Hcp Transformation Research Articles

Cryogenic deformation strengthening mechanisms in FeMnSiNiAl high-entropy alloys

The mechanical properties and deformation mechanisms of a newly developed Co-free FeMnSiNiAl high entropy alloy (HEA) at room and cryogenic temperatures were systematically investigated. The initial tensile deformation at room temperature was dominated by dislocation slipping, with modest strengthening from the Transformation-Induced Plasticity (TRIP) effect due to the deformation-induced FCC → HCP martensitic transformation. Subsequently, the TRIP effect was markedly enhanced during the middle and later stages of deformation, leading to an excellent combination of yield strength (σy, 315.1 MPa), ultimate tensile strength (σu, 773.4 MPa), and fracture elongations (εf, 78.3%). The strengthening by the TRIP effect was significantly enhanced at cryogenic temperatures as a result of enhanced FCC → HCP martensitic transformation. This resulted in a synergetic improvement in strength and ductility at 223 K, with σy of 363.6 MPa, σu of 832.1 MPa, and εf of 87.2%. The enhanced ductility at 223 K was linked to the FCC → HCP → BCC sequential martensitic transformation during the middle and later stages of deformation, which acted as an additional way to accommodate plastic strain and delay strain localization. However, the rapid FCC → HCP transformation at the early stage of deformation at 173 K and 77 K impeded the FCC → HCP → BCC sequential martensitic transformation during subsequent deformation stages, thus remarkably enhancing strength but reducing ductility. Our findings provide new insights into the design and development of TRIP-assisted single-phase FCC HEAs for cryogenic applications.

In-situ analysis of the effect of residual fcc phase and special grain boundaries on the deformation dynamics in pure cobalt

Polycrystalline hcp metals – an important class of engineering materials – typically exhibit complex plasticity because of a limited number of slip systems. Among these metals, deformation is even more complicated in cobalt as it commonly contains residual fcc phase due to the incomplete martensitic fcc→hcp transformation upon cooling. In this work, we employ a combination of in-situ (acoustic emission, AE) and ex-situ (scanning electron microscopy, SEM) techniques in order to examine deformation dynamics in pure polycrystalline cobalt varying in grain size and the content of residual fcc phase prepared using systematic thermal treatment and cycling. We reveal that the presence of the fcc phase and special ∼71∘ grain boundaries between different hcp martensite variants brings about higher deformability and strength. The fcc phase provides additional slip systems and also accommodates deformation via the stress-induced fcc→hcp transformation during loading. On the other hand, special boundaries enhance structural integrity and suppress the formation of critical defects. Both these non-trivial effects can dominate over the influence of grain size, being a traditional microstructural variable. The ex-situ SEM experiments further reveal that the stress-induced fcc→hcp transformation is sluggish and only partial even at high strains, and it does not give rise to detectable AE signals, unlike in other materials exhibiting martensitic transformation. In turn, these insights into cobalt plasticity provide new avenues for the microstructure and performance optimization towards the desired applications through the modern concept of grain boundary engineering.

Atomistic Simulations of the Shock and Spall Behavior of the Refractory High-Entropy Alloy HfNbTaTiZr

AbstractUsing molecular dynamics simulation, we study the effect of a shock wave on the refractory high-entropy alloy HfNbTaTiZr. A single-crystalline sample, shocked along the [001] direction, is considered. The initial compression leads to only weak dislocation activity and a bcc $$\rightarrow$$ → hcp transformation in some regions of the sample. After the shock wave is reflected from the free back surface of the sample, hcp transforms back to bcc, and twins are formed in the bcc phase. The sample spalls under the high tensile pressures developing after wave reflection. In this stage, we observe dislocation activity from the twin boundaries and inside the nanograins generated by twinning. Under the large tensile stresses, some fcc phase appears together with disordered amorphous regions where voids nucleate and lead to spall. The fracture surfaces follow the twin boundaries set up in the compression phase. The spall strength is similar to the one found in simulations of other bcc metals at similar strain rates. Similar simulations for the equiatomic HfNbTaZr HEA show the same qualitative behavior, with twins and reduced dislocation activity, but without phase transformations.

On the fcc to hcp transformation in a Co-Ru alloy: Variant selection and intervariant boundary character

Ru is a common addition to the Co-binder in WCCo hardmetals for advanced cutting tool applications. Internal Co-Co interfaces control many properties such as hot-hardness, toughness, and creep resistance. Hence, phase transformations determining the internal interface character can be harnessed to achieve superior properties. We investigate the γ (face-centred cubic) to α (hexagonally closed packed) martensitic phase transformation of a model Co-Ru binder alloy. We describe the crystallography of γ to α phase transformations and the resulting α/γ and α/α interface character distributions. The stabilisation of the γ-phase results in the formation of low energy (0 0 0 1) α/γ interphase boundary planes. Preferred formation of α/α intervariant and twin-related boundaries with symmetrical tilt configurations are also observed. We discuss how crystallographic constraints of the transformation promote the formation of grain boundary planes other than those of the lowest energy configurations.

High-pressure phase transformations and lattice distortions in industrial AISI 1070 steel: Insights from Debye-Scherrer ring integration

In this study, the behavior of the industrially prominent AISI 1070 pearlitic steel under pressures up to 35 GPa was meticulously explored. Leveraging the 1D integration patterns from the Debye-Scherrer rings, we introduced a groundbreaking method to quantify dislocation densities, providing a novel lens to understand lattice distortions in such widely-used steels. At 5 GPa, the decomposition of cementite became evident, corroborated by changes in lattice parameters and dislocation densities. As the pressure neared to 10 GPa, the emergence of Hägg carbide was observed. The hallmark discovery was the phase transition from BCC to HCP around 20 GPa. This transformation bore witness to further distortions in the HCP structure with mounting pressures. Finally, molecular dynamic simulations revealed a mostly BCC to HCP transformation via an intermediary FCC phase at crystalline defects, elucidating observed orientations and phase transformation pathways.

Co50Cr20Ni20Fe5Mn5 high entropy alloy: Overcoming the strength-ductility trade-off of Cantor alloy

In this study, we aimed to design and investigate a non-equiatomic high strength and ductile high entropy alloy (HEA) by controlling the FCC phase stability. A new Co-rich Co50Cr20Ni20Fe5Mn5 HEA was synthesized by vacuum arc remelting and processed through homogenization, cold rolling, and recrystallization. The alloy’s phase composition, mechanical properties, and plastic deformation behavior were characterized using X-ray diffraction (XRD), tensile testing, and Transmission Electron Microscopy (TEM) analysis. The tensile properties of the alloy were compared with those of the conventional Co20Cr20Ni20Fe20Mn20 HEA produced and processed using the same technique and route. The results showed that reducing the Fe and Mn concentrations while increasing the Co concentration reduced the FCC phase’s stability, leading to the occurrence of partial FCC to HCP phase transformation upon cooling from high temperature and TWIP-TRIP effects during tensile deformation. The Co50Cr20Ni20Fe5Mn5 HEA exhibited improved mechanical properties, including higher yield strength (502 MPa), ultimate tensile strength (1002 MPa), and total elongation (50%) compared to the reference Cantor HEA (405 MPa, 766 MPa, and 40%, respectively). The principal strengthening mechanisms contributing to the increased yield strength with higher Co concentration were identified as initial HCP phase strengthening and grain boundary strengthening. The enhancement in ultimate tensile strength and ductility is attributed to the TWIP-TRIP effects during the tensile deformation of the Co50Cr20Ni20Fe5Mn5 HEA.

Probing plastic mechanisms in gradient dual-phase high-entropy alloys under nanoindentation

The plastic deformation mechanisms of gradient nanostructured high entropy alloys (HEAs) subjected to nanoindentation tests have been examined. First, we have constructed gradient glass/crystal (GGC) dual-phase HEAs by embedding crystalline CoCrFeNi HEA grains into CoCrFeNiAlx (x = 0.5, 1, and 2) metallic glass (MG) matrix in a gradient manner, leading to a gradient change in both the grain size and MG matrix shell thickness. Then, nanoindentation tests on the specimens under proper boundary and loading conditions are simulated by molecular dynamics software. It is found that the gradient-distributed MG shell effectively reduces the degree of strain localization within the region around the indenter, which results in a considerable hardening effect, thanks to the coexistence of both dislocation motion in the grain phase and plastic flow activated simultaneously in the MG phase. The results also show that the damage tolerance of GGC specimens is higher than that of the polycrystalline specimen because gradient-distributed MG phases can effectively prevent FCC to HCP transformation. Besides, the Al addition in the gradient-distributed MG shell can reduce its deformation propagation capability, weakening the hardening effect. The proposed method of constructing GGC can be promisingly helpful, for example, in the surface engineering of high-entropy alloys.

Bidirectional transformation enabled improvement in strength and ductility of metastable Fe50Mn30Co10Cr10 complex concentrated alloy under dynamic deformation

High strain rate compression experiments performed on a non-equiatomic metastable fcc + hcp Fe50Mn30Co10Cr10 high entropy alloy using split Hopkinson pressure bar setup shows improved flow stress and compression ductility compared to quasistatic deformed samples. The deformation response was characterised by the occurrence of hardening and softening stages compared to sustained strain hardening for quasistatic deformation. Detailed EBSD, BSE imaging analysis coupled with TEM shows significant bi-directional transformation (B-TRIP) and increased fcc γ phase stability in high strain rate regime while only forward (fcc to hcp) transformation dominates with increasing fraction of hcp ε phase in the quasistatic regime of deformation. Bidirectional transformation aided by adiabatic heating and heterogeneous deformation in the high strain rate regime leads to optimal stress and strain partitioning between the two phases and delays the initiation of damage at the interface. The presence of concomitant strain rate hardening and improvement in ductility in the dynamic deformation regime opens up avenues for microstructural tunability to achieve simultaneous improvement in strength and ductility using the metastability paradigm in complex concentrated alloys.

Kinetics of FCC to HCP Transformation During Aging Heat Treatment of Co–28Cr–6Mo Alloy Fabricated by Laser-Powder Bed Fusion

Kinetics of FCC to HCP Transformation During Aging Heat Treatment of Co–28Cr–6Mo Alloy Fabricated by Laser-Powder Bed Fusion

Insight into the FCC→HCP Transformation in Co-Rich Co-Cr-Fe-Mn-Ni High-Entropy Alloys

The existence of an HCP phase in FCC-type high-entropy alloys can improve the alloy’s mechanical properties. In many cases, an HCP phase is induced by deformation. In the present work, an FCC to HCP transition was detected during the cooling of Co1.5CrFeMnNi0.5 and Co1.75CrFeMnNi0.25 alloys. Therefore, arc-melted annealed CoxCrFeMnNi2−x (x = 0.25–1.75) alloys that were then subjected to long-term vacuuming were investigated using XRD, DSC, HT-XRD, thermodynamic calculation, and first-principle calculation. It was confirmed that the FCC to HCP transition occurred at ~450 °C during the cooling of the alloys with x ≥ 1.5. The volume fraction of the HCP phase increased with Co content. It was proven that the HCP phase was not stable above 600 °C. First-principle calculations further indicated that the HCP structure was more stable than the FCC structure for Co1.75CrFeMnNi0.25 alloy, and there was a likelihood of an FCC to HCP transition. Moreover, experimental tests confirmed that the microhardness of the Co1.75CrFeMnNi0.25 alloy reached 213 HV because it contained a substantial HCP phase. This value is much higher than those of other non-HCP-containing alloys, either in their as-cast states or after annealing. These results provide guidance for the design of FCC-type high-entropy alloys with desirable mechanical properties through HCP phase strengthening.

Microstructure and texture of heavily cold-rolled and annealed extremely low stacking fault energy Cr26Mn20Fe20Co20Ni14 high entropy alloy: Comparative insights

Microstructure and crystallographic texture of an extremely low stacking fault energy (SFE) FCC single-phase Cr26Mn20Fe20Co20Ni14 high entropy alloy (HEA) was investigated and compared with selected low SFE FCC HEAs. The Cr26Mn20Fe20Co20Ni14 HEA was 90% cold-rolled and annealed between 750 °C and 1200 °C. The microstructural evolution revealed nano-twins, extensive shear bands, and gradual evolution of deformation-induced lamellar nanostructure. Evidence of deformation-driven FCC→HCP transformation was indicated after heavy cold-rolling. Concomitant to microstructural evolution, the formation of a predominant brass-type ({110}<112 >) texture after heavy cold-rolling was confirmed. Annealing resulted in the formation of (Co, Cr) rich σ phase precipitates stable up to 1000 °C but dissolved in the FCC matrix at higher annealing temperatures. The pinning effect exerted by the precipitates resulted in considerable hindrance to grain growth compared to other FCC HEAs, whereas dissolution of the precipitates resulted in extensive grain growth. Annealing textures retained α-fiber (normal direction (ND)//< 001 > ) components and also showed a high volume of random components. These observations highlighted the limited contributions of preferential nucleation and growth. Broadly, the annealing texture showed remarkable resemblance with other low SFE HEAs. An appreciable strength-ductility balance could be observed in the suitably annealed HEA. Meanwhile, Hall-Petch analysis indicated a significantly lower lattice friction stress (∼53 MPa) compared to FCC equiatomic HEAs.

The role of 9R structures on deformation-induced martensitic phase transformations in dual-phase high-entropy alloys

The occurrence of transformation induced plasticity (TRIP) effects can be affected by the 9R structure, thus improving the mechanical properties of the alloy. However, the mechanism of the 9R structure during the phase transformation is still unclear. A new microscopic mechanism is proposed in this work, and a crystallographic model is established to explain the FCC→9R→HCP transformation processes. The 9R structure is obtained by adjusting the stacking fault energy (SFE) in Fe40Mn40-xCo20Crx high entropy alloy (HEAs) based on first-principles calculation. The calculations reveal that the interfacial synergism derived from the FCC and 9R structures lowers the cohesive energy and enhances stability. Moreover, the 9R structure provides more HCP phase nucleation cores and enhances the TRIP effect. This work proposes a valuable design concept for developing high-strength ductile structural materials.

Activation of non-basal slip in coarse-grained Mg-Sc alloy

Mechanical properties of a coarse-grained hexagonal close packed (hcp) single phase Mg-Sc alloy with two different textures, randomized (through body centered cubic (bcc) to hcp transformation) or weakened (through only cold rolling) texture, was investigated. Alloys of both the textures showed high elongation of over 30% with a moderately high ultimate tensile strength of about 240 MPa. Transmission electron microscopy (TEM) observations showed that both a- and c- component dislocations were activated in the grain interiors, over 20 µm away from grain boundaries. Thus, pyramidal <c+a> slip can be activated in grain interiors of a coarse-grained Mg-Sc alloy.

Defects induced through rapid solidification in a Co–20 Cr alloy

Martensitic transformation in cobalt alloys has been widely explored due to its complex nature. Recent studies demonstrated that rapid solidification enhances the γ–FCC → ε–HCP transformation through a high density of ε–martensite nucleating defects formation (i. e. vacancy supersaturation, sub – micron sized voids, staking faults/ε – martensite plate intersections, among others), resulting in almost 100% ε–martensite. In this work, the rapid solidified (RS) alloy exhibits a complex microstructure consisting of an athermal ε–martensite matrix with residual austenite (γ–phase) as the γ → ε transformation was kinetically dominant. Thus, a variety of defects are exposed, and critical theoretical modeling is derived from TEM experimental evidence, consequently, their interactions as a stair–rod dislocations are proposed. A disclosure concerning how these defects’ interaction influence the mechanical behavior of the studied Co–20 Cr alloy in the as – cast and heat – treated (at 1023 K/750 °C for 1 h) conditions is presented. In particular, annealing was found to be highly effective in promoting alloy strength and ductility. Apparently, internally straining is considerably reduced through thermally activated recovery mechanisms including ε–plate thickening and alloy recrystallization.

Temperature-dependent universal dislocation structures and transition of plasticity enhancing mechanisms of the Fe40Mn40Co10Cr10 high entropy alloy

The FCC Fe40Mn40Co10Cr10 (at.%) high entropy alloy exhibits deformation twinning for room temperature deformation and deformation-induced HCP transformation for subzero deformation. Since the systematic investigation of temperature-dependent dislocation structures is not available, we present an in-depth characterization of the defects involved in deformation at room temperature (298 K) and subzero temperature (223 K) of the cold-rolled and annealed Fe40Mn40Co10Cr10 alloy. The material deformed at 223 K shows a higher strain hardening rate than the material deformed at room temperature while both materials show large ductility, 48% for the 223 K deformation and 55% for the 298 K deformation. The main deformation mechanisms of the investigated HEA include the development of inhomogeneous dislocation structures and interaction between dislocations and deformation twin/mechanically induced HCP martensite. The stacking fault energy measured using TEM weak-beam dark-field imaging of dissociated dislocations is 20±9 mJ/m2 at 298 K. The Fe40Mn40Co10Cr10 alloy exhibiting a positive temperature dependence of SFE leads to a decrease of SFE as deformation temperature decreases from 298 K to 223 K. The decrease of SFE results in the transition from deformation twinning to deformation-induced HCP transformation. Further, at higher strains at 223 K, kink banding of HCP and reverse transformation from HCP to FCC were observed, which could account for strain accommodation and stress relaxation, and the large ductility. The 298 K deformation leads to various types of dislocation structures: Extended dislocations, Taylor lattice of perfect dislocations, dislocation loops, highly dense dislocation walls, cell blocks, and cell structures. The observed dislocation structures at 298 K and 223 K are similar suggesting the minor effect of SFE on dislocation structures.

Heat treatment of laser powder-bed-fused Co–28Cr–6Mo alloy to remove its microstructural instability by massive FCC→HCP transformation

Laser powder bed fusion ( L -PBF) of Co–28Cr–6Mo alloy is a powerful tool for many engineering applications but one has to be familiar with its microstructural instability in the as-produced state. In our previous study [Roudnicka et al., Additive Manuf. 44 (2021) 102025], we depicted the phenomenon of this instability when exposing the material to temperatures above 673 K. Microstructural changes bringing about hardening of the material were studied thoroughly. In the present study, we follow up by revealing the effect of this hardening on mechanical properties in tension and show the beneficial effect of heat treatment on removing the observed instability. Hardening associated with increasing the proportion of HCP ε-phase and with the precipitation of σ-phase has a detrimental effect on material plasticity and can be negatively affect the fatigue too. We thus applied a two-stage heat treatment comprising solution annealing at 1473 K/2 h and aging at 1173 K/5 h. Not only it induced material stability but also increased the material strength with not such a detrimental decrease in material plasticity as was brought by direct aging at 1173 K/5 h. • L -PBF Co–28Cr–6Mo shows microstructural instability above 673 K. • The stability can be induced by massive FCC → HCP transformation. • Appropriate solution annealing and aging (STA) treatment stabilized the material. • STA enhanced material strength and preserved 70% of its plasticity.

Annealing treatment induced ε martensite formation and evolution in TWIP steel

Crystallographic analyses on the orientation relationships of ε martensite variants were carried out by electron backscatter diffraction and transmission electron microscope in TWIP steel. Results show that the ε phases are mainly distributed in single or multiple initial variants with 70.5° angle on {111}γ planes during short-time annealing. Importantly, secondary variants, which are generally considered difficult to form by thermal-induced and are also not derived directly from matrix by the well-known simple shear process for the FCC to HCP transformation, were observed at the intersection between two initial variants with 19.5° rotation angle after long-time annealing. These successfully interpret the microstructural features of ε variants and their relationships.

Stacking fault energy and fcc→hcp transformation driving force in Fe-Mn-C-Cr-Si high manganese steels and experimental investigation

In order to improve the strength and work hardening capacity of high manganese steel, the effects of alloying elements on the stacking fault energy (SFE) and driving force of fcc→hcp transformation in Fe-Mn-C-Cr-Si high manganese steels were explored in detail. Based on the thermodynamic calculations, the Fe-0.6C-15Mn-(4,6)Cr-(0,3)Si (wt.%) steels were prepared to investigate the microstructure and mechanical properties. The calculated results show that the Cr reduces the SFE of Fe-C-Mn-Cr high Mn steels linearly and the reduction rate of SFE is greater as the C content increasing. With increasing the Si concentration, the SFE of Fe-C-Mn-Si steels decreases when the Mn content is higher than 18 wt.%. However, when carbon content is less than 1 wt.% and Mn content less than 18 wt.%, the SFE reaches to a maximum value and then declines with the increase of Si content. The combined effect of Cr and Si on the SFE of Fe-C-Mn-Si-Cr steels appears the similar behaviors to that of Si. Generally, the value of driving force, of fcc→hcp transformation in Fe-Mn-C-2Cr-Si (wt.%) steels increases with the increase of C and Mn and decreases with the Si content increasing. However, for 0.6 wt.% C and 7 wt.% Mn steels, the value of first increases and then decreases with the increase of Si content. The results of tensile test indicated that the Fe-15Mn-0.6C-6Cr-3Si (wt.%) high manganese steel demonstrates a better combination of the ultimate strength of 987 MPa, yield strength of 470 MPa, fracture elongation of 41.5% and high work hardening capacity because of the ε-martensitic transformation caused by the great (large negative value) and low SFE under stress.

Mechanical properties and deformation mechanisms in CoCrFeMnNi high entropy alloys: A molecular dynamics study

We used molecular dynamics simulations to investigate the relationship between mechanical properties and deformation mechanisms in CoCrFeMnNi alloys having different compositions. According to deformation twinning activity, the deformation mechanisms in the different CoCrFeMnNi compositions under tensile loading can be classified into three major categories: easy-shear (ES), inter-locks (IL) and bulk hexagonal closed packed (hcp) transformation (BH). We found that the compositions with ES were more likely to rupture early because of the short and fragmented twins. The compositions following the IL pattern promoted the movement of interlocking stacking faults in the twin boundaries that could prolong tensile deformation. Moreover, BH could further increase ductility because the hcp transformation could absorb and dissipate the stacking faults. Experimental observations of immobile stacking-fault networks that impede dislocation motion and further provide preferred sites for the formation of the hcp phase were commonly found in the ES, IL and BH patterns. The calculated intrinsic stacking fault energies of CoCrFeMnNi and CoCrFeNi were consistent with the experimental measurements. The combination of high unstable stacking fault energy with the formation of the IL pattern results in high strength and high ductility in the CoCrFeMnNi HEA system.

Effect of Heat Treatment on the Microstructure and Mechanical Properties of a Co-Based Superalloy Strip

In this paper, the effect of heat treatment on the microstructure and mechanical properties of a deformed Co-Cr-Ni-W superalloy strip was investigated. The cold-rolled superalloy was annealed at different temperatures 500°C~1240°C for 10min. It was found that hardness increased in the range temperature 500°C~800°C, the annealing had aging strengthening effect, this was due to the fcc~hcp transformation. As the annealing temperature is higher than 800°C, the hardness decreased with the increase of the annealing temperature. There were four inflection points in the influence of annealing temperature on the hardness of the alloy, which were about 800°C,900°C,1050°Cand 1150°C. From 800°C to 900°C, the hardness of the alloy decreased rapidly, recovery and recrystallization was initial. From 900°C to 1050°C, the hardness of the alloy decreased slower, recrystallization was finished and grain growth slowly. The temperature reaches 1050°C to 1150°C, the hardness curve decreased rapidly, carbides dissolution and grain growth was considerable. Above 1150°C, the hardness tended to be stable, the grain growth of the alloy was obvious, and more annealing twins were also formed. As the annealing temperature increased, the tensile strength decreased and the elongation increased. Keywords Cobalt-base superalloy; Heat treatment; Microstructure; Mechanical Properties Yong-Ji Niu,Yue Zhang University of Science and Technology Beijing,Beijing 100083,china; e-mail: niuyongji@163.com Yong-Ji Niu, Zhi-Wei Zhang, Ning An, Yang Gao Beijing Beiye Functional Materials Corproration, Beijing 100192, China