Search

Ferritic-martensitic low-carbon steels can be produced by inter-critical treatment with fast cooling, and are commonly referred to as dual-phase steels. The soft ferrite with hard martensite provides higher mechanical resistance than normalized steel. In this work, specimens from a low-carbon steel with 0.158%C were submitted to inter-critical heat treatments in three temperatures (770 °C, 750 °C and 730 °C) followed by rapid cooling. The martensite and ferrite phases were quantified by light optical microscopy and electron backscattered scanning diffraction using band slope, with good match between the results from both techniques. Martensite derived from the austenite phase present in the inter-critical annealing temperature, which can be predicted by computational thermodynamics. In agreement, the martensite volume fraction increased with the increase of the inter-critical annealing temperature. Fine carbides were observed by high-resolution scanning electron microscopy in the ferrite phase, for all heat treatment conditions. Kernel average misorientation (KAM) and geometrically necessary dislocation (GND) analysis were also calculated by electron backscattered scanning diffraction. Impact toughness, hardness and microhardness were determined, and the results were correlated to the microstructural features. The mixture rule was not verified with the microhardness tests in specimens treated at 730 °C and 750 °C, but matched the experimental values for the specimen treated at 770 °C. The effects of tempering at 300 °C on the microstructure (precipitation, KAM and GND) were investigated and correlated to the mechanical properties.

In this work an alternative technique to produce shape memory wires directly from the molten was developed. In this method, the alloy is melted in a high frequency induction furnace with the levitation melting technique and suctioned through a quartz tube connected to a low-pressure chamber. Cu-AI-Nb wires with a diameter of 1.3 mm and length of about 150mm were obtained by this procedure. Their microstructure and physical properties were studied using X-ray diffraction, optical microscopy and DSC thermal analysis.

This work focuses on the necessary conditions for obtaining Cu-Al-Nb based shape memory alloys by means of mechanical alloying powder metallurgy. The Cu, AI, and Nb elemental powders were pre-alloyed using a pulverisette high energy ball milling for Cu-13. 0wt%AI-2.0wt%Nb alloy. The microstructural evolution during the milling was investigated by using X-ray diffraction. It was observed that the Cu(AI) solid solution can be obtained with short milling times. A relative sintered density on the order of 93% on the theoretical value was achieved. Additionally, after quenching the microstructure of the sintered compacts was formed by the 18R type martensite and Cu(AI) solid solution.

Nb modified Cu-Al-Ni shape memory alloys with different Ni contents (1.0, 2.0 and 3.0wt%) were processed using the melt spinning technique. The microstructures and the characteristic transformation temperatures were detennilled by x-ray diffraction and thennal analysis (differential scanning calorimeter and thennomechanical analysis) respectively. It was observed that all as-melt spun ribbon microstructures present the $\gamma_2$phase and some peaks that are thought to be from the 18R type martensite. After water quenching the microstuctures of the ribbons are fully 18R and 2H type martensites. The typical Ms, Mf, As and Af temperatures are shifted to lower values with increasing the Ni concentration.

In this study, alloys with a Cu-13.84Al-1.0Nb (wt. %) composition and different content of nickel (1.0, 2.0 and 3.0 wt%) were cast in a high frequency induction furnace, and reprocessed by the suction method for obtaining wires. The effect of Ni additions and the different cooling rates on the martensite stability and shape recovery were studied. It was observed that for alloys with a low nickel content the addition of Nb strongly influences the effect of tNi element by hindering the formation of the $\gamma_2$(Al4Cu9) phase. ln alloys with a higher Ni content the XRD analysis clearly shows that the microstructure consists of 18R and 2H martensite.

In the last decades many investigations have been oriented toward the development of nanostructured biomaterials such as calcium phosphate ceramics, particularly those composed of stoicheiometric hydroxyapatite (Ca10(PO4)6(OH)2) and a-and β- tricalcium phosphate (Ca3(PO4)2), which present chemical and crystallographic similarities with hard tissues (bones and teeth). Because of these characteristics, these materials can be used for reconstitution and regeneration of bone tissues. The odontological and biomedical applications are still limited due to their brittle behaviour. This study was focused in the synthesis and characterization of a bone matrix of calcium phosphate (β-TCP) and hydroxyapatite (HAP-101 e HAp-201). The results presented here are related to the morphological characterization of nanostructured powders from scanning electron microscopy viewpoint. X-ray diffraction was used to identifier the present phases in the powders and the infrared spectrometry (FTIR) was used to analyse OH bonds from hydroxyapatite and PO4 from calcium phosphates.

The addition of amorphous Fe–Si–B particles to Fe powder increases the shrinkage of sintered components resulting in higher densification rates. Consequently, several research groups worldwide have studied the properties of such systems in an attempt to produce superior structural alloys. In the present work, Fe75Si10B15ribbons obtained by melt spinning were milled in a high energy Spex mill for times varying from 2 to 32 h. The resulting powders were characterised by differential thermal analysis and X-ray diffraction. The results showed that the amorphous characteristics of the ribbons persisted after the milling process. Next, samples consisting of a mixture of Fe powder and 4 wt-% milled amorphous phase were uniaxially pressed and sintered following a series of thermal cycles. High temperature microstructures were obtained for compacts subjected to rapid cooling from the sintering temperature. The results of scanning electron microscopy and energy dispersive spectroscopy revealed substantial precipitation of fine Fe2B particles before α → γ allotropic transformation. In addition, an oxide phase was observed in the interface between Fe and the additive particles. Preliminary analysis suggested that the oxide particles can be easily reduced by adding small amounts of carbon to the system. PM/0765