The present study deals with the warm temperature deformation behavior of the Ti–6Al–4V alloy throughout investigating the flow behavior and the correlated microstructure evolutions. Toward this end, ...the tension and compression tests were conducted at temperatures of 100, 300, 500, and 600 °C under the strain rate of 0.001s−1. The low compressive formability of the material and premature fracture at 100 and 300 °C was attributed to the insufficient number of operative slip systems at this relatively low temperature regime. In contrast, in tensile mode the work hardening capacity was increased and the portion uniform elongation was considerable. In fact, the higher activity of pyramidal slip systems and higher dislocation maneuverability in tensile mode provided a proper condition for substructure development, which resulted in higher strain accommodation. Interestingly, the tensile deformed microstructures at 100 and 300 °C were strongly texturized and most of α-grains were oriented around the midline of 0001--1100 or even toward the -1100. This was attributed to the lower critical resolved shear stress of pyramidal slip in tension mode, which caused the rotation of the axis toward extension direction. The significant flow softening in compressive mode and appreciable post-necking deformation (due to diffused necking) in tensile mode were the main characteristics of the flow curves at 500, and 600 °C. This was attributed to the extended dynamic recovery and continuous recrystallization, spheroidization, and geometric softening. It was believed that the intensified substructure development facilitated the fragmentation and boundary splitting mechanism, therefore, trigger the spheroidization of α phase.
Abstract
The use of electrochemical methods allows fast and inexpensive corrosion measurements of bulk materials with high significance. In the case of thermal spray coatings, electrolyte penetration ...into open pores up to the substrate material can cause undesired mixed potentials. Furthermore, the implementation of complex geometries or rough surfaces remains a problem. Preconditioning of the surface or the use of the electrochemical cell is required to eliminate leakage. Therefore, reliably measuring corrosion is still a challenging task. This undermines fast monitoring of corrosion performance as a part of the production process. Gelling agents are investigated to increase the viscosity of many electrolytes. A procedure has been developed to determine the concentration level and the mixing conditions. Passivation and pitting-corrosion testing are performed on thermal spray AISI 316L coatings. The electrochemical potential curves as well as the corroded surface layers were studied in comparison to a liquid electrolyte. The suitability of the test on rough surfaces in the sprayed condition was investigated. The results prove the novel approach as an alternative to established electrochemical test methods with extended application range.
Abstract
Increasing demands on component performance and efficiency require continuous development efforts in existing material systems and concepts. In addition to the specific material properties, ...economic aspects have to be taken into account. Thermochemical treatments of iron-and nickel-base alloys allow for a distinct improvement of hardness and wear resistance. The process of boriding enables the highest hardness values and the formation of thermally stable precipitates. Especially nickel-base alloys are suitable for high temperature applications. An economic application of these alloys can be achieved by applying coating technologies and limiting the material usage to the surface. High-velocity oxygen fuel thermal spraying of the nickel-base alloy Inconel 718 and subsequent powder-pack boriding is conducted. Furthermore, the influence of a solution annealing step prior to the boriding process is investigated with the motivation to achieve a homogenisation of the coating. A successful diffusion enrichment and the formation of a precipitation layer could be achieved. The investigation of the resulting properties revealed a distinct increase of hardness and an improvement of wear resistance tested under reciprocating conditions in a wide temperature range.
Abstract
The surface integrity of parts is strongly impacted by the surface-layer properties, which are modified by machining processes. In particular, it is advantageous if the finish machining ...process generates a resilient residual-stress state without additional post-treatment. Thus, this paper describes relationships between the forces and temperature which are measured in-situ/during the process and the residual-stress profile for the turning of the aluminum alloy EN AW-2017. The residual-stress depth profiles are measured by X-ray diffraction after electrochemical removal of material by means of jet-electrochemical machining. The characteristic features of the residual-stress profile (value and depth under the surface of the local minimum of the residual stress) are determined and modeled using multiple regression. The predictions of the models are validated by test samples. An excellent agreement between experiments and the model is achieved. Thus, the models can be applied to predict the expected residual-stress profiles during the machining process, which allows for an in-process adjustment of the machining parameters in order to generate an advantageous residual-stress state.
The embedding of ceramic particles of a certain size and concentration in a ductile matrix is a strategy for producing wear-resistant coatings. In wire arc spraying, flux-cored wires with a metallic ...sheath and particle-filled core can be used for this purpose. The influence on wear properties of wire diameter and particle fraction has hardly been investigated so far. Due to the high kinetics of arc and gas flow, there are considerable challenges with regard to reproducibility, homogeneity and material efficiency. As a basis for a significantly improved understanding of the process, the relationships between the wire diameter, the particle fraction used and the wear resistance of the produced coatings have been investigated. The objects of investigation were flux-cored wires with an iron sheath and a particle filling of Al2O3/ZrO2. By using the ball-on-disc test and rubber wheel test, the wear resistance of the respective coatings was investigated.
Different manufacturing approaches have been investigated regarding their suitability to process high-entropy alloys (HEAs). However, comprehensive investigations on the influence of the production ...route on the microstructure, phase formation and properties have not been conducted yet. For the current study the alloy AlCoCrFeNiTi0.5 is considered. Previous investigations have proven the formation of phases with predominantly body centred cubic structure for this alloy. Castings are produced by arc-melting. Feedstock material for coating deposition and powder metallurgical processing is produced by inert gas atomisation. For the processing high-velocity-oxygen-fuel (HVOF) thermal spraying and spark plasma sintering (SPS) are applied. Due to the significantly differing process conditions and temperature-time profiles, differences of microstructure, phase formation and resulting properties can be observed. Wear investigations under various conditions have been conducted. Especially under sliding and reciprocating wear conditions the structural defects formed for the thermally sprayed coating cause a reduction of wear resistance. The formation of structural defects could be avoided by SPS. However, the additional tetragonal phase causes a reduction of the wear resistance. The current study contributes to a better understanding of the interaction between process, microstructure and properties.
In the present study, 35vol% SiCp/AlSi7Mg0,6 composites were prepared using field assisted sintering technique in order to investigate the effect of different particle fractions and size ...distributions of the AlSi7Mg matrix powder on the tensile properties of the produced composite material. In most usecases the size of the reinforcement phase is given by the application and is only variable within narrow limits (< 20pm particle size in this work). On the other hand, there is potential for optimization of the matrix powder. In this investigation, fine (d50 = 25 µm), coarse (d50 = 52 µm), bimodal (50wt% of fine + 50wt% of coarse, d50 = 36 µm) and as received (d50 = 40 µm) aluminum powder was used as the matrix powder. Using fine matrix powder has improved yield strength by 5 % and ultimate tensile strength by 7 % compared to the as received condition. This is largely due to the lower porosity of the composite produced under the use of the fine matrix powder ((0.07 ± 0.04) %) in contrast to the composite using the as received aluminum powder ((0.62 ± 0.35) %). At the same time, the consumed heating energy of the composite was decreased by almost a third when using the fine matrix powder in comparison to the use of the as-received matrix powder. This paper presents results of an optimization approach for mechanical properties of aluminum matrix composites without any changes of the sintering parameters.
•An EPD process has been established for yttrium oxide coating on tungsten weaves.•The EPD prepared yttrium oxide coating gives a porous structure.•The coating process has been optimized in terms of ...deposition thickness.•The yttrium oxide coating prepared by EPD process serves well as the interface between W fiber and W matrix in Wf/W composites.
As the interface material for tungsten fiber reinforced tungsten composites (Wf/W), yttrium oxide thin films were prepared via an electrophoretic deposition (EPD) method in this study. The yttrium oxide dispersion is optimized based on its Zeta potential. The coating process was performed on the tungsten weaves (anode), and stainless steel was used as the counter electrode (cathode). The coating structure obtained from the EPD process was homogeneously distributed on all the fibers of the weave, but possess a porous structure. In addition, to optimize the coating process, the voltage influence on the coating process is demonstrated. Using the EPD prepared yttria interface, Wf/W is produced based on a chemical vapor deposition process. Initial mechanical test shows a promising property with extrinsic toughening mechanisms and high damage resilience.
Abstract
The plasma electrolytic oxidation (PEO) is an innovative method for providing light metals and their alloys with protective ceramic surfaces. However, for iron-based materials, the process ...requires very high current densities and results in the formation of coatings which consist of less stable iron compounds. Therefore, it was the aim of this study to design a PEO procedure on low-carbon steel at moderate current densities, which allows for the formation of ceramic coatings whose chemical composition is dominated by the electrolyte constituents. The electrolyte used was based on aluminate and preselected by systematic electrochemical passivation experiments. The PEO treatment was monitored by electrical and optical process diagnostics. As a result of this, it was possible to obtain alumina layers of 80 micrometers in thickness, with a high corundum content of approximately 50 to 90%, after 37 minutes of treatment time, at a current density below 25 A/dm
2
on C8C-steel. However, the coating’s microstructure was inhomogeneous and showed poor substrate bonding. Based on the results of the experimental work, explanatory approaches were provided and a course of action is suggested for counteracting these problems.
Ethylene glycol-functionalized Co(II) carboxylates Co(CO
2
CH
2
(OC
2
H
4
)
n
OMe)
2
(
2a
,
n
= 1;
2b
,
n
= 2), Co(CO
2
CHPh(OC
2
H
4
)
2
OMe)
2
(
2c
) and Co(CO
2
CMe
2
(O–C
2
H
4
)
2
OMe)
2
(
...2d
) have been synthesized by the reaction of Co(OAc)
2
·4H
2
O with the corresponding acids MeO(C
2
H
4
O)
n
CRR′CO
2
H (
n
= 1, 2; R=H, R′=Me; R=H, R′=Ph). Based on the IR spectroscopic studies, the binding motif of the carboxylato ligands to cobalt is discussed. Thermogravimetry and mass spectrometry studies were carried out in order to investigate the thermal decomposition mechanism of
2a
–
2d
in the solid state. Based on the result obtained, complex
2b
was chosen as single-source precursor for the generation and stabilization of Co
3
O
4
nanoparticles (NPs) by solid-state thermal decomposition in air. Depending on the decomposition time, NPs with different chemical composition (consisting of Co
3
O
4
, CoO and Co) and with crystallite sizes ranging from 9 to 18 nm were obtained. Furthermore, the preparation of cobalt oxide-based nanocomposites by twin polymerization of 2,2′-spirobi4
H
-1,3,2-benzodioxasiline (
3
) in the presence of
2b
is reported. After treatment of the as-prepared hybrid material by either oxidation or etching, the respective mesoporous carbon/silica (IUPAC type IV isotherms) matrices were obtained. Quenched solid and nonlinear density functional theory calculations gave a surface area of 1040 cm
2
g
−1
for the respective carbon and 336 cm
2
g
−1
for the appropriate silica material. Powder X-ray diffraction measurements confirm the formation of Co
3
O
4
NPs in both components. High-angle annular dark field scanning transmission electron microscopy revealed well-distributed particles within the silica matrix, whereas no particles were found in the carbon material. The electrochemical properties of the composite materials have been investigated by cyclic voltammetry. The respective silica material shows five reduction events (Co
3
O
4
to CoO, CoO to Co), while no redox potentials occurred for the Co
3
O
4
NPs embedded in the carbon matrix.
PDF
