Fine Dimples Research Articles

Influence of Temperature Rise during Finishing Rolling on the Mechanical Properties of T8 Cord Steel

Different temperature rises during finishing rolling of T8 cord steel were obtained through normal procedure (process 1) and controlled cooling procedure (process 2), and its effects on the microstructure and the mechanical properties were studied. The microstructure of T8 cord steel obtained by process 1 consisted of clusters of coarser and inhomogeneity sorbite colonies, and that obtained by process 2 consisted of equiaxed and finer sorbite colonies, with the similar interlamellar spaces of sorbite cementite in the two microstructures. The results of tensile tests showed that the tensile strength of T8 cord steel obtained by process 2 was higher than that obtained by process1 by 20MPa, with higher reduction of area and tensile elongation. In addition, the tensile fracture of T8 cord steel obtained by process 1 consisted of a lot of lotus cleavage plane, while that of T8 cord steel obtained by process 2 mainly consisted of finer dimples. That indicated that the smaller temperature rise of process 2 during finishing rolling can improve the mechanical properties of T8 cord steel.

The Influence of Li on the Tensile Properties of Al-Mg<sub>2</sub>Si Metal Matrix Composite

The present research is undertaken to study the influence of Li on the tensile properties of Al-15%Mg2Si metal matrix composite. The tensile test results show that adding 0.15%Li to the composite raises the UTS and elongation values, insignificantly. In the point of fracture behaviour of the composite, the addition of 0.15%Li converts the fracture behaviour from brittle, with cleavage facets, to ductile, with fine ductile dimples.

The effect of solution temperature on the microstructure and tensile properties of Al–15%Mg 2Si composite

The effect of different solution temperatures has been investigated on the microstructure and tensile properties of in situ Al–Mg 2Si composite specimens were subjected to solutionizing at different temperatures of 300 °C, 350 °C, 400 °C, 450 °C, 500 °C, 550 °C and 580 °C for holding time of 4 h followed by quenching. The microstructural studies of the polished and etched samples by scanning electron microscopy (SEM) in the solution condition indicated that the increase in the temperature changes the morphology of both the primary and secondary Mg 2Si phases. Solutionizing led to the dissolution of the Mg 2Si particles and changed their morphology. Tensile test results indicated that ultimate tensile strength (UTS) gradually decreased upon solutionizing from 300 to 550 °C while further increase in the temperature followed by a sharp decrease in UTS up to 580 °C solutionizing temperature. It was found that the elongation has become three times greater in comparison to the as-cast state. Elongation results showed an increase up to 500 °C and then reduced temperatures of 550 and 580 °C. Fractographic analysis revealed a cellular nature for the fracture surface. On the cellular fracture surface, the features of both brittle and ductile fracture were present simultaneously. As a result of solution treatment the potential sites for stress concentration and crack initiation areas were reduced due to softening of the sharp corners and break up of eutectic network respectively, while increase in the number of fine dimples rendered the nature of fracture to ductile and also increased elongation.

Effects of High-Temperature Solutionizing on Microstructure and Tear Toughness of A356 Cast Aluminum Alloy

A356 alloy was cast into permanent molds and subjected to T6 heat treatment. Two solutionizing (solution heat treatment) temperatures (540°C and 560°C) were used, with the holding time for each varied between 0 min and 240 min. Tear test specimens of 4 mm in thickness were machined from the castings. Tear toughness (UEp: unit crack propagation energy) was obtained from load-displacement curves. With increasing solutionizing holding time, eutectic silicon particles were found to become larger and mean interparticle distance was found to increase. The UEp of specimens solutionized at the higher temperature (560°C) increased with holding time until 120 min but decreased with longer holding times. SEM observation of fracture surfaces after tear testing revealed dimples originating from dispersed silicon particles within the eutectic phase. Fine dimples were also observed in the eutectic aluminum region in specimens with reduced UEp. These fine dimples were different from the dimples found to originate from the eutectic silicon particles.

Effect of Cooling Rates during Solidification of Al-5.5%Mg-2.3%Si-0.6%Mn and Al-13%Mg<SUB>2</SUB>Si Pseudo-Binary Alloys on Their Secondary-Particle Morphology and Tear Toughness

Al–5.5%Mg–2.3%Si–0.6%Mn alloy and Al–13%Mg2Si pseudo-binary alloy were cooled at various cooling rates during solidification. Morphological change of secondary particle was examined. Solidified structure of Al–5.5%Mg–2.3%Si–0.6%Mn alloy consisted of primary α–Al dendrites, Al–Mg2Si eutectic structure and Al6Mn particles. The most significant morphological change was from lamella Al–Mg2Si eutectic structure in the permanent mold cast and the die-cast product to refined and globular Mg2Si phase in the high-speed twin-roll cast product. In contrast, no morphological change was observed for Al6Mn particles. In order to investigate the effect of morphological change of Mg2Si phase on tear toughness, tear toughness tests were performed for Al–13%Mg2Si pseudo-binary alloy cast products produced by permanent mold casting and high-speed twin-roll casting. Unit crack propagation energy (UEp) of the high-speed twin-roll cast products was 5 times higher than that of permanent mold cast products. For permanent mold cast specimens, crack propagation occurred along the interface of plate-like Mg2Si/matrix of the eutectic solidified region and in Al-matrix. But fracture surface of high-speed twin-roll cast specimens was covered with fine dimples. Such difference of fracture mode is due to morphological change of Mg2Si phase from plate-like to globular by rapid cooling.

Effect of cooling rates during solidification of Al–5.5%Mg–2.3%Si–0.6%Mn and Al–13%Mg2Si pseudo-binary alloys on their secondary-particle morphology and tear toughness

Al–5.5%Mg–2.3%Si–0.6%Mn alloy and Al–13%Mg2Si pseudo-binary alloy were cooled at various cooling rates during solidification. Morphological change of secondary particle was examined. Solidified structure of Al–5.5%Mg–2.3%Si–0.6%Mn alloy consisted of primary α–Al dendrites, Al–Mg2Si eutectic structure and Al6Mn particles. The most significant morphological change was from lamella Al–Mg2Si eutectic structure in the permanent mold cast and the die-cast product to refined and globular Mg2Si phase in the high-speed twin-roll cast product. In contrast, no morphological change was observed for Al6Mn particles. In order to investigate the effect of morphological change of Mg2Si phase on tear toughness, tear toughness tests were performed for Al–13%Mg2Si pseudo-binary alloy cast products produced by permanent mold casting and high-speed twin-roll casting. Unit crack propagation energy (UEp) of the high-speed twin-roll cast products was 5 times higher than that of permanent mold cast products. For permanent mold cast specimens, crack propagation occurred along the interface of plate-like Mg2Si/matrix of the eutectic solidified region and in Al-matrix. But fracture surface of high-speed twin-roll cast specimens was covered with fine dimples. Such difference of fracture mode is due to morphological change of Mg2Si phase from plate-like to globular by rapid cooling.

Tensile fracture characteristics of nanostructured ferritic alloy 14YWT

High temperature tensile fracture behavior has been characterized for the nanostructured ferritic alloy 14YWT (SM10 heat). Uniaxial tensile tests were performed at temperatures ranging from room temperature to 1000 °C in vacuum at a nominal strain rate of 10 −3 s −1. Comparing with the existing oxide dispersion strengthened (ODS) steels such as Eurofer 97 and PM2000, the nanostructured alloy showed much higher yield and tensile strength, but with lower elongation. Microstructural characterization for the tested specimens was focused on the details of fracture morphology and mechanism to provide a feedback for process improvement. Below 600 °C, the fracture surfaces exhibited a quasi-brittle behavior presented by a mixture of dimples and cleavage facets. At or above 600 °C, however, the fracture surfaces were fully covered with fine dimples. Above 700 °C dimple formation occurred by sliding and decohesion of grain boundaries. It was notable that numerous microcracks were observed on the side surface of broken specimens. Formation of these microcracks is believed to be the main origin of the poor ductility of 14YWT alloy. It is suggested that a grain boundary strengthening measure is essential to improve the fracture property of the alloy.

The reaction mechanism and mechanical properties of the composites fabricated in an Al–ZrO 2–C system

The in situ composites with the reinforcement volume fraction of 30 vol.% and the C/ZrO 2 mole ratio of 0, 0.5 and 1.0 have been fabricated by using exothermic dispersion synthesis in an Al–ZrO 2–C system. The reaction mechanism and mechanical properties of the composites have also been studied. When the reinforcement volume fraction of the composites is 30 vol.% and the C/ZrO 2 mole ratio is zero, the Al first reacts with ZrO 2 to produce the α-Al 2O 3 particles and the active Zr atoms, and then the Zr atoms react with Al to form the Al 3Zr blocks, which are distributed uniformly throughout the aluminum matrix. The ultimate tensile strength and elongation of the composites at room temperature are 215.2 MPa and 3.0%, respectively. The fracture mechanism of the composite can be characterized by a crack nucleus initiating in the Al 3Zr blocks and then propagating to the interface because of the poor properties of Al 3Zr. With increasing the C/ZrO 2 mole ratios, the ZrC is formed previous to the Al 3Zr due to its lower Gibbs free energy, and its formation peak becomes bigger in the DSC curve. The amount of the Al 3Zr blocks decreases, which leads to the improvement in the tensile properties of the composites. When the C/ZrO 2 mole ratio is up to 1, the Al 3Zr blocks have almost disappeared in the composites. The reinforcements are composed of α-Al 2O 3 and ZrC. At the same time, the ultimate tensile strength and elongation increase to 245.4 MPa and 8.0%, respectively. The tensile fracture surface is composed of fine ductile dimples.

Mg–Cu–Y–Ag bulk metallic glasses with enhanced compressive strength and plasticity

Novel Mg–Cu–Y–Ag bulk metallic glasses (BMGs) were prepared by water cooled copper mold casting. The compressive test shows that, by partial substitution of Mg with Ag, the compressive fracture stress, plastic strain, and Young's modulus of Mg 58Cu 25Y 10Ag 7 BMG alloy reach 1330 MPa, ∼0.2% and 74.8 GPa, respectively. The compressive strength of this BMG is the highest among those of Mg-based BMGs and crystalline alloys reported to date. Fine vein patterns and dimples are observed on the fracture surface of these BMGs. It is suggested that the improvement in the mechanical properties of Mg–Cu–Y–Ag BMG alloys is mainly attributed to the high moduli and Poisson's ratio of Ag.

Evolution of Surface Morphology in Ni(γ)/Ni<sub>3</sub>Al(γ´) Two-Phase Foil during Electrochemical Etching

Evolution of surface morphology in Ni(γ)/Ni3Al(γ´) two-phase foil of binary Ni-18 at.%Al was examined during the electrochemically selective etching in the electrolyte of distilled water including 1 wt.% (NH4)2SO4 and 1 wt.% citric acid. In the early stage (0.5 h), only the γ matrix was etched and the outmost γ´ particles were protected by a preexisting surface product. As the γ matrix was etched more, the side surfaces of the outmost γ´ particles and the γ´ particles that were located inside were exposed in the electrolyte. They were dissolved, and had a high density of fine dimples. However, the dissolution rate of the γ´ particles was slower than that of the γ matrix and thus the selective etching was retained in this stage. Finally, at 5h, more γ´ particles were exposed and the flat and smooth surfaces of the outmost γ´ particles were completely eliminated by the dissolution on the side surfaces. From these observations plus the saturation of the current density observed in the electrochemical test, we concluded that the change in the surface morphology was finished at this stage. Thus, the surface became more rough and irregular, which resulted from the original two-phase microstructure and the fine dimple structure by transpassivation.

Low Temperature Failure of Fe<sub>76</sub>Ni<sub>2</sub>Si<sub>9</sub>B<sub>13</sub> Compacted from Amorphous Glass Powder

The fracture surface morphology of Fe76Ni2Si9B13 bulk amorphous alloys failed in compression at temperatures from 4.2 to 300 K was investigated. The samples were prepared by the explosive compaction technique from amorphous powder. It has been found that fracture stress decreases with temperature from 300 to 4.2 K. In this temperature range, the brittle failure prevails. The failure propagates across particles and along particle boundaries too. The fracture micromorphology is riverlike pattern with fine dimples.

Effects of RE oxide on the microstructure of hardfacing metal of the large gear

The Rare Earth (RE) oxide was added into the electrode coating for hardfacing large gear and the microstructures of the hardfacing specimens with and without RE oxide were observed by using optical microscopy (OM). Meanwhile, the matrix phase of the hardfacing metals was determined by using X-ray diffraction (XRD) and the fractographs as well as inclusions were observed and analyzed by using scanning electron microscopy (SEM) with energy dispersive spectrum (EDS). The results show that, the microstructure of the hardfacing metal is mainly composed of the fine acicular ferrite, and the fracture surface is uniform and fine dimple exists on the fractograph of the specimen with RE oxide. The inclusions become spherical ones, which are distributed in hardfacing metal dispersively. However, the microstructure of the hardfacing metal is composed of coarse acicular ferrite and pearlite, and the fractograph is composed with dimple and quasi-cleavage in the specimen without RE oxide.

Mechanical properties of QFP micro-joints soldered with lead-free solders using diode laser soldering technology

Soldering experiments of quad flat package(QFP) devices were carried out by means of diode laser soldering system with Sn-Ag-Cu and Sn-Cu-Ni lead-free solders, and competitive experiments were also carried out not only with Sn-Pb eutectic solders but also with infrared reflow soldering method. The results indicate that under the conditions of laser continuous scanning mode as well as the fixed laser soldering time, an optimal power exists, while the optimal mechanical properties of QFP micro-joints are gained. Mechanical properties of QFP micro-joints soldered with laser soldering system are better than those of QFP micro-joints soldered with IR reflow soldering method. Fracture morphologies of QFP micro-joints soldered with laser soldering system exhibit the characteristic of tough fracture, and homogeneous and fine dimples appear under the optimal laser output power.

Study of the microstructure and mechanical properties of composites fabricated by the reaction method in an Al–TiO 2–B 2O 3 system

This paper investigates the microstructure, mechanical properties and fracture mechanism of composites fabricated by the exothermic dispersion method in the Al–TiO 2–B 2O 3 reaction system. When the B 2O 3/TiO 2 mole ratio is zero, the reinforcements of the composites are composed of α-Al 2O 3 and Al 3Ti. The α-Al 2O 3 particulates are segregated at matrix boundaries and cannot enter the aluminum matrix grains due to poor wetting with the matrix and the Al 3Ti rods distribute uniformly throughout the matrix. The ultimate tensile strength and elongation of the composites at room temperature are 250.4 MPa and 4.0%, respectively. The fracture mechanism of the composite can be characterized by a crack nucleus initiating in the Al 3Ti rod and then propagating to the interface because of the poor properties of Al 3Ti. When the B 2O 3/TiO 2 mole ratio reaches 1, the Al 3Ti phase is almost eliminated, the ultimate tensile strength and elongation increase to 320.8 MPa and 10.6%, respectively, and the tensile fracture surface is composed of fine ductile dimples. When the test temperature is about 723 K and the B 2O 3/TiO 2 mole ratio is 1, the elongation increases to 20.5% and the ultimate tensile strength decreases to 85.6 MPa.

Charpy impact energy of nanocrystalline and polycrystalline cobalt

Charpy impact energy and microhardness of electrodeposited nanocrystalline cobalt (average grain size ∼18 nm), as well as electrowon polycrystalline cobalt in as-deposited (average grain size ∼1 μm) and annealed (average grain size ∼10 μm) conditions, were studied. At room temperature nanocrystalline, as-deposited and annealed polycrystalline cobalt had impact energies of about 2, 6 and 9 J, and hardness values of about 5, 3 and 2.5 GPa, respectively. Finer dimples were observed on the fracture surfaces of nanocrystalline cobalt in comparison with polycrystalline cobalt.

Influence of modified processing on structure and properties of hot isostatically pressed superalloy Inconel 718

Inert gas atomized (IGA) superalloy Inconel 718 powder was consolidated by hot isostatic pressing (HIPing) at 1200 °C under 120 MPa pressure for 3 h. The HIPed alloy heat treated as per the aerospace materials specification (AMS) 5662J standard schedule, viz. solution treatment (ST) at 980 °C for 1 h/water quenching (WQ) to room temperature (RT) and a two-step ageing treatment (AT) at 720 °C for 8 h/furnace cooling (FC) at 55 °C h −1 to 620 °C and holding at 620 °C for 8 h and air cooling (AC) to room temperature has exhibited the yield strength (YS) and ultimate tensile strength (UTS) comparable to that of the conventionally processed (forged and heat treated) IN 718. However, its ductility and stress rupture properties at 650 °C were found to be poor due to the presence of prior particle boundary (PPB) networks decorated with highly stable oxides (Al 2O 3 and TiO 2) and brittle MC (Nb, Ti)C carbides. To mitigate this problem, the HIPed alloy was subjected to solution treatment at 1270 °C for 1 h followed by re-HIPing at 1100 °C/130 MPa/3 h before heat-treating it as per AMS 5662J standard schedule. Optical and scanning electron microscopy (SEM) of the alloy processed under modified HIPing and heat treatment conditions have shown the dissolution of MC-carbides, breaking up of PPB networks and formation of equiaxed grains with an average diameter of about 50 μm. Transmission electron microscopy (TEM) of this alloy has revealed uniform distribution of γ″ and γ′ strengthening precipitates in the γ-matrix and the presence of δ(Ni 3Nb) phase as well as very fine oxide particles near the grain boundaries. The tensile properties of the alloy processed under modified conditions have shown quite satisfactory levels of YS and UTS combined with a significantly improved elongation (EL) values at room temperature (19.5%) and at 650 °C (8.0%). The improvement in alloy ductility was found to correlate well with the fractography of the tensile tested specimens, which showed the predominance of transgranular fracture with fine dimples at room temperature and fine dimples together with the particle boundary facets at 650 °C. The stress rupture properties of modified processed alloy at 650 °C and at a stress level of 690 MPa have shown a vastly improved rupture life of 80 h combined with 5% ductility. The improvement in tensile and stress rupture properties accomplished by the modified processing makes it possible to explore the near net shape capability of HIP technology to its full potential in the development of alloy 718 components.

Effect of Boron and Carbon on the Fracture Toughness of IN 718 Superalloy at Room Temperature and 650 °C

The effect of B and C microadditions on the fracture toughness of IN 718 superalloy was investigated at room temperature (RT) and at 650 °C. At RT, the fracture toughness was observed to increase with increasing B and C concentrations. C had a relatively weak effect on the fracture toughness at 650 °C, but the influence of B was significant. At RT the highest fracture toughness value was obtained for the alloy with 29 ppm B and 225 ppm C at RT, and at 650 °C the alloy with 60 ppm B and 40 ppm C had the highest fracture toughness. An increase in the concentration of B to 100 ppm, however, resulted in a reduction in the fracture toughness at 650 °C. Fractographic observations showed that the formation and coalescence of microvoids was the predominant fracture mechanism at RT. In contrast, at 650 °C, the fracture surface exhibited intergranular cracking in the alloy with lower B concentrations and transgranular cracking coupled with fine dimples in the alloy with higher B concentrations. It is suggested that B impedes intergranular cracking by increasing the cohesion of grain boundaries and improving the grain boundary stabilization. The RT increase in the fracture toughness of the material caused by the addition of C is attributed to the formation of intergranular and intragranular carbides that increased the resistance to the plastic deformation.

Deformation processing and mechanical properties of Cu–Cr–X (X=Ag or Co) microcomposites

The strength of Cu–Cr–X (X=Ag or Co) improved by intermediate heat treatments during the rolling process. The strength (798 MPa) of Cu–Cr–Ag microcomposites with intermediate heat treatments at 550 °C (IH 550) was found to be better than that of Cu–Cr–Ag microcomposites (423 MPa) with no intermediate heat treatment (NH). The improvement of the strength is attributed to the refinement of the microstructural scale by thermo-mechanical processing involving intermediate heat treatments. Severely elongated Cr filaments along with relatively coarse elongated Cr particles are clearly observed on the longitudinal sections. The Cr particles in Cu–Cr–Ag microcomposites were observed to be larger than that of Cu–Cr–Co microcomposites because of the initially coarser Cr dendrites. On the other hand, the width between filaments in Cu–Cr–Ag (2.4 μm) was found to be slightly smaller than that of Cu–Cr–Co (2.44 μm). The finer filamentary structure in Cu–Cr–Ag can be attributed to easier refinements due to the stronger Cu matrix strengthened by Ag precipitates. Cu–Cr–Ag and Cu–Cr–Co microcomposite plates exhibited ductile fractures. The surfaces of the microcomposite plates show many fine dimples and relatively coarse dimples. Examination of the coarse dimples on the fracture surfaces revealed fractured Cr filaments or particles in the center.

Microstructure and mechanical properties of rapidly solidified Al–7wt.%Mg–X (X=Cr, Zr or Mn) alloys

In this study, the effect of the transition elements on the microstructure and mechanical properties of rapidly solidified Al–Mg–X alloys was investigated. As a result of the rapid solidification processing, fine equiaxed grains with a mean diameter of 2 μm were observed in these alloys. Many fine particles were found to be distributed rather homogeneously throughout the matrix with relatively big particles occasionally observed at grain boundaries. The ultimate tensile strengths of Al–Mg–X alloys were found to decrease rather remarkably at 150°C although the ductility was not increased, which may result from segregation of β(Al 3Mg 2) precipitates. Fine dimples were observed on the fracture surfaces for all alloy systems and the variation of the size and shape of dimples was not observed. The ductility at 530°C was found to be ∼100%, suggesting that grain boundary sliding did not contribute despite fine grain size stabilization. The absence of superplastic behavior may be associated with low boundary misorientation in rapidly solidified Al–Mg–X alloys.

Microstructure and mechanical properties of friction welds of an α+β titanium alloy

The mechanical properties of friction welds of an α+β titanium alloy (Ti–6.5Al–1.9Zr–3.3Mo–0.25Si) were evaluated in the stress relieved and in the post weld heat treated (PWHT) conditions to understand the effect of post weld heat treatments on the microstructure and mechanical properties. Stress relieved welds exhibited mechanical properties comparable to the base metal except impact toughness. The impact toughness was ∼65% of the base metal. This was mainly due to a mixed microstructure of martensite and thin alpha+beta. Post weld heat treatment at 700°C that led to beta precipitation together with silicides exhibited poor impact toughness and trangranular fracture with shallow and under developed fine dimples. A PWHT at 960°C for 1 h followed by air cooling (AC) that led to the decomposition of α 1 to equilibrium alpha+beta and coarsening of the transgranular alpha improved the toughness. This treatment improved all other properties. 960°C/1 h/water quenching (WQ) PWHT reduced the impact toughness and exhibited quasi cleavage fracture possibly due to α 1 microstructure. Prolonged soaking at 960°C marginally reduced the toughness. This is thought to be due to a lean distribution of alpha consequent to its coarsening and possible compostional differences between the α and β phases that led to smooth and flat fracture in the transgranular locations. The smooth and flat fracture features were due to poor resistance of the microstructure to crack propagation.