Development of the high-strength ductile ferritic alloys via regulating the intragranular and grain boundary precipitation of G-phase

A typical G-phase strengthened ferritic model alloy(1Ti:Fe-20Cr-3Ni-1Ti-3Si,wt.%)has been carefully studied using both advanced experimental(EBSD,TEM and APT)and theoretical(DFT)techniques.During the classic“solid solution and aging”process,the superfine(Fe,Ni)_(2)TiSi-L2_(1)particles densely precipitate within the ferritic grain and subsequently transform into the(Ni,Fe)_(16)Ti_(6)Si_(7)-G phase.In the meanwhile,the elemental segregation at grain boundaries and the resulting precipitation of a large amount of the(Ni,Fe)_(16)Ti_(6)Si_(7)-G phase are also observed.These nanoscale microstructural evolutions result in a remarkable increase in hardness(100-300 HV)and severe embrittlement.When the“cold rolling and aging”process is used,the brittle fracture is effectively suppressed without loss of nano-precipitation strengthening ef-fect.Superhigh yield strength of 1700 MPa with 4%elongation at break is achieved.This key improvement in mechanical properties is mainly attributed to the pre-cold rolling process which effectively avoids the dense precipitation of the G-phase at the grain boundary.These findings could shed light on the further exploration of the precipitation site via optimal processing strategies.

Enhanced thermoelectric performance of n-type TiCoSb halfHeusler by Ta doping and Hf alloying

The p-type TiCoSb-based half-Heuslers are widely studied due to the good electrical transport properties after hole doping,while the pristine TiCoSb is intrinsically n-type.It is thus desired to obtain a comparable n-type counterpart through optimization of electron concentration.In this work,n-type Ti_(0.9-x)HfxTa_(0.1)CoSb half-Heuslers were fabricated by arc melting,ball milling,and spark plasma sintering.An optimized carrier concentration,together with a decreased lattice thermal conductivity,was obtained by Ta doping at the Ti site,leading to a peak figure of merit(ZT)of 0.7 at 973 K in Ti_(0.9)Ta_(0.1)-CoSb.By further alloying Hf at the Ti site,the lattice thermal conductivity was significantly reduced without deteriorating the power factor.As a result,a peak ZT of 0.9 at 973 K and an average ZT of 0.54 in the temperature range of 300-973 K were achieved in Ti_(0.6)Hf_(0.3)Ti_(0.1)CoSb.This work demonstrates that n-type TiCoSb-based halfHeuslers are promising thermoelectric materials.

Novel and durable composite phase change thermal energy storage materials with controllable melting temperature

The development of high temperature phase change materials(PCMs)with great comprehensive performance is significant in the future thermal energy storage system.In this study,novel and durable Al-Si/Al_(2)O_(3)-Al N composite PCMs with controllable melting temperature were successfully synthesized by using pristine Al powder as raw material and tetraethyl orthosilicate as SiO_(2)source.The Al_(2)O_(3)shell and Al-Si alloy were in-situ produced via the substitution reaction between molten Al and SiO_(2).Importantly,the crack caused by the incomplete encapsulation of the Al_(2)O_(3)shell could repair itself by the nitridation reaction of internal molten Al and thereby forming a highly dense Al_(2)O_(3)-Al N composite shell.The produced dense Al_(2)O_(3)-Al N composite shell could significantly improve the thermal cycling stability of composite PCMs,and thus,the thermal storage density decrease of the Al-Si/Al_(2)O_(3)-Al N(59.8 J/g to77.7 J/g)was far less than that of the Al-Si/Al_(2)O_(3)(118.5 J/g)after 3000 thermal cycles.Moreover,the synthesized Al-Si/Al_(2)O_(3)-Al N still exhibited a controllable melting temperature(571.5-637.9℃),relatively high thermal storage density(105.6-150.7 J/g),great dimensional stability and structural stability after3000 thermal cycles.Hence,the synthesized Al-Si/Al_(2)O_(3)-Al N composite PCMs,as promising preferential thermal energy storage materials,can be stably used in the energy utilization efficiency improvement of various systems for more than 6 years.

Characterization and modeling studies towards Al_(3)Ti/Mg interfaces in Ti reinforced AZ31 alloys

The interfaces between in-situ Al3Ti particles and magnesium(Mg)matrix are crucial role in highperformance titanium(Ti)reinforced AZ31 alloy.Herein,the interfaces between Al3Ti particles and the Mg matrix are fabricated and investigated using advanced characterization tools and first-principles calculations.The orientation relationship(OR)and atomic interface structure between the Al_(3)Ti particles and matrix are characterized using a high-resolution transmission electron microscope.The OR is determined to be(1010)Mg//(001)Al3Ti;[1213]Mg//[100]Al3Ti.Based on the characterized OR,the interface properties(including atomic structure,work of adhesion,interface energy,and fracture mechanism)are investigated using first-principles calculations.The relaxed interface structure indicates that the TiAl-terminated bridge site configurations(MT1)and hollow site configurations(HCP1)are unstable and would convert into other bridge site configurations(MT).Furthermore,the work of adhesion and interface energy suggests that Al-terminated hollow site configurations(HCP)and bridge site configurations(MT)are more stable than other configurations.In addition,the calculations of work of fracture show that fracture of the interfaces with Al-MT1,Al-HCP,and TiAl-MT configurations may initiate from bulk Mg interior.The findings may help to understand and tailor the deformation mechanisms and mechanical properties of Ti-reinforced Mg alloys.

One-step synthesis of novel core-shell bimetallic hexacyanoferrate for high performance sodium-storage cathode

Recently, the design of core-shell hierarchical architecture plays an important role in improving the electrochemical performance of Prussian blue analogue cathodes(PBAs). Unfortunately, the inconvenient stepwise preparation and the strict lattice-matching requirement have restricted the development of coreshell PBAs. Herein, we demonstrate a one-step synthesis strategy to synthesize core-shell manganese hexacyanoferrate(MnFeHCF@MnFeHCF) for the first time. And the formation mechanism of the core-shell hierarchical architecture is investigated by first-principles calculations. It is found that the as-obtained Mn FeHCF@MnFeHCF act out the superior intrinsic natures, which not only can obtain a larger specific surface area and lower Fe(CN)_(6) vacancies but also can activate more Na-storage sites. Compared with the manganese hexacyanoferrate(MnHCF), the iron hexacyanoferrate(FeHCF), and even the traditional coreshell nickel hexacyanoferrate(FeHCF@NiHCF) prepared by a stepwise method, the Mn Fe HCF@MnFeHCF demonstrates a superior rate performance, which achieves a high capacity of 131 mAh g^(−1) at 50 mA g^(−1) and delivers a considerable discharge capacity of about 100 mAh g^(−1) even at 1600 mA g^(−1). Meantime, the capacity retention can reach up to nearly 80% after 500 cycles. The improved performances could be mainly originated from two aspects: on the one hand, Mn substitution is helpful to enhance the material conductivity;on the other hand, the core-shell structure with matched lattice parameters is more favorable to enhance the diffusion coefficient of sodium ions. Beside, the structural transformation of MnFeHCF@MnFeHCF upon the extraction/insertion of sodium ions is instrumental in releasing the interior stress and effectively maintaining the integrity of the crystal structure.

Phase equilibria in Co-Ti-Re ternary system

The phase equilibria in the Co-Ti-Re ternary system at 800,1000,1100 and 12000C were studied combined with electron probe microanalyzer(EPMA),X-ray diffraction(XRD),scanning electron microscope(SEM)and transmission electron microscope(TEM).The new phaseτwith a large solubility was discovered at all four isothermal sections,and its structure still needs a further study.A large solubility of Re in the CoTi phase was observed.The solubility of Re in the Co_(3)Ti phase is6.1%at 800℃,8.7%at 1000℃and 7.7%at 1100℃.The present work could be significant for alloy design of Co-based alloys and thermodynamic analysis of the Co-TiRe ternary system.