Brittle Quasi-cleavage Fracture Research Articles

Relationship between microstructure and mechanical properties of V-N microalloyed high strength ship plate steel

V-N microalloying treatment is an important way to improve the service performance of non-quenched and tempered ship plate steel. Herein, the influence of V(C, N) on the evolution of microstructure and improvement of mechanical properties was studied. In addition, the relationship between microstructure and mechanical properties of V-N microalloyed high strength ship plate steel was revealed. The results showed that the composite addition of V and N not only formed a fine dispersed precipitated phase, but more importantly, significantly refined the ferrite/pearlite microstructure, promoted the formation of intragranular acicular ferrite, increased the proportion of high angle grain boundaries, and decreased the kernel average misorientation value. The optimization of microstructure brought about by V-N microalloying achieved synchronous improvement of strength and cryogenic toughness. The impact energy of V-N microalloying ship plate steel increased from 97 J of V-N-free ship plate steel to 239 J at −40 °C, and the impact fracture mode changed from brittle quasi-cleavage fracture to microvoid coalescence fracture with a large number of equiaxial dimples.

Microstructure and mechanical property of Ti/Cu ultra-thin foil lapped joints with different weld depths by nanosecond laser welding

In present study, 50 μm thick Ti/Cu pure metal ultra-thin foils are successfully lap-welded by a nanosecond fiber laser. The joints with adjustable depths are obtained by controlling the laser energy input precisely. The microstructure and mechanical properties of Ti/Cu joints with different depths are well analyzed, respectively. In the joint with smaller penetration depth of 10 μm, thin band-like Ti/Cu intermetallic compounds (IMCs) are formed, while in the joint with a larger penetration depth of 40 μm, thicker band-like Ti/Cu IMCs and sizable bright dendritic Ti2Cu phases are formed, since more Cu element are involved in the chemical reaction in the joint with penetration depth increasing. The tensile test results show that the weld with 10 μm penetration depth has the best tensile strength up to 240 MPa, which is mainly due to less IMCs and low heat input in the weld. In addition, the fracture mode of the joint is classified into two models, including brittle quasi-cleavage fracture on the Ti side fusion zone (FZ) and ductile fracture on the Cu heat affected zone (HAZ), respectively, depending on the penetration depth above or below 30 μm.

Influence of La addition on Fe-rich intermetallic phases formation and mechanical properties of Al-7Si-4Cu-0.35Mg-0.2Fe alloys prepared by squeeze casting

Brittle β-Al5FeSi (β-Fe) intermetallic phases directly affect the mechanical properties of Al–Si alloys; thus, it is essential to modify these phases. Scanning electron microscope analysis showed that rare earth Lanthanum (La) additions (0, 0.15, 0.30 and 0.50 wt%) had a refining effect on β-Fe phases in Al–7Si–4Cu-0.35Mg-0.2Fe (wt.%) alloys prepared by squeeze casting. The underlying refinement mechanisms of rare earth La on the β-Fe were discussed in terms of experimental observations, nucleation thermodynamics and growth kinetics. For nucleation thermodynamics, La additions could induce the formation of La-rich intermetallic phases Al4Cu2SiLa with orthorhombic structure. Because β-Fe nuclei had a good lattice matching and epitaxial relationship with the Al4Cu2SiLa phases, the Al4Cu2SiLa provided energetically favorable nucleation sites for β-Fe and greatly promoted the nucleation of β-Fe in alloys. Meanwhile, the added La atoms could restrain the diffusion of atoms to the solid/liquid interface front and alter the β-Fe interfacial structure and energy, therefore restricting the β-Fe growth. Furthermore, β-Fe/Al4Cu2SiLa grew into side-by-side and cross-like morphologies, and its growth behaviors were controlled by solute concentration and anisotropy of the solid-liquid interfacial energy in a cooperative eutectic reaction. In addition, different fracture behaviors from brittle fracture to mixed ductile fracture and quasi-cleavage brittle fracture were also illustrated with increasing La addition, and these fracture behaviors were influenced by the characteristics of brittle La-rich and β-Fe intermetallic phases. When the addition of La was increased to 0.3 wt% and 0.5 wt%, it occurred quasi-cleavage brittle fracture, which was caused by the precipitation of coarse plate-like La-rich phases.

Enhancing strength and ductility in a near medium Mn austenitic steel via multiple deformation mechanisms through nanoprecipitation

High-Mn austenitic steels, usually deformed via the TWIP (twinning induced plasticity) mechanism, suffer high cost and other technical issues (e.g., hot-dip galvanizing, welding, etc.) from the high Mn content. Nevertheless, decreasing the Mn content would result in a shift of deformation mechanisms from TWIP to TRIP (transformation induced plasticity), usually causing quasi-cleavage brittle fracture. Herein, we report that by massive nanoprecipitation of coherent disordered particles, the grain sizes of a near medium Mn austenitic steel are successfully refined to 0.9 ± 0.4 μm, leading to a significant stacking fault energy increment of 6.4–7.9 mJ m–2 and accordingly, the transition of deformation mechanism from TRIP to multiple deformation mechanisms, namely, stacking faults, dislocation slip, nanotwinning and ԑ-martensite transformation. Moreover, the high-density of coherent nanoprecipitates effectively refines ԑ martensite and nanotwins from 20–500 nm and 10–50 nm to a few atomic columns and 1–15 nm, respectively. More importantly, these deformation mechanisms were sequentially activated at different deformation stages, resulting in a consistently high work hardening rate. Based on the synergistic refinement effects of grains, twins and martensite and the transition of deformation mechanism, a near medium Mn ultrafine-grained (UFG) austenitic steel with 15 wt.% Mn is developed, which shows a unique combination of high tensile strength (1210 ± 19 MPa) and large elongation (72 ± 6 %). These findings provide a new route to addressing the trade-off between the Mn content and mechanical performance of high-Mn austenitic steels which could facilitate their widespread applications.

Effects of Ti addition and heat treatment on the microstructure and mechanical properties of NiAl-Cr(Mo) hypereutectic alloy

The effects of Ti addition and heat treatment on the microstructure evolution and mechanical properties of NiAl-Cr(Mo) hypereutectic alloy were studied. The microstructure of NiAl-32Cr-6Mo alloy consists of Cr(Mo) dendrites and NiAl + Cr(Mo) eutectic. Ti addition would lead to the formation of Ni2AlTi precipitated phase. With the increasing Ti content, the number of Ni2AlTi phase increases. After heat treatment, the phase constituent of the alloy does not change. The hardness, room-temperature and high-temperature yield strength of the as-cast NiAl-32Cr-6Mo-1Ti alloy obviously increase due to the precipitation strengthening and solid solution strengthening, but the increasing primary phase and irregular microstructure result in the decreasing strength of NiAl-32Cr-6Mo-3Ti alloy. The yield strength and hardness of the alloy decline obviously after heat treatment while the ductility increases due to the uniform microstructure and increasing number of primary phase. All alloys show typical brittle quasi-cleavage fracture at room temperature. The number of cleavage steps and tearing ridges in the Ti-doped alloys increases significantly, and the fracture surface becomes more rugged. In addition, multiple cleavages are observed in the NiAl phase, which shows that the NiAl phase also plays a role in strengthening the alloy.

Strengthening and toughening effect of laser melting deposited Nb–16Si–20Ti–3Al with nano-ZrC additions

The phase composition, microstructure, and properties of Laser Melting Deposited (LMD) Nb–16Si–20Ti–3Al alloys with different nano-ZrC (0, 2.5, 5.0, 10.0 wt%) addition were detailed investigated in this study. Results showed that the LMDed Nb–16Si–20Ti–3Al alloy was composed Nb-based solid solution (Nbss) and Nb3Si. The addition of nano-ZrC promoted the decomposition of the Nb3Si phase and the eutectic transformation from Nb3Si to (Nbss+γ-Nb5Si3) eutectic structure. With the content of nano-ZrC increased from 2.5 wt% to 5.0 wt%, the (Nbss+γ-Nb5Si3) eutectic structure was significantly refined. Nb-rich TiC phase precipitated at the grain boundaries of (Nbss+γ-Nb5Si3) eutectic structure by the titanium in the alloy reacted with nano-ZrC additions. With the content of nano-ZrC increased to 10.0 wt%, coarse and irregular primary γ-Nb5Si3 phase and dendritic TiC were observed in the Nb–Si alloy. The fracture toughness was improved from 5.1 MPa m½ to 17.1 MPa m½ by adding an optimized amount of nano-ZrC (5.0 wt%) due to the refinement of (Nbss+γ-Nb5Si3) eutectic structure and dispersion distributed nano-ZrC in the alloys. The Nb–16Si–20Ti–3Al alloy with 5.0 wt% nano-ZrC addition has the maximum compressive strength of 1836 MPa and a compressive strain of 12.6%. The crack deflection seems an important toughening mechanism for Nb–16Si–20Ti–3Al alloys containing nano-ZrC. The fracture mode of each Nb–16Si–20Ti–3Al alloys with (0, 2.5, 5.0, 10.0 wt%) addition exhibited a brittle quasi cleavage fracture.

Microstructure, segregation and fracture behavior of 6061 aluminum alloy samples formed by semi-solid or traditional high pressure die casting

A serpentine channel pouring (SCP) process for efficiently preparing semi-solid 6061 aluminum alloy slurry and its rheological high pressure die casting (Rheo-HPDC) technology were introduced. The microstructure, segregation and fracture behavior of 6061 aluminum alloy samples obtained via the traditional HPDC or Rheo-HPDC process were investigated and compared. Furthermore, the effects of pouring temperature and Si content on the distribution, grain size and volume fraction of the primary phase were analyzed. The results indicated that the Rheo-HPDC process can effectively refine and improve the morphology of the primary phase compared with the traditional HPDC samples, which evolved from coarse dendrites into fine spherical grains. The pouring temperature had a significant effect on the primary α1-Al grains during slurry preparation, but had little effect on the secondary solidified α2-Al grains in the Rheo-HPDC samples. Moreover, with increasing Si content, the casting capability of the alloy was significantly enhanced, while the grain size of the primary phase was no obviously changed. Additionally, the liquid segregation led to varying microstructures, elemental distributions and fracture mechanisms of the samples. The volume fraction of the primary phase increased with increasing the distance from the sample surface, while the Si, Mg, Fe and Cu contents were enriched at edge of the testing bars. Besides, the liquid segregation tendency gradually increased along the filling direction in the Rheo-HPDC samples. Under the same die casting process parameters, the Rheo-HPDC samples had excellent mechanical properties than that of the traditional HPDC samples, and its ultimate tensile strength (UTS) increased from 90.8 MPa to 106.9 MPa. The fracture mechanism of the traditional HPDC samples was mainly brittle quasi-cleavage fracture, while the Rheo-HPDC samples was mainly mixed-fracture mechanism of dimple and quasi-cleavage fracture.

Vacuum diffusion bonding of α‑titanium alloy to stainless steel for aerospace applications: Interfacial microstructure and mechanical characteristics

In present study dissimilar metal joints between austenitic stainless steel (modified SS321) and α-Ti alloy (Ti-5Al-2.5Sn ELI) were produced by solid-state diffusion bonding process performed at different temperatures (750–950 °C) in vacuum. The effects of bonding temperature, time and pressure on interface microchemistry and microstructure were analyzed and subsequently the local hardness of interface layer(s) and the bond (shear) strength of the joints were evaluated. The results indicate the formation of four layers comprised of intermetallic phases viz. σ, χ, λ + Fe2Ti + FeTi and β-Ti at the joint interface, and their width increased with increase in the bonding temperature and time. The maximum shear strength of ~348 MPa, which is ~65% of the shear strength of SS321, was noted for the joint formed at the bonding temperature of 850 °C. This was achieved primarily due to complete collapse of surface asperities making intimate contact at the joint surface with formation of optimal width of interface layers having minimum embrittlement effect. The fractography of the specimens failed under shear loading revealed the transition of failure of the joints from featureless brittle fracture to mixed mode of ductile-brittle failure to quasi-cleavage brittle fracture with increasing the bonding temperature. Furthermore, it was noticed that the bonded joint specimens failed either through χ (layer 2) or λ + Fe2Ti + FeTi (layer 3). Interestingly, the optimized diffusion bonding parameters of the dissimilar metal joints of SS321/Ti-5Al-2.5Sn ELI have yielded different shear strength levels for the similar metal joints of SS321/SS321 (~35% of base alloy) and Ti-5Al-2.5Sn ELI/Ti-5Al-2.5Sn ELI (75% of base alloy).

Microstructure evolution and mechanical properties of ZrC-added Nb–16Si–20Ti alloys

The effect of ZrC addition on the phase composition, microstructure, and room-temperature mechanical properties of arc melted Nb–Si–Ti based alloys has been systematically investigated. The results indicated that the addition of ZrC promotes the decomposition of (Nb,X)3Si phase. The addition of ZrC promotes the microstructure transformation from hypoeutectic to hypereutectic, and the eutectoid reaction of (Nb,X)3Si to Nbss/(Nb,X)5Si3 eutectic structure. The KQ value of Nb–16Si–20Ti–1ZrC alloy is the highest, reaching 13.9 MPa m1/2. The decomposition of (Nb,X)3Si phase can increase the room-temperature fracture toughness. However, the precipitation of primary γ-(Nb,X)5Si3g-b phase deteriorates the room-temperature fracture toughness. Meanwhile, the more the content primary γ-(Nb,X)5Si3g-b is, the lower the room-temperature fracture toughness is. The maximum compressive strength of Nb–16Si–20Ti–1ZrC alloy is 2331 MPa, and the fracture strain of Nb–16Si–20Ti–3ZrC alloy is 11.4%. The fracture morphology of Nb–16Si–20Ti based alloys with ZrC additions is a brittle quasi-cleavage fracture.

The grain size-dependent control of the phase composition in ion-plasma treated 316L stainless steel

We study the influence of different grain size (density of grain boundaries) on the way of phase transformations in the surface layers of 316 L-type austenitic stainless steel under ion-plasma treatment. Using thermomechanical treatments, we fabricated a series of specimens possessing a single-phase austenitic structure, close density of the defects of the crystal lattice and different grain sizes (fine-grained with d ≈ 3–6 μm and coarse-grained with d ≈ 55 μm). These specimens were subjected to ion-plasma surface treatment at 550 ± 10 °C in N2+C2H2+Ar gases mixture to provoke a precipitation hardening. Although fine-grained and coarse-grained specimens possess similar penetration depth of interstitial atoms (N, C) under ion-plasma treatment (≈40–48 μm), the distribution of interstitials and phase composition are different in them. After ion-plasma treatment, specimens with low density of grain boundaries (coarse-grained structure) maintain a high level of N, C atoms in the solid solution of austenite (a = 0.3653–0.3674 nm) with a strip-like arrangement of Fe4(N,C) particles within grains, while precipitation of Cr(N,C) phase is suppressed. For these specimens, tensile diagrams have the extended linear stages typical of nitrogen-bearing austenitic steels, and the loss of ductility assisted with ion-plasma treatment is the smallest among studied specimens. Ion-plasma treated specimens with high density of grain boundaries (fine-grained structure) are prone to a decomposition of Fe-γN,C phase with the formation of grain-boundary and intragranular Cr(N,C) and Fe-α phases and partial preservation of a solid-solution strengthening of austenite (a = 0.3597–0.3622 nm). Precipitation hardening is more characteristic of these specimens and their flow curves are parabolic. The complex fracture mode of the specimens subjected to ion-plasma treatment is caused by the surface solid-solution strengthening and precipitation hardening. In the surface-hardened region (where the concentrations of N, C atoms are the highest), brittle quasi-cleavage fracture occurs due to the presence of Fe-based and Cr-based precipitates and austenite oversaturated with interstitials.

Different effects of pure hydrogen vs. hydrogen/natural gas mixture on fracture toughness degradation of two carbon steels

This work aims to investigate the fracture toughness of X80 and GB20# pipeline steel in hydrogen and actual natural gas/hydrogen mixture. It is found for the first time that pure hydrogen and natural gas/hydrogen mixture had different deteriorating effects on the fracture toughness of the two kinds of materials. The fracture toughness of X80 deteriorated the most in pure hydrogen gas compared with that in natural gas and the mixture. However, for GB20#, the fracture toughness degradation in the mixture was the most serious, approximately 2.5 times of that in hydrogen. The fracture surfaces observation showed that X80 evolved from ductile dimple fracture in natural gas to brittle quasi-cleavage fracture in hydrogen-containing environment, while GB20# in lower pressure was always ductile dimple fracture, resulting in its higher fracture toughness and slighter deterioration.

Bending-fatigue performance of the dissimilar joints of Fe-Mn-Si memory alloy and 304 stainless steel

The dissimilar joints of Fe-17Mn-5Si-10Cr-5Ni memory alloy and 304 stainless steel were welded by 5 kw transverse flow CO2 laser. The fracture morphology, microhardness of the welding joint was analyzed by field emission scanning electron microscopy, micro hardness tester, and bending fatigue test. The results showed that the fracture location of dissimilar joint is in the welding joint, and the fusion zone to weld areas prone to stress concentration while the formation of cracks. The full cyclic fatigue fracture of dissimilar joints consisted of the extension zone and instantaneous fracture zone. In the former, there were fatigue striations, dissociation steps and some arc tearing ridges, showing typical quasi-cleavage brittle fracture characteristics, and in the latter, there were a lot of dimples, showing plastic fracture characteristics. The toughness of dissimilar joint is better than that of the 304 stainless steel joint.

Effects of Homogenized Treatment on Microstructure and Mechanical Properties of AlCoCrFeNi2.2 Near-Eutectic High-Entropy Alloy

A 4 kg AlCoCrFeNi2.2 near-eutectic high-entropy alloy ingot was prepared by vacuum medium frequency induction melting. The effects of homogenized treatment on microstructure and mechanical properties of AlCoCrFeNi2.2 were studied. The results showed that all the alloys consisted of the primary FCC phases and eutectic FCC/B2 phases. After homogenized treatment, lots of precipitated phases appeared in the primary phase. The hardness of the as-cast alloy was HV296. The hardness values of samples were decreased and were around HV250 after homogenized treatment. The tensile fracture strength of the as-cast alloy reached 900 MPa, while the elongation was 18%. After homogenized treatment at 900 °C, the alloy showed the most excellent mechanical properties with the fracture strength 880 MPa and the elongation was 29%, respectively. All the alloys displayed a mixture fracture mechanism, including ductile fracture in primary FCC phases and eutectic FCC phases, and brittle quasi-cleavage fracture in eutectic B2 phases. Through a simple heat treatment method, the strength of the alloy was not reduced but the plasticity was greatly enhanced, which was more conducive to the industrial application prospects.

Multiphase microstructure formation and its effect on fracture behavior of medium carbon high silicon high strength steel

This work investigated the evolution of multiphase microstructure and impact fracture behavior of medium carbon high silicon high strength steel subjected to the austempering treatment at 240, 360, and 400 ℃. The results show that martensite, bainite, and retained austenite (RA) are the main microstructural phases. The austempering treatments at 360 and 400 ℃ caused the formation of carbon-poor ferrite in the matrix, and the transformation of ultrafine bainite into coarse lath bainite and granular bainite, respectively. Thick filmy RA was distributed between bainite laths. The polygonal martensite-austenite islands and blocky RA formed along the grain boundaries. The average carbon concentration in the matrix decreased with the temperature increase, while the impact toughness initially increased and then dropped with temperature. The quasi-cleavage brittle fracture dominated the impact fracture mechanism of the sample austempered at 240 ℃ by forming tearing surfaces and tearing steps. The microcracks disappeared in the RA on the prior austenite grain boundaries. On the other side, the fracture surface of the sample austempered at 360 ℃ exhibited ductile fracture with deep dimples and brittle fracture with cleavage river patterns. The polygonal martensite-austenite islands or blocky RA constrained the microcracks. After austempered at 400 ℃, the brittle fracture was dominant, showing river patterns, and the microcracks propagated through the granular bainite without any resistance.

Comparison Study of Low-Heat-Input Wire Arc-Fabricated Nickel-Based Alloy by Cold Metal Transfer and Plasma Arc

A comparative study of Inconel 625 surfacing on low-carbon steel produced by cold metal transfer (CMT) welding and plasma arc welding (PAW) was carried out. CMT surfacing weld with large reinforcement was produced as compared with PAW, and PAW generated smaller contact angle and larger dilution rate. Regression analysis based on weld forming features indicated that the main factor affecting dilution rate and weld width was welding speed, and dilution ratio decreased with increasing welding speed. Good surfacing forming was produced by plasma arc welding, and CMT surfacing process was more suitable for wire arc deposition. The characteristic of fast solidification and epitaxial growth was presented on the microstructure of surfacing layer of CMT and PAW, and that mainly consisted of γ-Ni columnar dendrites. Shear strength of PAW surfacing layer was 53% higher than that of CMT. Fracture morphology of CMT shear specimens was quasi-cleavage brittle fracture and that of PAW showed ductile fracture characteristics.

Effects of V addition on the solidification microstructures and room temperature compression properties of NiAl–Cr(Mo) hypereutectic alloy

The solidification microstructures and compression properties of NiAl–32Cr–6Mo and NiAl–36Cr–6Mo hypereutectic alloys with different V additions were studied. The results show that V addition did not change the constitute phase. However, the primary Cr(Mo) phase began to show obvious dendritic morphology, the formation of the eutectic lamellae was disturbed, and the microstructure of the eutectic cell was degraded. The EDS result indicated that all V solubilized in the Cr(Mo) phase. As the V addition increased, the volume fraction of the primary Cr(Mo) phase and the solid solution content of V in the Cr(Mo) phase continued to increase. Due to the solid solution strengthening of V in the alloy, the compression properties of V-doped alloys were obviously improved. Especially, the NiAl–32Cr–6Mo–1V alloy simultaneously had the highest compression strength of 1816 MPa and compression strain of 18.4%. When the V content further increased, although the positive effect due to the solid solution strengthening increased, the increasing number of primary Cr(Mo) dendrites in the alloy would result in decreased compression properties. In addition, all alloys showed brittle quasi-cleavage fracture pattern, while crack bridging, crack deflection and shear ligament were observed in the NiAl–32Cr–6Mo–1V alloy.

Effect of processing parameters on the microstructure and mechanical properties of Co–Cr–Mo alloy fabricated by selective laser melting

Selective laser melting (SLM) is an additive manufacturing process that builds metal components according to computer-aided design (CAD), selectively fusing and consolidating metal powders one layer at a time. This study systematically investigates the influence of various laser conditions on the microstructure and mechanical properties of Co–Cr–Mo alloy fabricated by SLM. Hot isostatic pressing (HIP) was applied after SLM to further densify the fabricated components, and their microstructures and mechanical properties were compared with those of as-built samples. Parts with relative density of ∼97.17% were obtained with a high-energy laser density and double laser scanning, and subsequent HIP effectively eliminated the majority of pores. X-ray diffraction revealed that γ and ε phases co-existed owing to the high solidification rate during SLM. Directional columnar grains and fine cellular dendrites that were parallel and normal, respectively, to the build direction were observed. Although variations in microstructure resulting from different as-built samples were insignificant, the mechanical properties remarkably changed. The improved mechanical behavior was attributed mainly to increased densification and grain-boundary strengthening. However, the HIP treatment increased the tensile ductility and reduced the yield strength, as a result of the increase in grain size. The tension-fractured surface suggests that mostly quasi-cleavage brittle fracture took place in the SLM-processed parts.

Inhomogeneous interface structure and mechanical properties of rotary friction welded TC4 titanium alloy/316L stainless steel joints

The inhomogeneous interface structure and mechanical properties along the radius direction of rotary friction welded TC4 titanium alloy/316 L stainless steel dissimilar joints were investigated under as-welded and post-weld heat treated conditions. The interface in the joint is convex-shaped on TC4 titanium alloy side and concave-shaped on 316 L stainless steel side. The mechanical properties of joints are highly correlated with the inhomogeneous interface structure and the dimension of tensile samples. The tensile strength of the as-welded entire joint was 117 MPa, while it dramatically increased to 419 MPa after post-weld heat treatment at 600 °C for 2 h due to homogenization of aggregated atoms. However, the sliced samples was relatively weak even after post-weld heat treatment, because the internal defects became surface crack sources after machining, and caused stress concentration and sharply reduced the strength of samples. All tensile samples failed through the interface. The X–ray diffraction patterns of TiC, Cr23C6, Fe2Ti, FeTi, FeNi3, AlTi3, CrTi4, NiTi and TiCr phases and the river-like features on fracture surfaces indicate a brittle quasi-cleavage fracture mode. C and Cr aggregated around the interface under as-welded condition. Nonetheless, elemental homogenization occurred in the joint after post-weld heat treatment.

Investigations on the solidification microstructures and room temperature compression properties of Nb-doped NiAl-32Cr-6Mo hypereutectic alloy

Solidification microstructures and compression properties of NiAl-32Cr-6Mo hypereutectic alloys containing different contents of Nb were investigated. The addition of Nb did not change the basic phase constituents, though a new phase of Laves Nb(Cr1-x-yNixAly)2 was observed. However, the primary Cr(Mo) phase no longer showed obvious dendritic morphology, the formation of lamellae was disturbed and the eutectic cellular microstructure degenerated. When the Nb content was 5%, eutectic lamellae coarsened and became spherical and poorly aligned. Fully eutectic microstructures were obtained in the directionally solidified hypereutectic alloys grown at 6 µm/s regardless of the addition amount of Nb. With the increasing content of Nb addition, the volume fraction of Laves phase located at the NiAl/Cr(Mo) boundary became more. The yield strength of directionally solidified NiAl-32Cr-6Mo-3Nb alloy reached to 2072 MPa, which is the highest value in the NiAl-based eutectic alloy at present. Well-aligned microstructure and more strengthening phase are conducive to the improvement of properties. All the alloys exhibited brittle quasi-cleavage fracture mode, cleavage in the NiAl phase and debonding of the NiAl/Cr(Mo) interface accompanied by Cr(Mo) lamellae and Cr2Nb phase pulled out were observed in the fracture surface.

Morphology and properties of CoCrMo parts fabricated by selective laser melting

Due to the rapid melting and solidification mechanisms involved in selective laser melting (SLM), CoCrMo alloys fabricated by SLM differ from the cast form of the same alloy. In this study, the surface morphologies, enhancement of mechanical properties and change to microstructure by heat treatment were discussed. Results showed that the microstructure of sample fabricated by SLM consisted mainly of face-centered-cubic (FCC) cobalt-rich solid solutions and carbide scattered cobalt-rich solid solutions, where Cr and Mo were dissolved in the Co-base phase and M23C6. The carbide scattered Co-base phase existed in the form of even margins. The tensile fracture of SLM-fabricated parts was mostly quasi-cleavage brittle fracture. Heat treatment transformed the SLM-fabricated parts such that failure occurred through ductile rather than brittle fracture, causing an increase in the tensile strength and elongation of the SLM-fabricated parts in addition to reduced yield strength and hardness. Some annealing twins were formed after heat treatment, which were demonstrated by TEM. These results will contribute to the development of biomedical CoCrMo alloys fabricated by SLM.