The development of two texture variants and their effect on the mechanical behavior of a high strength P/M aluminum alloy,X7091

The influence of texture on the deformation behavior and monotonic and cyclic properties of two recrystallized P/MX7091-T651 plates was investigated. Thermal mechanical treatments were used to produced two different textures which varied in intensity by a factor of four. The two plates had similar grain and precipitate structures. The deformation behavior and mechanical properties were correlated with grain orientation and grain boundary misorientation. Differences in fracture surface roughness and crack deflection frequencies were observed for the two texture variants during fatigue crack propagation studies. Deformation behavior, crack closure, and crack deflection affected the fatigue crack growth rates. A small but measurable improvement in tensile strength, fatigue strength, and fatigue crack growth resistance was obtained in the sharply textured material when compared with the weakly textured counterpart.     

Fatigue crack initiation and microcrack propagation in X7091 type aluminum P/M alloys

Fatigu crack initiation in extruded X7091 RSP-P/M aluminum type alloys o°Curs at grain boundaries at both low and high stresses. By a process of elimination this grain boundary embrittlement was attributed to Al₂O₃ particles formed mainly during atomization and segregated to some grain boundaries. It is not due to the small grain size, to Co₂Al₉, to η precipitates at grain boundaries, nor to a precipitate free zone. Thermomechanical processing after extrusion of X7091 with 0.8 pct Co was done by Alcoa to produce large recrystallized grains. This resulted in initiation of fatigue cracks at slip bands, and the resistance to initiation of fatigue cracks at low stresses was much greater. Microcrack growth is, however, much faster in the thermomechanically treated samples, as well as in ingot alloys, than in extruded and aged X7091.     

Transformation behavior in a thermomechanically cycled TiNiCu alloy

The effect of thermomechanical cycling under 150 MPa on the transformation behavior in a TiNi₄₀Cu₁₀ (at pct) alloy annealed at different temperatures was investigated using electrical resistivity measurements and differential scanning calorimetry (DSC). It was found that thermomechanical cycling to failure could increase or decrease the transformation temperature for specimens annealed below or above the recrystallization temperature, respectively, but there was no obvious change of the transformation temperature for specimens annealed at the recrystallization temperature. The DSC and electrical-resistance experiments show that the B2 ⇋ B19 and B19 ⇋ B19′ two-stage transformations occurred in cold-worked and thermomechanically cycled specimens and that the electrical-resistance change due to the B2 → B19 transformation is larger than that of annealed specimens.     

TA32钛合金板材的超塑性性能研究

研究了真空环境中TA32钛合金在950℃,初始变形速率在5.32×10-4~2.08×10-2s-1条件下的超塑性变形行为。结果表明,不同应变速率条件下,板材的流变应力曲线特征和显微组织演变呈现显著不同。在应变速率较低条件下(5.32×10-4 ~3.33×10-3s-1),拉伸真应力-应变曲线呈传统超塑变形的稳态流动特征,变形后的板材中初生α相晶粒尺寸较大;在高应变速率（8.31×10-3 s-1~2.08×10-2 s-1）条件下,拉伸真应力-应变曲线中流变应力增大到峰值后快速单调递减直到断裂,变形后的板材中初生α相发生动态再结晶,晶粒尺寸与低应变速率条件拉伸的板材相比显著细化。在950℃下,TA32钛合金板材均具有超塑性变形能力,超塑性延伸率在145%~519%之间,当应变速率为5.32×10-4s-1时,板材具有最佳的超塑性性能,拉伸延伸率可达519%。断裂区分析发现,TA32钛合金板材的超塑性断裂模式为空洞聚集-连接-长大型断裂。     

Fatigue Crack Initiation and Microcrack Propagation in X7091 Type Aluminum P/M Alloys

Fatigue crack initiation in extruded X7091 RSP-P/M aluminum type alloys occurs at grain boundaries at both low and high stresses. By a process of elimination this grain boundary embrittlement was attributed to A1₂O₃ particles formed mainly during atomization and segregated to some grain boundaries. It is not due to the small grain size, to Co₂Al₉, to 17 precipitates at grain boundaries, nor to a precipitate free zone. Thermomechanical processing after extrusion of X7091 with 0.8 pct Co was done by Alcoa to produce large recrystallized grains. This resulted in initiation of fatigue cracks at slip bands, and the resistance to initiation of fatigue cracks at low stresses was much greater. Microcrack growth is, however, much faster in the thermomechanically treated samples, as well as in ingot alloys, than in extruded and aged X7091.     

Effect of Co content on the stress-corrosion cracking behavior of 7091-type aluminum powder alloys

An investigation of the stress-corrosion cracking (SCC) behavior of three aluminum powder alloys, containing 0.0, 0.4, and 0.8 wt pct Co, using double cantilever beam specimens has shown a significant increase in SCC resistance with increasing Co content. This resistance to cracking takes the form of both a decrease in plateau crack velocity and an increase in the threshold stress intensity factor for cracking (K _ISCC ) as the Co content increases. The SCC fracture is intergranular and the crack path is tortuous because of the oxides and Co₂Al₉ intermetallic particles contained within the powder metallurgy alloys. We propose that the improvements in SCC resistance result from the Co₂Al₉ particles, which catalyze the recombination and evolution of hydrogen, thereby reducing hydrogen absorption and embrittlement. Formerly with Martin Marietta Laboratories     

Structure and properties of rapidly solidified 7075 P/M aluminum alloy modified with nickel and zirconium

Aluminum alloy 7075 was modified by additions of 1.1 wt pct nickel and 0.8 wt pct zirconium, rapidly solidified by ultrasonic gas atomization, canned, cold compacted, hot extruded, and evaluated in terms of structure and properties. Significant improvements in tensile strength (627 MPa YS and 680 MPa UTS) and crack growth rates were realized, along with a decrease in fracture toughness (23.7 MPa√m) while maintaining ductility (10 pct elong.) as compared to nominal I/M 7075 behavior. The stress for 10⁷ cycles fatigue life was greater than 275 MPa, which represents a 73 pct increase over that of I/M 7075. A variety of experiments was performed to evaluate effects on strength, ductility, and on structure. The variables were: powder size distribution, extrusion ratio, extrusion profile, different size fractions from the same lot of powder, and different locations of test bars in the several extrusions. Tensile properties, toughness, and fatigue properties were not importantly influenced by the location of test bars in the cross section or length of rectangular extruded bars. A comparison of mechanical properties from extruded bars prepared from ?53 μm powdersvs 53 to 250 μm powders showed a small loss of ductility and fatigue stress for 10⁷ cycles for the fine powder product. Higher extrusion ratios were beneficial for mechanical properties.     

Rapidly solidified P/M X2020 aluminum alloys

X2020 aluminum alloys were produced with variations in the Li/Cu ratio by the ultrasonic gas atomization process. In alloy 68 (Al-4.9Cu-l.2Li) and 69 (Al-4.4Cu-l.55Li) alloys, the Θ′ and T₁, phases are dominant with evidence of the T_B phase. In the 70 (Al-3.5Cu-2.8Li) alloy, the δ′ phase is dominant with a trace of T₁. It was found that Θ′ andT ₁ are effective strengtheners whereas δ′ provides excellent fatigue crack initiation resistance. Overall results indicate that the fracture behavior of three RS-PM X2020 alloys is closely related to alloy production route as well as to the phases present in the alloys. Formerly Research Assistant, Massachusetts Institute of Technology.     

P/M Al-Zn-Mg-Cu系铝合金中的微量元素

总结P／M Al-Zn-Mg-Cu系铝合金中常用的微量元素和最新进展情况，以及各微量元素在合金中的主要作用，并提出了一些观点和建议。     

粉末冶金低合金钢的制备和性能研究进展

从粉末制备、成形工艺、烧结工艺和组织控制等方面,系统的介绍了国内外粉末冶金低合金钢的制备技术,并对其研究方向和应用前景进行了展望。