The present work reports on the effect of hydrogen on mechanical properties of aluminum AA 6061 hardened by precipitation. Test specimens are solution treated in an electric induction furnace at 803 K during 1 hour and then precipitation hardened during 4 hours at 423, 448 and 473 K. The heat treated specimens are subjected to a hydrogen cathodic charging process during 0, 24 and 96 hours and then tested under monotonic uniaxial tensile loading according to ASTM E8-03. Scanning electron microscopy characterization of tensile fracture surfaces reveals that hydrogen charging of precipitation hardened specimens enhances intergranular cracking by microvoid coalescence and intragranular cleavage fracture. The results show that, although hydrogen charging causes embrittlement of precipitation hardened aluminum AA 6061, the effect is a function of time; short time cathodic charging increases the ductility of the alloy.

Cobalt nanomaterials with promoters have been prepared by precipitation followed by calcination and impregnation method. The obtained materials are characterized by ICP, H2-TPR, BET and XRD. A small addition of manganese increases the specific surface area of cobalt nanomaterials and thermostability under reduction atmosphere. Bulk Co3O4 has been reduced in two steps (Co3+→Co2+→Co). A small addition of manganese to the cobalt oxide shifts the reduction peaks to high temperatures.

Titanium diboride has been sintered using spark plasma sintering (SPS). With the addition of tetragonal Al3Ti or cubic (Al,Ni)3Ti, the Vickers hardness, Hv, of TiB2 increases up to as high as 2000 by the sintering at 1273K, while that of the sample sintered without additives is as low as 20. Such a remarkable improvement is caused by the improvement of direct link between TiB2 grains in addition to the effect that Al3Ti or (Al,Ni)3Ti fills in the space between TiB2 grains and act as a binder.

In this work a methodology to measure kinetic parameters to describe grain growth during equiaxed eutectic solidification is proposed. This methodology includes the numerical processing of two cooling curves and requires input data concerning the number of grains per unit volume. In addition, free grain growth before impingement and an exponential dependence of the grain growth rate on undercooling are assumed. The evolution of solid fraction of the sample as a function of time is obtained by applying the Fourier thermal analysis (FTA) method. Information collected is processed numerically in order to find numerical values for the pre-exponential and exponential parameters that characterize the grain growth kinetics as a function of undercooling. To validate this methodology a mechanistic model that simulates the cooling and solidification of eutectic Al-Si alloy in a sand mold is used. The results suggest that this methodology can be used to measure the kinetic parameters of equiaxed growth from the numerical processing of cooling curves and grain density data.

Four Celsian (Ba0.75Sr0.25Al2Si2O8)/Mullite (Al6Si2O13) composites, with potential structural applications at high temperatures, are synthesized from coal fly ash (byproduct of a Mexican coal-burning power plant, constituted mainly by SiO2 and Al2O3). Nominal Celsian/Mullite weight ratios studied are 80/20, 60/40, 40/60 and 20/80. Mullite is synthesized separately at 1600ºC/2h and then mixed with a Celsian precursor mixture previously calcined at 900°C/5h. During this process the Celsian phase is formed by a solid state reaction at 1100-1400ºC/5h. Prior to this, the reacting mixture is milled in a planetary mill during 1 or 2h and then compacted by uniaxial and cold isostatic pressing. The microstructure and phase composition of the synthesized composites are characterized by X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM/EDS). Their dynamic Young’s modulus is measured by an ultrasonic technique, and their mechanical strength is evaluated from flexural tests carried out at room temperature. The expected phases are obtained in all cases, although with some differences with respect to their expected relative proportions, according to the studied nominal compositions. In general, the longest milling time employed produced samples with the largest degree of crystallinity and density, as well as with the best microstructural characteristics and mechanical properties.

Macrosegregation is harmful to the material properties (strength, corrosion resistance, etc.) and it must be avoided or eliminated from castings. In this work, macrosegregation behavior is evaluated in terms of silicon concentration in cast ingots of steels with 0.5, 1.5 and 3 wt% Si. The dimensions of the castings are 25x30x12.5 cm. The Si segregation profiles are evaluated in three zones: upper, middle and bottom of each ingot. The characterization is carried out by energy dispersive spectrometry in at least 10 positions of each zone of the ingot. Silicon analysis is evaluated from the surface to the centre of each ingot. The experimental results are compared with predictions using an Experimental Segregation Index (ESI) and the Scheil Segregation Index (SSI). The experimental results show a strong deviation from the profiles predicted by the Scheil model. The evaluation of the degree of segregation by "ESI" indicates little variation of silicon from the surface to the center of the ingot showing a uniform distribution across the ingot analyzed.

Celsian with a chemical composition of Ba0.75Sr0.25Al2Si2O8, is synthesized by using coal fly ash (byproduct of a Mexican coal-burning power plant, composed mainly by SiO2 and Al2O3) as main raw material. The thermal behavior of the synthesized material is evaluated by differential (DTA) and gravimetric (TGA) thermal analyses as well as by heating microscopy; its coefficient of linear thermal expansion (CTE) is also determined. Heating microscopy shows that cylinders of compacted powdered Celsian start sintering at ∼1140 ºC, which is associated with a considerable contraction occurring up to 1500 ºC. The mean CTE value of the material in the temperature range of 30-1100ºC is slightly affected by the synthesis conditions employed. Synthesis at 1400 or 1300 ºC during 10 h, with a pre-calcination step at 900 ºC/5h in both cases, produce mean CTE values of 5.15 x 10-6 and 5.43 x 10-6 ºC-1, respectively. On the other hand, Celsian synthesized at 1400 ºC/10 h, without the pre-calcination step, has a mean CTE value of 5.25 x 10-6 ºC-1. Lastly, the DTA/TGA analysis of the synthesized material shows that a slight weight gain takes place from room temperature to 1100ºC, which is followed by a slight weight loss up to 1300ºC. This is attributed to oxidation and evaporation of some of the impurities present in the material.

The usual process to produce NiTi shape memory alloys is vacuum induction melting (VIM). Currently a new alternative process to produce NiTi shape memory alloys by rapid solidification structures called Melt Spinning has been studied. In this work, results of ribbons with a chemical composition Ti-55.2 Ni (wt %) alloy prepared by this method are presented. The ribbons are prepared at two different linear velocities: 30 m/s and 50 m/s. After that, samples are heat treated at 350 °C during 1 hour. The alloys are characterized by Scanning Electron Microscopy (SEM), Differential Scanning Calorimetry (DSC) and X-Ray Diffraction. According to the cycled DSC test, transformation peaks are associated with the B2→R→B19´ transformation during cooling and B19´→R→B2 during heating, showing transformation in multi-peaks. The martensite B19´ start (Ms) is varying from 35 to 39°C and the martensite finish (Mf) from 15 to 21°C, 42-47°C for austenite B2 start (As) and 65-69°C for austenite finish (Af) approximately. All analyzed ribbons show very similar values of transformation hysteresis temperatures at 50% of transformation of around 28°C. In order to change solidification rate, linear velocity is varied during the melt spinning process. Results indicate that linear velocity affects directly the temperature of transformation. When the linear velocity is increasing, crystallographic Ti-rich precipitates are developed, but dendritic growth disappears, changing the microstructure and decreasing these transformation temperatures. Then changes in linear velocity can dramatically affect shape memory properties, and in this case a velocity of 50 m/s produces a more homogenous alloy.

The aim of this research work is to study the effect of boron addition on mechanical properties and microstructure of a new family of low carbon NiCrVCu advanced high strength steels (AHSS). Experimental steels are thermo-mechanically processed (TMP) (hot-rolled+quenched). Results show that the microstructure of these steels contains bainite and martensite, predominantly, which nucleate along prior austenite grain boundaries (GB). On the other hand, tensile tests reveal that the TMP steels have YS (0.2% offset) of 978 MPa, UTS of 1140 MPa and EL of 18%. On the basis of exhibited microstructure and mechanical properties, these experimental steels are classified as bainitic-martensitic complex phase (CP) advanced ultra-high strength steels (UHSS).

Payload and high-tech are important characteristics when the goals are aerospace applications. The development of the technologies associated to these applications has interests that transcend national boundaries and are of strategic importance to the nations. Ultra lightweight mirrors, supports and structures for optical systems are important part of this subject. This paper reports the development of SiC substrates, obtained by pressing, to be applied on embedded precision reflective optics. Different SiC granulometries, having YAG as sintering additive, were processed by: ball milling, drying and deagglomeration, sift, uniaxial and isostatic pressing, and, finally, argon atmosphere sintering at 1900°C. Different porosities were obtained according to the amount of organic material added. Into one side of the samples pellets of organic material were introduced to generate voids to reduce the weight of samples as a whole. The substrates were grinding and polished, looking for a SiC surface having low porosity, as porosity is directly related to light scattering that should be avoided on optical surfaces. Laser surface treatments were applied (using or not SiC barbotine) as a method to improve the surface quality. The samples were characterized by optical and laser confocal microscopy, roughness measurements and mechanical tests. The results are very promissory for future applications.

Tantalum carbide is a technologically important material for use in ultra-high temperatures and corrosive environments. In this report, we describe the scalability of a low temperature solvothermal method for the preparation of this useful material. X-ray diffraction shows phase-pure powders with no change in average crystallite size or compound stoichiometry compared to synthesis in smaller batches, remaining at 25 nm and 0.94 respectively. Dynamic light scattering shows a slight decrease in particle size distribution with scale-up. Thermogravimetric analysis (TGA) in air shows a decrease in surface species on the powders, but the powders oxidize at a lower temperature when scaling the synthesis. Mass spectrometry performed alongside TGA in a helium atmosphere reveals that water is the most abundant species on the surface of the powders, but oxygen, carbon monoxide, carbon dioxide, and nitrogen are also detected. Oxygen analysis reveals that the oxygen content of the powders is high (>6%). The oxygen source and methods of decreasing oxygen content are discussed. Initial sintering trials were performed and demonstrate the need for further powder processing.

Ribbons of the Ni-44.8wt%Ti shape memory alloy are prepared through the melt spinning technique. The study is focused on investigating the effect of the rapid solidification and grain size at characteristic start martensitic (Ms), final martensitic (Mf), start austenite (As) and final austenite (Af) transformation temperatures. Changes on martensitic transformation temperatures in Ti45Ni55 melt spun ribbons are observed as grain size is reduced. Results of optical microscopy and differential scanning calorimetry (DSC) are used to associate grain size with transformation temperatures.

In this research we have focused on the influence of thermal treatment for periods of time on the thermal and mechanical properties of extruded films of a series of high-performance thermotropic liquid crystal polymers (LCPs). The dependence of microstructure, thermal and mechanical properties on the extent of thermal treatment is investigated. Especially synthesized wholly aromatic LCPs based on hydroxybenzoic acid (B), hydroxynaphthoic acid (N), terephthalic acid (TA) and biphenol (BP) are kindly supplied by Hoechst Celanese Research Corp in the form of 50 μm thick extruded films. Thus, the influence of monomer composition is also studied in order to contrast the influence of molecular conformation. Thermal treatments are carried out at temperatures close to the solid-to-nematic transition (Ts→n) for up to several hours under dry air conditions. The results show a profound influence of thermal annealing on morphology and mechanical modulus when annealing is carried out c.a. 40ºC below Ts→n, where solid-to-nematic transition and Young’s modulus are significantly increased.