Search

In order to develop high strength Mg-Gd-Y-Zn alloys, key experiments coupled with CALPHAD (CALculation of PHAse Diagrams) calculations were carried out in the current work to provide critical understanding of this important alloy system. Three Mg-10Gd-xY-yZn (x = 4 or 5, y = 3 or 5, wt.%) were mapped on Mg-Gd-Y-Zn phase diagrams for phase equilibria and microstructure investigation. Electron microscopy was performed for phase identification and phase fraction determination in as-cast and solution treated conditions. In all three alloys, the major phases were Mg-matrix and long period stacking order (LPSO) 14H phase. With ST at 400 and 500 °C, the phase fraction of LPSO 14H increased, particularly the fine lamellar morphology in the Mg matrix. The as-cast and 400 °C Mg10Gd5Y3Zn samples had Mg5(Gd,Y) present. At 500 °C, Mg5(Gd,Y) is not stable and transforms into LPSO 14H. The Mg10Gd5Y5Zn alloy included the W-Phase, which showed a reduction in phase fraction with solution treatment. These experimental results were used to validate and improve the thermodynamic database of the Mg-Gd-Y-Zn system. Thermodynamic calculations using the improved database can well describe the available experimental results and make accurate predictions to guide the development of promising high-strength Mg-Gd-Y-Zn alloys.

We revisit the meaning of stacking fault energy (SFE) and the assumptions of equilibrium dissociation of lattice dislocations in concentrated alloys. SFE is a unique value in pure metals. However, in alloys beyond the dilute limit, SFE has a distribution of values depending on the local atomic environment. Conventionally, the equilibrium distance between partial dislocations is determined by a balance between the repulsive elastic interaction between the partial dislocations and a unique value for SFE. This assumption is used to determine SFE from experimental measurements of dislocation splitting distances in metals and alloys, often contradicting computational predictions. We use atomistic simulations in a model NiCo alloy to study the dislocation dissociation process in a range of compositions with positive, zero, and negative average SFE and surprisingly observe a stable, finite splitting distance in all cases at low temperatures. We then compute the decorrelation stress and examine the balance of forces on the partial dislocations, considering the local effects on SFE, and observe that even the upper bound of SFE distribution alone cannot satisfy the force balance in some cases. Furthermore, we show that in concentrated solid solutions, the resisting force caused by interaction of dislocations with the local solute environment becomes a major force acting on partial dislocations. Here, we show that the presence of a high solute/dislocation interaction, which is not easy to measure and neglected in experimental measurements of SFE, renders the experimental values of SFE unreliable., The stacking fault energy is connected to the response of crystals to deformation. Here the authors report a computational study in a model NiCo system to demonstrate the key importance of the dislocation/solute interaction for the accurate assessment of stacking fault energy in alloys beyond dilute limit.

The precipitation of a recently introduced γ′-strengthened, Powder Metallurgy (PM) nickel superalloy is characterized and modeled. A range of experimental techniques are employed to capture aspects of the alloy microstructure necessary to calibrate a supersolvus, continuous cooling precipitation model. The proposed precipitation model framework incorporates a computationally efficient addition to the classical mean-field modeling approach that increases its ability to model dynamic, multi-modal γ′ burst events. The γ′ size predicted by the model shows good agreement with the experimental results spanning several orders of cooling rate magnitudes. The scalability of the modeling framework is then demonstrated in a quench trial on a diskette made from another PM nickel superalloy. The precipitation calculation is applied to the element integration points of a continuum Finite Element (FE) heat conduction simulation, where the latent heat generated from the precipitate evolution is accounted for. The results are compared to experimental findings using embedded thermocouple measurements and indicate that this approach is suitable for quantifying effects of γ′ precipitate evolution at the meso-scale and continuum length scales.

Sheet metal forming operations in the automotive industry, including stamping, flanging, bending, hemming and trimming, are dominantly done at room temperature (RT). Unfortunately, the poor RT formability of magnesium due to its hexagonal close-packed structure and generally strong texture has limited the use of these processes in high-volume automotive production. However, the formability of magnesium can be improved via fine grain structure and random texture to enable some RT forming operations. This paper presents the latest magnesium alloy development and evaluation by the United States Automotive Materials Partnership (USAMP) in collaboration with its university partners. A new sheet alloy developed in a recent USAMP project, ZAXME11100 (USAMP Alloy 2 Plus), offers excellent ductility (31% tensile elongation) and RT formability (7.8 mm Erichsen index) in a solution-treated condition (T4), and a high yield strength (270 MPa) upon post-forming aging treatment (T6), promising RT forming for automotive applications.

An equiatomic CuFeMnNi high-entropy alloy (HEA) was synthesized and homogenized at 1000 °C to achieve a microstructure of single face centered cubic phase based on CALPHAD (CALculation of PHAse Diagrams) prediction. The strength and ductility of this Co-free (low cost) HEA can be tuned through thermomechanical processing (cold rolling and annealing at temperatures ranging from 650 °C to 950 °C). Electron backscattered diffraction, transmission electron microscopy and scanning transmission electron microscopy techniques were used to characterize the deformation microstructure of this new HEA. The results show that plastic deformation was achieved through the slip of full dislocations with Burgers vector of a/2 . No dissociated dislocations and deformation twins were observed, indicating that this alloy poses high stacking-fault energy, which is supported by first principle calculations. The research suggests that further improvement on the mechanical properties of this alloy can be achieved by reducing stacking-fault energy via alloying, providing a promising low-cost alternative to CoCrFeMnNi HEAs for engineering applications.

A new Mg-2Zn-0.3Ca-0.2Ce-0.1Mn 1 (ZXEM2000) alloy and a multi-stage homogenization process have been designed and developed based on CALculation of Phase Diagram (CALPHAD) modeling results and experimental validation. The new sheet alloy offers an excellent combination of ductility (about 29% elongation) and yield strength (157 MPa), promising room-temperature forming applications. Microstructure characterization reveals that the exceptional properties of this alloy are mainly attributed to the weak basal texture, fine average grain size and high density of nano-scale Mn and Mg6Zn3Ca2 precipitates achieved through multi-stage heat treatment and thermomechanical processing.

The microstructural evolution and microsegregation of a series of magnesium alloys (Mg-Al, Mg-Al-Ca and Mg-Al-Sn) at various cooling rates were investigated using a CALPHAD-based solidification model called PanSolidification. Experimental validations were carried out in a wide range of cooling rate (0.12 ~ 55 K/s) using both directional solidification and die casting techniques. The back-diffusion effect in solidified solid was included in the solidification model. Good agreements have been achieved between the simulated and measured solidification microstructure parameters (phase fraction and secondary dendrite arm spacing) and microsegregation within the α(Mg) phase. This modeling approach demonstrated the reliability of the CALPHAD-based models for the prediction of solidification microstructure of magnesium alloys and their applicability for the optimization of magnesium castings.

Grain structure is one of the most important factors that affect the mechanical properties of metallic alloys. Accurate prediction of as-cast grain structure is critical in the design and manufacturing of metal castings using an integrated computational materials engineering (ICME) approach. In this paper, a three-dimensional (3-D) model based on cellular automaton (CA) and process simulation was developed for predicting grain size of components produced by high pressure die casting (HPDC) of aluminum alloys. The concurrent nucleation and growth of grains were considered in the model. A test specimen casting, consisting of different wall thicknesses, was simulated using a finite element based casting process simulation software ProCAST®. The thermal history of the simulated casting was extracted and used in subsequent mesoscale CA modeling. Different thermal conditions were used to simulate grain nucleation and growth during HPDC. The grain morphology, grain density and grain size were obtained by CA modeling. Electron backscatter diffraction (EBSD) analysis was performed on aluminum castings of different wall thicknesses under various cooling rates to validate the simulation results. The 3-D grain structure model is in excellent agreement with the experimental results, and can be used in ICME design and development of aluminum castings.

Despite having otherwise outstanding mechanical properties, many single-phase medium and high entropy alloys are limited by modest yield strengths. Although grain refinement offers one opportunity for additional strengthening, it requires significant and undesirable compromises to ductility. This work therefore explores an alternative, simple processing route to achieve strength by cold-rolling and annealing an equiatomic CrCoNi alloy to produce heterogeneous, partially recrystallized microstructures. Tensile tests reveal that our approach dramatically increases the yield strength (to ∼1100 MPa) while retaining good ductility (total elongation ∼23%) in the single-phase CrCoNi alloy. Scanning and transmission electron microscopy indicate that the strengthening is due to the non-recrystallized grains retaining their deformation-induced twins and very high dislocation densities. Load-unload-reload tests and grain-scale digital image correlation are also used to study the accumulation of plastic deformation in our highly heterogeneous microstructures.

Lightweight sheet alloys with superior mechanical performance such as high strength, ductility and formability at room temperature (RT) are desirable for high volume automotive applications. However, ductility or formability of metallic alloys at RT are generally inversely related to strength, thereby making it difficult to optimize all three simultaneously. Here we design a new magnesium sheet alloy-ZAXME11100 (Mg-1.0Zn-1.0Al-0.5Ca-0.4Mn-0.2Ce, wt. pct.) via CALPHAD (CALculation of PHAse Diagram) modeling and experimental validation. This new sheet alloy offers an excellent RT formability with a high Index Erichsen (I.E.) value of 7.8 mm in a solution-treated condition (T4), due to its weak and split basal texture and fine grain structure. The new ZAXME 11100 alloy also shows a rapid age-hardening response during post-forming artificial aging treatment at 210 °C for 1 hour (T6), resulting in a significant increase of yield strength from 159 MPa (T4) to 270 MPa (T6). The excellent combination of T4 ductility (31%), T4 formability (7.8 mm) and T6 yield strength (270 MPa) in this new magnesium alloy is comparable to that of common 6xxx series aluminum sheet alloys. Thus, this new magnesium sheet alloy is highly attractive for sheet applications in automotive and other industries.

In this paper, the effects of a small addition of Si (about 0.4 wt pct) on the microstructure and mechanical properties of a Mg-Al-Sn alloy produced with vacuum assisted high pressure die casting (HPDC) are reported. CALculation of PHase Diagrams-based modeling was used to design the heat treatment schedules to optimize the precipitation of Mg2Sn and Mg17Al12 in the Mg-7Al-2Sn-0.4Si alloy (in wt pct, designated as ATS). The HPDC test specimens were studied in the as-cast condition as well as in multiple heat-treated conditions to improve their tensile properties. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques were used to characterize the microstructure in each condition. The results suggest that the small addition of Si is effective in enhancing and accelerating the precipitation hardening of Mg-7Al-2Sn alloy AT72, due to enhanced precipitation of Mg17Al12 and Mg2Sn phases as well as the precipitation of Mg2Si phase. In as-cast conditions, the new ATS alloy has similar strengths but significantly improved ductility compared with AZ91D, and significantly improved strength but reduced ductility compared with AM60B. The ATS-T6 after a one-stage solution treatment provides further improvement of mechanical properties: yield strength 151 MPa, ultimate tensile strength 253 MPa, and elongation 4.9 pct.

A cost-effective α-β titanium casting alloy, Ti-6Al-5Fe-0.05B-0.05C, 1 has been designed using Calculation of Phase Diagrams (CALPHAD) method and the workhorse alloy Ti-6Al-4 V as a baseline. The substitution of iron for vanadium significantly reduces the raw material cost and improves the castability compared to the Ti-6Al-4 V alloy. The very fine α phase in the microstructure of the new alloy, likely due to Fe partitioning, provides exceptionally high strength (1023 MPa yield strength and 1136 MPa ultimate tensile strength) and reasonable ductility (3.71% elongation) for structural applications.

There is an increasing need to develop lower cost titanium alloys. In this investigation, Ti-Al-Mn ternary alloy system was experimentally and computationally investigated using Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDXS), Transmission Electron Microscopy (TEM), and Differential Scanning Calorimetry (DSC) to assess its thermodynamic models and parameters. The Ti-rich region was accurately assessed in this investigation, including multiple phase region boundaries and the identifications of all related phases. Based on the experimental results from this investigation and various literature articles, the thermodynamic description of Ti-Al-Mn in this work is in good agreement with experimental results and can be applied as a basis to develop higher-order multicomponent titanium alloys.

Texture evolution during room-temperature tensile testing of recrystallized equimolar CrCoNi was studied using electron backscatter diffraction and electron channeling contrast imaging on specimens from interrupted tests. Dominant deformation mechanisms included slip at low strains and deformation twinning at larger strains, which were accompanied by the development of a strong texture parallel to the tensile axis. Highly deformed material also contained nanotwin/hcp lamellae, which have previously been hypothesized to act as potent barriers for non-coplanar dislocations. To examine this hypothesis, mean-field modeling was performed using the viscoplastic self-consistent framework with varying ratios for hardening by slip and twinning. In the optimal model, twinning produced approximately three times as much non-coplanar hardening as slip, which is larger than previous observations in other twinning-induced plasticity materials that do not form twin/hcp lamellae. Additional full-field elasto-viscoplastic simulations were performed using the fast Fourier transform (EVP-FFT) method to examine intragranular rotation and the effect of initial grain orientation on the deformation mode. Grains with initial orientations near had the greatest propensity for deformation twinning while grains near were more likely to deform by slip even at large strains. Excellent quantitative agreement was obtained between the experiments and EVP-FFT model.

A new processing route is demonstrated for producing a nickel-based alloy with 1099 MPa tensile yield strength and 30% elongation. A chromium- and cobalt-rich commercial alloy was solution-annealed, cold-rolled, and aged at different temperatures and times to develop partially-recrystallized microstructures with ordered Ni3(Al,Ti) γ′ precipitates. These were tested in uniaxial tension and compared to alloys given the same heat treatment without rolling to assess the contributions of different strengthening mechanisms. Orientation mapping showed the development of a modest 〈111〉 and 〈100〉 double-fiber texture. Scanning transmission electron microscopy also revealed the development of exceptionally large dislocation densities including wall structures.

Aging-hardening response of Mg-7Al-2Sn (wt%) alloy can be remarkably accelerated by a small addition of silver (Ag). Using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) coupled with high resolution energy dispersive X-ray spectroscopy (EDS) mapping, for the first time, segregation of solute atoms (Ag in this case) was observed at the interphase boundary between continuous Mg17Al12 precipitate and magnesium matrix in Mg-7Al-2Sn alloy with 0.7 wt% Ag addition. Substantial size refinement and increased number density of Mg17Al12 precipitates may be responsible for the enhanced aging-hardening response in Mg-7Al-2Sn alloy with Ag addition.

Cyclic fatigue experiments in the high and very high cycle fatigue regimes have been performed on a Rene 88DT polycrystalline nickel-based superalloy. The microstructural configurations that favor early strain localization and fatigue crack initiation at high temperature from 400 °C to 650 °C have been investigated. Competing failure modes are observed in the high to the very high cycle fatigue regime. Fatigue cracks initiate from non-metallic inclusions and from intrinsic internal microstructural features. Interestingly, as stresses are reduced into the very high cycle regime, there is a transition to initiation only at crystallographic facets. At higher stress in the high cycle fatigue regime, a significant fraction of specimens initiate cracks at non-metallic inclusions. This transition is analyzed with regard to microstructural features that favor strain localization and accumulate damage early during cycling.

The microstructural configurations that favor early strain localization and fatigue crack initiation at intermediate and high temperature (400 °C–650 °C) have been investigated using novel experimental techniques, including high resolution digital image correlation and transmission scanning electron microscopy. Cyclic fatigue experiments in the high and low cycle fatigue regimes have been performed on a Rene 88DT polycrystalline nickel-base superalloy at temperatures up to 650 °C and compared to previous fatigue results obtained from tests in the very high cycle fatigue regime. Competing failure modes are observed along with an inversion in the temperature fatigue life dependence of fatigue strength from the low to high cycle fatigue regime. Oxidation-assisted processes are dominant at high applied stresses while cyclic plastic localization and accumulation govern fracture at low applied stresses. In addition, a second competing mode exists in the high and very high cycle fatigue regime from non-metallic inclusions as compared to internal intrinsic initiation sites. The grain-scale features that exhibit strain localization and crack initiation were investigated in detail. Transmission electron microscopy (TEM), transmission scanning electron microscopy (TSEM) and electron channeling contrast imaging have been conducted on samples removed from targeted regions with microstructural configurations that favor crack initiation to characterize the associated dislocation sub-structure and its evolution with temperature. Plasticity is observed to be less localized during cyclic loading at high temperature compared to room temperature. The microstructural features that drive initiation across the temperature range investigated are: twin-parent grains pairs that are at the upper end of the size distribution, are oriented for near maximum elastic modulus mismatch, and have high stresses along planes parallel to the twin boundaries.

Dislocation dynamics simulations were used to predict the strengthening of a commercial magnesium alloy, AZ91, due to β-Mg17Al12 formed in the continuous precipitation mode. The precipitate distributions used in simulations were determined based on experimental characterization of the sizes, shapes, and number densities of the precipitates for 10-hour aging and 50-hour aging. For dislocations gliding on the basal plane, which is expected to be the dominant contributor to plastic deformation at room temperature, the critical resolved shear stress to bypass the precipitate distribution was 3.5 MPa for the 10-hour aged sample and 16.0 MPa for the 50-hour aged sample. The simulation results were compared to an analytical model of strengthening in this alloy, and the analytical model was found to predict critical resolved shear stresses that were approximately 30 pct lower. A model for the total yield strength was developed and compared with experiment for the 50-hour aged sample. The predicted yield strength, which included the precipitate strengthening contribution from the DD simulations, was 132.0 MPa, in good agreement with the measured yield strength of 141 MPa.

The equiatomic NiCoCr alloy exhibits an excellent combination of strength and ductility, even greater than the FeNiCrCoMn high entropy alloy, and also displays a simultaneous increase in strength and ductility with decreasing the testing temperature. To systemically investigate the origin of the exceptional properties of NiCoCr alloy, which are related to the evolution of the deformation substructure with strain, interrupted tensile testing was conducted on the equiatomic NiCoCr single-phase solid solution alloy at both cryogenic and room temperatures at five different plastic strain levels of 1.5%, 6.5%, 29%, 50% and 70%. The evolution of deformation substructure was examined using electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD), conventional transmission electron microscopy (CTEM), diffraction contrast imaging using STEM (DCI-STEM) and atomic resolution scanning transmission electron microscopy. While the deformation substructure mainly consisted of planar dislocation slip and the dissociation of dislocations into stacking faults at small strain levels (≤6.5%), at larger strain levels, additional substructures including nanotwins and a new phase with hexagonal close packed (HCP) lamellae also appeared. The volume fraction of the HCP lamellae increases with increasing deformation, especially at cryogenic temperature. First principles calculations at 0 K indicate that the HCP phase is indeed energetically favorable relative to FCC for this composition. The effects of the nanotwin and HCP lamellar structures on hardening rate and ductility at both cryogenic and room temperature are qualitatively discussed.

Aluminium alloys can be strengthened significantly by nano-scale precipitates that restrict dislocation movement. In this study, the evolution of inhomogenously distributed trialuminide precipitates in two multi-component alloys was characterised by synchrotron small-angle X-ray scattering (SAXS). The appropriate selection of reference sample and data treatment required to successfully characterise a low volume fraction of precipitates in multi-component alloys via SAXS was investigated. The resulting SAXS study allowed the analysis of statistically significant numbers of precipitates (billions) as compared to electron microscopy (hundreds). Two cast aluminium alloys with different volume fractions of Al3ZrxV1-x precipitates were studied. Data analysis was conducted using direct evaluation methods on SAXS spectra and the results compared with those from transmission electron microscopy (TEM). Precipitates were found to attain a spherical structure with homogeneous chemical composition. Precipitate evolution was quantified, including size, size distribution, volume fraction and number density. The results provide evidence that these multi-component alloys have a short nucleation stage, with coarsening dominating precipitate size. The coarsening rate constant was calculated and compared to similar precipitate behaviour.

The application of titanium components is generally limited by their high raw material and manufacturing costs. In this paper, a lower cost cast titanium alloy based on the Ti-Al-Fe system has been designed using an ICME approach. The new alloy Ti-6Al-5Fe-0.05B-0.05C (all wt.%) significantly reduces raw material cost and demonstrates improved castability compared with the baseline Ti-6Al-4V alloy. The fine primary and secondary α phase microstructure in the new alloy, due to Fe partitioning, provides exceptionally high strength (1023 MPa yield strength and 1136 MPa ultimate tensile strength) and reasonable ductility (3.7% elongation) for structural applications. On the manufacturing front, the high cost multi-step investment casting process currently used can now be replaced with a low-cost permanent mold casting process using steel molds and a novel ceramic coating. An experimental casting setup, including an induction skull melting (ISM) system, a gravity tilt-pour system and a ceramic-coated H13 steel mold, has been used to produce near-net-shape permanent metallic mold castings with the new titanium alloy developed. Using this setup, and aided by casting process simulation, a prototype automotive connecting rod was cast successfully. The ZrO2 ceramic coating applied to the H13 steel mold was proven effective in minimizing the metal-mold reactions.

A CrCoNi based medium entropy alloy with small additions of Ti, Al and Nb (denoted as (CrCoNi)93Al4Ti2Nb) in the as-quenched condition, exhibits tensile properties comparable to those of the equiatomic CrCoNi alloy at room temperature. Dark field transmission electron microscopy (TEM), atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) together with atom probe tomography (APT) show that spatially-localized long range ordering (LRO) L12 domains exist in this alloy. The evolution of deformation substructure with plastic deformation in this alloy was characterized using electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and STEM based techniques including the recently developed weak beam dark field STEM imaging. Plastic deformation occurs by the slip of a/2 dislocations, which are narrowly dissociated into Shockley partial dislocations on {111} slip planes. Their dissociation distances in the (CrCoNi)93Al4Ti2Nb alloy are much smaller than the widths of the corresponding partials in the equiatomic CrCoNi alloy due to one or more of the minor alloying elements (Al, Ti, Nb). Dislocation slip in this alloy has a pronounced planar character. The leading dislocations in slip bands glide as pairs due to the existence of LRO domains. Multipoles were formed through the slip of dislocations with opposite signs on adjacent {111} slip planes. Those multipoles serve as building blocks for the formation of subgrain structures consisting of fine slip bands. The distances between slip bands were continuously refined during plastic deformation and dynamic refinement of slip bands plays a crucial role in strain hardening. The effects of LRO domains on planar dislocation slip, the deactivation of deformation twinning and strain hardening of this alloy are discussed.

The effects of initial microstructure on hot compression behavior of a new Al–Li–Cu alloy AA2070 were studied in the temperature range of 250 °C–450 °C and strain rate range of 0.001 s−1 - 0.1 s−1. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used to characterize microstructure prior to and post deformation. Two types of major strengthening precipitates were identified: Al2CuLi (T1) phase with habit planes parallel to {111}Al and Al2Cu/Al3Li (Ɵ'/δ′) composite precipitates with habit planes parallel to {100}Al, in the initial microstructure. The combination of these precipitates provides strong resistance to dislocation slip. While dynamic recovery is the main softening mechanism at compression temperatures below 450 °C, dynamic recrystallization dominates softening at 450 °C. T1 precipitate bands consisting of multiple fine T1 precipitates exist in microstructure after hot deformation at 250 °C and 350 °C. The formation mechanisms of T1 precipitate bands and fine T1 precipitates are discussed in light of shearing deformation of T1 precipitates. After hot deformation at 350 °C, Ɵ'/δ′ composite structure is lost and only Ɵ′ precipitates exist in the post-deformation microstructure. The interface between Ɵ′ precipitate and aluminum matrix becomes incoherent. The optimal processing conditions of this alloy have been identified as a temperature of 450 °C and a strain rate of 0.001 s−1, with significant grain refinement and the weakest deformation texture, which are consistent with the analysis results of hot work efficiency and instability using pseudo processing maps.

Correlative high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy are used to study deformation-induced planar faults in the single-crystal superalloy MD2 crept at 800 °C and 650 MPa. Segregation of Cr and Co at microtwins, anti-phase boundaries (APB), and complex/superlattice extrinsic and intrinsic stacking faults (CESF/SESF and CISF/SISF) is confirmed and quantified. The extent of this is found to depend upon the fault type, being most pronounced for the APB. The CESF/SESF is studied in detail due to its role as a precursor of the microtwins causing the majority of plasticity under these conditions. Quantitative modeling is carried out to rationalize the findings; the experimental results are consistent with a greater predicted velocity for the lengthening of the CESF/SESF—compared with the other types of fault—and hence confirm its role in the diffusion-assisted plasticity needed for the microtwinning mechanism to be operative.

Damage initiation during cycling loading of polycrystalline metallic alloys involves localized damage at the scale of individual grains. To better understand damage processes and to build models for material behavior, there is a need for quantitative assessment of the microstructural configurations that favor fatigue crack initiation. In materials that form annealing twins during processing, these special interfaces are often locations of particular interest for their role in strain and damage accumulation. In the present study, fatigue experiments in the very high and low cycle fatigue regime on a Rene 88DT polycrystalline nickel-base superalloy were performed to statistically evaluate grain-scale features that favor crack initiation. Combined elastic and plastic criteria at the grain scale have been developed. A crack distribution function is defined to compare and assess the effect of the microstructural parameters for the two fatigue regimes.

CrCoNi alloy exhibits a remarkable combination of strength and plastic deformation, even superior to the CrMnFeCoNi high-entropy alloy. We connect the magnetic and mechanical properties of CrCoNi, via a magnetically tunable phase transformation. While both alloys crystallize as single-phase face-centered-cubic (fcc) solid solutions, we find a distinctly lower-energy phase in CrCoNi alloy with a hexagonal close-packed (hcp) structure. Comparing the magnetic configurations of CrCoNi with those of other equiatomic ternary derivatives of CrMnFeCoNi confirms that magnetically frustrated Mn eliminates the fcc-hcp energy difference. This highlights the unique combination of chemistry and magnetic properties in CrCoNi, leading to a fcc-hcp phase transformation that occurs only in this alloy, and is triggered by dislocation slip and interaction with internal boundaries. This phase transformation sets CrCoNi apart from the parent quinary, and its other equiatomic ternary derivatives, and provides a new way for increasing strength without compromising plastic deformation., Medium entropy alloy CoCrNi has better mechanical properties than high entropy alloys such as CrMnFeCoNi, but why that is remains unclear. Here, the authors show that a nanostructured phase at lattice defects in CoCrNi causes its extraordinary properties, while it is magnetically frustrated and suppressed in CrMnFeCoNi.

The effect of a small addition of silicon (Si) was studied to increase the mechanical properties of Mg–Al–Sn alloys. Test specimens were produced by a vacuum die casting process which makes it possible to perform various heat treatments on the alloy which was not possible with conventional high pressure die casting (HPDC). The addition of Si leads to the formation of a binary Mg2Si phase which contributes to an increase in strength in the as-cast condition. Artificial aging treatments were performed to further increase the strength of the alloys by precipitation of fine Mg2Sn, Mg2Si, and Mg17Al12 phases. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) techniques were used to observe these fine precipitates at different stages of the heat treatment and mechanical testing was performed to compare strength and ductility to previous magnesium alloys.

The fatigue behavior of the nickel-base superalloy Rene 88 DT has been investigated at room temperature with fully reversed loading in an ultrasonic fatigue apparatus operating at a frequency close to 20 kHz. A characterization protocol based on the electron backscatter diffraction technique has been developed to identify the combination of microstructural features within crack initiation sites and surrounding neighborhoods that leads to the transition from initiation to early small crack growth. Surface grains that were more than three times the average grain size, that were favorably oriented for cyclic slip localization and that also contained Σ3 twin boundaries inclined to the loading axis were most favorable for fatigue crack initiation. Fatigue cracks subsequently grew in grain clusters within which grains are misoriented by less than 20° relative to the initiation grains. More highly misoriented neighboring grains resulted in crack arrest. The material characteristics that promote crack initiation and small crack growth exist only at the extreme tails of the microstructural distributions. The implications for modeling of fatigue life and fatigue life variability are discussed.

The fatigue behavior of a polycrystalline nickel-based superalloy Rene 88DT was examined in the lifetime regime of 105–109 cycles at 593 °C in air using an ultrasonic fatigue apparatus operating at frequencies close to 20 kHz. Three experimental techniques were combined to obtain new insights into the crack initiation process: serial sectioning, electron backscatter diffraction and quantitative fractographic analysis. Most fatigue failures initiated from internal microstructural sites comprised of large grains. Large crystallographic facets formed at crack initiation sites due to cyclic strain localization on {1 1 1} slip planes in the region close to Σ3 twin boundaries in large grains having high Schmid factors. The micromechanical mechanism of crystallographic fatigue crack initiation was analyzed in terms of both resolved shear stress and elastic incompatibility stresses in regions close to Σ3 twin boundaries. The influence of critical microstructure features on fatigue crack initiation and fatigue life variability is discussed.

Fatigue crack initiation in the polycrystalline nickel-based superalloy, Rene 88 DT, was investigated at 593 °C in air using an ultrasonic fatigue testing instrument. Within the testing stress range of 500 - 760MPa, all fatigue failures initiated from internal regions. Large crystallographic facets were observed at the crack initiation sites in most failed fatigue samples. Critical microstructure features controlling fatigue crack initiation and the early stage of small crack growth were identified by the combination of serial sectioning, orientation imaging microscopy (OIM) and quantitative fractographic analysis. Large crystallographic facets at crack initiation sites were formed in regions close to Σ 3 annealing twin boundaries within favorably oriented large grains. The facet planes are parallel to Σ 3 annealing twin boundaries and are of {111} type. Cyclic deformation substructures were studied by transmission electron microscopy. At the low cyclic stresses and long lives examined, fatigue failure at 593°C is controlled by strain localization in large, favorably oriented grains and subsequent propagation of small cracks through neighborhoods of favorably oriented grains. Annealing twin boundaries, prevalent in the microstructure, appear to be important in cyclic strain localization.