Ductile Dimple Research Articles

Effect of Friction Stir Processing on Gas Tungsten Arc-Welded and Friction Stir-Welded 5083-H111 Aluminium Alloy Joints

This paper presents the analysis of the friction stir-processed aluminium alloy 5083-H111 gas tungsten arc-welded and friction stir-welded joints. The comparative analysis was performed on the processed and unprocessed gas tungsten arc-welded and friction stir-welded joints of similar aluminium alloy 5083-H111. The results showed a clear distinction between the friction stir processed joints and unprocessed joints. There is a good correlation observed between the microstructural results and the tensile results. Ultrafine grain sizes of 4.62 μm and 7.177 μm were observed on the microstructure of the friction stir-processed friction stir-welded and gas tungsten arc-welded joints. The ultimate tensile strength for friction stir-welded and gas tungsten arc-welded before friction stir processing was 153.75 and 262.083 MPa, respectively. The ultimate tensile strength for friction stir processed friction stir-welded joint was 303.153 MPa and gas tungsten arc-welded joints one was 249.917 MPa. The microhardness values for the unprocessed friction stir-welded and gas tungsten arc-welded joints were both approximately 87 HV, while those of the friction stir-processed ones were 86.5 and 86 HV, respectively. The application of friction stir processing transformed the gas tungsten arc morphology from brittle to ductile dimples and reduced the ductile dimple size of the unprocessed friction stir-welded joints from the range of 4.90–38.33 μm to 3.35–15.59 μm.

Vacuum Brazing Ti–15–3 with a TiNiNb Braze Alloy

Among all types of brazing fillers, Ti-based fillers show satisfactory joint strengths in brazing titanium alloys. However, the major concern in using such fillers is the formation of Cu/Ni/Ti intermetallic compound(s) in the joint. In this study, a Ti–15–3 alloy was vacuum brazed with a clad Ti–35Ni–25Nb foil. The brazed zone consisted of a Ti2Ni intermetallic compound in a (β-Ti,Nb)-rich matrix for specimen brazing at 1000 °C/600 s. Raising brazing temperature and time resulted in the Ti2Ni dissolving into the (β-Ti,Nb)-rich matrix. For the specimen brazing at 1100 °C/600s, Ti2Ni could only be observed at the grain boundaries of the (β-Ti,Nb)-rich matrix. After further raising it to 1200 °C/600 s, the Ti2Ni intermetallic compound was all dissolved into the (β-Ti,Nb)-rich phase. The average shear strength was significantly raised from 140 (1000 °C/600 s) to 620 MPa (1100 °C/3600 s). Crack initiation/propagation in the brittle Ti2Ni compound with the cleavage fractograph were changed into the Ti–15–3 base metal with a ductile dimple fractograph. The advantage of using Nb in the TiNiNb filler foil was its ability to stabilize β-Ti, and most of the Ni in the braze alloy was dissolved into the β-Ti matrix. The brazed joint could be free of any intermetallic phases with a proper brazing cycle applied, and the joint was suitable for a few harsh applications, e.g., repeated stresses and impact loadings.

Influence of tempering treatments on mechanical properties and hydrogen embrittlement of 13 wt% Cr martensitic stainless steel

The influence of tempering treatments on mechanical properties and hydrogen embrittlement (HE) of 13 wt% Cr martensitic stainless steel (MSS) have been investigated by Charpy impact tests and slow strain rate tensile (SSRT) tests followed by fracture surface examination. The austenitized and quenched specimens were tempered at 300, 550 and 700 °C for 2.5 h. The MSS tempered at 550 °C showed brittle intergranular (IG) fracture after impact tests indicating its susceptibility to temper embrittlement. The experimental results showed that the 13 wt% Cr MSS is sensitive to HE. The as-quenched condition showed cracking during hydrogen pre-charging itself. Hydrogen pre-charging duration increased the susceptibility to HE of tempered MSS. The maximum HE susceptibility was observed for specimen tempered at 550 °C with a drastic reduction in strength and strain to failure. Tempering at 300 and 550 °C showed brittle IG fracture with hydrogen pre-charging in SSRT tests whereas increased IG region with ductile dimples was observed for specimen tempered at 700 °C with increase of pre-charging duration. The reason for maximum susceptibility to HE of specimen tempered at 550 °C is due to synergistic interaction of hydrogen and impurities segregated at prior austenitic grain boundaries.

Stress Corrosion Cracking Resistance of Cold-Sprayed Al 6061 Deposits Using a Newly Developed Test Fixture

The stress corrosion cracking (SCC) response of Al 6061 bulk deposits produced by high-pressure cold spray (HPCS) was investigated and compared to commercial wrought Al 6061-T6 material. Representative tensile coupons were stressed to 25%, 65% and 85% of their respective yield strength and exposed to ASTM B117 salt fog for 90 days. After exposure, the samples were mechanically tested to failure, and subsequently investigated for stress corrosion cracking via optical and scanning electron microscopy with energy-dispersive X-ray spectroscopy (EDS). The results were compared to the wrought Al 6061-T6 properties and correlated with the observed microstructures. Wrought samples showed the initiation of stress corrosion cracking, while the cold-sprayed deposits appeared to be unaffected or affected by general corrosion only. Optical microscopy revealed evidence of stress corrosion cracking in the form of intergranular corrosion in the wrought samples, while no significant corrosion was observed in the cold-sprayed deposits. Fractography revealed wrought samples failed due to multiple mechanisms, with predominant cleavage and intergranular failure, but cold-sprayed samples only failed by ductile dimple rupture. The difference in SCC response between the differently processed materials is attributed to the documented benefits of the cold spray process, which includes maintaining fine grain structure of the feedstock powder and high density after consolidation, low oxidation, and work hardening effect.

Unraveling the effects of Zn addition and hot extrusion process on the microstructure and mechanical properties of as-cast Mg–2Al magnesium alloy

The effects of zinc addition and hot working via extrusion process on the microstructure and mechanical properties of as-cast Mg–2Al-xZn (x = 0, 0.5, 1, 2, and 5 wt%) magnesium alloys were studied. It was revealed that the simultaneous addition of 2 wt% Al and 0.5 wt% Zn (known as AZ20 alloy) is an effective way for refining of α-Mg in the as-cast condition, which resulted in a significant enhancement of tensile properties and changing the fracture mechanism from brittle cleavage facets to fibrous appearance characterized by ductile dimples. Further addition of Zn had negligible effect on the grain size but resulted in the appearance of Mg21(Zn,Al)17 intermetallic known as φ-phase, where the latter resulted in the deterioration of tensile properties of as-cast ingot due to its unfavorable morphology. After extrusion process, fine and recrystallized microstructures were observed along with the fractured and dispersed second phase particles. Besides the effect of fine grain size as verified by the Hall-Petch relationship, the presence of finely dispersed particles in the ZA52 (Mg–5Zn–2Al) alloy was found to be favorable in improving the mechanical properties and tensile toughness of the extruded alloy. Based on these findings, it was recommended to use these alloys in the wrought condition.

Effect of Shock and Vibration Preloading on the Deformation and Fracture Behavior of 17G1S-U Steel

The procedure of evaluating the crack resistance of 17G1S-U sheet steel after shock and vibration loading was advanced and experimentally tested using the method of complete deformation diagrams. The technical potential was employed to provide the growth of a mixed mode (I+III) macrocrack on the specimens with an identical central circular opening, which guaranteed the self-similar macrocrack propagation. The complete deformation diagrams displayed initial almost straight descending portion. In real constructions of gas mains, rather long macrocracks can arise after this fracture mode. The advanced procedure permits of reliable assessment of energy variations spent for a (I+III) mode crack propagation under any complex combined loading. The straight descending branch slope of the deformation diagram is established to be used for evaluating crack resistance variations of pipe steel subject to thermomechanical loading. Shock and vibration loading of a high frequency (1–2 kHz) is shown to essentially influence the crack resistance of pipe steel and plastic strain in the vicinity of a stress raiser. The impact of a power pulse on the material is dependent on its prestrain level through static tension and damage of its initial structure correspondingly. The controlling factor influencing the change in mechanical properties is the intensity of the power pulse. Basic fracture mechanisms of steel were established on the basis of examination of specimen fractures with scanning electron microscopy. The shock and vibration loading is evidently accompanied by energy contribution not only to the existing damages of the material but also to the initiating ones, which causes the localization of deformation and growth of pores in their vicinity. Since the energy accumulation can contribute to the modification of the material in the vicinity of those damages, the shape and sizes of ductile tear dimples are the informative parameters for evaluating the strength and plasticity of examined steel.

Recrystallization-induced transition from brittle to ductile fracture in severe plastic deformed copper

Severe plastic deformation (SPD) has been widely used to improve the strength of metallic materials. The improvement in strength comes with sacrifice of ductility. The poor ductility of SPD-materials is generally accompanied by brittle fracture. Here, we demonstrate that, once heat treatment (HT) is conducted over the recrystallization temperature, SPD-copper under quasi-static uniaxial tension can fracture via ductile dimples, showing a transition from brittle to ductile fracture. This transition is reflected by the diagram of ultimate strength versus uniform elongation, as well as the changes in the macroscopic fracture angle from incline to perpendicular and in the microscopic fracture morphology from shear dimples to tensile dimples. Microstructural observations show that the inhomogeneous microstructure of recrystallization surrounded by elongated substructure affects the fracture behavior. An exponential expression describing the relationship between fracture parameter and the volume fraction of recrystallization is obtained by performing numerical simulations. Combining the exponential expression and the ellipse criterion, we proposed a theoretical model to analyze the brittle-to-ductile transition in materials processed by SPD and HT. Comparative studies show that the proposed failure criterion can accurately predict the fracture angles. It is revealed that normal stress sensitivity of failure in SPD-materials depends on HT temperature.

The Effects of Sc Addition on the Microstructure and Mechanical Properties of Be-Al Alloy Fabricated by Induction Melting

This study investigated the alloying effects of 0.4 wt.% Sc on the microstructure, mechanical properties, and fracture behavior of a cast Be-Al alloy prepared by vacuum induction melting. The micromechanics of Sc-containing secondary-phase particles (SPs) were characterized by nanoindentation, and the fracture behavior of the alloy was investigated via tensile fractography and optical metallography. The addition of 0.4 wt.% Sc significantly modified the alloy microstructure by decreasing the secondary dendritic arm spacing (SDAS), weakening the dendritic structure of Be, and producing an equiaxed/cellular alloy microstructure with refined Be grains. Sc alloying increased the Vickers hardness by 36%, elastic modulus by 28%, yield strength by 35%, ultimate strength by 52%, but decreased the elongation from 3.6 to 2.9%. Sc-containing SPs had a higher elastic modulus, yield strength, microhardness, and a lower plasticity index than the Be and Al matrices, resulting in interface separation during the fracture process. SPs located at the Be/Al interfaces did not act as microcrack sources for the matrices, whereas highly faceted SPs within the Be grains induced microcrack formation and decreased the elongation. The failure mode of the cast Be-Al-0.4Sc alloy still involved the brittle cleavage failure of Be regions and ductile dimple failure of Al regions, which was not the case with the cast Be-Al alloy.

Evolution of damage and failure in an additively manufactured 316L SS structure: experimental reinvestigation of the third Sandia fracture challenge

The third Sandia Fracture Challenge (SFC3) was a benchmark problem for comparing experimental and simulated ductile deformation and failure in an additively manufactured (AM) 316L stainless steel structure. One surprising observation from the SFC3 was the Challenge-geometry specimens had low variability in global load versus displacement behavior, attributed to the large stress-concentrating geometric features dominating the global behavior, rather than the AM voids that tend to significantly influence geometries with uniform cross-sections. This current study reinvestigates the damage and failure evolution of the Challenge-geometry specimens, utilizing interrupted tensile testing with micro-computed tomography (micro-CT) scans to monitor AM void and crack growth from a virgin state through complete failure. This study did not find a correlation between global load versus displacement behavior and AM void attributes, such as void volume, location, quantity, and relative size, which incidentally corroborates the observation from the SFC3. However, this study does show that the voids affect the local behavior of damage and failure. Surface defects (i.e. large voids located on the surface, far exceeding the nominal surface roughness) that were near the primary stress concentration affected the location of crack initiation in some cases, but they did not noticeably affect the global response. The fracture surfaces were a combination of classic ductile dimples and crack deviation from a more direct path favoring intersection with AM voids. Even though the AM voids promoted crack deviation, pre-test micro-CT scan statistics of the voids did not allow for conclusive predictions of preferred crack paths. This study is a first step towards investigating the importance of voids on the ductile failure of AM structures with stress concentrations.

Deformation and fracture mechanisms of gradient nanograined pure Ti produced by a surface rolling treatment

Gradient nanograined Ti (GNG Ti) samples were produced by a surface rolling treatment. The mechanical properties were evaluated by microhardness and tensile tests. The results indicated that GNG Ti achieved an optimized combination of strength and ductility relative to the free-standing nano/ultrafine-grained Ti. The deformation and fracture mechanisms were studied using transmission electron microscope (TEM) and scanning electron microscope (SEM). As the grain size reduced from the coarse-grained (CG) matrix to the nano/ultrafine-grained (NG/UFG) layer, the transition from conventional plastic deformation mechanisms, such as dislocation activity and twinning deformation, to grain boundary-mediated deformation mechanisms, such as stress-inducing grain growth, was identified. At the early stage of tensile deformation, the microcracks were first initiated in the CG and deformed grain (DG) regions rather than in the NG/UFG region and the CG/DG interface had a deflection effect on crack propagation. After tensile failure, the fracture morphology showed a mixture of ductile dimples and shear band-like regions.

Impact Toughness and Fracture Structure about Welded Joints of Low Chromium Nickel Ferritic Stainless Steel Containing Ce/Ti

In order to improve the impact toughness and reduce inclusion, rare earth element Ce and stable element Ti are added in the low chromium nickel ferritic stainless steel welded joints and heat treatment. The Charpy test and tensile test are carried out and fracture structure are analyzed. The results show that when the ratio of Ce is 0.24% and Ti is 0.72% in welded joints of ferritic stainless steel, the mechanical properties is the best. The impact energy of welded joint is 9.4 times more than the samples without Ce and Ti, the impact toughness is also increased by 123.6% after heat treatment and the tensile strength increases by 41.9% The fracture structure is main ductile fracture and dimple, the shape of dimples is parabolic. The dimple is large and deep, and in the bottom of dimple has second phase particles. There is a few cleavage facets and river patterns in the fracture. After thermal cycling, the number of oxide particles reduce and the second phase particles are refined, so the impact toughness is improved.

Influence of temperature and strain rate on tensile deformation and fracture behaviour of boron added P91 steel

Tensile tests were performed at the strain rates ranging from 3.16 × 10−5 to 1.26 × 10−3 s−1 over a temperature range 300–873 K to examine the influence of temperature and strain rate on tensile deformation and fracture behaviour of boron added P91 ferritic steel. The variations of flow stress/strength values, work hardening rate and tensile ductility with respect to temperature exhibited three distinct temperature regimes. The steel exhibited anomalous variations in tensile properties due to dynamic strain ageing occurring at intermediate temperatures and the dominance of dynamic recovery at high temperatures. The fracture mode remained transgranular. Presence of large amount of chisel tip appearance along with dimples at 300 K and dominance of ductile dimples at intermediate and high temperatures were observed. Boron added P91 steel with reduced nitrogen exhibited strength values comparable to those reported for the plates and tubes of P91 steel.

Effect of tensile strain rate on high-temperature deformation and fracture of rolled Al-15 vol% B4C composite

Deformation behavior of a rolled Al-15 vol% B4C composite was studied at high temperatures, using single- tensile tests over a wide range of strain rates. The deformation of the composite is characterized by high apparent stress exponent, na and high activation energy, Qa, which are higher than those reported in pure Al. The analysis showed the presence of threshold stress that its value increases with decreasing the testing temperature. Using the threshold stress in the analysis, the obtained values of the true stress exponents, nt of ~ 5 and the true activation energy, Qt of 130 kJ mol−1, were similar to those reported for pure Al. TEM results of subgrain formation along with the mechanical data (nt and Qt) suggest that dislocation climb in the Al matrix is the rate controlling mechanism. The elongation (ef %) showed a maximum value at 500 °C at intermediate value of Zener-Hollomon parameter, Z. The fracture surfaces of tested samples are characterized by mixed modes of ductile (dimple formation) and brittle (cleavage) failures, which were dependent on the deformation conditions of temperature and strain rate.

Comparison of the Cracking Behavior of Powder Metallurgy and Ingot Metallurgy Ti-5Al-5Mo-5V-3Cr Alloys during Hot Deformation.

The hot workability of metallic materials is significantly dependent on its ability to form plastic without cracking and fracturing. In this work, the cracking behavior of powder metallurgy (PM) Ti-5Al-5Mo-5V-3Cr (Ti-5553) alloy, consolidated from powder mixture, at a deformation temperature range of 600 °C–850 °C and strain rate of 0.1 s−1–10 s−1, was investigated through isothermal compression tests. The cracking behavior of the as-cast ingot metallurgy (IM) Ti-5553 alloy, at a deformation temperature of 700 °C was also investigated for comparison. Results suggested that the PM Ti-5553 alloy had a better hot workability, with a larger cracking-free processing window, and a lower deformation resistance than the IM counterpart. 45° shear fracture occurred in the PM alloy, compressed at the condition of 600 °C/10 s−1, and edge cracking was observed at the 700 °C/10 s−1. 45° shear fracture was also significant in the IM alloy specimen tested at 700 °C/10 s−1, and all the other IM alloy specimens compressed at 700 °C displayed longitudinal cracking. Moreover, the microscopic cracking observation showed that ductile dimple cracking can be found in the IM alloy, but brittle cleavage fracture was dominant in the cracking surface of PM alloy with a relatively low cracking ductility.

Dynamic behavior and constitutive modeling of Ti-10-2-3 alloy

To investigate the dynamic mechanical behavior of Ti-10-2-3 alloy, the material was subjected to quasi static and dynamic compression testing. The results confirm that material exhibits strain rate sensitivity at low and high strain rates. The deformation mechanisms are elucidated by thermal activation theory. Voyiadjis-Abed (V-A) model is proposed to establish the thermo mechanical response of Ti-10-2-3 alloy. The developed model is compared with Johnson-Cook (J-C) model at different strain rates and temperatures. The V-A model well established the quasi-static and dynamic behavior of the Ti-10-2-3 alloy, mainly the effects of strain hardening and thermal softening. This model is embedded in ABAQUS/Explicit code in the form of a VUMAT user-subroutine to perform ballistic impact simulations. It is found that the predicted residual velocities are well matching with the experimental results. Adiabatic shear bands were observed in specimens deformed at higher strain rates. The tensile specimen is broken in a manner of ductile fracture presenting typical dimple structure, while the compressive specimen fails in a manner of localized shearing failure.

PM versus IM Ti-5Al-5V-5Mo-3Cr Alloy in Mechanical Properties and Fracture Behaviour

The comparisons of mechanical properties and fracture behaviour between as-consolidated PM Ti-5553 alloy and as-cast IM Ti-5553 alloy were investigated through tensile, fracture toughness and impact toughness tests in this research. The slightly higher strength but much higher ductility and toughness can be identified in IM alloy specimens, which is also confirmed by the fracture behaviour of the specimens after mechanical tests. IM alloy specimens always exhibit the ductile dimple fracture mechanism in the different tests, while the fracture mechanism of PM alloy specimens indicates a high loading rate sensitivity, changing from the mixed ductile-brittle quasi-cleavage fracture into the brittle cleavage fracture mechanism accompanied by the remarkable decrease of impact toughness. The relatively low mechanical properties, especially the ductility and the brittle fracture behaviour of as-consolidated PM Ti-5553 alloy, are mainly explained by the differences in the initial microstructures between these two alloys.

Microstructure evolution and mechanical properties of 316L austenitic stainless steel with aluminum addition by warm rolling

Microstructure evolution and mechanical properties of 316L austenitic stainless steel with aluminum addition by warm rolling at 550 °C were investigated. It is found that sample is composed of an ashen austenite matrix, a gray black ferrite phase and a small number of NiCx. The average grain sizes are 21.62, 19.66 and 19.49 μm for samples with the rolling deformation of 30%, 50% and 70%, respectively. The yield strength and tensile strength of samples with solid solution time of 30 min and deformation of 70% are higher. The fracture modes are similar and belong to toughness fracture. The fracture surfaces of the samples are composed of relatively large equal-axis ductile dimples (5–15 μm) and fine scattered ones around the dimples ( orientation is more prominent. {001} rotation-cube textures are present in ferrite phase of samples and weak Goss texture is formed in austenite pole images.

Evolution of the tensile properties of the tempered martensitic steel Eurofer97 after spallation irradiation at SINQ

Small disk tensile specimens of the reduced activation tempered martensitic steel Eurofer97 were tested from −100 °C up to 370 °C before and after irradiation in the spallation source SINQ at Paul Scherrer Institute. The calculated irradiation dose and helium content are respectively 11 dpa and 540 appm. Even for such a large dose, the tensile curves showed a significant total elongation at all testing temperatures. Fractographic observations revealed a predominance of cleavage facets at −100 °C for unirradiated and irradiated specimens while at higher temperature the fracture surfaces were dominated by ductile dimples. True stress/strain curves were derived using a finite element based inverse method for simulating small disk tensile tests. The finite element simulations indicated that the local fracture stress in the neck is consistent with that calibrated based on fracture tests and that the micro-mechanisms of fracture were not affected by helium up to 540 appm.

Initial microstructure effect on the mechanical properties of Ti-6Al-4V ELI alloy processed by high-pressure torsion

effect of initial microstructure on the mechanical properties and fracture mode of the Ti-6Al-4V ELI alloy subjected to the high-pressure torsion (HPT) processing at room temperature and 500 °C was investigated. In this purpose, the four different microstructures (fully lamellar, martensitic, equiaxed and globular microstructures) were developed by the proper heat treatments. The results showed that the application of the HPT processing provided substantial microstructural refinement, independent on the alloy initial microstructure. As a consequence, the alloy hardness, tensile strength and ductility significantly increased. At the same time, the elastic modulus of the HPT-processed alloy is almost 2 times lower as compared to that of the alloy in the initial heat treated state. The obtained improvements strongly depend on the alloy initial microstructure and HPT processing temperature. It was found that the initial martensitic microstructure is beneficial to improve the hardness, tensile and fracture properties. The HPT processing at 500 °C enhanced them more effectively. After the warm HPT processing, alloy with the initial martensitic microstructure exhibited exceptional hardness (455 MPa), high ultimate tensile strength (1546 MPa), high elongation to failure (18.8%) and low elastic modulus (78.6 GPa). Although the alloy with initial fully lamellar microstructure displayed the lowest elastic modulus (68 GPa), the ultrafine and homogeneous (α + β) two-phase microstructure produced by the HPT processing of the alloy with initial martensitic microstructure offered the best combination of the strength-ductility balance and strength-to-elastic modulus ratio (19.8 × 10−3). The complex fracture process, involving transgranular quasi-cleavage and ductile dimple fracture mode, corresponded to the changes in ductility.

Effect of Powder Recycling on the Fracture Behavior of Electron Beam Melted Alloy 718

Abstract Understanding the effect of powder feedstock alterations during multicycle additive manufacturing on the quality of built components is crucial to meet the requirements on critical parts for aerospace engine applications. In this study, powder recycling of Alloy 718 during electron beam melting was studied to understand its influence on fracture behavior of Charpy impact test bars. High resolution scanning electron microscopy was employed for fracture surface analysis on test bars produced from virgin and recycled powder. For all investigated samples, an intergranular type of fracture, initiated by non-metallic phases and bonding defects, was typically observed in the regions close to or within the contour zone. The fracture mode in the bulk of the samples was mainly moderately ductile dimple fracture. The results show a clear correlation between powder degradation during multi-cycle powder reuse and the amount of damage relevant defects observed on the fracture surfaces. In particular, samples produced from recycled powder show a significant amount of aluminum-rich oxide defects, originating from aluminum-rich oxide particulates on the surface of the recycled powder.