phases was observed. Crystal size observed in TEM samples were a little different with those values calculated from XRD spectra. During TEM observations were found small concentration gradients in different particles. However because the particle size is in the range of 10 to 30 microns, it is not considered important. At this moment, deep microstructural characterization is carrying out to know the effect of different elements additions in the multi-component systems evolution.

TEM Characterization on the Nanocomposite Al 7075 and Silver Nanoparticles Synthesized by Powder Metallurgy

Aluminum-based nanocomposites have been produced by mechanical milling, introducing silver nanoparticles within the matrix of a 7075 aluminum alloy using a high energy ball mill. The milled products were compacted by uniaxial load and pressure-less sintered under argon atmosphere, and finally hot extruded. Silver nanoparticles are well dispersed into the matrix of the powder particles as well as in the matrix of the extruded material. Transmission electron microscopy (TEM) analyses are used to corroborate and understand the hypothesis that second-phase particles finely and homogeneously dispersed in the matrix give greater strength to the material. In addition to the strengthening effect, the nanoparticles act like a process control agent (PCA) since the crystallite size of the nanocomposite is smaller at higher contents of nanoparticles.

A Green Method for Graphite Exfoliation Using High-Energy Ball Milling

Graphite, an allotropic and stable form of carbon, is a useful material constituted by multiple layers joined with covalent bonds linked together by a weak Van Der Walls interaction. The single units named graphenes have attracted considerable attention, because of their excellent mechanical, chemical, thermal, electrical properties and low thermal expansion coefficient [1]. The strong covalent (sp) bonds in this unique honeycomb structure of graphene and their atomic scale thickness impart them with these unusual properties [2]. Taking advantage of the weakness of these interactions, it is possible to insert ions, atoms or molecules between the layers in order to obtain graphenes from graphite. In the exfoliation process, elimination of the intercalated species leads to a significant expansion up to hundreds of times along the c-axis, forming a highly porous material. Exfoliated graphite (EG) has been synthesized by galvanic, chemical and thermal treatments of the natural graphite. However, the chemical method is widely used because its simplicity and versatility. Usually EG is produced intercalating acid guest species between the stacked graphene layers via liquid-phase by reaction between graphite and 18M H2SO4 in the presence of strong chemical oxidants such as KMnO4, HNO3 or H2O2 [3]. However, reaction leftovers are highly toxic and corrosive materials that required careful manipulation and a special confinement.

Nanohardness and Microstructure of NiCoAlFeCu and NiCoAlFeCuCr Alloys Produced by Mechanical Alloying

The development of high-entropy (HE) alloys expanded the path of advanced materials research. These alloys with at least 5 principal elements in equiatomic or near equiatomic ratios have the tendency to form solid solutions (SS) phases, usually with FCC and/or BCC structures, instead of intermetallic compounds [1]. This effect is reflected with superior mechanical properties compared with conventional alloys which are composed for one or two principal elements [2]. The structures and microstructures in HE alloys, as well as their mechanical properties depend on the nature and number of the alloying elements. The mechanical properties of several high-entropy alloys have been reported. However, evaluations by nanoindentation in HE alloys have not been deeply investigated. The AlCoCrCuFeNi alloy has been the most widely investigated in terms of microstructure and mechanical properties, but at this moment there are no reports about hardness determinations of individual phases. This paper first reviews the microstructure of high entropy alloys in relation to their chemical composition, particularly to the contribution of Cr adittion, and its effect on microhardness of bulk samples and the mechanical properties of individual phases by nanoindentation tests.

A Green Method for Graphite Exfoliation, Effect of Milling Intensity.

Graphite is a natural allotropic form of carbon, being the most stable allotrope under standard conditions; multiple layers or foils named graphitic carbon and graphenes are considered single or double layers of carbon with a honey comb structures that constitute this material. Graphenes have attracted considerable attention of the scientific community, because of their excellent mechanochemical properties, high thermal conductivity and low thermal expansion coefficient [1]. A chemical process to produce single layers graphenes is by inserting ions, atoms or molecules between the layers. During the exfoliation process, elimination of the intercalated species leads to an important material expansion, forming a highly porous material commonly named exfoliated graphite (EG).

Effect of Solubilization Temperature, Zn Addition and Thermo-Mechanical Treatments in the Microstructure of the Aluminum 2024 Alloy

Al-2024 is a Cu-rich heat treatable alloy which is used in aerospace applications because of its high strength/weight ratio, as well good fatigue resistance [1]. Precipitation heat treatments are an important procedures that directly affects the mechanical properties of the material. An important step of such treatments is solubilization, where the main objective consist in maintain the maximum amount of solute phases in solid solution, where the processing time is a critical variable to control to obtain a satisfactory degree of solubilized [2]. The addition of Zn to the 2024 alloy has been investigated and reported by various authors, Zn and Al have a difference of 7% in the atomic radius [3], which represents some advantages from structural viewpoint due the mechanism of hardening by solid solution.

Effect of Ti and W Additions on the Microstructural Behavior of a Nanocrystalline CoCrFeMoNi High Entropy Alloy

Over the past decade, a new alloy design, the high entropy alloys (HEAs), has attracted significant attention due to their unusual properties. Due to the fact that HEAs are composed of at least five principal elements, the compositional design is one of the most critical factors in the selection and development of these materials. The selection of chemical compositions of HEAs has been focused in the aim to obtain solid solution with simple structures, in most of the cases, single FCC or BCC solid solution phases [1].

An Analysis of Nanoindentation in a NiCoAlFeMo High Entropy Alloy Produced by Sintering

High entropy alloys (HEA) consist of five or more elements in equiatomic or near equiatomic composition, they might exhibit unique microstructural features, such as solid solution phases and nanostructures. With the proper composition design, high entropy alloys can get excellent properties such as high strength and hardness, god ductility and resistance to wear. Some researchers have studied the high entropy alloys based on the elements and their content, processing conditions, microstructure and properties, but there are still some knowledge to be known. For a better information on the mechanical behavior of these materials with a microstructure formed by phases that are around or below the micrometric scale, the knowledge of the local mechanical properties can be carried out by techniques as nanoindentation and can represent a great advance in the understanding and prediction of its properties. The aim of this investigation was to evaluate the mechanical properties of individual phases by nanoindentation thecnique in a NiCoAlFeMo high entropy alloy produced by mechanical alloying and conventional sintering, to establish a relationship between microstructure, chemical composition and properties.