High-temperature creep in an Al-8.5Fe-1.3V-1.7Si alloy processed by rapid solidification

The creep behavior of an Al-8.5Fe-1.3V-1.7Si alloy processed by rapid solidification is investigated at three temperatures ranging from 623 to 723 K. The measured minimum creep strain rates cover seven orders of magnitude. The creep behavior is associated with the true threshold stress, decreasing with increasing temperature more strongly than the shear modulus of aluminum. The minimum creep strain rate is controlled by the lattice diffusion in the alloy matrix, and the true stress exponent is close to 5. The apparent activation energy of creep depends strongly on both applied stress and temperature and is generally much higher than the activation enthalpy of lattice self-diffusion in aluminum. Also, the apparent stress exponent of minimum creep strain rate depends on applied stress as well as on temperature and is generally much higher than the true stress exponent. This behavior of both the apparent activation energy and apparent stress exponent is accounted for by the strong temperature dependence of the threshold stress-to-shear modulus ratio. The true threshold creep behavior of the alloy is interpreted in terms of athermal detachment of dislocations from fine incoherent Al₁₂(Fe, V)₃Si phase particles, admitting a temperature dependence of the relaxation factor characterizing the strength of the attractive dislocation/particle interaction.     

The interrelationship of fracture toughness and microstructure in a Ti-5.25Al-5.5V-0.9Fe-0.5Cu alloy

Fracture toughness of anα-Β titanium alloy heat treated to a constant yield strength has been found to depend upon the morphology of α produced or remaining after the initial solution treatment. In equiaxed α structures, fracture toughness depends linearly upon the grain boundary area per unit volume,S _v, and is independent of equiaxed α particle size or spacing. In a grain boundary α structure fracture toughness depends both onS _v and, within limits, linearly on the thickness of the α. Explanations are offered for the observed propagation of cracks at α-@#@ Β interfaces and for the observation that high fracture toughness can accompany low tensile ductility. This paper is based on a thesis to be submitted by M. A. Greenfield in partial fulfillment of the requirements for the Ph.D. degree in metallurgy at New York University.     

Solidification microstructure and temperature field during direct chill casting of Al-16Si alloy

By properly controlling casting parameters such as pouring temperature, casting velocity and water flux, direct chill (DC) casting can be employed to produce refined microstructure in the hypereutectic Al-Si alloys without chemical modification. This refined microstructure is characterized of fine primary Si particles, fully developed dendritic Al halos and fine coupled eutectics. In this work, in situ measurements of temperature field in the mould during DC casting of Al-16Si alloy at casting velocity of 2.17 mm/s, 3.5 mm/s and 4.34 mm/s at a pouring temperature of 800°C were performed. The results show that the primary Si phase nucleated at considerable undercooling (about 27°C to 38°C) and the growth temperature of dendritic Al halos was 7°C to 8°C below the equilibrium eutectic temperature. In the center regions of the DC cast billet, halos are fully developed because the G_l/R value is low.     

锻造对喷射成形Al-8.5Fe-1.3V-1.7Si合金组织和性能的影响

采用喷射成形和锻造工艺制备了Al-8.5Fe-1.3V-1.7Si合金．通过金相、扫描电镜和力学性能测试等实验，对锻件组织和性能进行了分析。结果表明：“闷车＋包套锻造”工艺对喷射成形Al-8.5Fe-1.3V-1.7Si合金坯件的致密化效果好于用自由锻造和包套锻造致密化的合金，采用该工艺可以制备出组织和性能优良的耐热铝合金材料。“闷车＋包套锻造”锻件在室温下的抗拉强度（σb）达到407MPa，屈服强度（σ0.2）达到344MPa，延伸率（δ5）为7．6％；在315℃，锻件的σb，σ0.2，σ5分别为222，216MPa，7．2％。     

喷射成形Al-8.5Fe-1.1V-1.9Si耐热铝合金的组织演变及性能分析

研究了喷射成形Al—8．5Fe—1．1V—1．9Si耐热铝合金组织结构的演变规律，测试了挤压态合金在室温及高温条件下的力学性能。与铸态组织特征相比较，喷射成形工艺有效地消除了铸态合金中粗大的富Fe析出相，获得了细小均匀的组织结构。利用OM，SEM，TEM，XRD等材料分析测试手段，探索了材料中可能的组织演变过程及规律，结果表明：喷射成形制备的Al—Fe—V—Si耐热铝合金中，形成了大量的弥散分布的球状相，有效的保证了合金在室温及高温下的力学性能。室温下，合金的抗拉强度可以达到445MPa，屈服强度也达到了398MPa，延伸率为16％；在315℃，合金的抗拉强度和屈服强度分别为229和209MPa。     

Phase transformation during annealing of rapidly solidified Al- rich Al- Fe- Si alloys

Thermal decomposition of rapidly solidified microstructure of three Al-Fe-Si alloys (Al-10,12, 14Fe-2Si wt pct) has been studied by DTA and TEM. The initial microstructure consists mostly of aluminum and a smaller volume fraction of an amorphous phase. During heating the amorphous phase first transforms to the metastable α(AlFeSi) cubic phase (Im ,a = 1.25 nm) at ∼ 380°C. At higher temperature ∼ 430 °C the α(AlFeSi) phase transforms by ordering to a trigonal phase (two modifications, α′ and α ″,were found). The crystallography of the α′(AlFeSi) and α″(AlFeSi) phases is analyzed using selected area and convergent beam electron diffraction technique.     

Microstructures and mechanical properties of dispersion-strengthened high-temperature Al-8.5Fe-1.2V-1.7Si alloys produced by atomized melt deposition process

Dispersion-strengthened high-temperature Al-8.5 pct Fe-pct Si-pct V alloys were produced by atomized melt deposition (AMD) process. The effects of process parameters on the evolution of microstructures were determined using optical metallography and scanning and transmission electron microscopy. The extent of undercooling and the rate of droplet solidification were correlated with process parameters, such as melt superheat, metal/gas flow rates, and melt stream diameter. The size distribution and morphology of silicide dispersoids were used to estimate the degree of undercooling and the cooling rate as functions of process parameters. The tensile properties at 25 °C to 425 °C and fracture toughness at 25 °C of these alloys produced with wide variations in dispersoids size and grain size were determined. Under optimum conditions, the alloy has ultimate tensile strength of 281 MPa and 9.5 pct ductility in the as-deposited condition. Upon hot-isostatic pressing and extrusion, the ultimate tensile strength increased to 313 MPa and ductility increased to 18 pct.     

Fatigue crack growth rates and fracture toughness of rapidly solidified Al-8.5 pct Fe-1.2 pct V-1.7 pct Si alloys

The room-temperature fatigue crack growth rates (FCGR) and fracture toughness were evaluated for different crack plane orientations of an Al-8.5 Pct Fe-1.2 Pct V-1.7 Pct Si alloy produced by planar flow casting (PFC) and atomized melt deposition (AMD) processes. For the alloy produced by the PFC process, properties were determined in six different orientations, including the short transverse directions S-T and S-L. Diffusion bonding and adhesive bonding methods were used to prepare specimens for determining FCGR and fracture toughness in the short transverse direction. Interparticle boundaries control fracture properties in the alloy produced by PFC. Fracture toughness of the PFC alloy varies from 13.4 MPa√m to 30.8 MPa√m, depending on the orientation of the crack plane relative to the interparticle boundaries. Fatigue crack growth resistance and fracture toughness are greater in the L-T, L-S, and T-S directions than in the T-L, S-T, and S-L orientations. The alloy produced by AMD does not exhibit anisotropy in fracture toughness and fatigue crack growth resistance in the as-deposited condition or in the extruded condition. The fracture toughness varies from 17.2 MPa√m to 18.5 MPa√m for the as-deposited condition and from 19.8 MPa√m to 21.0 MPa√m for the extruded condition. Fracture properties are controlled by intrinsic factors in the alloy produced by AMD. Fatigue crack growth rates of the AMD alloy are comparable to those of the PFC alloy in the L-T orientation. The crack propagation modes were studied by optical metallographic examination of crack-microstructure interactions and scanning electron microscopy of the fracture surfaces.     

Al-5Ti-B对过共晶Al-18Si合金的变质效果

采用Al-5Ti-B变质剂对过共晶Al-18Si合金进行反向变质处理,用光学显微镜观察合金的组织与形貌,研究变质剂加入量、变质温度和冷却速度对初晶硅的尺寸、形态和面积分数以及共晶组织的影响。研究表明:当Al-5Ti-B加入量(质量分数)为0.3%时,变质处理后Al-18Si合金中的初晶硅和共晶硅尺寸明显减小,初晶硅的面积分数减小;与其相比,变质剂加入量增加到0.6%时,初晶硅尺寸变化不明显,但共晶硅进一步细化;随冷却速率降低,变质处理后Al-18Si合金中初晶硅相的数量减少,但Si颗粒尺寸明显增大,并且共晶硅细化;与Al-18Si合金在720℃变质相比,该合金在780℃变质处理时,初晶硅的尺寸增大,但初晶硅的面积分数显著减小;合金在850℃变质处理后初晶硅的尺寸、面积分数都比720℃变质处理后明显减小;随变质温度升高,Al-Si合金中的共晶硅明显细化。     

喷射成形Al-8.5Fe-1.1V-1.9Si耐热铝合金中弥散强化相体积分数的确定

利用喷射成形工艺制备了Al 8.5Fe 1.1V 1.9Si耐热铝合金 ,观察了合金中耐热相的形貌 ,发现弥散强化相的尺寸在 5 0～ 10 0nm之间。采用Rietveld全谱拟合的方法初步测定了铝合金中弥散强化相的重量百分数为 2 8.4% ,通过换算得出弥散强化相的体积分数为 2 2 .4%。同时分析了与平面流铸造 (PFC)所制备的合金中耐热相体积分数的差异 ,并探讨了合金耐热相体积分数的变化对合金力学性能的影响     

Phase transition in an Fe-23.2Al-4.1Ni alloy

The phase transition in an Fe-23.2 at. pct Al-4.1 at. pct Ni alloy has been investigated by means of transmission electron microscopy. In the as-quenched condition, the microstructure of the alloy is a mixture of (A2 + B2) phases. When the as-quenched alloy is aged at temperatures ranging from 500 °C to 1050 °C, the phase transition sequence is found to be (A2 + B2∗) → (B2 + B2∗) → B2 → A2, where B2∗ is also a B2-type phase. It is worthwhile to note that the coexistence of two kinds of ordered B2 phase has not previously been observed by other workers in the Fe-Al-Ni ternary alloy system.     

Fe- Si合金钢氧化铁皮的结构对酸洗行为的影响

通过扫描电镜观测、拉曼光谱分析、电化学酸洗模拟等方法,研究了Fe- Si合金钢的氧化铁皮结构和酸洗动力学行为。结果表明,Fe- Si合金钢氧化铁皮- 基体分界面上生成的Fe2SiO4层能提高Fe- Si合金钢的抗氧化性,使得其氧化铁皮厚度小于DC- 03。Fe- Si合金钢的氧化铁皮存在贯穿至基体分界面的原始裂缝,HCl溶液通过原始裂缝渗入基体分界面,导致其酸洗过程跳过“孕育期”,直接进入“加速期”和“平稳期”,使得Fe- Si合金钢的酸洗时间比DC- 03短。超声辅助酸洗和酸洗温度的提高,均能提高Fe- Si合金钢酸洗后的表面质量,减少“红色氧化铁皮”的残留。     

Fatigue Crack Growth Behavior of Ti-4.5Al-3V-2Fe-2Mo Laser Welds

Fatigue crack growth tests of Ti-4.5Al-3V-2Fe-2Mo (SP-700) laser welds after various postweld heat treatments (PWHTs) were investigated. The welds and the mill-annealed base metal had similar fatigue crack growth rates (FCGRs) at a stress ratio (R) of 0.1. After increasing the stress ratio to 0.5, the peak-aged (482 °C) weld exhibited higher FCGRs due to increased notch brittleness of the material. The tough microstructure as well as tortuous crack path of the overaged (704 °C) weld could account for the reduced FCGRs, particularly at a higher R. The fatigue fracture appearance of the welds varied from transgranular to intergranular failures, depending on the stress intensity factor ranges and PWHTs. Experimental results also demonstrated that the 704 °C-aged weld with coarsened α + β structures had better impact toughness than the base metal with banded structures.     

Influence of Ti and C on the Solidification Microstructure of Fe-10Al-5Cr Alloys

The influence of Ti and C on the solidification microstructure of Fe-10Al-5Cr (all composition values in weight percent) alloys was examined with solidification modeling and a variety of experimental techniques. Several Fe-10Al-5Cr-Ti-C alloys were fabricated using the arc button melting process and characterized using quantitative image analysis and electron microscopy techniques. The experimental alloys exhibited primary ferrite dendrites with an interdendritic ferrite/TiC eutectic constituent, and the amount of eutectic was affected by the Ti and C concentrations. A liquidus projection and primary solidification paths were calculated for the Fe-10Al-5Cr-Ti-C system in order to estimate the amount of TiC that is expected to form during solidification. The range in the calculated amount of TiC-containing eutectic matched the experimentally measured values reasonably well. The ability to control the amount of TiC that forms during solidification of an Fe-10Al-5Cr-Ti-C-based alloy shows promise for developing corrosion-resistant weld overlay claddings with resistance to hydrogen cracking.