孔隙和α-Al(Fe/Mn)Si相对高压压铸Al-7Si-0.2Mg合金塑性的影响

采用高压压铸工艺制备两批次不同尺寸的Al-7Si-0.2Mg(质量分数,%)合金,获得显微组织和孔隙非均匀分布的薄壁铸件,并对比研究孔隙和显微组织对铸态合金塑性的影响。结果表明:不同铸件和不同位置样品的伸长率有较大波动(9.7%~17.9%)。当合金存在大面积孔隙时,有效承载面积减小导致由孔隙产生的应力集中使合金伸长率显著降低。当合金只存在小面积孔隙时,塑性变形过程中合金中的α-Al(Fe/Mn)Si相先于共晶硅相发生脆性断裂,α-Al(Fe/Mn)Si相的数量密度对伸长率的波动起主导作用,具有高数量密度α-Al(Fe/Mn)Si相试样的伸长率显著降低。此外,局部较高的冷却速率导致铸件α-Al(Fe/Mn)Si相数量密度的增加。

Mechanism of cryogenic,solid solution and aging compound heat treatment of die-cast Al alloys considering microstructure variation

To develop a new compound heat treatment process for improving the mechanical properties of die-cast Al alloys,this study investigated the effects of cryogenic,solution and aging compound treatment on the microstructure and mechanical properties of die-cast Al alloys.The characterization methods used were optical microscopy(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM),electron backscatter diffraction(EBSD),and tensile tests;and the Image Pro Plus software was used for statistical analysis.The results indicated that compared with T6 heat treatment,the compound heat treatment process consisting of cryogenic treatment(-196.C for 12 h),solid solution treatment(476.C for 22 min),and aging(159.C for 403 min)significantly enhanced the mechanical properties of the diecast Al alloys.For instance,the tensile strength increased from 224.3 to 249.5 MPa;the hardness increased from HV110.5 to HV 124.6,and the elongation increased from6.28%to 7.72%,which in relative terms corresponds to11.2%,12.8%and 22.9%,respectively.The compound heat treatment process of the alloy led to significant refinement of its a-Al phases.In addition,Si phases tended to be more ellipsoidal or granular,while the tips of Fecontaining phases became rounded,which played a key role in enhancing the mechanical properties and microstructure stability of the alloys.

Dynamic recrystallization behavior and coincidence site lattice evolution in thermal deformation of 316H stainless steel used in nuclear systems

The hot deformation behavior of 316H stainless steel used in the 4th-generation nuclear systems was investigated by thermal compression tests at 1000–1150 C and 0.01–10 s^(-1).It was found that true stress firstly increased and then decreased with the increasing strain rate with a threshold of 1 s^(-1).Electron backscatter diffraction was used to analyze the microstructure evolution.Discontinuous dynamic recrystallization(DDRX)was the dominant dynamic recrystallization(DRX)mechanism,while continuous dynamic recrystallization(CDRX)was the supplementary one.DDRX happened before CDRX and provided additional nucleation sites for the latter.Twin grain boundaries(R3)appeared in DRX grains due to growth accidents.As the length fraction of R3 increased,the coincidence site lattice(CSL)boundary transition began to occur,forming R9 and R27.After the occurrence of full DRX,the growth and annexation of DRX grains were easy to be promoted,in which progress both equiaxed grains and CSL boundaries disappeared.The ideal deformation microstructure with fine and uniform DRX grains,which was accompanied by a high length fraction of CSL boundaries,appeared at 1000℃–0.01 s^(-1),1050℃–0.01–0.1 s^(-1),1100℃–0.1–1 s^(-1) and 1150℃–1–10 s^(-1).That is,the deformation conditions mentioned above were the preferable thermal forming parameters for 316H stainless steel in actual productions.

Micromechanical behavior of TiNb/NiTi composite during a pre-straining process:an in situ synchrotron investigation

The micromechanical behavior of a recently developed TiNb/NiTi composite during a pre-straining process was investigated to broaden the understanding of deformation mechanisms of the martensitic-transforming composites.It was found that during loading,besides inherent elastic elongation,the TiNb/NiTi composite also experienced two different categories of stress-induced martensitic transformations(SIMTs,including B2→B19’andβ→α")and slight plastic deformation.Upon the following unloading,this composite recovered elastically,and underwent simultaneously a fully reversibleα"→βSIMT as well as a partially reversible B19’→B2 SIMT.It was the incomplete B19’→B2 SIMT and the permanent plastic deformation that led to the~4.6%macroscopic residual strain after unloading.In the entire loading-unloading procedure,the growth and shrinkage of(001)compound twins in B19’martensite also contributed to the large nearlinear-elastic deformation of the present composite.

Near-Iinear elastic deformation behavior of a novel Zr-Ti-Nb-Sn alloy with multi-phase microstructures ofβ,α"andα'phases

In this work,a multi-phase Zr-30Ti-7Nb-4Sn alloy with large near-linear elastic deformability was prepared by the solution treatment plus pre-straining(S TP)treatment,and the underlying mechanism accountable for the near-linear elastic deformation was systematically clarified.It was found that the Zr-30Ti-7Nb-4Sn alloy was composed ofβ,α″andα′phases,and numerous dislocations were formed in the alloy specimen after STP treatment.With the retarding effect of high-density dislocations,the stress-induced martensitic(SIM)transformation fromβtoα″phases took place homogeneously and continuously during loading,in conjunction with the occurrence of elastic deformation ofβ,α″andα′phases.Under the entire tensile procedure,no observable phase transformation occurred betweenα′martensite andβphase.Consequently,the near-linear elastic deformation capability in STP Zr-30Ti-7Nb-4Sn alloy is mainly ascribed to the coupling actions of consecutiveβ→α″SIM transformation and intrinsic elastic deformation ofβ,α″andα′phases.These experimental results provide a basis for designing and developing novel multi-phase Zr-based alloys that possess large near-linear deformability.

Temperature dependence in tensile properties and deformation behavior of GH4169 alloy

The effect of temperature on the tensile properties and deformation mechanism of GH4169 alloy has been systematically studied over a wide range of room temperature(RT)to 1000℃.The results indicate that the stress–strain curve of the alloy shows serrations at 200–600℃,and the character of the serrations changes from type A to type B and then to type C at different temperatures.The ultimate tensile strength of the alloy decreases gradually from RT to 600℃.The yield strength decreases slowly from RT to 700℃ but decreases rapidly above 800℃.Transmission electron microscopy analysis relieves that the primary deformation mechanism of the alloy below 500℃ is Orowan bypass mechanism.At temperatures between 600 and 700℃,the coordinated deformation of twins and cross-slip of dislocations are activated.The transformation of\upgamma^{\prime\prime}phase toδphase above 650℃ will decrease the strength.The primary deformation mechanism above 800℃ transforms into the repeated shearing of\upgamma^{\prime\prime}by dislocations to form multiple stacking faults.Recrystallized grains were observed above 800℃,and continuous dynamic recrystallization and discontinuous dynamic recrystallization were observed.The stress concentration caused by Nb-rich carbides is the cause of intracrystalline crack nucleation.At 700℃,grain boundary crack sprouting is caused by the combined effect of slip band impact on grain boundaries and grain boundary dislocation plugging.The relationship between the serrated flow behavior and the deformation mechanism has been discussed based on the experimental results.

Dynamic recrystallization behavior of Fe–20Cr–30Ni–0.6Nb–2Al–Mo alloy

Single-pass compression tests of an aluminaforming austenite(AFA) alloy(Fe–20Cr–30Ni–0.6Nb–2Al–Mo) were performed using a Gleeble-3500 thermal–mechanical simulator. By combining techniques of electron back-scattered diffraction(EBSD) and transmission electron microscopy(TEM), the dynamic recrystallization(DRX) behavior of the alloy at temperatures of 950–1100 ℃ and strain rates of 0.01–1.00 s^(-1) was investigated. The regression method was adopted to determine the thermal deformation activation energy and apparent stress index and to construct a thermal deformation constitutive model. Results reveal that the flow stress is strongly dependent on temperature and strain rate and it increases with temperature decreasing and strain rate increasing. The DRX phenomenon occurs more easily at comparably higher deformation temperatures and lower strain rates. Based on the method for solving the inflection point via cubic polynomial fitting of strain hardening rate(h) versus strain(e) curves, the ratio of critical strain(ec) to peak strain(ep) during DRX was precisely predicted. The nucleation mechanisms of DRX during thermal deformation mainly include the strain-induced grain boundary(GB)migration, grain fragmentation, and subgrain coalescence.

In situ synchrotron X-ray diffraction analysis of deformation behavior of a Nb/NiTi composite for biomedical applications

The deformation behavior involved in Nb/NiTi composite for biomedical applications within a large macroscopic strain range was investigated by tensile loading-unloading test and in situ synchrotron X-ray diffraction(SXRD).Experimental results show that during loading,the Nb/NiTi composite experiences the elastic elongation of B2-NiTi austenitic,B19’-NiTi martensitic and β-Nb phases,B2→ B19’ stress-induced martensitic(SIM) transformation and tensile plastic deformation of β-Nb phase.During unloading,the deformation behavior involved in Nb/NiTi composite includes the elastic recovery of B2-NiTi austenitic,B19’-NiTi martensitic and β-Nb phases,reverse phase transformation B19’→B2 and compressive deformation of p-Nb phase.The martensitic transformation in this composite is almost reversible and occurs in a localized manner.These results might contribute to a comprehensive understanding of the deformation mechanism involved in Nb/NiTi composite and shed some light on design and development of novel composites with a combination of good biocompatibility and excellent superelasticity for biomedical applications.

Wear response of metastable b-type Ti–25Nb–2Mo–4Sn alloy for biomedical applications

The wear response of a newly developed metastable b-type Ti–25Nb–2Mo–4Sn(abbreviated as Ti-2524) alloy was investigated and compared with that of(a+b)-type Ti–6Al–4V alloy. Experimental results show that solution-treated(ST) Ti-2524 specimen has the lowest wear rate due to the combined effects of excellent ductility and lubricative Nb2O5. Although similar Nb2O5 forms on the surface of the cold rolled plus annealed(CRA) Ti-2524 specimen, the beneficial effect of Nb2O5 on the wear resistance is counteracted by the increase in wear rate caused by low elongation. Thus, the wear rate of the CRA Ti-2524 alloy is higher than that of ST Ti-2524 specimen.As for the ST Ti–6Al–4V alloy, no lubricative Nb2O5 forms on its worn surface owing to the absence of Nb. In addition, the ST Ti–6Al–4V alloy exhibits an elongation roughly similar to the CRA Ti-2524 specimen. Therefore,the ST Ti–6Al–4V specimen possesses a higher wear rate than the CRA Ti-2524 specimen.

Uniaxial tensile deformation behavior of a sandwich-like structural TiNb-NiTi composite for biomedical applications

The uniaxial tensile deformation behavior of a sandwich-like structural TiNb-NiTi composite was investigated by uniaxial tensile test and in situ high-energy synchrotron X-ray diffraction(SXRD).It is found that below 1.2%macroscopic strains,the elastic deformations of the B2,β,B19'andα"phases take place in the TiNbNiTi composite.During the subsequent loading,theβ→α"and B2→B19'stress-induced martensitic transformations(SIMTs)occur within the macroscopic strains of 0.5%-4.2%and the macroscopic strains of 0.7%-6.2%,respectively.At the macroscopic strain of about 4.2%,the outer TiNb layer of the TiNb-NiTi composite experiences a partial fracture,as proved by the disappearance of(040)_(α")and a sudden jump in the(110)_(B19')d-spacing caused by load transfer.With further uniaxial tensile deformation,the TiNbNiTi composite finally fractures at a strain of~6.2%.Our results might provide some valuable information for understanding the deformation behavior of novel sandwich-like structural shape memory composites in more depth.