Cold Spray Deposition of Heat-Treated Inconel 718 Powders

In this work; Inconel 718 gas-atomized powder was successfully heat treated over the range of 700-900°C. As-atomized and as-heat treated powders were cold sprayed with both nitrogen and helium gasses. Cold spray of high strength materials is still challenging due to their resistance to particle deformation affecting the resulting deposit properties. Powder heat treatment to modify its deformation behavior has recently been developed for aluminum alloy powders; however; there is no literature reported for Inconel 718 powders. The microstructural evolution of the powder induced by the heat treatment was studied and correlated with their deformation behavior during the cold spray deposition. Deposits sprayed with heat-treated powders at 800 and 900 °C and nitrogen showed less particle deformation and higher porosity as compare to as-atomized deposit associated to the presence of delta phase in the powders precipitated by the heat treatment. In contrast; deposits sprayed with helium using both powder conditions; as-atomized and as heat-treated powders; showed high particle deformation and low porosity indicating that the type of gas has a greater effect on the particle deformation than the delta phase precipitated in the heat-treated powders. These results contribute to understanding the role of powder microstructure evolution induced by heat treatment on the cold spray deposits properties.

Context and Phase Dependent Effects of Delta transcranial Alternating Current Stimulation on Dynamic Attending

Attention requires the allocation of limited resources to properly interpret our environment, making it ultimately unsustainable. Dynamic Attending Theory suggests that, in order to realistically maintain vigilance to our surroundings, attention likely fluctuates between high and low energetic states, such that information can be processed more quickly and accurately during attentional peaks and vice versa. Additionally, prior studies have suggested that the phase of delta oscillations (1-4 Hz) are critically involved in the entrainment of attention. We investigated the physiological and behavioral entrainment of attention and the role that delta phase plays to moderate the benefits of this attending. Participants (N=28) passively listened to a background auditory rhythm and were required to complete a visual discrimination task while undergoing 2 Hz transcranial alternating current stimulation (tACS). The task involved identifying an image, either upright or inverted, presented either on or before the final beat, while receiving delta stimulation that was either aligned or unaligned with image presentation. As expected, reaction times (RTs) were faster for on-beat than off-beat stimuli, and for upright images than inverted. Crucially, tACS phase-aligned with the beat led to faster RTs over out-of-phase stimulation, but only for upright images; remarkably, this pattern was reversed for inverted images presented on-beat, with slower RTs for inverted stimuli during in-phase tACS. These results suggest that the effects of delta tACS are both phase and context dependent, and mediate a potential form of speed-accuracy tradeoff in the allocation of attentional resources during rhythmic entrainment.

The SEM and TEM Analysis of IN718 Alloy after Fatigue Push-Pull Loading at 700°C

Specimens from Ni-Cr-Fe wrought superalloy INCONEL 718 were used for fatigue push-pull test at elevated temperature 700 °C. Fatigue loading was with the coefficient of cycle asymmetry R = -1. Temperature of fatigue test was chosen from two reasons; one is that limit operating temperature for this type of alloy is 650 – 700 °C due to precipitation of stable but incoherent orthorhombic Ni3Nb delta phase; the second reason for this temperature is fact that metastable body-centred tetragonal Ni3Nb gamma double prime phase starts to transform to delta phase and from that reason there is an expectation for mechanical properties decreasing due to increased volume of delta phase. For evaluation of fatigued specimens were Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) observation used. Also methods of quantitative metallography – coherent testing grids were used for evaluation of delta phase volume. Some references describe that higher volume of delta phase, mostly situated at grain boundaries, act as initiation sites for cracks and therefore decreasing mechanical properties. Employed SEM and TEM analysis confirm the increased volume of delta phase in specimens after fatigue loading but a negative influence on dynamic properties, such fatigue lifetime, for this alloy was not confirmed.