Rodinei Medeiros Gomes

Self-lubricating, low-friction, wear-resistant Al-based quasicrystalline coatings

Abstract After gas atomization, a quasicrystalline powder based on aluminium was used to prepare a thick coating by high-velocity oxygen-fuel flame torch spraying. This layer was deposited on top of a bond-coat layer on a steel plate. A post-spraying annealing treatment turned the two layers to their stable state, a γ-brass crystal and an icosahedral quasicrystal, respectively. The projection parameters were selected in such a way that the coating behaved like a self-lubricating material, which offered very good wear resistance (duration of pin-on-disk tests superior to 5 km with negligible material loss) and low friction (µ ≤ 6% against sintered tungsten carbide), in contrast to the state of the art. This property was achieved thanks to, on the one hand, excellent bonding to the substrate via the bound coat, and on the other hand, presence at the boundaries between quasicrystalline flakes of a mixture of both threefold and fourfold coordinated carbon originating from spray processing. Application to hard materials used in mechanical devices is appealing, especially because soft, lubricating additives may not be needed, thus considerably increasing the lifetime of the devices and reducing waste of materials.

Investigation of Quasicrystal-Reinforced Aluminium Metal Matrix Composite by Hot Extrusion

The use of quasicrystalline alloys as reinforcement material is due to the fact that they posses high hardness and low coefficient of friction. For this purpose was used compaction/extrusion equipment with which it was possible to observe a tendency toward increase in the mechanical strength from 72MPa (0% reinforcement) to 129Mpa (6% reinforcement).

Effect of heat treatment on Fe-B-Si-Nb alloy powder prepared by mechanical alloying

The effect of heat treatment on crystallization behavior of Fe73.5B15Si10Nb1.5 alloy powder prepared by mechanical alloying was studied. The powder samples were prepared by mechanical alloying (MA) and for different milling times (1, 5, 25, 70 and 100 hours). Crystalline powders of iron, boron, silicon and niobium were sealed with tungsten carbide balls in a cylindrical vial under nitrogen atmosphere. The ball-to-powder weight ratio was 20 to 1. A Fritsch Pulverizette 5 planetary ball mill was used for MA the powders at room temperature and at 250 rpm. To study the microstructural evolution, a small amount of powder was collected after different milling times and examined by X-ray diffraction, using CuKa radiation (l = 0.15418 nm). The crystallization behavior was studied by differential thermal analysis, from 25 up to 1000 °C at a heating rate of 25 °C min-1.

Mechanical Strength Evaluation of a CuAlBe Shape Memory Alloy under Different Thermal Conditions

CuAl shape memory alloys containing 0.6wt% and 0.65wt% was casted by induction melting at room temperature without protective atmosphere and their mechanical strength evaluated as a function of the temperature. It was observed that the melting at room temperature does not promote any difficulty to control de beryllium content. The ultimate stress to rupture drop significantly with decreasing the test temperature is such way that the alloys became brittle irrespective to beryllium content.

Microstructural Characterization of Equiatomic NiTi Alloy Prepared by High Energy Milling

NiTi alloys with equiatomic composition of NiTi have the highest technological interest for its potencial application in differents areas such as biomedical, naval, aerospace, nuclear, automobilist , robotic,etc. In this work , it was used a 50Ni50Ti at % powder mixture, comercially pure, prepared by mechanical alloying in a Attritor with the following conditions: the milling speed and the ball charge were 1500 rpm and 10:1 respectively. The milling time was 2,4,8 and 16h, under an argon atmosphere at room temperature. After milling it was determined the particle size distribution, the phases by X-ray diffractions (XRD) and the powder morphology by scanning electron microscopy (SEM). The milling promotes dissolution of Titanium in Nickel and continuous amorphization by increasing the milling time. After 16h milling the alloy was almost amorphous. The powders after milling were compacted and heat treated at high temperature and microstructural evolution was characterized. In the heat treated samples were detected different phases showing heterogeneity in the alloy. The detected phases were Ni3Ti, NiTi, Ni2Ti and Ni2Ti4O. Contamination by milling was detected in the powder after milling and in the heat treated samples.

Study of the stability of the NiTi wire applied to thermomechanical actuators

Shape memory alloys (SMA) have been the subject of several studies due to their excellent physical and mechanical properties. For this reason, a wire SMA is used in various applications as thermomechanical actuators that are subject to repeated thermal cycling in a range of temperatures and variable loads. The mechanical stability is a fundamental behavior that need to be understood to project well accurate actuators. Thus, the total recoverable strain must be constant under certain external conditions. In this research, an apparatus was built to monitor the recoverable strain in Ni-Ti SMA wire with differentiated thermomechanical training during prolonged thermal cycles under constant loads. Experimental results have shown that the recoverable strain decreases with cycling. In addition, an acceptable stabilization can be found for a specific condition of thermomechanical training and constant load.

Supercritical carbonation of lightweight aggregate containing mortar: Thermal behavior

Lightweight concrete shows good insulation properties, depending on several parameters such as mix design and aggregate type. Perlite aggregate is one of the most effective aggregates for such a purpose, mainly because of its low thermal conductivity (0.04 W/m.°C), but is not available globally. This paper explores the potential use of another source of thermal efficient aggregate, vermiculite (0.058 W/m.°C) which is available in Brazil and other countries where perlite is absent. Cylindrical samples were cast by using two lightweight aggregates, perlite and vermiculite, and treated with supercritical carbon dioxide. Supercritical carbonation (SCC) of concrete can improve mechanical, thermal and durability features. In this paper, the effect of SCC on the thermal behavior of lightweight mortars was investigated with regards to physical and microstructure features and thermal behavior due to cooling.

Effect of the Cutting Velocity and Heat Treatment on Turning Cutting Forces of an SMA Cu-Al-Be Alloys

This work studies the effect of heat treatment and cutting velocities on machining cutting forces in turning of a Cu-11.8%Al-0.55%Be shape memory alloys. The heat treatment was performed to obtain samples with austenite and martensite microstructures. Cutting force was investigated using a 3-component dynamometer in several revolutions and data were analyzed using statistic tools. It was found that the resultant forces were higher in quenched alloy due to the presence of Shape Memory Effect. Chip formation occurred in a shorter time in the sample without the Shape Memory Effect.

Study of the Elasticity Module through Indentation Instrumented Technical of AlCuFeB Quasicrystalline Alloy

Quasicrystalline materials have unique properties such as high hardness, excellent surface properties, good resistance to oxidation and corrosion and low electrical and thermal conductivities. These materials can be obtained by conventional methods of metallurgy. However, quasicrystals are quite weak and this characteristic complicates their use in the form of billets for the manufacture of mechanical components. For this reason, the evaluation of mechanical properties of quasicrystalline materials using conventional methods, such as tension, compression, fatigue, among others, is not feasible. One method to evaluate the mechanical properties of quasicrystals is through instrumented indentation, once it is a very efficient tool for the calculation of properties such as hardness, fracture toughness and modulus of elasticity. The latter was the property studied in this work and that it is an important design criterion for the manufacturing of quasicrystalline alloys for their use in industry.

Effect of the Milling Time and Addition of a Grinding Agent of Quasicrystalline Powder Based on AlCuFeB Alloys

Quasicrystalline (QC) materials represent a new class of alloys differing from amorphous and crystalline materials due to quasicrystalline periodicity and therefore unusual properties. Applications of quasicrystals range from surface coatings, thin films to reinforcements of ductile matrix composites such aluminum and, more recently, polymers. Quasicrystalline alloys show fundamentally different behavior compared to crystalline alloys even when their compositions are very similar, including low friction coefficient, high hardness and high brittleness. Due to this brittle behavior there are some limitations with respect to what methods can be used to process QC materials restricting their applications to powder form. One of the techniques for particle size reduction is mechanical milling which, however may lead to destabilization of the icosahedral phase. Therefore, there is a need to study the stability of quasicrystalline alloys during this comminution process. In the present study, AlCuFeB alloys were milled with the aid of a grinding agent that helps reduce overheating and thus controlling the QC powder stability. It was found that QC phase was destabilized after 10 h while the addition of a grinding agent led to milling times of 20 h without destabilizing the QC phase.