Effect of tensile loading on crystallisation behaviour of as-cast and laser thermal treated Fe–Si... more Effect of tensile loading on crystallisation behaviour of as-cast and laser thermal treated Fe–Si–B metallic glass foils was investigated. Tensile loading lacked any marked influence on the crystallisation behaviour of as-cast and structurally relaxed laser-treated metallic glass foils. Furthermore, the average crystallite/grain size in partially crystallised laser-treated metallic glass foil was nearly equal to the average crystallite/grain size in the region away from the fracture of the same partially crystallised laser-treated metallic glass foil after tensile loading. However, a significant crystallite/grain growth/coarsening of the order of two and half times was observed in the fractured region compared to the region around it for the laser-treated partially crystallised metallic glass foils. The simultaneous effects of stress generation and temperature rise during tensile loading were considered to play a key role in crystallite/grain growth/coarsening.
A B S T R A C T The surface of Ti-6Al-4V was treated mechanically by applying ultrasonic nanocrys... more A B S T R A C T The surface of Ti-6Al-4V was treated mechanically by applying ultrasonic nanocrystal surface modification. The effect of this treatment on the hardness, compressive residual stresses and fatigue performance were investigated. It is shown that in terms of the measured nanoindentation hardness values and the presence of compressive residual stresses, the treated sample only differed from the as-received sample in the first 200– 300 µm area far from the surface. Also, the microstructure very close to the treated surface (< 5 µm) was characterized using a relatively new transmission orientation microscopy technique named ASTAR™/preces-sion electron diffraction. Based on different types of results (e.g., index map and virtual bright field image) acquired by this technique, it is concluded that titanium grains smaller than 10 nm exist within the distance of less than 1 µm from the treated surface. Difficulties associated with ASTAR™/precession electron diffraction technique to characterize this challenging near-surface area are discussed.
Grain boundary engineering and other fundamental materials science problems (e.g., phase transfor... more Grain boundary engineering and other fundamental materials science problems (e.g., phase transformations and physical properties) require an improvement in the understanding of the type and population of grain boundaries in a given system e yet, databases are limited in number and spare in detail, including for hcp crystals such as zirconium. One way to rapidly obtain databases to analyze is to use small-grained materials and high spatial resolution orientation microscopy techniques, such as ASTAR™/precession electron diffraction. To demonstrate this, a study of grain boundary character distributions was conducted for a-zirconium deposited at room temperature on fused silica substrates using physical vapor deposition. The orientation maps of the nanocrystalline thin films were acquired by the ASTAR™/precession electron diffraction technique, a new transmission electron microscope based orientation microscopy method. The reconstructed grain boundaries were classified as pure tilt, pure twist, 180-twist and 180-tilt grain boundaries based on the distribution of grain boundary planes with respect to the angle/axis of misorientation associated with grain boundaries. The results of the current study were compared to the results of a similar study on a-titanium and the molecular dynamics results of grain boundary energy for a-titanium.
To date, Zn has not been used as an alloying addition in structural Ti alloys. The main obstacle ... more To date, Zn has not been used as an alloying addition in structural Ti alloys. The main obstacle has been the disparity between their melting and vaporization temperatures. A novel processing technique was developed to create a Ti-Zn compound. The equilibrium phases and microstructures were studied by electron microscopy and x-ray diffraction techniques. Results show the presence of pearlitic domains of a-Ti (hexagonal closed packed crystal structure) and Ti 2 Zn (body center tetragonal structure) in regions that have a near eutectoid composition. Solutionizing and water quenching results in the formation of martensite along with intermetallic laths, suggesting that the eutectoid transformation is active.
A direct comparison between the oxidation behavior of Ti-6Al-4V and Ti-6Al-4V + 1B has been condu... more A direct comparison between the oxidation behavior of Ti-6Al-4V and Ti-6Al-4V + 1B has been conducted to elucidate whether the addition of boron to Ti-6Al-4V impacts the oxidation behavior. Industrially prepared sheet of Ti-6Al-4V and Ti-6Al-4V + 1B were oxidized at temperatures between 650 and 950 • C for holding times of 25 and 50 h. Weight-gain measurements and characterization of surface and near-surface microstructures showed that the addition of 1 wt% B increased the material's oxidation resistance. Additionally, the ingress of oxygen tends to decrease the solubility of other alloying species in-Ti and leads to the formation of a distinctive and atypical microstructure with a distinct morphology.
The presence and interaction of nanotwins, geometrically necessary dislocations, and grain bounda... more The presence and interaction of nanotwins, geometrically necessary dislocations, and grain boundaries play a key role in the mechanical properties of nanostructured crystalline materials. Therefore, it is vital to determine the orientation, width and distance of nanotwins, the angle and axis of grain boundary misorientations as well as the type and the distributions of dislocations in an automatic and statistically meaningful fashion in a relatively large area. In this paper, such details are provided using a transmission electron microscope-based orientation microscopy technique called ASTAR™/precession electron diffraction. The remarkable spatial resolution of this technique (~ 2 nm) enables highly detailed characterization of nanotwins, grain boundaries and the configuration of dislocations. This orientation microscopy technique provides the raw data required for the determination of these parameters. The procedures to post-process the ASTAR™/PED datasets in order to obtain the important (and currently largely hidden) details of nanotwins as well as quantifications of dislocation density distributions are described in this study.
Fe-Si-B metallic glass foils were treated with a linear laser track using a continuous wave Nd-YA... more Fe-Si-B metallic glass foils were treated with a linear laser track using a continuous wave Nd-YAG laser and its effect on the overall tensile behavior was investigated. Microstructure and phase evolutions were evaluated using X-ray diffraction, resistivity measurements, and transmission electron microscopy. Crystallization fraction was estimated via the differential scanning calorimetry technique. Metallic glass foils treated with the lower laser fluences (<0.49 J/mm2) experienced structural relaxation, whereas higher laser fluences led to crystallization within the laser treated region. The overall tensile behavior was least impacted by structural relaxation, whereas crystallization severely reduced the ultimate tensile strength of the laser treated metallic glass foils.
The influence of composition on microstructural phase stability has been assessed for an emerging... more The influence of composition on microstructural phase stability has been assessed for an emerging class of Co-based superalloys. This has been achieved using a combinatorial approach that incorporates both material synthesis and state-of-the-art characterization techniques. A compositionally graded Co–11Al–xW–yTa alloy (x 6 10, y = 10 x, all in wt.%) was produced using Laser Engineered Net Shaping (LENSe) and subsequently solutionized and water quenched. At selected locations along the gradient, the microstructures present were characterized in detail, and phase identification was conducted on site-specific thin specimens using transmission electron microscopy (TEM), providing details regarding the earliest stages of microstructural evolution in these alloys. The partitioning behavior of the alloying elements W, Ta and Al was characterized using a 3-D local electrode atom probe, revealing the existence of Co-rich clusters. In addition, high-resolution TEM and precession electron diffraction have revealed and confirmed that these clusters are a pseudomorphic body-centered cubic (bcc) structure that has resulted from the decomposition of a B2 ordered region. Although bcc Co has previously been reported in some thin-film multilayers, owing to the relative stabilities of the bcc phase and the face-centered cubic (fcc) phase, this observation has not been previously reported in bulk structural materials. It is argued that this nanoscaled pseudomorphic bcc Co phase may be a precursor to the fcc phase.
While it is useful to predict properties in metallic materials based upon the composition and mic... more While it is useful to predict properties in metallic materials based upon the composition and microstructure, the complexity of real, multi-component, and multi-phase engineering alloys presents difficulties when attempting to determine constituent-based phenomenological equations. This paper applies an approach based upon the integration of three separate modeling approaches, specifically artificial neural networks, genetic algorithms, and Monte Carlo simulations to determine a mechanism-based equation for the yield strength of α+β processed Ti-6Al-4V (all compositions in weight percent) which consists of a complex multi-phase microstructure with varying spatial and morphological distributions of the key microstructural features. Notably, this is an industrially important alloy yet an alloy for which such an equation does not exist in the published literature. The equation ultimately derived in this work not only can accurately describe the properties of the current dataset but also is consistent with the limited and dissociated information available in the literature regarding certain parameters such as intrinsic yield strength of pure hexagonal close-packed alpha titanium. In addition, this equation suggests new interesting opportunities for controlling yield strength by controlling the relative intrinsic strengths of the two phases through solid solution strengthening.
Poor oxidation performance of Ti-based alloys is an important life-limiting factor for high tempe... more Poor oxidation performance of Ti-based alloys is an important life-limiting factor for high temperature applications. In this paper, a combinatorial approach is used to investigate systematically the influence of composition and time on the oxidation of Ti–Cr system. A compositionally graded Ti–xCr specimen (0 ⩽ x ⩽ 40 wt%) was prepared and oxidized at 650 °C. The structure and composition of the oxide and near-surface region were studied and a critical composition of ∼20 wt% Cr was identified above which the oxidation resistance is enhanced. Below the critical composition transition to a rapid breakaway oxidation was observed for extended exposure times.
To understand the influence of composition on the oxidation of the Ti–Mo system, a combinatorial ... more To understand the influence of composition on the oxidation of the Ti–Mo system, a combinatorial approach was adopted whereby a compositionally graded specimen, Ti–xMo (0 ⩽ x ⩽ 12 wt%), was prepared and oxidized at 650 °C. The results show that the scale thickness is independent of the composition and a parabolic law is obeyed in the composition-time domain. An increase in the Mo content degrades the adhesion of the scale by forming a TiO transition layer. Some of the observations including the homogeneous nucleation of β for compositions <∼7.5 wt% Mo and discontinuous precipitation for compositions >∼7.5 wt% Mo in the subsurface region have not been previously reported.
Effect of tensile loading on crystallisation behaviour of as-cast and laser thermal treated Fe–Si... more Effect of tensile loading on crystallisation behaviour of as-cast and laser thermal treated Fe–Si–B metallic glass foils was investigated. Tensile loading lacked any marked influence on the crystallisation behaviour of as-cast and structurally relaxed laser-treated metallic glass foils. Furthermore, the average crystallite/grain size in partially crystallised laser-treated metallic glass foil was nearly equal to the average crystallite/grain size in the region away from the fracture of the same partially crystallised laser-treated metallic glass foil after tensile loading. However, a significant crystallite/grain growth/coarsening of the order of two and half times was observed in the fractured region compared to the region around it for the laser-treated partially crystallised metallic glass foils. The simultaneous effects of stress generation and temperature rise during tensile loading were considered to play a key role in crystallite/grain growth/coarsening.
A B S T R A C T The surface of Ti-6Al-4V was treated mechanically by applying ultrasonic nanocrys... more A B S T R A C T The surface of Ti-6Al-4V was treated mechanically by applying ultrasonic nanocrystal surface modification. The effect of this treatment on the hardness, compressive residual stresses and fatigue performance were investigated. It is shown that in terms of the measured nanoindentation hardness values and the presence of compressive residual stresses, the treated sample only differed from the as-received sample in the first 200– 300 µm area far from the surface. Also, the microstructure very close to the treated surface (< 5 µm) was characterized using a relatively new transmission orientation microscopy technique named ASTAR™/preces-sion electron diffraction. Based on different types of results (e.g., index map and virtual bright field image) acquired by this technique, it is concluded that titanium grains smaller than 10 nm exist within the distance of less than 1 µm from the treated surface. Difficulties associated with ASTAR™/precession electron diffraction technique to characterize this challenging near-surface area are discussed.
Grain boundary engineering and other fundamental materials science problems (e.g., phase transfor... more Grain boundary engineering and other fundamental materials science problems (e.g., phase transformations and physical properties) require an improvement in the understanding of the type and population of grain boundaries in a given system e yet, databases are limited in number and spare in detail, including for hcp crystals such as zirconium. One way to rapidly obtain databases to analyze is to use small-grained materials and high spatial resolution orientation microscopy techniques, such as ASTAR™/precession electron diffraction. To demonstrate this, a study of grain boundary character distributions was conducted for a-zirconium deposited at room temperature on fused silica substrates using physical vapor deposition. The orientation maps of the nanocrystalline thin films were acquired by the ASTAR™/precession electron diffraction technique, a new transmission electron microscope based orientation microscopy method. The reconstructed grain boundaries were classified as pure tilt, pure twist, 180-twist and 180-tilt grain boundaries based on the distribution of grain boundary planes with respect to the angle/axis of misorientation associated with grain boundaries. The results of the current study were compared to the results of a similar study on a-titanium and the molecular dynamics results of grain boundary energy for a-titanium.
To date, Zn has not been used as an alloying addition in structural Ti alloys. The main obstacle ... more To date, Zn has not been used as an alloying addition in structural Ti alloys. The main obstacle has been the disparity between their melting and vaporization temperatures. A novel processing technique was developed to create a Ti-Zn compound. The equilibrium phases and microstructures were studied by electron microscopy and x-ray diffraction techniques. Results show the presence of pearlitic domains of a-Ti (hexagonal closed packed crystal structure) and Ti 2 Zn (body center tetragonal structure) in regions that have a near eutectoid composition. Solutionizing and water quenching results in the formation of martensite along with intermetallic laths, suggesting that the eutectoid transformation is active.
A direct comparison between the oxidation behavior of Ti-6Al-4V and Ti-6Al-4V + 1B has been condu... more A direct comparison between the oxidation behavior of Ti-6Al-4V and Ti-6Al-4V + 1B has been conducted to elucidate whether the addition of boron to Ti-6Al-4V impacts the oxidation behavior. Industrially prepared sheet of Ti-6Al-4V and Ti-6Al-4V + 1B were oxidized at temperatures between 650 and 950 • C for holding times of 25 and 50 h. Weight-gain measurements and characterization of surface and near-surface microstructures showed that the addition of 1 wt% B increased the material's oxidation resistance. Additionally, the ingress of oxygen tends to decrease the solubility of other alloying species in-Ti and leads to the formation of a distinctive and atypical microstructure with a distinct morphology.
The presence and interaction of nanotwins, geometrically necessary dislocations, and grain bounda... more The presence and interaction of nanotwins, geometrically necessary dislocations, and grain boundaries play a key role in the mechanical properties of nanostructured crystalline materials. Therefore, it is vital to determine the orientation, width and distance of nanotwins, the angle and axis of grain boundary misorientations as well as the type and the distributions of dislocations in an automatic and statistically meaningful fashion in a relatively large area. In this paper, such details are provided using a transmission electron microscope-based orientation microscopy technique called ASTAR™/precession electron diffraction. The remarkable spatial resolution of this technique (~ 2 nm) enables highly detailed characterization of nanotwins, grain boundaries and the configuration of dislocations. This orientation microscopy technique provides the raw data required for the determination of these parameters. The procedures to post-process the ASTAR™/PED datasets in order to obtain the important (and currently largely hidden) details of nanotwins as well as quantifications of dislocation density distributions are described in this study.
Fe-Si-B metallic glass foils were treated with a linear laser track using a continuous wave Nd-YA... more Fe-Si-B metallic glass foils were treated with a linear laser track using a continuous wave Nd-YAG laser and its effect on the overall tensile behavior was investigated. Microstructure and phase evolutions were evaluated using X-ray diffraction, resistivity measurements, and transmission electron microscopy. Crystallization fraction was estimated via the differential scanning calorimetry technique. Metallic glass foils treated with the lower laser fluences (<0.49 J/mm2) experienced structural relaxation, whereas higher laser fluences led to crystallization within the laser treated region. The overall tensile behavior was least impacted by structural relaxation, whereas crystallization severely reduced the ultimate tensile strength of the laser treated metallic glass foils.
The influence of composition on microstructural phase stability has been assessed for an emerging... more The influence of composition on microstructural phase stability has been assessed for an emerging class of Co-based superalloys. This has been achieved using a combinatorial approach that incorporates both material synthesis and state-of-the-art characterization techniques. A compositionally graded Co–11Al–xW–yTa alloy (x 6 10, y = 10 x, all in wt.%) was produced using Laser Engineered Net Shaping (LENSe) and subsequently solutionized and water quenched. At selected locations along the gradient, the microstructures present were characterized in detail, and phase identification was conducted on site-specific thin specimens using transmission electron microscopy (TEM), providing details regarding the earliest stages of microstructural evolution in these alloys. The partitioning behavior of the alloying elements W, Ta and Al was characterized using a 3-D local electrode atom probe, revealing the existence of Co-rich clusters. In addition, high-resolution TEM and precession electron diffraction have revealed and confirmed that these clusters are a pseudomorphic body-centered cubic (bcc) structure that has resulted from the decomposition of a B2 ordered region. Although bcc Co has previously been reported in some thin-film multilayers, owing to the relative stabilities of the bcc phase and the face-centered cubic (fcc) phase, this observation has not been previously reported in bulk structural materials. It is argued that this nanoscaled pseudomorphic bcc Co phase may be a precursor to the fcc phase.
While it is useful to predict properties in metallic materials based upon the composition and mic... more While it is useful to predict properties in metallic materials based upon the composition and microstructure, the complexity of real, multi-component, and multi-phase engineering alloys presents difficulties when attempting to determine constituent-based phenomenological equations. This paper applies an approach based upon the integration of three separate modeling approaches, specifically artificial neural networks, genetic algorithms, and Monte Carlo simulations to determine a mechanism-based equation for the yield strength of α+β processed Ti-6Al-4V (all compositions in weight percent) which consists of a complex multi-phase microstructure with varying spatial and morphological distributions of the key microstructural features. Notably, this is an industrially important alloy yet an alloy for which such an equation does not exist in the published literature. The equation ultimately derived in this work not only can accurately describe the properties of the current dataset but also is consistent with the limited and dissociated information available in the literature regarding certain parameters such as intrinsic yield strength of pure hexagonal close-packed alpha titanium. In addition, this equation suggests new interesting opportunities for controlling yield strength by controlling the relative intrinsic strengths of the two phases through solid solution strengthening.
Poor oxidation performance of Ti-based alloys is an important life-limiting factor for high tempe... more Poor oxidation performance of Ti-based alloys is an important life-limiting factor for high temperature applications. In this paper, a combinatorial approach is used to investigate systematically the influence of composition and time on the oxidation of Ti–Cr system. A compositionally graded Ti–xCr specimen (0 ⩽ x ⩽ 40 wt%) was prepared and oxidized at 650 °C. The structure and composition of the oxide and near-surface region were studied and a critical composition of ∼20 wt% Cr was identified above which the oxidation resistance is enhanced. Below the critical composition transition to a rapid breakaway oxidation was observed for extended exposure times.
To understand the influence of composition on the oxidation of the Ti–Mo system, a combinatorial ... more To understand the influence of composition on the oxidation of the Ti–Mo system, a combinatorial approach was adopted whereby a compositionally graded specimen, Ti–xMo (0 ⩽ x ⩽ 12 wt%), was prepared and oxidized at 650 °C. The results show that the scale thickness is independent of the composition and a parabolic law is obeyed in the composition-time domain. An increase in the Mo content degrades the adhesion of the scale by forming a TiO transition layer. Some of the observations including the homogeneous nucleation of β for compositions <∼7.5 wt% Mo and discontinuous precipitation for compositions >∼7.5 wt% Mo in the subsurface region have not been previously reported.
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