Resumo Este estudo aborda a microestrutura, ensaio de tração quasi-estático e modos de falha de um aço TWIP soldado a ponto por resistência. Pontos de solda foram produzidos variando os principais parâmetros de soldagem: corrente de soldagem, tempo de soldagem, e força do eletrodo. Para todas as amostras, as diferenças de dureza, observadas via mapas de dureza por microindentação, entre a zona fundida, a zona termicamente afetada e o metal-base foram consideravelmente elevadas. A dureza da zona fundida é menor que esta da ZTA e do metal-base, facilitando falhas interfaciais durante ensaio de tração-cisalhamento. Porém, elevado aporte térmico acarreta na mudança do modo de falha para parcialmente interfacial e arrancamento, como confirmado via microscopia eletrônica de varredura. Esta mudança é também acompanhada por um notável aumento na resistência em tração-cisalhamento e capacidade de absorção de energia.

Abstract This study discusses the microstructure, quasi-static mechanical strength and failure modes of TWIP steel weld spots. Weld spots were produced by varying the main resistance spot welding parameters: welding current, welding time and electrode compression force. All the samples showed a remarkable material hardness mismatch between the fusion zone, the heat affected zone and the base material, as evidenced by microindentation maps. Hardness at the fusion zone is lower than that of heat-affected zone and base metal, which facilitates interfacial failure mode during tensile-shear tests. However, high heat inputs promoted the failure mode changes to partial interfacial mode and then to pullout mode during tensile-shear tests as confirmed by Scanning Electron Micrographs. These changes in failure mode were accompanied by a notable increase in tensile-shear strength and energy absorption capability.

The present work studied the influence of annealing on a Ni-Ti alloy submitted to uniaxial compression. The stress-strain curve under compression showed the absence of the Stress-Induced-Martensite (SIM) plateau and 42% of remaining austenitic phase (B2). identified by the Electron Backscattered Diffraction (EBSD) technique on a Scanning Electronic Microscope (SEM). Annealing at 150 ºC increased the volume fraction of austenite to 70%. Annealing at 200 ºC reduced the volume fraction of austenite to 33% and increased the classical Vickers hardness and the dynamic elastoplastic hardness. This increase suggests the presence of a phase with higher indentation resistance, which can be the R phase reported in the literature. Annealing at 250 ºC decreased the classical Vickers hardness and the dynamic elastoplastic hardness, probably due to recovery. Annealing at 300 ºC advanced the recovery process and promoted some recrystallization, as suggested by the large standard deviation observed in the hardness results.

In the current work, XRD, SEM, EBSD and TEM techniques were used to evaluate the microstructure of stainless Fe-Mn-Si-Cr-Ni-Co shape memory steel processed by ECAE and annealed for one hour at temperatures ranging from 650ºC to 950ºC. The results were then correlated with the mechanical and shape-memory properties of the steel. It was observed that the samples containing large grains and a microstructure free of defects or precipitates presented a high volume fraction of multi-variant thermal martensite and stress-induced martensite, resulting in good shape recovery, owing to the memory effect. The grain refinement and precipitation of second-phase particles decreased the volume fraction and number of martensite variants and considerably increased the mechanical resistance, enhancing the elastic shape recovery. It was shown that shape memory properties were essentially related to the mechanical resistance of the matrix, which in turn was related to the microstructure.

Plasma nitriding of a solution annealed and aged 300 grade maraging steel was studied aiming to increase the creep resistance. The surface microhardness reached 1,140 HV, producing 50 µm layer composed of ε-Fe3N and γ'-Fe4N nitrides at the uppermost sample layer. The inner core remained unaltered presenting typical plate-like martensite microstructure of maraging steels with average microhardness of 604 HV. Surface RMS roughness in the nanometric scale increase from 52 nm to 71 nm. The continuous layer of iron nitrides seems to behave as a barrier for oxidation and for inward oxygen diffusion improving the creep resistance by reducing the steady-state creep rate (εs) in 52-65% when compared with the literature results. Dominant creep mechanism is controlled by dislocations climb. Fracture surfaces of specimens presented ductile failure consisting of equiaxed and bi-modal dimples in the fibrous zone surrounded by 45º shear lip. Nitrided sample presented a reduced ductility, associated to the hard surface layer.

Ti‒50.13Ni and Ti‒49.62Ni (at.%) shape memory alloy ribbons were fabricated by melt-spinning method at different circumferential wheel velocities. The effects of wheel velocity, chemical composition and heat treatments on microstructure and Transformation temperature were investigated. Differences in wheel velocity led to differences in cooling rate and sample dimension, as well as in phase transformation temperatures. Two heat treatment conditions were studied, 350°C for 1h and 350°C for 5h. In the samples produced at high wheel velocity and heat-treated at 350°C for 5h, nanosized Ti-rich precipitates were observed in both chemical compositions. Cross-sectional microstructure was studied by optical microscopy; SEM was used to study the nanometric grains and nano precipitation. The transformation temperatures were analyzed by DSC.

ABSTRACT In order to quantify ferrite content, three techniques, XRD, ferritoscope and optical metallography, were applied to a duplex stainless steel UNS S31803 solution-treated for 30 min at 1,000, 1,100 and 1,200 °C, and then compared to equilibrium of phases predicted by ThermoCalc® simulation. As expected, the microstructure is composed only by austenite and ferrite phases, and ferrite content increases as the solution treatment temperature increases. The microstructure presents preferred grains orientation along the rolling directions even for a sample solution treated for 30 min at 1,200 °C. For all solution treatment temperatures, the ferrite volume fractions obtained by XRD measurements were higher than those achieved by the other two techniques and ThermoCalc® simulation, probably due to texturing effect of previous rolling process. Values obtained by quantitative metallography look more assertive as it is a direct measurement method but the ferritoscope technique should be considered mainly for in loco measurement.

This work deals with structural, electrical and mechanical characterization of Ti‒50.13Ni and Ti‒49.62Ni (at.%) shape memory alloys (SMAs) fabricated at different circumferential wheel velocities. The effect of wheel velocity, chemical composition and heat treatments are investigated. The characterization of crystallographic phases of the Ti‒Ni ribbons was carried out using X-ray diffraction. Electrical resistance variations as function of temperature (∆R/R %) were analyzed using a non-commercial technique, which consists in a thermal-adjustable bath apparatus revealing the temperatures of B2→R→B19´ two stage transformation, whereby the presence of R‒phase can be definitively confirmed. The Stress-Assisted Two-Way Memory Effect was measured by an own designed apparatus with an Linear Variable Differential Transformer captor and a current controlled heating, and results indicate that the as-spun condition, promotes the Stress-Assisted Two-Way Memory Effect. On the other hand, increments in Ni content tend to decrease transformation temperatures and high wheel velocities help to the R‒phase formation.

ABSTRACT: The main goal of this paper is to analyze if it is feasible to employ a trained shape memory alloy wire as a linear actuator to modify the camber of a morphing wing rib. In order to achieve this purpose, a morphing rib with a compliant trailing edge was proposed, developed, and subjected to structural analyses to ensure its flexibility. After the rib configuration was set, it was manufactured by a 3-D printer. The NiTi wire used as actuator was trained by a thermomechanical procedure based on a cycling process with a constant load application to present the two-way shape memory effect. In that way, the wire presents a determined length at its low-temperature phase and a shorter one at its high-temperature phase. Since the wire contraction and the torque applied by it are two crucial factors to define the camber curvature, it was decided to study two different wire lengths: 103.5 and 152.1 mm. The aerodynamic performance of the morphing cambered airfoils was studied using XFOIL software and compared to that of conventional airfoils with single hinged flap. The results show that both morphing airfoils present better aerodynamic performance for small angles of attack.

ABSTRACT: Maraging steels have low carbon content and are highly alloyed, having as main feature the ability to increase the mechanical strength after thermal aging. Therefore, the objective of this work is to analyze the effect of aging in a Co-free maraging steel, Fe - 0.014% C - 0.3% Mn - 3.9% Mo - 2.1% Cu - 0.19% Si - 11.8% Cr - 9.1% Ni - 1.0% Ti (wt), at different heat treatment times (10 min - 960 min) at constant temperature (550°C), after cold rolling up to 66% and 77% area reduction. Microstructures were analyzed by scanning electron microscopy (SEM - EDS) and mechanical properties by Vickers hardness measurements. The results showed a significant increase in the hardness of the material after aging heat treatment on the solution treated and deformed samples. The aging heat treatment which was harder about 650 HV was 550°C/60 min.

The current work evaluated the microstructures and martensitic transformation temperatures of NiTi shape memory alloy (SMA) deformed by equal channel angular extrusion (ECAE). The Ti-55.27wt.%Ni alloy was processed by 1 ECAE pass at 250 °C using a die with an intersection angle of 120°. After processing, samples were annealed at 300 °C, 400 °C and 500 °C for 1h to evaluate the microstructural changes. Microstructural characterization was performed by scanning electron microscopy (SEM) equipped with an energy dispersive spectrometer (EDS) device, and Vickers hardness measurement. Martensitic transformations temperatures were analyzed by differential scanning calorimetry (DSC). Results show that the annealing treatments presented no significant change in the microstructure of the ECAE processed samples. Meanwhile, the DSC curves corresponding to the annealing treatments performed at 300 °C and 400 °C show two step martensitic transformation related to B2→R→B19'. For the annealing at 500 °C, the martensitic transformation temperatures returned to the ST condition, indicating a reduction of the processing defects.

The effect of the microstructure on the shape recovery in stainless Fe-8Mn-5Si-13Cr-6Ni-12Co shape memory steel (SSMS) was evaluated using tensile tests and reversion temperature of 600ºC for a pre-strain of 4%. The tests were performed for a solution treated and annealed conditions at different temperatures after wire drawing of 57% area reduction. The best total shape recovery (TSR) was 83% for a sample deformed and annealed at 850ºC. It was concluded that the elastic (or, superelastic) shape recovery (ESR) is high when the austenitic matrix strength is high surpassing the value of shape recovery due to memory effect (SR) and once the austenitic matrix becomes softer, the contribution of SR increases and that of ESR decreases.

As propriedades de recuperação de forma da liga à base de Fe-Mn-Si-Cr-Ni-Co foram estudadas usando ensaio de compressão. Foram analisadas medidas de recuperação elástica (Re), recuperação de forma (Rf) e recuperação de forma total (R T = Re + Rf) em função do número de ciclos de treinamento. Os resultados indicaram que 3,3 foi a melhor razão entre altura (h o = 20 mm) e diâmetro (φo = 6 mm) para se conseguirem uma deformação homogênea e curvas de carregamento bem definidas. A maior contribuição para a R T foi atribuída à recuperação de forma. No último ciclo de treinamento, foi obtida uma R T = 90%, sendo 25% atribuída a Re.

The shape recovery properties of an Fe-Mn-Si-Cr-Ni-Co based alloy were studied using compression tests. Analyzed were the elastic recovery (Er), shape recovery (Sr) and total shape recovery (T SR = Er + Sr) measurements as a function of training cycles. The results indicated that 3.3 was the best ratio between height (h o = 20 mm) and diameter (φo = 6 mm) to obtain a homogeneous deformation and defined loading curves. A major contribution of T SR was attributed to shape recovery. In the last training cycle, a T SR = 90 % was obtained, where 25% was attributed to Er.

RESUMO Esse trabalho apresenta um estudo da dureza e do mee austenita-γ da liga inoxidável à base de ferro com efeito de memória de forma. Utilizando indentação instrumentada, foi possível determinar a dureza e o módulo de elasticidade das fases austenita-γ e martensita-ε separadamente. A fase martensítica apresentou uma dureza de 7,0 GPa, enquanto que a dureza da fase austenítica foi de 3,0 GPa. Os valores de módulo de elasticidade foram 202 e 137 GPa, para as fases martensita e austenita, respectivamente.

ABSTRACT This work presents a hardness study and elastic modulus for the ε-martensite and γ-austenite phases of an iron based shape memory alloy. Using instrumented indentation, it was possible to determine the hardness and elasticity modulus of these phases separately. The martensitic phase presented a hardness of 7.0 GPa and the austenitic phase presented a hardness of 3.0 GPa. The elastic modulus values were 202 and 137 GPa for the martensitic and austenitic phases, respectively.

No presente trabalho, três ligas inoxidáveis Fe-Mn-Si-Cr-Ni com efeito de memória de forma foram estudadas variando o tempo e a temperatura de austenitização - de 600s até 57600s a 1050ºC e de 800ºC até 1050ºC por 2400s - ação seguida por têmpera em água. Os resultados de microscopia ótica mostraram que não existiu variação significativa do tamanho de grão austenítico até a temperatura de 900ºC, atingindo-se valor médio de 30µm. Entre 950ºC e 1050ºC notou-se um aumento significativo do tamanho médio de grão austenítico quando comparado à temperatura de 900ºC ou menor. É possível observar um aumento de duas vezes no tamanho de grão ao serem comparadas as temperaturas de 950ºC e 1050ºC. Os resultados mostram que a influência da temperatura de austenitização no crescimento de grão parece ser mais significativa do que o tempo de austenitização. Ainda foi observado que o crescimento do grão depende da composição química, sendo menor para a liga que apresentava menor teor de manganês e maior teor de cromo.

In this work, three stainless steel alloys (Fe-Mn-Si-Cr-Ni) with shape memory effects were studied by varying austenitizing time and temperature - from 800ºC to 1050ºC for 2400s and from 600s to 57600s at 1050°C - followed by a water quenching. Optical microscopy results showed no grain size variation up to 900ºC with its average dimensions around 30µm. From 950ºC to 1050ºC, an increase of grain size was noted when compared to 900ºC and less. It is possible to note an increase of twice the grain size from 950ºC to 1050ºC. The results also show that the influence of the treatment temperature on grain growth seems to be more significant than the time elapsed. Also, it was observed that the grain growth depended on the chemical composition, being lower for the alloy with lower man-ganese and higher chromium content.

Entre os aços inoxidáveis, os austeníticos podem ser considerados os de melhor soldabilidade e de maior aplicação na indústria. No entanto, as propriedades desses aços podem ser facilmente degradadas em operações de soldagem ou em serviço. As trincas de solidificação podem ser controladas através da presença comedida de ferrita delta no depósito, fato que é amplamente discutido na literatura. No entanto, raramente se discute a presença da ferrita δ nos metais de base, uma vez que os aços inoxidáveis austeníticos solubilizados e temperados deveriam apresentar apenas traços dessa fase. Componentes forjados em inoxidáveis austeníticos têm apresentado quantidades elevadas de ferrita δ, chegando mesmo, em alguns casos, a apresentar precipitados de fases intermetálicas, trazendo sérias implicações na fabricação de equipamentos e tubulações. Nesse trabalho, é apresentado um estudo de caso onde a presença de ferrita δ e precipitados intermetálicos, particularmente a fase σ, tornaram impraticável a aplicação de flanges forjados de aço ASTM A182 F 317L em plantas de processo.

Within the stainless steel family, austenitic steel can be considered as the one with the best weldability and greatest application in the processing industry. However, the properties of this type of steel can be easily degraded during welding operations or plant service. The solidification cracks can be avoided when there is a moderate presence of δ ferrite in the welded metal, as described in technical literature. On the other hand, δ ferrite is seldom discussed when applied to base metals, since austenitic stainless steel, annealed and quenched, probably presents only traces of it at this phase. Forged austenitic stainless steel components have presented high amounts of δ ferrite, sometimes appearing in the intermetallic phases, creating serious concerns for equipment and pipe manufacturers. Herein, a case study is presented where δ ferrite and intermetallic precipitates, especially the σ phase, hindered the application of ASTM A182 F 317L forged flanges in processing plants.