Correlation of work function and stacking fault energy through Kelvin probe force microscopy and nanohardness in diluteα-magnesium

Electronic interactions of the Group 2A elements with magnesium have been studied through the dilute solid solutions in binary Mg-Ca,Mg-Sr and Mg-Ba systems.This investigation incorporated the difference in the‘Work Function'(ΔWF)measured via Kelvin Probe Force Microscopy(KPFM),as a property directly affected by interatomic bond types,i.e.the electronic structure,nanoindentation measurements,and Stacking Fault Energy values reported in the literature.It was shown that the nano-hardness of the solid-solutionα-Mg phase changed in the order of Mg-Ca>Mg-Sr>Mg-Ba.Thus,it was shown,by also considering the nano-hardness levels,that SFE of a solid-solution is closely correlated with its‘Work Function'level.Nano-hardness measurements on the eutectics andΔWF difference between eutectic phases enabled an assessment of the relative bond strength and the pertinent electronic structures of the eutectics in the three alloys.Correlation withΔWF and at least qualitative verification of those computed SFE values with some experimental measurement techniques were considered important as those computational methods are based on zero Kelvin degree,relatively simple atomic models and a number of assumptions.As asserted by this investigation,if the results of measurement techniques can be qualitatively correlated with those of the computational methods,it can be possible to evaluate the electronic structures in alloys,starting from binary systems,going to ternary and then multi-elemental systems.Our investigation has shown that such a qualitative correlation is possible.After all,the SFE values are not treated as absolute values but rather become essential in comparative investigations when assessing the influences of alloying elements at a fundamental level,that is,free electron density distributions.Our study indicated that the principles of‘electronic metallurgy'in developing multi-elemental alloy systems can be followed via practical experimental methods,i.e.ΔWF measurements using KPFM and nanoindentation.

Effect of stacking fault energy on mechanical properties of ultrafine-grain Cu and Cu-Al alloy processed by cold-rolling

Cu,Cu-2.2%Al and Cu-4.5%Al with stacking fault energies（SFE） of 78,35 and 7 mJ/m2 respectively were processed by cold-rolling（CR） at liquid nitrogen temperature（77 K） after hot-rolling.X-ray diffraction measurements indicate that a decrease in SFE leads to a decrease in crystallite size but increase in microstrain,dislocation and twin densities of the CR processed samples.Tensile tests at room temperature indicate that as the stacking fault energy decreases,the strength and ductility increase.The results indicate that decreasing stacking fault energy is an optimum method to improve the ductility without loss of strength.

Interplay between temperature-dependent strengthening mechanisms and mechanical stability in high-performance austenitic stainless steels

The effects of deformation temperature on the transformation-induced plasticity(TRIP)-aided 304L,twinning-induced plasti-city(TWIP)-assisted 316L,and highly alloyed stable 904L austenitic stainless steels were compared for the first time to tune the mechan-ical properties,strengthening mechanisms,and strength-ductility synergy.For this purpose,the scanning electron microscopy(SEM),electron backscattered diffraction(EBSD),X-ray diffraction(XRD),tensile testing,work-hardening analysis,and thermodynamics calcu-lations were used.The induced plasticity effects led to a high temperature-dependency of work-hardening behavior in the 304L and 316L stainless steels.As the deformation temperature increased,the metastable 304L stainless steel showed the sequence of TRIP,TWIP,and weakening of the induced plasticity mechanism;while the disappearance of the TWIP effect in the 316L stainless steel was also observed.However,the solid-solution strengthening in the 904L superaustenitic stainless steel maintained the tensile properties over a wide temper-ature range,surpassing the performance of 304L and 316L stainless steels.In this regard,the dependency of the total elongation on the de-formation temperature was less pronounced for the 904L alloy due to the absence of additional plasticity mechanisms.These results re-vealed the importance of solid-solution strengthening and the associated high friction stress for superior mechanical behavior over a wide temperature range.

Temperature-jump tensile tests to induce optimized TRIP/TWIP effect in a metastable austenitic stainless steel

In the present work,plastic deformation mechanisms were initially tailored by adjusting the deformation temperature in the range of 0 to 200℃ in AISI 304L austenitic stainless steel,aiming to optimize the strength-ductility synergy.It was shown that the combined twinning-induced plasticity(TWIP)/transformation-induced plasticity(TRIP)effects and a wider strain range for the TRIP effect up to higher strains by adjusting the deformation temperature are good strategies to improve the strength-ductility synergy of this metastable stainless steel.In this regard,by consideration of the observed temperature-dependency of plastic deformation,the controlled sequence of TWIP and TRIP effects for archiving superior strength-ductility trade-off was intended by the pre-designed temperature jump tensile tests.Accordingly,the optimum tensile toughness of 846 MJ/m^(3) and total elongation to 133% were obtained by this strategy via exploiting the advantages of the TWIP effect at 100℃ and the TRIP effect at 25℃ at the later stages of the straining.Consequently,a deformation-temperature-transformation(DTT)diagram was developed for this metastable alloy.Moreover,based on work-hardening analysis,it was found that the main phenomenon constraining further improvement in the ductility and strengthening was the yielding of the deformation-induced α′-martensite.

Stacking fault energy,yield stress anomaly, and twinnability of Ni_3Al:A first principles study

Using first principles calculations combined with the quasiharmonic approach, we study the effects of temperature on the elastic constants, generalized stacking fault energies, and generalized planar fault energies of Ni3Al. The antiphase boundary energies, complex stacking fault energies, superlattice intrinsic stacking fault energies, and twinning energies decrease slightly with temperature. Temperature dependent anomalous yield stress of Ni3Al is predicted by the energybased criterion based on elastic anisotropy and antiphase boundary energies. It is found that p increases with temperature and this can give a more accurate description of the anomalous yield stress in Ni3Al. Furthermore, the predicted twinnablity of Ni3Al is also decreasing with temperature.

Stacking fault energy and electronic structure of molybdenum under solid solution softening/hardening

Ab initio calculations are used to understand the fundamental mechanism of the solid solution softening/hardening of the Mo-binary system.The results reveal that the Mo-Ti,Mo-Ta,Mo-Nb,and Mo-W interactions are primarily attractive with negative heats of formation,while the interactions of Mo-Re,and Mo-Zr would be mainly repulsive with positive heats of formation.It is also shown that the addition of Re and Zr would cause the solid solution softening of Mo by the decrease of the unstable stacking fault energy and the increase of ductility.On the contrary,the elements of W,Ta,Ti,and Nb could bring about the solid-solution hardening of Mo through the impediment of the slip of the dislocation and the decrease of ductility.Electronic structures indicate that the weaker/stronger chemical bonding due to the alloying elements should fundamentally induce the solid solution softening/hardening of Mo.The results are discussed and compared with available evidence in literatures,which could deepen the fundamental understanding of the solid solution softening/hardening of the binary metallic system.

EFFECT OF Cr AND Al CONTENT ON THE STACKING FAULT ENERGY IN r-Fe-Mn ALLOYS

The effects of Cr and Al content were investigated on the stacking fault energy in austenitic Fe-31Mn-(0-7.26)Cr-0.96C and Fe-31Mn-(0-8.68)Al-0.85C alloys by the thermodynamic analysis. The results show that the additions of chromium or aluminum increase the non-magnetic component of the stacking fault energy in the γ-Fe-Mn alloys, and the effect of aluminum is larger than that of chromium. The change in the magnetic entropy caused in the antiferromagnetic transition increases the free energy difference between the γ and s phases in the γr-Fe-Mn alloys. The effects of chromium and aluminum on the magnetic component were discussed on the basis of the influence of both upon the antiferromagnetic transition in the γ-Fe-Mn alloys.

Stacking fault probability and stacking fault energy in CoNi alloys

The stacking fault probability of CoNi alloys with different contents of Ni was measured by X ray diffraction methods. The results show that the stacking fault decreases with increasing Ni content and with increasing temperature. The thermodynamical calculation has found an equation that can express the stacking fault energy γ of CoNi at temperature T . The phase equilibrium temperature depends on the composition of the certain alloy. The relationship between stacking fault energy γ and stacking fault probability P sf is determined.

Variation in Creep Rupture of γ′ Strengthened Superalloys with Stacking Fault Energy of Matrices

The correlation between the creep rupture behaviour and the stacking fault energy of matrices of γ′strengthened superalloys has been studied dur- ing constant load creep.At high temperature and intermediate stress,the creep rupture time and strain strongly depend on the stacking fault energy of matrices rather than the creep friction stress,but at higher stress,the role of grain boundary carbides becomes more obvious. However,in the considerably extensive stress range investigated here,the mean creep rate is a power function of the stacking fault energy of matrices and the power index decreases with in- creasing initial applied stress.Particularly,at inter- mediate stresses the product of this index and the initial applied stress compensated by the shear modulus is same for two series of superalloys. Hence,this product may be a criterion predicting that the matrix deformation controls high tempera- ture creep rupture.

New insights into the properties of high-manganese steel

In the Collaborative Research Centre 761’s“Steel ab initio-quantum mechanics guided design of new Fe based materials,”scientists and engineers from RWTH Aachen University and the Max Planck Institute for Iron Research conducted research on mechanism-controlled material development with a particular focus on high-manganese alloyed steels.From 2007 to 2019,a total of 55 partial projects and four transfer projects with industrial participation(some running until 2021)have studied material and process design as well as material characterization.The basic idea of the Collaborative Research Centre was to develop a new methodological approach to the design of structural materials.This paper focuses on selected results with respect to the mechanical properties of high-manganese steels,their underlying physical phenomena,and the specific characterization and modeling tools used for this new class of materials.These steels have microstructures that require characterization by the use of modern methods at the nm-scale.Along the process routes,the generation of segregations must be taken into account.Finally,the mechanical properties show a characteristic temperature dependence and peculiarities in their fracture behavior.The mechanical properties and especially bake hardening are affected by short-range ordering phenomena.The strain hardening can be adjusted in a never-before-possible range,which makes these steels attractive for demanding sheet-steel applications.

Impact of replacement of Re by W on dislocation slip mediated creeps of γ'-Ni_(3)Al phases

The anomalous flow behavior of γ'-Ni_(3)Al phases at high temperature is closely related to the cross-slip of 1/2<110>{111}super-partial dislocations.Generalized stacking fault energy curves(i.e.,Γ-surfaces)along the lowest energy path can provide a great deal of information on the nucleation and movement of dislocations.With the first-principles calculation,the interplay between Re and W,Mo,Ta,Ti doped at preferential sites and their synergetic influence on Γ-surfaces and ideal shear strength(τ_(max))in γ'-Ni_(3)Al phases are investigated.Similar to single Re-addition,the Suzuki segregation of W at stacking faults is demonstrated to enable to impede the movement of 1/6<112>{111} Shockley partial dislocations and promote the cross-slip of 1/2<110>{111}super-partial dislocations.With the replacement of a part of Re by W,a decreased γ_(APB)^(111)/γ_(APB)^(001) indicates that the anomalous flow behavior of γ'phases at high temperature is not as excellent as the double Re-addition,but an increasedτmax means that the creep rupture strength of Ni-based single crystal superalloys can be benefited from this replacement to some extent,especially in the co-segregation of Re and W at Al−Al sites.As the interaction between X1_(Al) and X2_(Al) point defects is characterized by an correlation energy function ΔE^(X1_(Al)+X2_(Al))(d),it is found that both strong attraction and strong repulsion are unfavarable for the improvement of yield strengths of γ'phase.

Twinnability of Al-Mg alloys:A first-principles interpretation

Al-Mg alloys are considered to have potentials to form twins during deformation because Mg can reduce the intrinsicstacking fault energy?ISFE of Al.Nevertheless,twinning has rarely been found in Al-Mg alloys even subjected to various severeplastic deformation(SPD)techniques.In order to probe the twinning propensity of Al-Mg alloys,first-principles calculations werecarried out to investigate the effects of Mg and vacancies on the generalized planar fault energy(GPFE)of Al.It is found that bothMg and vacancies exhibit a Suzuki segregation feature to the stacking fault,and have the influence of decreasing the?ISFE of Al.However,?ISFE does not decrease and the twinnability parameterτa of Al does not increase monotonically with increasing Mgconcentration in the alloy.On the basis ofτa evaluated from the calculated GPFE of Al-Mg alloys,we conclude that deformationtwinning is difficult for Al-Mg alloys even with a high content of Mg.Besides,the decrease of?ISFE caused by the introduction ofMg and vacancies is supposed to have the effect of improving the work-hardening rate and facilitating the formation of bandstructures in Al-Mg alloys subjected to SPD.

Determination of stacking fault energies in a high-Nb TiAl alloy at 298 K and 1273 K

The stacking fault energies of Ti-46Al-8.5Nb-0.2W alloy at 298 K and 1273 K were determined. The principle for the determination of the stacking fault energies is based on the fact that the stacking fault energy and the elastic interaction energy acting on the dissociated partial dislocations are equal. After the compress deformations with the strain of 0.2% at 298 K and 1273 K, and water quench to maintain the dislocation structures deformed at 1273 K, the dissociation distances between two partial dislocations were determined by weak beam transmission electron microscopy (WBTEM) technique. Based on these dissociation distances and the corresponding calculation method, the stacking fault energies were determined to be 77-81 mJ/m2 at 298 K and to be 57-60mJ/m2 at 1273 K respectively.

First-principles study of the effects of selected interstitial atoms on the generalized stacking fault energies, strength, and ductility of Ni

We analyze the influences of interstitial atoms on the generalized stacking fault energy （GSFE）, strength, and ductility of Ni by first-principles calculations. Surface energies and GSFE curves are calculated for the （112） （111） and / 101） （ 1 1 1） systems. Because of the anisotropy of the single crystal, the addition of interstitials tends to promote the strength of Ni by slipping along the （10T） direction while facilitating plastic deformation by slipping along the （115） direction. There is a different impact on the mechanical behavior of Ni when the interstitials are located in the slip plane. The evaluation of the Rice criterion reveals that the addition of the interstitials H and O increases the brittleness in Ni and promotes the probability of cleavage fracture, while the addition of S and N tends to increase the ductility. Besides, P, H, and S have a negligible effect on the deformation tendency in Ni, while the tendency of partial dislocation is more prominent with the addition of N and O. The addition of interstitial atoms tends to increase the high-energy barrier γmax, thereby the second partial resulting from the dislocation tends to reside and move on to the next layer.

Comparison of formation and evolution of radiation-induced defects in pure Ni and Ni-Co-Fe medium-entropy alloy

High-entropy alloys(HEAs)and medium-entropy alloys(MEAs)have attracted a great deal of attention for developing nuclear materials because of their excellent irradiation tolerance.Herein,formation and evolution of radiation-induced defects in Ni Co Fe MEA and pure Ni are investigated and compared using molecular dynamics simulation.It is observed that the defect recombination rate of ternary Ni Co Fe MEA is higher than that of pure Ni,which is mainly because,in the process of cascade collision,the energy dissipated through atom displacement decreases with increasing the chemical disorder.Consequently,the heat peak phase lasts longer,and the recombination time of the radiation defects(interstitial atoms and vacancies)is likewise longer,with fewer deleterious defects.Moreover,by studying the formation and evolution of dislocation loops in Ni-Co-Fe alloys and Ni,it is found that the stacking fault energy in Ni-Co-Fe decreases as the elemental composition increases,facilitating the formation of ideal stacking fault tetrahedron structures.Hence,these findings shed new light on studying the formation and evolution of radiation-induced defects in MEAs.

Determination of stacking fault energies in a high-Nb TiAl alloy at 298 K and 1273 K

The stacking fault energies of Ti-46Al-8.5Nb-0.2W alloy at 298 K and 1273 Kwere determined. The principle for the determination of the stacking fault energies is based on thefact that the stacking fault energy and the elastic interaction energy acting on the dissociatedpartial dislocations are equal. After the compress deformations with the strain of 0.2% at 298 K and1273 K, and water quench to maintain the dislocation structures deformed at 1273 K, thedissociation distances between two partial dislocations were determined by weak beam transmissionelectron microscopy (WBTEM) technique. Based on these dissociation distances and the correspondingcalculation method, the stacking fault energies were determined to be 77-81 mJ/m^2 at 298 K and tobe 57-60mJ/m^2 at 1273 K respectively.

Microstructures and mechanical properties of high Mn and high Al steels

Super-ductile and high strength manganese TWIP steels have received much attention in recent years.In the present work,two high Mn high Al steels with different stacking fault energies were investigated, microstructures were examined and mechanical properties were measured.The results showed that Fe-26Mn-6Al- 1C steel（6Al steel） exhibits high tensile strength and Fe-26Mn-12Al-1C steel（12Al steel） possesses a very high strength.Both steels exhibit good ductility.For 6Al steel with a moderate stacking fault energy（60 mJ/m;）, twinning is the major deformation mechanism;while dislocation slip dominates the deformation process of 12Al steel with stacking fault energy of 90 mJ/m;.

Household energy stacking and structures in Pakistan-Results from a multiple-energy study in Azad Kashmir and Punjab

Solid fuel use(SFU)is common in most developing countries and would release many hazardous air pollutants posing high risks on human health.The Global Burden of Disease(GBD)study highlighted risks associated with household SFU in Pakistan,however,high uncertainties prevail because of scanty data on SFU and unaccounted energy stacking.This study conducted a field campaign aiming at collecting first-hand data on household energy mix in Pakistan.The first survey was in Punjab and Azad Kashmir,and revealed that stacked energy use was pervasive,especially for cooking.The stacking was found to be much more obvious in SFU households(defined as those using SFU dominantly)compared to those non-SFU.There were significantly substantial differences between Azad Kashmir and Punjab because of distinct resources available and economic conditions.Woody materials comprised up to nearly 70% in Azad Kashmir,but in Punjab,gas was frequently used for cooking.Only investigating primary household energy would probably overestimate main energy types that being used for a longer time but underestimated other supplements,suggesting the preference of multiple-energy surveys in household energy studies.

Simultaneous enhancement of strength and ductility via microband formation and nanotwinning in an L1_(2)-strengthened alloy

Metallic alloys with high strength and large ductility are required for extreme structural applications.However,the achievement of ultrahigh strength often results in a substantially decreased ductility.Here,we report a strategy to achieve the strength-ductility synergy by tailoring the alloy composition to control the local stacking fault energy(SFE)of the face-centered-cubic(fcc)matrix in an L1_(2)-strengthened superlattice alloy.As a proof of concept,based on the thermodynamic calculations,we developed a non-equiatomic CoCrNi_(2)(Al_(0.2)Nb_(0.2))alloy using phase separation to create a near-equiatomic low SFE disordered CoCrNi medium-entropy alloy matrix with in situ formed high-content coherent Ni_(3)(Al,Nb)-type ordered nanoprecipitates(∼12 nm).The alloy achieves a high tensile strength up to 1.6 GPa and a uniform ductility of 33%.The low SFE of the fcc matrix promotes the formation of nanotwins and parallel microbands during plastic deformation which could remarkably enhance the strain hardening capacity.This work provides a strategy for developing ultrahigh-strength alloys with large uniform ductility.

Predicting the variation of stacking fault energy for binary Cu alloys by first-principles calculations

The variation of stacking fault energy(SFE)in a number of binary Cu alloys is predicted through considering the Suzuki segregation by the full potential linearly augmented plane wave(FPLAPW)method.The calculated results show that some solute atoms(Mg,Al,Si,Zn,Ga,Ge,Cd,Sn,and Pb),which prefer to form the Suzuki segregation,may decrease the value of SFE;while the others(Ti,Mn,Fe,Ni,Zr,Ag,and Au),which do not cause the Suzuki segregation may not decrease the SFE.Furthermore,it is interesting to find that the former alloying elements are located on the right of Cu group while the latter on the left of Cu group in the periodic table of elements.The intrinsic reasons for the new findings can be traced down to the valences electronic structure of solute and Cu atoms,i.e.,the similarity of valence electronic structure between solute and Cu atoms increases the value of SFE,while the difference decreases the value of SFE.