Search

Due to increasingly stringent regulations governing fuel economy and emissions, new technological developments towards automobile efficiency are in play including lightweighting by reducing weight of the structural components. The historical development of steel grades for autos has resulted in strength increases, including in recent advanced high strength steels (AHSS), but it has come with commensurate decreases in ductility and formability. NanoSteel 3rdGeneration AHSS overcomes the trade-off in ductility due to novel structural changes during cold deformation through a complex Nanophase Refinement and Strengthening (NR&S) mechanism leading to material strengthening. During stamping, the ability of a steel blank to be formed into complex parts is paramount and it has to retain sufficient ductility for energy absorption during a subsequent crash event. In this paper, the specific characteristics of the NR&S mechanism in two NanoSteel grades will be detailed including structure and property changes during stamping analyzed by utilizing destructive and non-destructive approaches to predict localized yield strength changes in the final stampings.

Over the past decade extensive development of advanced high strength steel (AHSS) was driven by the demand from the automotive industry for stronger materials that can enable lightweighting to meet increasing fuel efficiency requirements. However, achievement of higher strength in many AHSS grades comes with reductions in ductility, leading to geometric constraints on formability and limiting their application. In this paper, a 3rd Generation AHSS with a compelling property combination of high tensile strength of ~1200 MPa and total elongation > 40% was used for laboratory and stamping studies. Various auto related laboratory tests were done including tensile testing, 180 degree bending, bulge testing, and cup drawing to estimate the steel’s formability under different applied conditions. Additionally, since laboratory testing provides only an estimation of the potential stamping response, the 3rd Generation AHSS sheet was stamped into B-pillars under industrial stamping conditions. Non-destructive and destructive analysis of the resulting stampings were done to evaluate the microstructural and property changes occurring during stamping. Significant strengthening of material in the stamped part is attributed to the structural changes through the complex Nanophase Refinement and Strengthening mechanism.

Existing amorphous Fe-based microwires are used primarily for non-structural magnetic applications due to limitations in tensile ductility which is less than 2% including elastic strain. In this research, exceptionally ductile microwires with tensile ductility up to 8% and with high strengths up to 4.8 GPa were produced by Taylor–Ulitovsky method from glass forming Fe-based alloys. Tensile ductility up to 8% is reported. The underlying reason for such unique property combination in the microwires is a formation of enabling type of microstructure with ability for shear band arresting and blunting. Specific features of multiple shear band interaction with microwire microstructure and with each other resulting in global ductility of the materials are discussed.

Reliable tensile data are crucial in the automotive design process, particularly for the implementation of new Advanced High Strength Steel grades that can be used in new intricate shapes. The property evaluation of specific subsections of a complex part, such as a metal stamping, requires deviation from standard-sized specimens because of material quantity limitations which force the use of smaller specimens. A new tensile geometry is introduced for testing regions within parts that were previously difficult to characterize. However, the specimen size effect is not consistent between different steels, as demonstrated for this new geometry herein. Tensile property measurements taken with noncontact extensometers from specimens of three different sizes from three different sheet steels showed varying material responses to changes in specimen geometry.

One of the largest controversial issues of mechanical testing is the interpretation of scaling laws on materials strength and ductility. With a general trend towards miniaturization based on recent development in nanomaterials and metallic glasses, the challenges confront not only with preparation, handling and testing of small volumes of materials, but, also with clear understanding of test results. The main question is whether mechanical behavior of small volumes is controlled by mechanics or by sample geometry. Analysis of mechanical behavior of well-known Ti–6Al–4V alloy at different length scales is presented in this study. The results are compared to that of metallic glasses at the same scale and the general trends are highlighted.

Nanocrystalline materials crystallized from metallic glasses usually have a very complicated phase composition and present a good example of nanocomposite materials. The high potential of nanostructured iron-based materials produced by controlling solidification and subsequent devitrification is demonstrated. A new Fe-based commercial alloy demonstrates strength of more than 6 GPa at room temperature. A strong effect of annealing conditions on the nature of crystallization products and, as a result, on the mechanical behavior of the alloy has been revealed. Hence, depending upon the microstructure that is produced and the deformation conditions that are chosen, a wide variety of strength/elongation properties can be generated for various potential applications.

A devitrification procedure by annealing was applied to a multicomponent Fe-based metallic glass in order to obtain nanocrystalline materials. Phase composition and phase morphology were strongly dependent on the annealing conditions. An elevated temperature mechanical behavior of nanostructures was evaluated by tensile testing. A strong effect of phase morphology on the mechanical response of the material was revealed. A most attractive combination of strength and plasticity was observed in the nanostructure with approximately equal grain sizes of crystallized phases.

The Palmqvist fracture toughness method was applied in a new fashion to measure the fracture toughness of a new wear-resistant alloy in both a powder product for plasma-transferred arc (PTA) welding and a wire product for metal inert gas (MIG) welding. Cracks were observed at high loads with a linear relationship between load and crack length indicating that only Palmqvist cracks occurred in the alloys. The fracture toughness for the no-preheat and preheat PT40 welds are 15.6±0.3 Mpa m1/2 and 17.6±0.1 MPa m1/2, respectively, and for the no-preheat and preheat MG40 welds are 12±1 MPa m1/2 and 20.5±0.2 MPa m1/2, respectively. Observations of the cracks show evidence of ligaments indicating that the toughness mechanism is due to crack bridging. The microstructural scale and morphology correlates with the fracture toughness, and as the structural scale decreases, the fracture toughness increases.

Steel in its various forms is the most widely utilized metallic alloy and comprises over 80 % by weight of all metallic alloys in industrial use. [1] The development of steel microstructures is based on manipulation of a very specific solid/solid state transformation called an eutectoid transformation (i.e. γ austenite → α ferrite + Fe 3 C cementite ). The control of this transformation is the primary factor resulting in wide variety of microstructures and resulting properties found in commercial steel alloys. However, the full benefit of its main constituent, iron has never been realized. Based on the metallic bonding in iron, the theoretical tensile strength has been calculated to be 13.2 GPa but ultra high strength steels, even today, only achieve maximum tensile strength levels from 1 to 1.5 GPa. Thus, our modern technological society has been established utilizing approximately only ∼ 10% strength level of iron. Here we demonstrate that a high level of strength (6.2 GPa) and strength to weight ratio of 8.3 x 10 4 /m 3 may be obtained in iron-based alloys by their solidification into metallic glasses, as well as, by employing another solid state transformation called glass devitrification.

New iron based alloys were developed for industrial hardfacing via plasma transferred arc-welding (PTAW). The alloys were designed with low critical cooling rates for metallic glass formation so that high undercooling could be obtained prior to the initiation of nucleation. The result was that conventional dendritic solidification could be avoided resulting in the refinement of metallurgical grain/phase sizes through a coupled lath eutectoid growth mode. High hardness up to R c 66 was developed from the fine structure consisting of a high volume fraction of borocarbides phases. Using a Palmqvist method, high toughness up to 74.7 MPa m 1/2 was measured which could adequately be described through a crack bridging model. The effective distribution of borocarbides phases with ductile iron in a coupled lath morphology appears to be especially potent for crack bridging and is believed enabling to achieve combinations of high toughness along with high hardness. Thus, this new class of PTAW hardfacing alloys is expected to be useful for applications requiring high abrasion resistance and high impact.

Variations in microstructure and chemical compositions of the metallic glasses found in the literature, as well as an overall lack of experimental data on inhomogeneous behavior of metallic glass make the evaluation of the effects of shear band/fracture behavior on mechanical properties of metallic glasses difficult. Investigating the effect of strain localization alone on inhomogeneous flow seems to be a first step in approaching this problem. Mechanical behavior of metallic glasses at room temperature and various strain rates in tension and compression was investigated. Formation of multiple shear bands was observed at high strain rates. An increase in strain rate leads to enhanced ductility in tension, whereas, the ductility of the material in compression decreases with increasing strain rate. Differences in deformation processes in tension and compression were compared.

A Nd–Dy–Fe–B alloy was designed to exhibit high coercivity and optimum loop shapes utilizing a combination of intrinsic and extrinsic alloy design principles. After achieving these initial goals, conventional structural analysis using X-ray and TEM could explain the salient features of the observed hysteresis behavior. However, greater understanding could be gained on the complex dynamic behavior of hysteresis through the exploitation of a new recoil technique that was developed, which focuses on switching field distributions arising from irreversible magnetic behavior. Utilizing this approach, we have successfully linked specific magnetic reversal behavior with distinct magnetic phases that have characteristic microstructural length scales. This new approach may become a powerful tool for the study of the structure–property–hysteresis behavior of hard magnetic materials.

Mechanical behavior of Dy3Al2 metallic glass at room temperature and different strain rates was investigated. Enhanced microscopic ductility was observed at the highest strain rate tested of 10−1 s−1 when multiple parallel shear bands on the gage and fracture surfaces were observed. Possible differences in deformation process by tension and compression were discussed.

The focus of this research was on gaining a fundamental understanding of the structure/property relationships that exist in an Fe-based multicomponent glass-forming alloy. While composition is one key parameter in determining the final microstructure, the specific transformation conditions (temperature and time) related to how a glass devitrifies were found to have significant effects on the development of the devitrified microstructure. Microstructural studies have revealed that recovery, relaxation, crystallization, and recrystallization phenomena are important effects which lead to large differences in the ensuing microstructure resulting in significant differences in properties such as devitrified hardness. The information gained in this study, combined with further investigations into the mechanistic processes governing these transformations, may allow for the further development of nanostructured materials with specific and targeted sets of properties.

This paper is focused on understanding hysteresis in Dy3Al2, which is a member of a class of high anisotropy compounds containing high fractions of rare earth elements with no 3-d transition metals. Analysis of the data suggests that the principal domain wall pinning mechanism does not involve the grain boundaries but instead arises from interactions between either the ferromagnetic glass or the ferromagnetic Dy3Al2 phase with the antiferromagnetic DyAl second phase through an exchange bias mechanism.

Through a unique methodology, novel nanocomposite microstructures were created in a bulk iron based alloy by first processing into a glass condition followed by devitrifying the glass through heat treating above the crystallization temperature. The as-crystallized microstructure was made up of three nanoscale phases; α-Fe, Fe 23 C 6 , and Fe 3 B phases. Vickers hardness testing revealed a maximum hardness of 16.2 GPa which is significantly harder than existing commercial steel alloys and hardmetals. Detailed structural studies uncovered two important factors which contribute to the development of this extreme hardness; reductions in microstructure scale to the nanometer regime and supersaturation of transition metal alloying elements significantly above their equilibrium solubility limits.

In this paper, the effect of alloy composition on the microstructural development of devitrified Nd–Fe–B-based alloys is investigated. While crystallization of simple ternary Nd2Fe14B resulted in a highly variable microstructure and relatively poor demagnetization behavior, alloying additions, notably Ti and C, were found to significantly refine and homogenize the crystallized microstructure. High spatial resolution energy dispersive X-ray spectroscopy revealed that partitioning to the grain boundaries led to the grain refinement by slowing interface kinetics. The more homogeneous grain structure resulted in improved demagnetization behavior, but segregating elements isolated grains and limited intergranular exchange coupling.

Partially ordered Sm 2 Fe 17 , closer to the SmFe 7 structure, formed upon melt spinning of Sm 11 Fe 89 . A strong dependence of grain size on the wheel speed was also observed, with wheel speeds less than 40 m/s generating grain sizes above the single domain limit for Sm 2 Fe 17 N x . The addition of Ti and C resulted in an order of magnitude grain refinement compared to the binary alloys. The Ti and C also effectively inhibited grain growth during annealing. The nitrided Sm–Fe–Ti–C alloys also displayed a rapid decrease in coercivity with decreasing grain size, which was attributed to increased exchange interactions.

In this article, we demonstrate high hysteresis in a well characterized homogeneous Tb–Al glass which contained no crystallites or crystalline embryos as verified using conventional and synchrotron diffraction, neutron diffraction, and direct observation in the transmission electron microscope. At low temperature (2 K), the metallic glass structure exhibited intrinsic coercivities approaching 23 kOe and high isotropic energy products of 12.4 MGOe. After crystallization into a three-phase nanoscale structure, the hard magnetic properties were found to be far inferior to that obtainable in the glass structure. From the well defined intrinsic magnetic properties (Msat,Tc), it is clear that the glass contains one or more types of well defined associations (i.e., clusters) and that these associations lead to ferromagnetic coupling/ordering. From the large random magnetic anisotropy, it is probable that the domain size is much larger than the structural cluster size. The measured single-phase loop shapes and the development of high coercivity in the glass state can be explained by an “exchange bias” mechanism resulting in a near perfect distribution of “fragile” pinning centers.

The microstructures of nanophase Pr-Co-C-(Ti) materials, which have improved magnetic properties, were investigated by means of transmission electron microscopy (TEM) to reveal their phase assemblage and grain-boundary structure. The phase assemblage was carefully controlled by the introduction of TiC nanoparticles and annealing. The optimal nanostructure contained uniformly distributed PrCo5 and PrCo2 nanophases without any magnetically soft phases, resulting in high coercivity and the characteristics of a single, hard magnetic phase. TEM analysis confirmed the presence of an amorphous grain-boundary phase surrounding the grains in alloys without TiC. In contrast, alloys with added TiC showed no amorphous phase and also showed higher coercivity compared to Co-Pr-C. Therefore, the variation of the grain boundary phases may be effective in changing the degree of exchange coupling. Controlling the formation of a uniform nanoscale microstructure, leading to improved magnetic properties, is discussed.

The magnetic properties and microstructures of a quaternary Pr–Co–Ti–C alloy were investigated. After heat treating at 800 °C, the quaternary alloy developed high coercivity (16.7 kOe) and high energy product (8.6 MGOe). Hysteretic loop shapes exhibiting single-phase magnetic character were achieved which was a problem which previously plagued development of Pr–Co alloys. The optimum microstructure contained 75–100 nm Pr1Co5 hard magnetic grains, 75–100 nm Pr1Co2 second phases, and

The demagnetization behavior of nanoassembled crystalline/amorphous microstructures was investigated. The microstructures consisted of isolated Nd2Fe14B crystallites surrounded by regions of remaining amorphous phase. The hard magnetic crystallites interacted with the surrounding amorphous phase over a length scale of approximately 1.5–2 nm. As the extent of the amorphous phase decreased to less than this, interactions between hard magnetic grains were observed.

Rapidly solidified Sm11Fe89 was observed to form partially ordered Sm2Fe17. The ordering increased systematically with annealing temperature, reaching an order parameter of 0.8 after annealing at 950 °C for 15 min. The coercivity of the corresponding interstitial compound Sm2Fe17Nx varied with annealing temperature and thus order parameter. The coercivity reached a maximum for an order parameter of 0.45–5, then dramatically decreased as the order increased. This behavior is characteristic of demagnetization controlled by domain wall pinning by antiphase boundaries.

Enhanced energy product and higher operating fields can be achieved by improving the squareness of the demagnetization loop. In isotropic materials, increasing loop squareness can be achieved by minimizing microstructural gradients and maximizing homogeneity. Achieving homogeneity by melt spinning can be difficult due to localized changes in cooling conditions from extrinsic factors such as gas entrapment between the melt and the chill surface and intrinsic effects resulting from the stochastic nature of nucleation on a surface. These localized variations are reduced by altering the solidification kinetics of the alloy, thus reducing its sensitivity to localized changes in cooling conditions. This was achieved through development of a nine-element modified Nd–Fe–B alloy which solidified with a uniform glass structure at much lower cooling rates. After crystallization, enhanced microstructural homogeneity was verified by electron microscopy and described by parameters based on the loop shape. The nine-elem...

Rapidly solidified samples of Nd9.5Fe84.5B6 with and without 3 at. % TiC were prepared by melt spinning and melt extraction and then annealed in vacuum (3×10−6 Torr) at temperatures from 600 to 750 °C. For alloys melt spun under similar conditions, the overquenched state was achieved at wheel speeds >10 m/s for the TiC added alloy while >20 m/s was necessary without TiC. The overquenched samples contained a smaller fraction of α-Fe in smaller grains than the undercooled samples where Fe dendrites formed near the free surface during solidification. These Fe dendrites were not removed by annealing. In addition, large orientated 2-14-1 grains nucleated on the Fe dendrites. This combination is detrimental to the magnetic properties. The addition of TiC results in improved control of the microstructure over a larger fraction of the ribbon volume enhancing the magnetic properties.

The microstructures of melt spun Sm11Fe89 and (Sm11Fe89)94Ti3C3 have been investigated under different solidification conditions. The magnetic properties of the nitrided materials have been related to the observed microstructures. Melt spinning of the Sm11Fe89 alloys resulted in grain sizes at or above the single domain limit. The addition of Ti and C resulted in an order of magnitude refinement in the microstructural scale. The magnetic properties of the samples with grain sizes below the single domain limit decreased with decreasing grain size due to increased intergranular exchange coupling.

For the Nd2Fe14B (2-14-1) compound, the optimal grain size should be smaller than the size of the single domain size of 150 nm. Transition metal carbides (TMC) also reduce the quench rate necessary to achieve the optimal or overquenched condition. This allows inert gas atomization (IGA) to produce viable magnetic materials. In this article we will demonstrate that optimal microstructure for the 2-14-1 can be produced by IGA with the addition of TiC. Moreover, a solidification model will be presented to show (1) how recalescence is a critical feature to the evolution of the microstructure in rapidly solidified materials and (2) the role TMC and other solute phases have on inhibiting grain growth so that lower quench rates can be employed.

It is known that the addition of alloying elements to the Nd2Fe14B-based alloys significantly inhibits the onset of the crystallization in particles produced by the inert gas atomization (IGA) process. Transmission electron microscopy (TEM) was performed on a selection of as-atomized particles synthesized by IGA methods with the addition of 3 wt % TiC. TEM samples were successfully prepared using the wedge technique. Large particles (>10 μm) tend to consist of well-defined grains of Nd2Fe14B while the submicron size particles are generally amorphous and particles with diameter d in the size range, 1 μm

Gas atomization processing is an alternative to melt spinning for the production of isotropic Nd2Fe14B powders for bonded magnet (and other) applications. A new approach is presented in which alloys are designed for gas atomization processing specifically, resulting in improved magnetic properties and corrosion resistance. The hysteresis properties of powders as a function of composition, particle size, and heat treatment will be discussed and compared with studies by electron microscopy, magnetic force microscopy, and x-ray diffraction.

Gas atomization (GA) processing does not generally have a high enough cooling rate to produce the initial amorphous microstructure needed to obtain optimal magnetic properties in RE{sub 2}Fe{sub 14}B alloys. Phase separation and an underquenched microstructure result from detrimental {alpha}-Fe precipitation, and the resulting magnetic domain structure is very coarse. Additionally, there is a dramatic dependence of the magnetic properties on the cooling rate (and therefore the particle size) and the powders can be sensitive to environmental degradation. Alloy compositions designed just for GA (as opposed to melt spinning) are necessary to produce an amorphous structure that can be crystallized to result in a fine structure with magnetic properties which are independent of particle size. The addition of titanium and carbon to the melt has been found to change the solidification process sufficiently to result in an ``overquenched`` state in which most of the powder size fractions have an amorphous component. Crystallization with a brief heat treatment produces a structure which has improved magnetic properties, in part due to the ability to use compositions with higher Fe contents without {alpha}-Fe precipitation. Results from magnetometry, magnetic force microscopy, and x-ray analyses will be used to contrast the microstructure, domain structure, and magnetic properties of this new generation of amorphous powders with their multiphase predecessors.

TiC is added as an alloying agent to Nd{sub 2}Fe{sub l4}B (2-14-1) system to both alter the solidification behavior of the melt and to act as pinning sites to control grain growth. The addition of TIC to 2-14-1 results in a factor of three reduction of the quench rate required to produce amorphous material. In addition, the crystallization temperature of the glass is enhanced leading to enhanced nucleation and finer grain size during crystallization. TIC additions to the stochiometric melt affect the range of primary solidification of the 2-14-1 phase. When TIC is added to the limit of its liquid solubility, the primary solidification range is move further from the stochiometric composition to the Nd rich region.

In bulk permanent magnets, the extrinsic properties relating to the microstructure determined the level of hard magnetic properties that is actually achieved. Many researchers have worked on the microstructure property relationships in the Nd2Fe14B (2‐14‐1) system but, in general, the approach has been to analyze the microstructure of materials which have been shown to have good magnetic properties rather than to study the effect of alloy additions on the metallurgical properties of the system. The microstructure which is obtained under a given set of processing conditions is highly dependent on the solidification behavior of the alloy, as well as grain growth phenomena. Alloy additions which effect the solidification behavior of the melt and then form precipitates which pin grain boundaries, and therefore control grain size, should be valuable in producing the uniform microstructure required for good magnetic properties. A number of refractory metal carbides perform both of these functions. The carbides ...