Precipitation behaviors and mechanical properties of a solution-treated Mg-Gd-Nd-Zn-Zr alloy during equal-channel angular pressing process

A magnesium alloy processed by equal-channel angular pressing (ECAP) exhibited excellent microstructure refinement and improved strength and hardness.The comprehensive mechanical properties of magnesium alloys have supported the expansion of their applications in the automotive,aerospace,and biomedical industries.Herein,pre-treatment of a solution-treated Mg-2.9Gd-1.5Nd-0.3Zn-0.3Zr alloy was conducted to investigate the precipitate behavior and microstructure evolution during the ECAP process.β;phase grains quickly precipitated from the solution-treated alloy,which accelerated grain refinement and enhanced the ductility after the ECAP process,as compared to the as-cast alloy reported in our previous study.Moreover,spherical precipitates (~200 nm) and fine phases (~100 nm) precipitated along the stripe-like Zn;Zr;phase,which formed a kabap-like structure dispersing homogeneously in the solution-treated alloy during the ECAP process.Owing to grain refinement,dislocations,sphericalβ;precipitates,and texture evolution,the solution-treated alloy after eight passes of ECAP exhibited good comprehensive mechanical properties,with the ultimate tensile strength,yield strength,and elongation values reaching210.9 MPa,263.9 MPa,and 27.9%,respectively.

Microstructure and mechanical properties of as-cast(CuNi)100-xCox medium-entropy alloys

Microstructure and mechanical properties of non-equiatomic(CuNi)_(100-x)Co_(x)(x=15,20,25 and 30,at.%)medium-entropy alloys(MEAs)prepared by vacuum arc-melting were investigated.Results show that all the as-cast MEAs exhibit dual face-centered cubic(fcc)solid-solution phases with identical lattice constant,showing typical dendrite structure consisting of(Ni,Co)-rich phase in dendrites and Cu-rich phase in inter-dendrites.The positive enthalpy of mixing among Cu and Ni-Co elements is responsible for the segregation of Cu.With the increase of Co content,the volume fraction of(Ni,Co)-rich phase increases while the Cu-rich phase decreases,resulting in an increment of yield strength and a decrement of elongation for the(CuNi)_(100-x)Co_(x) MEAs.Nano-indentation test results show a great difference of microhardness between the two fcc phases of the MEAs.The measured microhardness value of the(Ni,Co)-rich phase is almost twofold as compared to that of the Cu-rich phase in all the(CuNi)_(100-x)Co_(x) MEAs.During the deformation of the MEAs,the Cu-rich phase bears the main plastic strain,whereas the(Ni,Co)-rich phase provides more pronounced strengthening.

Mechanical behavior and deformation mechanism of shape memory bulk metallic glass composites synthesized by powder metallurgy

The synthesis of martensitic or shape-memory bulk metallic glass composites(BMGCs)via solidification of the glass-forming melts requires the meticulous selection of the chemical composition and the proper choice of the processing parameters in order to ensure that the glassy matrix coexists with the desired amount of austenitic phase.Unfortunately,a relatively limited number of such systems,where austenite and glassy matrix coexist over a wide range of compositions,is available.Here,we study the effective-ness of powder metallurgy as an alternative to solidification for the synthesis of shape memory BMGCs.Zr_(48)Cu_(36)Al_(8)Ag_(8)matrix composites with different volume fractions of Ni_(50.6)Ti_(49.4)are fabricated using hot pressing and their microstructure,mechanical properties and deformation mechanism are investigated employing experiments and simulations.The results demonstrate that shape-memory BMGCs with tun-able microstructures and properties can be synthesized by hot pressing.The phase stability of the glass and austenitic components across a wide range of compositions allows us to examine fundamental as-pects in the field of shape memory BMGCs,including the effect of the confining stress on the martensitic transformation exerted by the glassy matrix,the contribution of each phase to the plasticity and the mechanism responsible for shear band formation.The present method gives a virtually infinite choice among the possible combinations of glassy matrices and shape memory phases,expanding the range of accessible shape memory BMGCs to systems where the glassy and austenitic phases do not form simul-taneously using the solidification route.

A novel high-strength Al-La-Mg-Mn alloy for selective laser melting

Developing high-strength Al-(La,Ce)alloys for additive manufacturing(AM)would entail considerable economic benefits.In this work,a novel near-eutectic Al-La alloy containing 5.50 wt.%Mg and 0.60 wt.%Mn was designed and fabricated via selective laser melting(SLM).Submicron Al_(11)La_(3)intermetallics with 3D continuous cellular-dendritic and granular morphologies were observed at the interior and boundary of the melt pool,respectively.Interestingly,these intermetallics are hierarchical and contained numerous Al_(6)Mn and Mg_(2)Si secondary nanoprecipitates.The as-fabricated alloy exhibited a tensile yield strength(YS)of 334 MPa and ultimate tensile strength(UTS)of 588 MPa at room temperature,which is the high-est UTS reported for Al-(La,Ce)alloys with an appreciable failure strain of∼6.4%.The 3D continuous cellular dendritic intermetallic and high Mg content afford significant strengthening and work harden-ing ability.In addition,the hierarchical feature of the intermetallics generated additional microcracks to coordinate the deformation.

Warm compaction powder metallurgy of Cu

A series of experiments were carried out using different admixed lubricant contents, different compaction pressures and temperatures in order to study the warm compaction of copper powder. Results show that too much admixed lubricant will lead to the squeeze out of the lubricant from the compact during the warm compaction processing of Cu powder. Results also show that blisters can be found in sintered samples that contain lubricant less than 0.15%(mass fraction). Optimal warm compaction parameters for producing high density powder metallurgy copper material are obtained. Compacts with green density of 8.6 g/cm^3 and a sintered density of 8.83 g/cm^3 can be produced by warm compacting the Cu powder, which contains 0.2% admixed lubricant, and is compacted at 145 ℃ with a pressure of 700 MPa.

Achieving superior strength-ductility synergy in a heterostructured magnesium alloy via low-temperature extrusion and low-temperature annealing

A low-alloyed Mg-1.2Zn-0.1Ca(wt.%)alloy was fabricated via low-temperature extrusion and annealing at 250℃for different times(10,30,and 90 min)to attain heterostructures with different fine-grained fractions,focusing on the effect of heterostructure on the mechanical properties.Partial dynamic recrystallization(RX)occurred during extrusion at 150℃,and a lamellar structure consisting of fine RX grains and coarse unRX grains was obtained.The subsequent annealing promoted static RX in the as-extruded alloy,leading to an increased fine-grained fraction from 67%to 95%.Meanwhile,the co-segregation of Zn and Ca atoms impeded the migration of grain boundaries,thus achieving a fine grain size of 0.8–1.6μm.The sample annealed for 10 min with a fine-grained fraction of 73%and an average RX grain size of 0.9μm exhibited a superior combination of high yield strength(305 MPa)and good ductility(20%).In comparison,an excellent elongation of 30%was achieved in the alloy with a nearly fully-RXed microstructure and an average grain size of 1.6μm after 90 min annealing,despite a lower yield strength of 228 MPa.In unRX grains,the hard orientation with(01–10)parallel to the extrusion direction and high-density dislocations made it more difficult to deform compared with the RX grains,thus producing hetero-deformation induced(HDI)strengthening.Besides fine grains and high-density dislocations,HDI strengthening is the key to achieving the superior mechanical properties of the low-alloyed Mg alloy.

Microstructure and Properties of Micro-Alloyed Mg-2.0Nd-0.2Sr by Heat Treatment and Extrusion

Magnesium(Mg)and its alloys have broad application prospects in the fields such as biomedical materials and automobile manufacturing.A micro-alloyed Mg-2.0Nd-0.2Sr(wt.%)magnesium alloy is designed and obtained through semicontinuous casting.The evolution of microstructures and tensile properties are investigated with different heat treatments and extrusion treatments.The grain sizes decrease significantly after extrusion,thus changing the fracture mode during the tensile testing process.The ultimate tensile strength(UTS),yield strength(YS)and elongation(EL)of the properly processed extrusion alloy(referred to as MNS-E2)reach to 247 MPa,228 MPa and 24%,respectively.The dramatical improvement of mechanical properties results from the refined grains and interactions between dislocations and precipitates.Some nanoparticle bands blocking the slippage and movement of dislocations are also found in the MNS-E2 alloy.The above causes combined result in an integrated effect of grain boundary strengthening,dislocation strengthening,precipitation strengthening and nanoparticle band strengthening.The contribution of strengthening mechanisms of MNS-E2 alloy consists of grain boundary with around 96 MPa,dislocations with around 3.4 MPa,precipitation strengthening with around 45 MPa and the nanoparticle band with around 18 MPa,respectively.

Microstructure and Mechanical Behavior of FeNiCoCr and FeNiCoCrMn High-Entropy Alloys Fabricated by Powder Metallurgy

The present work reports a systematic investigation on phase evolution,microstructure and microstructure-property relationship of two typical face-centered cubic(FCC)structured high-entropy alloys(HEAs),FeNiCoCr and FeNiCoCrMn,prepared via mechanical alloying(MA)followed by spark plasma sintering(SPS).Following 50 h of MA,the two HEAs consisted of a mixture of FCC and body-centered cubic phases.Following SPS,the bulk FeNiCoCr alloy showed a primary FCC phase with a small amount of Cr_(23)C_(6) and Cr_(2)O_(3) contaminants,while the bulk FeNiCoCrMn alloy was composed of a primary FCC phase with some(Cr,Mn)23C6 and MnCr_(2)O_(4) contaminants.The average grain size of the primary FCC phase in the bulk FeNiCoCr alloy was~416 nm,while that of the primary FCC phase in the bulk FeNiCoCrMn alloy was~547 nm.The yield strength,compressive strength and strain-to-failure of the bulk FeNiCoCr alloy are 1525 MPa,1987 MPa and 24.4%,respectively,whereas those of the bulk FeNiCoCrMn alloy are 1329 MPa,1761 MPa and 21.9%,respectively.It suggests that the bulk FeNiCoCrMn exhibited lower strength and plasticity in comparison with the bulk FeNiCoCr alloy.Clearly,the smaller grain size of the primary FCC phase in the FeNiCoCr alloy is mainly responsible for the better mechanical performance.

Achieving strength-ductility synergy in a non-equiatomic Cr_(10)Co_(30)Fe_(30)Ni_(30)high-entropy alloy with heterogeneous grain structures

Cold rolling and post-deformation annealing(PDA)heat treatments were used to produce heterogeneous grain structures(HGS)in a single-phase face-centered cubic(fcc)Cr_(10)Co_(30)Fe_(30)Ni_(30)high-entropy alloy(HEA).The microstructural evolution and microstructure-property relationship of the HEA were systematically studied under different states.HGS could be achieved in PDA-treated samples at 875℃for 20 s and at 900℃for 20 s(PDA-900-20 s).PDA-900-20 s sample exhibits the most excellent combination of strength and ductility,showing a tensile yield strength of~590 MPa,an ultimate strength of~706 MPa and a total elongation of~23.9%.Additionally,compared with the homogenized counterpart exhibiting homogenous grains,PDA-900-20 s sample displays a notable increment of~413%in yield strength and simultaneously maintains a good ductility.The dominated strengthening mechanisms in PDA-900-20 s sample are grain-boundary strengthening and heterogeneous deformation-induced(HDI)strengthening,whereas the good ductility is mainly resulted from the HDI ductility.Accordingly,the present study provides an effective and simple pathway to overcome the strength-ductility tradeoff of typical fcc HEAs through heterogeneous microstructure.

Additive manufacturing of a Co-Cr-W alloy by selective laser melting:In-situ oxidation,precipitation and the corresponding strengthening effects

Additive manufacturing exhibits great potentials for the fabrication of novel materials due to its unique non-equilibrium solidification and heating process.In this work,a novel nano-oxides dispersion strengthened Co28 Cr9 W1.5 Si(wt.%)alloy,fabricated by laser powder bed fusion(LPBF),was comprehensively investigated.During the layer-by-layer featured process,in-situ formation of Si rich,amorphous,nano-oxide inclusions was observed,whose formation is ascribed to the high affinity of Si to oxygen.Meanwhile,distinctive body-centered cubic(BCC)Co5 Cr3 Si2 nano-precipitates with an 8-fold symmetry were also confirmed to appear.The precipitates,rarely reported in previous studied Co-Cr alloys,were found to tightly bond with the in-situ oxidization.Furthermore,the morphologies,the size distributions as well as the microstructure of the interface between the matrix and the inclusions were investigated in detail and their influence on the tensile deformation was also clarified.The existence of transition boundaries between these inclusions and the matrix strongly blocked the movement of dislocations,thereby increasing the strength of the alloy.It was understood that when the plastic deformation proceeds,the fracture occurs in the vicinity of the oxide inclusions where dislocations accumulate.A quantitative analysis of the strengthening mechanism was also established,in which an additional important contribution to strength(~169 MPa)caused by the effects of in-situ formed oxide inclusions was calculated.

Corrosion behavior of a spark plasma sintered Fe-2OMn-11Al-1.8C- 5Cr alloy in molten aluminum

The corrosion behavior of an Fe-20Mn-11Al-1.8C-5Cr alloy prepared by spark plasma sintering was investigated via immersion tests in molten aluminum at 750℃ for 1 and 4 h, respectively, and a hot work steel （AlSI H13） was included as a reference. The experimental results show that the corrosion rate of Fe-20Mn-11Al-1.8C-5Cr alloy is - 24% of that of H13 steel, suggesting that Fe-20Mn-11Al-1.8C-5Cr alloy in molten aluminum possesses better corrosion resistance than H13 steel. Detailed analysis show that k-carbide （（Fe, Mn）3AlCx） and Cr7C3 carbide precipitated in the matrix play a key role in enhancing the corrosion resistance of Fe-20Mn-11Al-1.8C-5Cr alloy in molten aluminum. Both of them show better corrosion resistance than 7-Fe matrix and H13 steel, and can also take on the role of roots in grasping the corrosion product and restrain them from spalling into the molten aluminum.