Temperature and load-ratio dependent fatigue-crack growth in the CrMnFeCoNi high-entropy alloy

Multiple-principal element alloys known as high-entropy alloys have rapidly been gaining attention for the vast variety of compositions and potential combinations of properties that remain to be explored. Of these alloys, one of the earliest, the ‘Cantor alloy’ CrMnFeCoNi, displays excellent damage-tolerance with tensile strengths of ∼1 GPa and fracture toughness values in excess of 200 MPa√m; moreover, these mechanical properties tend to further improve at cryogenic temperatures. However, few studies have explored its corresponding fatigue properties. Here we expand on our previous study to examine the mechanics and mechanisms of fatigue-crack propagation in the CrMnFeCoNi alloy (∼7 μm grain size), with emphasis on long-life, near-threshold fatigue behavior, specifically as a function of load ratio at temperatures between ambient and liquid-nitrogen temperatures (293 K–77 K). We find that ΔKth fatigue thresholds are decreased with increasing positive load ratios, R between 0.1 and 0.7, but are increased at decreasing temperature. These effects can be attributed to the role of roughness-induced crack closure, which was estimated using compliance measurements. Evidence of deformation twinning at the crack tip during fatigue-crack advance was not apparent at ambient temperatures but seen at higher stress intensities (ΔK ∼ 20 MPa√m) at 77 K by post mortem microstructural analysis for tests at R = 0.1 and particularly at 0.7. Overall, the fatigue behavior of this alloy was found to be superior, or at least comparable, to conventional cryogenic and TWIP steels such as 304 L or 316 L steels and Fe-Mn steels; these results coupled with the remarkable strength and fracture toughness of the Cantor alloy at low temperatures indicate significant promise for the utility of this material for applications at cryogenic environments

Controlling the relaxation versus rejuvenation behavior in Zr-based bulk metallic glasses induced by elastostatic compression

Elastostatic compression (ESC) has received considerable research attention as a tool to study rejuvenation and relaxation processes for bulk metallic glasses (BMGs). However, little is understood about the conditions that control whether rejuvenation or relaxation will occur, and whether conditions exist that can give structural stability. We address these questions by applying ESC at 90% of the yield stress to both cast and laser powder bed fusion (LPBF) manufactured Zr-based BMG samples in the as-cast, as-built, and different annealed states. The structural state and mechanical properties for each material condition were characterized by differential scanning calorimetry and microhardness, respectively, and two representative groups were also used for compression testing. Initial relaxation or rejuvenation was observed for elastostatically compressed as-cast samples, and the behavior reversed over 72 h of ESC. In contrast, no ESC effect was observed for the as-built LPBF samples. It was found that the onset of either relaxation or rejuvenation by ESC could be better predicted if samples were annealed into a controlled initial state. Five different types of initial response to ESC were observed, corresponding to different initial energy state ranges. Materials in the highest and lowest initial energy states were stable against structural changes by ESC. Close to the highest energy state, rejuvenation was dominant, while relaxation took place close to the lowest energy state. At intermediate initial energy states, both relaxation and rejuvenation were observed after ESC loading, suggesting that the glass structure easily finds different local minima in the potential energy landscape. In all cases, relaxation was associated with BMG hardening and rejuvenation was associated with softening. Overall, the results of this study provide new insights into how ESC impacts the structural state and mechanical properties of BMGs

Real-time observations of TRIP-induced ultrahigh strain hardening in a dual-phase CrMnFeCoNi high-entropy alloy

Strategies involving metastable phases have been the basis of the design of numerous alloys, yet research on metastable high-entropy alloys is still in its infancy. In dual-phase high-entropy alloys, the combination of local chemical environments and loading-induced crystal structure changes suggests a relationship between deformation mechanisms and chemical atomic distribution, which we examine in here in a Cantor-like Cr20Mn6Fe34Co34Ni6 alloy, comprising both face-centered cubic (fcc) and hexagonal closed packed (hcp) phases. We observe that partial dislocation activities result in stable three-dimensional stacking-fault networks. Additionally, the fraction of the stronger hcp phase progressively increases during plastic deformation by forming at the stacking-fault network boundaries in the fcc phase, serving as the major source of strain hardening. In this context, variations in local chemical composition promote a high density of Lomer-Cottrell locks, which facilitate the construction of the stacking-fault networks to provide nucleation sites for the hcp phase transformation

Deformation-induced medium-range order changes in bulk metallic glasses

Bulk metallic glasses (BMGs) naturally have excellent strength and elasticity while structural rejuvenation into higher energy glassy states is often required to improve ductility. However, our understanding of the detailed atomic ordering changes that occur during rejuvenation processes, such as plastic deformation, remains limited. This study utilizes nanobeam electron diffraction in a transmission electron microscope as an effective method to reveal the structural changes that occur after deformation in two Zr-based BMGs. Our findings indicate that heavy deformation from indentation or fracture causes an increase in the size of fcc-like medium-range order (MRO) clusters in a harder icosahedral dominated matrix, which corresponds to local softening of the BMGs. By examining the structure evolution at different points in the fracture process, we reveal that the mechanism of growth of MRO clusters is likely driven by enhanced diffusion from local temperature rise and/or free volume generation rather than deformation-induced nucleation and growth of new MRO sites

Size-dependent fracture toughness of bulk-metallic glasses

The fracture toughness is a critical material property that determines engineering performance. However, as is well known for crystalline materials, if certain sample geometry and size requirements are not met, test results become sample-size dependent and difficult to compare among different studies. Here, the room-temperature fracture toughness of the Zr-based bulkmetallic glass (BMG) Zr₅₂.₅Cu₁₇.₉Ni₁₄.₆Al₁₀Ti₅ (Vitreloy 105) was evaluated using compacttension, as well as single-edge notched bend, specimens of different sizes to measure KIc values according to ASTM standard E399 and JIc values according to ASTM standard E1820. It is concluded that the ASTM standard E399 sample-size requirements should be cautiously accepted as providing size-independent (valid) KIc results for BMGs; however, it is also concluded that small sized samples may result in a wider scatter in conditional toughness KQ values, a smaller yield of valid tests, and possibly somewhat elevated toughness values. Such behavior is distinct from crystalline metals where the size requirements of ASTM standard E399 are quite conservative. For BMGs, KQ values increase and show a larger scatter with decreasing uncracked ligament width b, which is also distinct from crystalline metals. Samples smaller than required by ASTM standards for KIc testing are allowed by the J-integral based standard E1820; however, in this study on BMGs, such tests were found to give significantly higher toughness values as compared to valid KIc results. Overall, the toughness behavior of BMGs is more sensitive to size-requirements than for crystalline metals, an observation that is likely related to the distinct size-dependent bending ductility and strain softening behavior found for metallic glasses. It is concluded that toughness values measured on BMG samples smaller than that required by the KIc standard, which are common in the literature, are likely sample size- and geometry-dependent, even when they meet the less restrictive valid JIc requirements

A Highly Fatigue-Resistant Zr-Based Bulk Metallic Glass

The strength-normalized fatigue endurance strength of the bulk metallic glass (BMG) Zr₅₂.₅Cu₁₇.₉Ni₁₄.₆Al₁₀Ti₅ (Vitreloy 105) has been reported to be the highest for any BMG; however, to date, there has been no explanation of why this material is so much better than other Zr-based compositions. In this study, the fatigue-crack growth behavior of Zr₅₂.₅Cu₁₇.₉Ni₁₄.₆Al₁₀Ti₅ was compared in ambient air vs dry nitrogen environment. The excellent fatigue life behavior is attributed to a relatively high fatigue threshold (Kₜₕ ≈ 2 MPa√m) and a lack of sensitivity to environmental effects on fatigue-crack growth in ambient air, as compared to other Zr-based BMGs. Fatigue life experiments conducted in ambient air confirmed the excellent fatigue life properties with a 10⁷-cycle endurance strength of ~0.24 of the ultimate tensile strength; however, it was also found that casting porosity, even in limited amounts, could reduce this endurance strength by as much as ~60 pct. Overall, the BMG Zr₅₂.₅Cu₁₇.₉Ni₁₄.₆Al₁₀Ti₅ appears to have excellent strength and fatigue properties and should be considered as a prime candidate material for future applications where good mechanical fatigue resistance is required

Interpretation of Fracture Toughness and R-Curve Behavior by Direct Observation of Microfracture Process in Ti-Based Dendrite-Containing Amorphous Alloys

Fracture properties of Ti-based amorphous alloys containing ductile beta dendrites were explained by directly observing microfracture processes. Three Ti-based amorphous alloys were fabricated by adding Ti, Zr, V, Ni, Al, and Be into a Ti-6Al-4V alloy by a vacuum arc melting method. The effective sizes of dendrites varied from 63 to 104 mu m, while their volume fractions were almost constant within the range from 74 to 76 pct. The observation of the microfracture of the alloy containing coarse dendrites revealed that a microcrack initiated at the amorphous matrix of the notch tip and propagated along the amorphous matrix. In the alloy containing fine dendrites, the crack propagation was frequently blocked by dendrites, and many deformation bands were formed near or in front of the propagating crack, thereby resulting in a zig-zag fracture path. Crack initiation toughness was almost the same at 35 to 36 MPaaem within error ranges in the three alloys because it was heavily affected by the stress applied to the specimen at the time of crack initiation at the crack tip as well as strength levels of the alloys. According to the R-curve behavior, however, the best overall fracture properties in the alloy containing fine dendrites were explained by mechanisms of blocking of the crack growth and crack blunting and deformation band formation at dendrites. (C) The Minerals, Metals & Materials Society and ASM International 2015ope

ICTEM 2018: Proceedings of the first International Conference on Theoretical, Applied and Experimental Mechanics

High strength in combination with low stiffness, high hardness, large elastic strain limits and near net-shape castability make bulk-metallic glasses (BMGs) candidate materials for many structural applications. Major drawbacks for their use in engineering service, however, are highly variable fracture toughness values and ductilities which can be entirely different for loading in tension, compression or bending. Specifically, whereas ductility is rather limited in tension/compression, BMGs can be quite ductile in bending. Due to the often-limited dimensions of cast BMGs, standard fracture-toughness tests are generally performed on smaller-sized samples with dimensions often comparable to the critical bending thickness of a glass. This critical bending thickness is defined as the dimension below which a glass can achieve the relevant number of shear bands to demonstrate significant bending ductility. To date, however, it is not clear how BMGs would behave in fracture toughness tests evaluated on samples with dimensions that are either below, above, or comparable to a glass’s critical bending thickness. Furthermore, while fracture toughness tests are often performed with “bending” geometries (three-point bending, compact-tension specimens), it has yet to be determined how the behavior of BMGs under these constrained stress-states relates to that in tension. Here, we report on a systematic study on Zr and Pd-based glasses to investigate the influence of sample size and loading condition on the fracture toughness of BMGs. Results show that with decreasing sample size the fracture behavior changes from brittle failure with low fracture toughness, via a semi-brittle failure regime, to fully ductile fracture and non-catastrophic failure with sub-critical crack growth, i.e., R-curve behavior. Our tests on samples subjected to different stress-states (three-point bending vs. tension loading) result in highly variable data which brings into question the extent of validity of nonlinear elastic fracture mechanics to characterize the toughness of BMGs

On the fracture behavior of bulk metallic glasses

High strength in combination with low stiffness, high hardness, large elastic strain limits and near net-shape castability make bulk-metallic glasses (BMGs) candidate materials for many structural applications. Major drawbacks for their use in engineering service, however, are highly variable fracture toughness values and ductilities which can be entirely different for loading in tension, compression or bending. Specifically, whereas ductility is rather limited in tension/compression, BMGs can be quite ductile in bending. Due to the often-limited dimensions of cast BMGs, standard fracture-toughness tests are generally performed on smaller-sized samples with dimensions often comparable to the critical bending thickness of a glass. This critical bending thickness is defined as the dimension below which a glass can achieve the relevant number of shear bands to demonstrate significant bending ductility. To date, however, it is not clear how BMGs would behave in fracture toughness tests evaluated on samples with dimensions that are either below, above, or comparable to a glass’s critical bending thickness. Furthermore, while fracture toughness tests are often performed with “bending” geometries (three-point bending, compact-tension specimens), it has yet to be determined how the behavior of BMGs under these constrained stress-states relates to that in tension. Here, we report on a systematic study on Zr and Pd-based glasses to investigate the influence of sample size and loading condition on the fracture toughness of BMGs. Results show that with decreasing sample size the fracture behavior changes from brittle failure with low fracture toughness, via a semi-brittle failure regime, to fully ductile fracture and non-catastrophic failure with sub-critical crack growth, i.e., R-curve behavior. Our tests on samples subjected to different stress-states (three-point bending vs. tension loading) result in highly variable data which brings into question the extent of validity of nonlinear elastic fracture mechanics to characterize the toughness of BMGs