Effect Of Microstructural Features Research Articles

The effects of microstructural features on the performance gap in corrosion resistance between bulk and HVOF sprayed Inconel 625

It is commonly observed that there is a performance gap between the corrosion resistance of thermally sprayed coatings and the equivalent bulk material. This is attributed to the significantly modified microstructure of the sprayed coatings. However, currently there is no detailed understanding of which aspects of microstructural modification are primarily responsible for this performance gap. In this work several deliberately microstructurally modified versions of the Ni-based superalloy Inconel 625 were produced. These were subjected to potentiodynamic electrochemical testing in 0.5 M H 2SO 4 to investigate the links between specific microstructural features and electrochemical behaviour. Samples were prepared by high-velocity oxy-fuel (HVOF) thermal spraying, laser surface remelting using a high power diode laser and conventional powder sintering. Microstructural features were examined by optical and scanning electron microscopy and X-ray diffraction. Potentiodynamic testing was carried out on the following forms of Inconel 625: wrought sheet; HVOF sprayed coatings; sintered powder compacts; laser melted wrought sheet and HVOF sprayed coatings. Using the corrosion behaviour, i.e. passive current density, of the wrought sheet as a baseline, the performance of different forms of Inconel 625 was compared. It is found that a fine dendritic structure (with associated microsegregation) produced by laser remelting wrought sheet has no significant effect on corrosion performance. Up to 12% porosity in sintered powder samples increases the passive current density by a factor of only around 2. As observed previously, the passive current density of HVOF sprayed coatings is 20–40 times greater. However, HVOF coatings subjected to laser surface remelting are found to have a passive current density close to that of wrought material. It is concluded that, whilst porosity in coatings produces some decrease in corrosion resistance, the main contributing factor is the galvanic corrosion of localised Cr-depleted regions which are associated with oxide inclusions within HVOF sprayed samples.

Dislocation theory based short crack model and its application for aircraft aluminum alloys

This paper presents a study on dislocation theory based short/small crack modeling, and its application for short crack growth life analysis on 2024-T351 aluminum specimens. The dislocation theory was applied to determine the crack tip opening displacement (CTOD) of a microstructurally short crack by taking into account the effects of microstructural features, such as grain size, orientation, and grain boundary. The CTOD was then used as the parameter for calculating the short crack growth rate. In this work, an existing CTOD model was modified for estimating the short crack life of 2024-T351 single edge notch tension (SENT) specimens, which were tested in an Advisory Group for Aerospace Research and Development (AGARD) program [Newman JC, Edward PR. Short-crack growth behavior in an aluminum alloy: an AGARD cooperative test programme. AGARD-R-732, NATO, Advisory Group for Aerospace Research and Development; 1988]. The analytical results matched the test results reasonably well.

Microstructure-based crystal plasticity modeling of cyclic deformation of Ti–6Al–4V

Ti–6Al–4V is a dual phase material with range of possible complex microstructures. It is well known that mechanical behavior of Ti–6Al–4V is significantly affected by its texture and microstructure morphology. A three-dimensional microstructure-based constitutive model for monotonic and cyclic deformation of duplex Ti–6Al–4V is developed and implemented. The model includes length scale effects associated with dislocation interactions with different microstructure features, and is calibrated using polycrystalline finite element simulations to fit the measured macroscopic responses (overall stress–strain behavior) of a duplex heat treated Ti–6Al–4V alloy subjected to a complex cyclic loading history. Representative microstructures are simulated using a three-dimensional finite element mesh with periodic boundary conditions imposed in all directions. The measured orientation and misorientation distributions of grains of this duplex Ti–6Al–4V are considered, and similar probability density distributions of the crystallographic orientations are assigned to the finite element mesh. The misorientation distributions are then fit using the simulated annealing method. Effects of microstructural features are examined and compared with the experimental data in terms of their influence on the material yield strength. The results are shown to be in good agreement with the experimental observations.

Deformation behavior of Al–Mg–Si alloy at the elevated temperature

Abstract Al–Mg–Si alloy is one of the most popular materials for light-weight automotive components production. Forging and extrusion are the usual processing routes to deal with Al–Mg–Si alloy. Specially, this alloy system has the possibility as the forging materials in the automotive industry as the market place has demanded the reduction of the total weight of car. Even though deformation behavior of Al–Mg–Si alloy has been investigated on the various aspects, however, most works on have focused on the extrusion process, which is relatively clear to understand the deformation characteristics compared to the forging process. In the present study, high temperature deformation behavior of Al–Mg–Si alloy has been investigated by constructing power dissipation map in which work efficiency can be related with microstructural evolution. The controlled compression tests have been done to obtain flow curves at a certain strain rate and temperature region. The effect of microstructural features on the deformation characteristics was carefully analyzed to discuss where the optimum processing windows for forging process is located. Finally, forging have been performed according to the processing window suggested by deformation map. The forging result was discussed in terms of the various features of thermomechanical response of Al–Mg–Si alloy.

High-stress abrasive wear response of 0.2% carbon dual phase steel: Effects of microstructural features and experimental conditions

Effects of heat treatment on the high-stress abrasive wear response of 0.2% carbon steel have been investigated at varying applied loads, abrasive (SiC) sizes and sliding distances. The heat treatment involved intercritical annealing at three different temperatures between Ac 1 and Ac 3 followed by ice water quenching in order to produce a dual phase microstructure consisting of varying quantities of ferrite plus martensite. The wear rate increased monotonically with applied load irrespective of the heat treatment schedule. Further, the wear rate increased drastically when the abrasive size was increased from 15 to 27 μm; a further increase in the abrasive size led to only a marginal increase in the wear rate. In general, the wear rate decreased with increasing sliding distance and attained a nearly stable value at longer sliding distances. Increasing intercritical annealing temperature resulted into higher martensite content, thereby leading to reduced wear rate. However, the extent of reduction in wear rate with martensite content has been found to change with the applied load and abrasive size. The present investigation clearly suggests that it is quite possible to attain desired combinations of bulk hardness and microstructure (ferrite plus martensite) that could greatly control abrasive wear properties in low carbon steel. The observed wear response of the samples has been explained on the basis of microconstituent–abrasive interaction during the course of abrasive action, degradation of the abrasive particles and the nature of various microconstituents, i.e. mechanical properties.

Plastic Deformation Characteristics of A356 Alloy with the Variation of Cast Microstructure

The effect of microstructural features of A356 alloys on plastic deformation behavior was studied in the present study. To authors' knowledge, the microstructural effect on mechanical properties of A356 alloy has not been well understood even though this alloy system is one of the most widely used alloys for the industrial purpose. Specially, quantitative relationship between properties like ductility and fracture toughness with microstructural features is lacking. In the present study, four different processing methods were used to investigate how the size and distribution of primary alpha and eutectic phases influence the plastic deformation characteristics. The prcessing routes for A356 control arm include low pressure casting, squeeze casting, rheo-casting and cast-forging processes. Special focus was given to understand which microstructural features do the beneficial or detrimental effect and how they work.

A stochastic model of damage accumulation in complex microstructures

A statistical approach for modeling fracture in brittle materials is presented. In particular, a microstructural-based finite element code called OOF is used in conjunction with a stochastic representation of failure that relies on the Weibull law. The OOF code, which maps materials microstructures onto finite element meshes, enables to calculate the local stress states; these stresses are used along with the statistical criterion for brittle fracture in order to determine microcrack formation and propagation. Computer simulations are performed on several microstructures of different materials types, e.g., laminates, particulate composites and polycrystals. The damage accumulation due to microcracking is characterized by the stereological measure of failed material and is investigated in order to assess the effect of microstructural features on the failure mechanism. Moreover, the approach allows to analyze the influence of the characteristic parameters for brittle materials on damage evolution.

Effect of Microstructural Features on Ductility of Drawn Pearlitic Carbon Steels

The effect of microstructural features on ductility of cold drawn pearlitic steels containing 0.52-0.92 wt% C was investigated. The relationship between ductility and microstructural features of interlamellar spacing, ferrite thickness and cementite thickness, was closely examined, compared with that with drawing strain. The variations of reduction of area (RA) with drawing strain well reflected the microstructural evolution occurred during wire drawing; RA increased initially with the progressive realignment of randomly oriented cementite, showed a maximum peak due to the completion of the alignment of most cementite, and decreased with thinning or fragmentation of the aligned cementite. Among factors on ductility, cementite thickness was found to be the most dominant microstructural feature for RA of drawn pearlitic wires, regardless of transformation temperature and carbon content in steels. Additionally, the presence of the specific cementite thickness for the maximum RA in drawn steel wires was observed as about 0.006-0.009 μm.

Strengthening in magnesium alloys by icosahedral phase

Strengthening effects of quasicrystalline icosahedral phase has been studied in two alloys Mg95Zn4.2Y0.8 and Mg92.5Zn6.5Y extruded at 250 and 400 8C. The quasicrystal particles are facetted and show definite orientation relationships with the matrix. Due to its high symmetry and quasiperiodicity, the icosahedral phase can form strong interfaces with the matrix in various orientations. The icosahedral phase particles have a strong pinning effect on the grain boundaries, which stabilizes grain size. The icosahedral particles are resistant to coarsening, and remain hard at higher temperatures, imparting good strength with ductility at 200 °C. Very few deformation structures such as high dislocation density and twins are observed after extrusion or tensile tests. Dislocations commonly observed are c-type. Due to the stability of microstructure, various post-extrusion treatments are possible. In the Mg92.5Zn6.5Y alloy upon annealing at 400 °C the icosahedral phase transforms to a hexagonal Mg25Zn58Y17 phase. The icosahedral phase then reprecipitates on its interface, forming a nano-composite. Effects of microstructural features on the deformation behavior are described.

신선가공한 고탄소 강선의 연성에 영향을 주는 미세조직 인자

The effect of microstructural features on ductility of cold drawn pearlitic steels containing 0.52 ～ 0.92 wt%C was investigated. During wire drawing, reduction of area (Rh) increased initially with the progressive realignment of randomly oriented cementite, showed a maximum peak due to the completion of the alignment of most cementite, and decreased with thinning or fragmentation of the aligned cementite. Among factors on ductility, cementite thickness was found to be the most dominant microstructural feature for Rh of drawn pearlitic wires, regardless of transformation temperature and carbon content in steels.

Fracture behavior of thixoformed 357-T5 Al alloys

The effects of microstructural features on the fracture behaviors, including impact, high-cycle fatigue, fatigue crack propagation, and stress corrosion cracking, of thixoformed 357-T5 (Al-7 pct Si-0.6 pct Mg) alloy were examined. The resistance to impact and high-cycle fatigue of thixoformed 357-T5 tended to improve greatly with increasing volume fraction of primary α. An almost threefold increase in impact energy value was, for example, observed with increasing volume fraction of primary α from 59 to 70 pct. The improvement in both impact and fatigue properties of thixoformed 357-T5 with increasing volume fraction of primary α in the present study appears to be related to the magnitude of stress concentration at the interface between primary α and eutectic phase, by which the fracture process is largely influenced. The higher volume fraction of primary α was also beneficial for improving the resistance to stress corrosion cracking (SCC) in 3.5 pct NaCl solution. The in-situ slow strain rate test results of thixoformed 357-T5 in air and 3.5 pct NaCl solution at various applied potential values demonstrated that the percent change in tensile elongation with exposure decreased linearly with increasing volume fraction of primary α within the range studied in the present study. Based on the fractographic and micrographic observations, the mechanism associated with the beneficial effect of high volume fraction of primary α in thixoformed 357-T5 alloy was discussed.

The Effect of Microstructural Features and Defects on the Ductility of High Pressure Die Cast AS21, AM60 and AZ91

The ductility of die cast AS21, AM60 and AZ91 tensile bars varied considerably depending on the alloy composition and the section thickness. The elongation to fracture in AM60 ranged from 1.8-15.0%, in AS21 from 3.2-10.3% and in AZ91 from 0.9-7.2%. In all alloys, the elongation increased as the section size increased from 1 mm to 5 mm, however the 10 mm thick samples had a lower elongation than the 5 mm thick samples. Fracture occurred along grain boundaries. The variation of ductility with section thickness appeared to be related to particular microstructural features. Observations using secondary electron imaging and optical metallography indicated that in the samples which failed most prematurely, a microstructural feature located close to the surface of the casting was involved. These features were surface interdendritic porosity, usually a defect band formed on the surface of the casting, or oxide and carbonate films. Entrained gas porosity, internal defect bands and externally solidified grains, caused a less significant reduction in the elongation to fracture.

Effect of carbon content on mechanical properties of fully pearlitic steels

The effects of microstructural features, such as interlamellar spacing, prior austenite grain size (AGS), and carbon content, on mechanical properties have been investigated for fully pearlitic steels containing 0.52 and 0.82 wt-%C. Interlamellar spacing and prior AGS had significant influence on strength and ductility of eutectoid steels. While interlamellar spacing and prior AGS had a significant influence on the ductility of eutectoid steels, the carbon content showed a stronger effect on ductility than interlamellar spacing and prior AGS for fully pearlitic steels with various carbon contents. The carbon content in steels controls the discontinuous nature, the thickness, and the volume fraction of cementite in pearlite. Therefore, hypoeutectoid steels show the superior reduction of area than eutectoid steels in all tested ranges of interlamellar spacing, in spite of the various prior AGS.

Effect of microstructural features on the electrical properties of TiO 2

The effect of microstructural features on the electrical properties of TiO 2 (rutile) was studied using impedance spectroscopy in conjunction with X-ray diffraction and scanning electron microscopy. The electrical properties of TiO 2 were measured at 350 °C after sintering in air at different temperatures. It was found that the electrical resistivity increased significantly after the interconnected pores closed in the TiO 2 due to sintering. However, further decrease of porosity in the TiO 2 had little effect on its resistivity prior to the closure of pores. On the other hand, after the sintered specimens were annealed in vacuum, the resistivity of TiO 2 decreased considerably without apparent change of the microstructure in TiO 2. Moreover, the resistivity, measured at room temperature, decreased after the interconnected pores closed due to sintering. These results suggest that the electrical conduction in TiO 2 after sintering in air is mainly due to ionic conduction, while the conduction in the sintered TiO 2 after annealing in vacuum was mainly due to electronic conduction. The closure of the open pores in the TiO 2 reduced the surface ionic conduction but increased the electronic conduction route. Overall, the microstructural features of TiO 2 have characteristic effects on its electrical properties.

Micromechanics simulation of spontaneous polarization in ferroelectric crystals

Based on a modified equivalent inclusion method and a thermodynamic theory, a micromechanics model is developed to predict the evolution of internal stress and the effects of microstructural features (i.e., new domain concentration and porosity) and the external hydrostatic pressure on the transition temperature for single crystal ferroelectric ceramics upon cooling. During the cubic-to-tetragonal transition the material is a composite, with an internal stress due to phase transformation. Consideration of the thermodynamic driving force and resistance force leads to a kinetic equation that provides the volume concentration of new domains at a given level of temperature and applied stress. This new domain concentration results in an increase of overall electric polarization. Numerical calculations using the properties of BaTiO3 indicate that internal stress in the parent cubic phase continues to develop during spontaneous polarization, and that increased porosity greatly facilitates the transition process. Hydrostatic pressure provides an additional contribution to the Gibbs free energy of the system and also can increase the transition temperature. Its effect generally follows the Clausius-Clapeyron equation, but weak nonlinearity is found at very high pressure. This departure is attributed to the change in heat capacity during the cubic–tetragonal transition. A comparison with some observed data for the evolution of overall polarization of a BaTiO3 crystal serves to support the rationality of the model.

Effect of Microstructural Features on the Austempering Heat Treatment Processing Window

Microstructural changes during the austempering heat treatment of alloyed ductile iron are described. The effect of heat treatment cycle and casting structure on mechanical properties of Austempered Ductile Iron, ADI, of composition 3.52%C, 2.64%Si, 0.25%Mo, 0.25%Cu, 0.67%Mn are studied. Comparison of EDX analysis with mechanical properties show that the optimised properties for an ADI are achieved when an ausferritic structure of high carbon austenite and ferritic plates with minimum martensite and carbides is formed. Processing Window is the period of austempering time over which the structure is fully ausferritic The results show that for a high Mn DI, single austempering process does not provide a fully ausferntic structure and usually further austempering process, double or stepped austempering process, is required. The experimental results demonstrate that such an ADI still might satisfy the ASTM standard with careful control over austempering parameters. This study confirms that some casting features like low nodules count and preferential segregation of alloying elements narrow the processing window This study also shows that the microstructural defects like microporosity and poor modularity have a detrimental effect on properties of ADI.

Theoretical approach to effective electrostriction in inhomogeneous materials

An analytical approach is developed for the effective electrostriction, a nonlinearly coupled electromechanical effect, in inhomogeneous materials based on Green's-function method. For an isotropic composite containing randomly oriented ferroelectric crystallites with cubic symmetry, we derive the first effective-medium-like formulas for calculating its effective electrostrictive coefficients. The effects of microstructural features (such as volume fraction, crystallite shape and orientation, and connectivity of phases) on the effective electrostrictive coefficients are illustrated numerically and discussed. The calculations show that it is possible to develop new electrostrictors combining large electrostriction with mechanical flexibility by choosing the best combination of ferroelectric ceramics and polymer.

Effect of microstructural features on ductility in hypo-eutectoid steels

Effect of microstructural features on ductility in hypo-eutectoid steels

Toughness and microstructural parameters in thermomechanically processed 2091 Al-Li alloy

The effect of microstructural features on the toughness of 2091 A1-Li alloys processed with various thermomechanical schemes was analyzed. Grain size and the distribution of precipitates were varied by thermomechanical processing. Dynamic fracture toughness tests were done to evaluate the toughness of these materials using a Computer Aided Instrumented testing system. The δ (A13Li) and S’ (Al2CuMg) precipitates, which are primary strengthening precipitates, were confirmed by TEM analysis in the under-aged 2091 Al-Li alloy. From analysis of the relation between toughness and the microstructural factors induced by thermomechanical processing, the toughening of the alloy was found to be associated with the refinement of the subgrains. The strength and toughness parametzrs were well related to subgrain size.

Microstructural control and mechanical properties of dual-phase TiAl alloys

Abstract This paper summarizes our recent work on the effects of microstructural features on the mechanical properties of TiAl alloys prepared by powder and ingot metallurgy. TiAl alloys based on Ti-47Al-2Cr-2Nb (at%) were alloyed with small amounts of Ta, W, and B additions for control of alloy phases and microstructure. The alloys were processed by hot extrusion above and below Tα, followed by short- and long-term heat treatments at temperatures to 1350 °C in vacuum. The microstructural features in the lamellar structures were characterized by metallography, SEM and TEM, and the mechanical properties were determined by tensile tests at temperatures to 1000 °C. The tensile elongation at room temperature is mainly controlled by the colony size, showing an increase in ductility with decreasing colony size. The yield strength, on the other hand, is sensitive to the interlamellar spacing. Hall-Petch relationships hold well for both yield strength and tensile elongation at room and elevated temperatures. TiAl alloys with refined colony size and ultrafine lamellar structures possess excellent mechanical properties for structural applications at elevated temperatures.