硼化物原位自生增强TiAl基复合材料的组织演化、热变形行为及加工工艺设计

硼元素添加造成的相转变和硼化物析出等因素会对原位TiAl基复合材料显微组织演化及热变形行为产生影响。利用等温压缩实验、扫描电子显微技术以及透射电子显微技术等研究材料的动态再结晶和动态回复机制,并计算出其表现变形激活能为691.506 k J/mol。在1100~1200℃温度区间,再结晶γ和α晶粒的形核长大分别主导α2→α相转变温度上、下的热变形行为。α相的动态回复主导材料在1250℃低应变速率下的热变形行为;同时,硼元素会提高α相含量,降低γ→α和α2→α相转变温度,进而促进加载过程中回复α相晶粒的形核长大。根据新建的本构模型,对TiAl基复合材料的变形机制和加工工艺进行详细阐述.

Human umbilical cord mesenchymal stem cells to treat spinal cord injury in the early chronic phase: study protocol for a prospective, multicenter, randomized, placebo-controlled, single-blinded clinical trial

Human umbilical cord mesenchymal stem cells(hUC-MSCs)support revascularization,inhibition of inflammation,regulation of apoptosis,and promotion of the release of beneficial factors.Thus,they are regarded as a promising candidate for the treatment of intractable spinal cord injury(SCI).Clinical studies on patients with early chronic SCI(from 2 months to 1 year post-injury),which is clinically common,are rare;therefore,we will conduct a prospective,multicenter,randomized,placebo-controlled,single-blinded clinical trial at the Third Affiliated Hospital of Sun Yat-sen University,West China Hospital of Sichuan University,and Shanghai East Hospital,Tongji University School of Medicine,China.The trial plans to recruit 66 early chronic SCI patients.Eligible patients will undergo randomization at a 2:1 ratio to two arms:the observation group and the control group.Subjects in the observation group will receive four intrathecal transplantations of stem cells,with a dosage of 1×106/kg,at one calendar month intervals.Subjects in the control group will receive intrathecal administrations of 10 mL sterile normal saline in place of the stem cell transplantations.Clinical safety will be assessed by the analysis of adverse events and laboratory tests.The American Spinal Injury Association(ASIA)total score will be the primary efficacy endpoint,and the secondary efficacy outcomes will be the following:ASIA impairment scale,International Association of Neural Restoration-Spinal Cord Injury Functional Rating Scale,muscle tension,electromyogram,cortical motor and cortical sensory evoked potentials,residual urine volume,magnetic resonance imaging–diffusion tensor imaging,T cell subtypes in serum,neurotrophic factors and inflammatory factors in both serum and cerebrospinal fluid.All evaluations will be performed at 1,3,6,and 12 months following the final intrathecal administration.During the entire study procedure,all adverse events will be reported as soon as they are noted.This trial is designed to evaluate the clinical safety and efficacy of subarachnoid transplantation of hUC-MSCs to treat early chronic SCI.Moreover,it will establish whether cytotherapy can ameliorate local hostile microenvironments,promote tracking fiber regeneration,and strengthen spinal conduction ability,thus improving overall motor,sensory,and micturition/defecation function in patients with early chronic SCI.This study was approved by the Stem Cell Research Ethics Committee of the Third Affiliated Hospital of Sun Yat-sen University,China(approval No.[2018]-02)on March 30,2018,and was registered with ClinicalTrials.gov(registration No.NCT03521323)on April 12,2018.The revised trial protocol(protocol version 4.0)was approved by the Stem Cell Research Ethics Committee of the Third Affiliated Hospital of Sun Yat-sen University,China(approval No.[2019]-10)on February 25,2019,and released on ClinicalTrials.gov on April 29,2019.

Effect of heat treatment and thermomechanical processing on microstructure and tensile property of Ti-44Al-8Nb-0.2W-0.2B-0.5Y alloy

High Nb-TiAl (Ti-44Al-8Nb-0.2W-0.2B-0.5Y,at.%) ingot was fabricated by vacuum arc remelting (VAR).The as-cast ingot was hot-isostatic pressed (HIP) and homogenizing annealing processed.The influence of heat treatment temperature and thermomechanical processing on the microstructure and tensile property of the alloy was investigated by X-ray diffractometry (XRD),scanning electron microscopy (SEM) and tensile tests.It was found that the high Nb-TiAl alloy after HIP and annealing was mainly composed of coarse α2/γ lamellae,β/B2 phase and γ phase and the solidification path of this alloy was:L→L+β→β→α+β→α→α+β+γ→α2+β+γ.The water quenching results showed that the alloy was in α single phase region at 1,340 °C.After heating at 1,340 °C for 30 min followed by furnace cooling,the alloy showed a full lamellar microstructure and its ultimate tensile strength was about 538 MPa,with an elongation of 0.3% at room temperature.Free-crack forged pancakes with fine-grained fully lamellar structure (FFLS) were obtained with an initial deformation temperature of 1,340 °C and the ultimate tensile strength of forged alloy was about 820 MPa,with an elongation of 0.9% at room temperature,which was much higher than that of alloy after HIP and annealing because of microstructural refinement.

Interactions between Ti Al alloy and AZC/AMT binder systems in investment casting

In this work, two kinds of binders, Ammonium Zirconium Carbonate(AZC) and Ammonium Metatungstate(AMT) hydrate, and three kinds of powders(ZrO 2, Al2O3 and Y2O3) were mixed to fabricate six kinds of face coating systems. The thermal behaviors of the AZC and AMT dried binders were investigated by TG-DTA, and the phase transformation of the two binders was determined by XRD. Monoclinic ZrO 2 phase was formed from AZC at 620 °C and WO3 at 700 °C, and the phase transformation was completed at lower than 1000 °C in both binders, and therefore, the sintering temperature for the molds was selected at 950 °C. The interaction between the ceramic molds with different face coatings and the Ti-48Al-2Cr-2Nb alloy during investment casting was studied. Results showed no α-case reaction in the TiA l-mold reaction, and the AMT + Y2O3 face coating appeared to be the best choice for investment casting of TiA l alloys under the experimental conditions.

Relationship between microstructure and tensile properties on a near-β titanium alloy after multidirectional forging and heat treatment

In this study, a new near-beta titanium alloy, Ti- 4Al-1 Sn-2Zr-5Mo-8V-2.5Cr, was prepared by induction skull melting (ISM) and multidirectional forging. The effect of aging heat treat me rn on microstructure and ten sile properties of the alloy after solution treatment in the twophase (α+β) region was investigated. The microstructure results show that the globular primary ot phase (cxp) and the needle-like secondary y. phase (ots) are precipitated in the P matrix. The size of ots increases with the increase in aging temperature, while the con tent of ots goes up to a peak value and then decreases. The tensile testing results show that the strength increases first and then decreases with the in crease in temperature. The variation of ductility presents the opposite way compared with the trend of strength level. When aged at 500℃, the alloy exhibits an excellebalance of tensile strength (1529 MPa) and elongation (9.22%). And the relative mechanism of sirengthening and toughening was analyzed and discussed.

Microstructure characterization and tensile properties of a Ni-containing TiAl-based alloy with heat treatment

The effects of Ni addition on solidification micro structure and tensile properties of Ti-48Al-2Cr-2Nb alloy were investigated using differential scanning calorimetry（DSC）,X-ray diffraction（XRD）,scanning electron microscopy（SEM） equipped with energy-dispersive spectroscope（EDS） and transmission electron microscopy（TEM）.Results show that with 3 at%Ni addition,the as-cast micro structure is mainly composed of fine lamellar colonies（~50 μm）,γ grains and Ni-ridied τ_3 phase.After heat treatment at 1380℃,the Ni-containing alloy is characterized by fine fully lamellar micro structure（~90 μm）.The heat-treated Ni-containing specimen exhibits superior room temperature tensile properties than other specimens.The tensile properties are discussed in light of the microstructure evolution and role of Ni-riched τ_3 phase.

Microstructure and properties of a beta-solidifying TiAl-based alloy with different refiners

This study systematically compared the influences of yttrium（Y）,boron（B）,and carbon（C） on the microstructural refinement and properties of a Ti-43Al-5Nb alloy.The microstructural refinement effect in the TiAl alloy closely depends on the refiner used.The refinement effects of the three elements on colony size and lamellar thickness can be arranged as B 〉 Y 〉 C and Y 〉 C 〉 B,respectively.Moreover,a microstructure with a small grain size and ultra-fine lamellar spacing can be obtained by adding B and Y or B and C.The mechanical properties of TiAl alloy are also influenced by the refiners.TiAl alloys with proper B and Y contents exhibit favorable hot workability,tensile properties,and fracture toughness,whereas the C-containing alloy displays poor tensile properties and low fracture toughness.These results indicate that Y and B are more suitable microstructure refiners than C.This study may serve as a reference for practical alloying design.

Microstructure evolution of Ti–46Al–6Nb–(Si,B) alloys during heat treatment with W addition

The cast ingots of Ti-46Al-6Nb-0.25Si-0.2B and Ti-46Al-6Nb-0.5W-0.25Si-0.2B（at%） were made by induction skull melting（ISM） technique.A series of heat treatments（HTs） were conducted to research the microstructure evolution of both alloys.Microstructure and tensile property were examined by scanning electron microscope（SEM）,X-ray diffraction（XRD）,transmission electron microscope（TEM）,and tensile testing machine.The results show that microsegregation（liquid segregation and solid segregation） is exacerbated by the addition of0.5 at%W;the addition of Nb,W in TiAl alloy makes the phase transition difficultly take place;then,the microstructures and tensile properties of both alloys are improved after certain HT processes;finally,the thicknesses of the γ/α_2 lamellae after a certain HT process are significantly affected by the number of residual γ phases before the furnace-cooling moment.

Microstructure and high-temperature tensile property of TiAl alloy produced by selective electron beam melting

The microstructure and high-temperature tensile property of a Ti-47Al-2Cr-2 Nb alloy fabricated via selective electron beam melting(SEBM) with hatch spacings of 85,100,and 115 μm were systematically investigated.When the hatch spacing increased from 85 to 115 μm,the microstructure comprising the horizontal cross section changed from coarse lamellar(y/B2) colonies to an inhomogeneous structure and the grain morphology transformed from elongated grains to inhomogeneous and equiaxed grains along the building direction of the vertical cross section.The boundary population of the SEBMproduced TiAl alloy samples was dominated by high-angle grain boundaries(≥ 15°),and the volume fraction of these boundaries decreased with hatch spacing increasing.Additionally,the as-built TiAl alloy sample produced under a spacing of 100 μm exhibited the highest room-and elevated-temperature tensile strengths,with the ultimate tensile strength at room temperature(642 MPa) increasing to 674 MPa at 700 ℃.Furthermore,the mechanism of anomalous strengthening at 700 ℃ was discussed in detail.