High-strength Cu-Ni-Sn alloys by thermomechanical processing

The influence of prior cold work on the aging characteristics and mechanical properties response for copper-rich alloys in the Cu-Ni-Sn system has been investigated. It has been established^15,16 that there exists a spinodal mode of decomposition below a critical temperatureT _R, 200 to 300°C below the equilibrium phase boundary in this system. Significant age hardening response is observed in this region; however, fracture ductility is severely impaired due to a grain boundary precipitate network which develops after relatively short aging times. Cold work prior to low temperature aging is found to have relatively little influence on the incubation time for this embrittling network. It does, however, profoundly enhance the kinetics of the continuous (spinodal) transformation. It is observed that for broad variations in composition, critical combinations of prior cold work, aging time and temperature yield material with unique combinations of. yield stress and fracture ductility (for example, a Cu-9 wt pct Ni-6 wt pct Sn alloy may be processed to exhibit an 0.01 pct offset yield of 174,000 psi in conjunction with a 55 pct R.A. on fracture; significantly higher 0.01 pct offset yield values may be achieved at some reduction in fracture ductility for other NiJSn ratios). It is concluded that the resultant ductileJbrittle properties response is a consequence of a critical compctitive balance between amplitude development in the modulated structure and nucleation of the grain boundary network. The minimum level of prior cold work required to effect this balance in the Cu-9 wt pct Ni-6 wt pct Sn alloy is 75 pct R.A. The present levels of yield stressJfracture ductility values reported, to the best of our knowledge, are unsurpassed by those of any other copper-base alloy system (at a significant cost reduction to the Cu-Be alloys) and suggest the potential yet to be realized in other systems exhibiting this mode of decomposition.     

Relationship between microstructure and mechanical properties of a spinodally decomposing Cu-15Ni-8Sn alloy prepared by spray deposition

A spinodally decomposing alloy of composition Cu-15 wt pct Ni-8 wt pct Sn processed by Osprey spray deposition was examined and the principal factors determining strength and ductility studied. Strengthening contributions are shown to arise from grain-size reduction by controlled recrystallization, and from the internal stress fields created by spinodal decomposition. Discontinuous precipitation takes place after the completion of spinodal decomposition and is partly responsible for the decrease of yield stress after such prolonged aging. At the same time, there is some loss of ductility, but the discontinuous precipitation is not believed to be the main cause. Instead, it is the appearance of additional phases, such as Ni₃Sn, in the matrix that is considered to be the main cause of the brittleness.     

Tensile strength of thermomechanically processed Cu-9Ni-6Sn alloys

The tensile properties of Cu-9Ni-6Sn alloys with different swaging amounts of 64, 77, and 95 pct, either solutionized and aged (S/A) or directly aged (D/A), were examined as a function of aging time. It was found that the aging response of Cu-9Ni-6Sn alloys varied greatly depending on the prior solution heat treatment before aging and/or different swaging amounts. The swaged S/A Cu-9Ni-6Sn alloys showed a multistage increase in tensile strength with respect to aging time, probably due to the sequential occurrence of spinodal decomposition, formation of metastable γ· precipitates, and recrystallization. The effect of different swaging amounts, ranging from 64 to 95 pct, was minimal on the aging response of S/A specimens. The prior cold working, however, appeared to favor the spinodal strengthening, comparing unswaged and swaged S/A Cu-9Ni-6Sn alloys. In 95 pct swaged D/A Cu-9Ni-6Sn alloys, the level of hardening was much less sensitive to aging time. A complex interaction between the reduction in dislocation density, the formation of equilibrium precipitates, and the reduction of Sn content in the Sn-rich segregates during an aging process is believed to be responsible for such a lean sensitivity. The increases in tensile strength of 64 and 77 pct swaged D/A Cu-9Ni-6Sn alloys were found to be much steeper than that in the 95 pct counterparts in the early and intermediate stages of aging, which is believed to be related to the relative contribution from work hardening and precipitation hardening to the strength level of D/A specimens.     

Effect of processing and heat treatment on behavior of Cu-Cr-Zr alloys to railway contact wire

A new series of Cu-Cr-Zr alloys to be used as railway contact wire, Cu-0.26 wt pct Cr-0.15 wt pct Zr, Cu-0.13 wt pct Cr-0.41 wt pct Zr, and Cu-0.34 wt pct Cr-0.41 wt pct Zr, were studied. The results indicated that processing and aging treatment had an effect on the microstructure, tensile strength, and electrical conductivity behavior of the Cu-Cr-Zr alloys. Process I (solution treatment + cold work + aging) was superior to process II (cold work + solution treatment + aging), because precipitation can occur heterogeneously at the dislocations and subcells. An appropriate processing and aging treatment may improve the properties of the alloys due to the formation of fine, dispersive, and coherent precipitates within the matrix. It is demonstrated that the best combination of tensile strength and electrical conducitivity, on the order of 599 MPa and 82 pct IACS (International Annealed Copper Standard), respectively, can be obtained in alloy Cu-0.34 wt pct Cr-0.41 wt pct Zr in the solution-heat-treated, cold-worked, and aged condition. The mechanism of tensile and conductive properties of Cu-Cr-Zr alloy is also discussed.     

The effect of prior deformation on spinodal age hardening in Cu-15 Ni-8 Sn alloy

The structure and kinetics of aged Cu-15 Ni-8 Sn with and without prior deformation was investigated by means of X-ray diffraction and transmission electron microscopy. Deformation prior to aging is known to accelerate the hardening process in this alloy. The purpose of the study was to determine whether the strengthing mechanisms are kinetically enhanced through modified alloy decomposition or through other mechanisms. Deformation does not affect most of the features of structure development, including DO₂₂ ordering in the tin-rich phase. However, for a given aging treatment, cold worked material shows a reduced strain modulation. The accelerated strengthening response in cold worked alloys cannot be attributed to an accelerated or altered decomposition process. It is suggested that the coarsened spinodal structure and the dislocation structure interact in a synergistic way to give the observed strengthening acceleration.     

High strength copper alloys by thermomechanical treatments

A thermomechanical processing technique for in creasing the strength of copper alloys is described. Alloys studied include phosphor bronze (5 pct Sn), nickel silver (12 pct Ni-28 pct Zn), tin-modified cupronickel (9 pct Ni-2 pctSn), and Cu?Be (2 pct Be). In this technique, the material is cold-rolled to about 95 pct reduction in thickness followed by heat treatment below the recrystallization temperature. The severe cold work results in increased strenght through strain hardening and texture strengthening, but at the expense of decreased ductility. The terminal heat treatment recovers the ductility while maintaining or increasing the strength imparted by cold work alone. Preliminary results indicate that cold work-accelerated precipitation is chiefly responsible for the strength increase during heat treatment. As a result of the present processing, the copper alloys exhibit higher yield strengths for given amounts of ductility than have heretofore been attained.     

Effects of combined high and low temperature deformation processing of β III titanium

The structure and properties of β III titanium alloy (nominal composition: 11.5 pct Mo, 6 pct Zr, 4.5 pct Sn, bal Ti) were studied as a function of combined high and low temperature ther-momechanical processing. A water quenched extrusion was deformed various amounts by swaging at room temperature prior to the aging treatment. No re-solution heat treatment was employed. The swaging introduced mechanical twinning and a small amount of stress induced orthorhombic martensite. Following a 900°F, 8 hr aging treatment, substantial increases in yield and tensile strength were observed, combined with a severe decrease in tensile ductility in samples with small amounts of swaging. The orientation and morphology of the deformation products have a critical influence on tensile ductility. A decrease in the plane strain fracture toughness accompanied the large increase in tensile strength.     

On the improvement of creep strength and ductility of Ni-20 pct Cr by small zirconium additions

The creep and fracture properties of high-purity Ni-20 pct Cr and Ni-20 pct Cr-0.11 pct Zr alloys are compared at 1073 K in vacuum. The Ni-20 pct Cr alloy cavitates at the grain boundaries and fractures intergranularly after strains of typically 20 pct. The observed cavity growth rates are in keeping with those predicted. Alloying with zirconium substantially increases the creep strength and ductility. Creep rupture associated with dynamic recrystallization occurs, and voids are observed only in heavily necked parts of the samples. In addition to Ni₅Zr and ZrO₂ inclusions, a Zr₄C₂S₂ carbo-sulfide was identified. Thus, the sulfur-gettering effect of zirconium even at very low residual sulfur levels (20 wt ppm) was confirmed. The zirconium-induced increase in the creep strength is discussed, and the inhibition of creep cavitation by zirconium is examined within the framework of thermal cavity nucleation. Lowering of the grain boundary diffusivity and the grain boundary free energy as well as dynamic recrystallization are likely to reduce cavity nucleation and growth rates in Ni-Cr-Zr and will thus increase its ductility. Finally, the results are used to illustrate the critical importance of minor alloying additions in constructing and using fracture mechanism maps.     

添加Ag对Cu-15Ni-8Sn 合金组织及性能的影响

通过对不同Ag含量的Cu-15Ni-8Sn合金铸态、固溶态和时效态的微观组织进行分析,研究了Ag添加对 Cu-15Ni-8Sn 合金组织及性能的影响。结果表明,适量Ag元素的添加能够减少铸态Cu-15Ni-8Sn合金 的枝晶间距,抑制显微偏析,改善合金的铸态组织。此外,当Ag含量为0.2 wt.%~ 0.5 wt.% 时,可有效抑制时效过程中不连续沉淀相的析出,从而改善合金的力学性能。但是,当Ag添加量大于等于0.5 wt.% 时,对不连续沉淀的抑制效果减弱。     

Age hardening in Cu-15Ni-8Sn alloy

Age hardening and accompanying phase transformations were studied in a Cu-15Ni-8Sn alloy using mechanical testing, X-ray diffraction and microscopy techniques on samples aged at 300 to 550°C. At 450°C and below, strengthening is accompanied by the formation of sideband peaks about the fundamental reflections corresponding to a modulated structure with A? 100Å. This structure is metastable and with further aging is displaced by a discontinuous (cellular) precipitation reaction which causes the yield strength to drop. A peak yield strength increment of ?450 MPa is achieved in samples aged at 350°C. Both the hardening and the discontinuous transformation growth rates are increased by extensive deformation prior to aging. Prior deformation decreases the time to peak strength but does not significantly affect the peak strength increment. The early stages of aging correlate well with a spinodal strengthening model proposed by Ditchek and Schwartz which considers squaring of the composition modulation wave.     

The effect of processing and microstructure development on the slip and fracture behavior of the 2.1 wt pct Li AF/C-489 and 1.8 wt pct Li AF/C-458 Al-Li-Cu-X alloys

Although Al-Li-Cu alloys showed initial promise as lightweight structural materials, implementation into primary aerospace applications has been hindered due in part to their characteristic anisotropic mechanical and fracture behaviors. The Air Force recently developed two isotropic Al-Li-Cu-X alloys with 2.1 wt pct Li and 1.8 wt pct Li designated AF/C-489 and AF/C-458, respectively. The elongation at peak strength was less than the required 5 pct for the 2.1 wt pct Li variant but greater than 10 pct for the 1.8 wt pct Li alloy. The objectives of our investigations were to first identify the mechanisms for the large difference in ductility between the AF/C-489 and AF/C-458 alloys and then to develop an aging schedule to optimize the microstructure for high ductility and strength levels. Duplex and triple aging practices were designed to minimize grain boundary precipitation while encouraging matrix precipitation of the T₁ (Al₂CuLi) strengthening phase. Certain duplex aged conditions for the AF/C-489 alloy showed significant increases in ductility by as much as 85 pct with a small decrease of only 6.5 and 2.5 pct in yield and ultimate tensile strength, respectively. However, no significant variations were found through either duplex or triple aging practices for the AF/C-458 alloys, thus, indicating a very large processing window. Grain size and δ′ (Al₃Li) volume fraction were determined to be the major cause for the differences in the mechanical properties of the two alloys.     

Cellular decomposition in a Cu-25Ni-15Co side-band alloy

The process of decomposition in a Cu-25 mass pct Ni-15 mass pet Co alloy has been investigated by X-ray diffraction, optical and electron microscopy, and hardness measurements. The alloy first decomposedvia spinodal decomposition, followed by a set of complex cellular-like reactions. X-ray diffraction results showed that side bands exist even after the spinodal microstructure has been completely replaced by the cellular-like microstructure. TEM showed that the cellular-like microstructure had a fiber-like appearance with high degree of periodicity. It is this structure which gives rise to the side bands in the X-ray diffraction pattern late in the transformation process. The primary cells engulfed the entire matrix after a short time of aging and were followed by secondary and tertiary cells. During these later reactions the fiber spacing increased and the composition of the phases reached their equilibrium values. This cellular-like transformation is different from the conventional cellular precipitation, and is interpreted as the preferential growth of part of the spinodal microstructure which is near the grain boundary, due to grain boundary migration. We call this type of cellular transformation in the spinodal alloys “cellular decomposition”.     

Influence of Aging and Thermomechanical Treatments on the Mechanical Properties of a Nanocluster-Strengthened Ferritic Steel

This study investigated the effect of aging and thermomechanical treatments on the mechanical properties of a nanocluster-strengthened ferritic steel, Fe-1.5Mn-2.5Cu-4.0Ni-1.0Al (wt pct). The effect of thermomechanical treatments on the microhardness and tensile properties were measured at room temperature and correlated with microstructural features. Cu-rich precipitates were characterized by transmission electron microscopy and were found to coarsen slowly during long-time aging. The microhardness measurements indicate a typical precipitation hardening behavior during aging at 773 K (500 °C). Tensile tests showed that thermomechanical treatments can improve the mechanical strength and ductility of the nanocluster-strengthened ferritic steel significantly compared with those without the treatments. Fractography results indicated that the high yield strength resulted from precipitation hardening makes the steel more susceptible to grain-boundary decohesion, which can be suppressed by grain refinement. Atmosphere adsorption and diffusion along grain boundaries were found to intensify brittle intergranular fracture, and this embrittlement can be avoided by vacuum heat treatment.     

粉末冶金Cu-15Ni-8Sn合金时效组织的演变行为

对粉末冶金Cu-15Ni-8Sn合金进行热挤压、固溶处理后,研究400℃时效不同时间对合金力学性能及其组织的影响。结果表明:随时效时间延长,Cu-15Ni-8Sn合金的抗拉强度先升高后降低,伸长率先降低后升高,合金断裂方式表现为由沿晶断裂为主向穿晶断裂为主转变。合金抗拉强度在400℃时效1.5 h时获得最大值918MPa。综合考虑合金的强度和韧性,400℃的最佳时效时间为2 h。借助扫描电子显微镜和透射电子显微镜研究合金在400℃时效不同时间(1~3 h)的组织结构变化及其对合金强度和塑性的影响,结果表明:在欠时效阶段,合金组织在富Sn区出现了有序排列的介稳态DO22相,基体的FCC结构开始向DO22有序化结构转变,合金强度大幅提升。而在峰时效阶段,DO22向L12转变,合金强度继续增加,随片层状不连续析出组织开始由晶界向晶内生长,合金强度下降。在过时效阶段,DO22进一步向L12转变,并且片层状不连续析出组织开始大肆侵蚀基体调幅组织,导致合金强度降低,塑性提高。在时效后期,调幅组织被片层状组织大幅侵蚀,出现片层状组织粗化并断裂的现象,粒状γ相(DO3)不断生成,由于这一过程需要更大的浓度起伏而进行得非常缓慢,合金组织仍以大量片层结构为主,因而合金强度和塑性变化不明显。     

Dynamic recrystallization during creep in a 45 Pct Ni-35 pct Fe-20 pct Cr alloy system

A combined 3.5 wt pct Mo + 1.2 wt pct Ti imparted dynamic recrystallization in a 35 wt pct Fe-45 wt pct Ni-20 wt pct Cr alloy system during creep at 700 °C, whereas 3.5 wt pct Mo addition alone did not initiate recrystallization. Dynamic recrystallization substantially increased the creep elongation and produced a high ductile fracture topography in the present alloy system. A subgrain coalescence nucleation mechanism for dynamic recrystallization mechanism was operative during creep. The critical initiation strain requirements are also discussed.     

Effect of aging and deformation on the microstructure and properties of Fe-Ni-Ti maraging steel

The age-hardening behavior of Fe-25.3Ni-1.7 Ti (wt pct) alloy both in undeformed specimens and in specimens cold deformed by 10 or 20 pct prior to aging was studied. The microstructural changes during aging were observed using transmission electron microscopy (TEM) and atom probe analysis and there were related to the mechanical properties as measured by microhardness and shear punch testing. An excellent combination of hardness, strength, and ductility was achieved after only 5 seconds aging at 550 °C. We propose that this rapid strengthening is due to a dislocation friction effect arising from the formation of a fine dispersion of Ni-Ti atomic co-clusters during this short aging time. The concomitant effects of a reverse transformation of martensite to austenite during aging and a gradual increase in both size of the clusters and distance between them contributed to a decrease in strength after aging for 15 seconds. This decline proceeded until aging for 300 seconds and was followed by a secondary hardening reaction toward peak hardness (at 10,800 seconds) and subsequent overaging. This secondary hardening was associated with fine-scale precipitation of Ni₃Ti and this process was accelerated by deformation prior to aging, leading to a reduction or elimination of hardness decline after the initial cluster hardening.     

Microstructure and mechanical behavior of spray-deposited High-Li Al-Li alloys

High-Li alloys, with the composition Al-3.8Li-XCu-1.0Mg-0.4Ge-0.2Zr, were synthesized using a spray deposition technique (wt. pct, X=0∼1.5). The microstructure of the spray-deposited Al-Li alloys consisted of equiaxed grains with an average grain size in the range from 20 to 50 μm. The grain-boundary phases were fine and discrete. The spray-deposited and thermomechanically processed materials were isothermally heat treated at 150 °C and 170 °C to investigate the age-hardening kinetics. It was noted that the spray-deposited Al-3.8Li-XCu-1.0Mg-0.4Ge-0.2Zr alloys exhibited relatively sluggish aging behavior. The peak-aged condition was achieved at 170 °C in the range from 20 to 90 hours. It was noted that Cu increases the hardness of alloys during aging. Moreover, the influence of Cu on age-hardening kinetics is marginal. The mechanical properties of the spray-deposited and extruded Al-Li alloys were studied in the underaged, peak-aged, and overaged conditions. For example, the peak-aged yield strength, tensile strength, and ductility of Al-3.8Li-1.0Cu-1.0Mg-0.4Ge-0.2Zr are 455 MPa, 601 MPa, and 3.1 pct, respectively. Moreover, an increase in the Cu content of the alloy led to improvements in strength, with only slight changes in ductility, for Cu contents up to 1.0 wt pct. Beyond this range, an increase in Cu content led to decreases in both strength and ductility.     

The influence of Mo and Co on the embrittlement of an Fe-Ni-Mn alloy

In the “as rolled” condition an Fe-6 Ni-5 Mn maraging type alloy was found to be brittle exhibiting intergranular fractures. The addition of 2.5 pct Mo and 5.0 pct Mo increased the impact toughness of the “as rolled” material and changed the mode of brittle fracture to transgranular cleavage. The addition of 9 pct Co embrittled the alloy. On aging Mo and Co raised the peak hardness of the base Fe-6 Ni-5 Mn alloy, however, aging led to rapid embrittlement. The base alloy and an alloy containing 2.5 pct Mo showed brittle intergranular fractures on aging. The addition of 5 pct Mo gave rise to brittle transgranular cleavage fractures on aging at 450°C, but at temperatures less than 450°C there was always up to 20 pct intergranular fracture present in brittle fractures. At temperatures greater than 475°C brittle intergranular failure occurred in the 5 pct Mo alloy due to a grain boundary film of M₆C and Fe₂Mo. This paper is based upon a thesis submitted by D. R. Squires in partial fulfilment for a higher degree of CNAA at Sheffield Polytechnic.     

Structure and properties of a β solution treated,quenched, and aged si-bearing near-α titanium alloy

The microstructure, tensile properties, and fractographic features of a near-α titanium alloy, IMI 829(Ti-6.1 wt pct Al-3.2 wt pct Zr-3.3 wt pct Sn-1 wt pct Nb-05 wt pct Mo-0.32 wt pct Si) have been studied after aging over a temperature range of 550°C to 950°C for 24 hours following solution treatment in the β phase field at 1050°C and water quenching. Transmission electron microscopy studies revealed that aging at 625°C and above produced discrete silicides at α′ interplatelet boundaries. However, aging at 900°C and above has also resulted in the precipitation of β phase along the lath boundaries of martensite. The silicides have been found to have a hexagonal structure withc=0.36 nm anda=0.70 nm (designated as S₂ by earlier workers). There is a significant improvement in yield and ultimate tensile strength after aging at 625°C, but there is less improvement at higher aging temperatures. The tensile ductility is found to be drastically reduced. While the fracture surface of the unaged specimen shows elongated dimples, the aged samples show a mixed mode of fracture, consisting of facets, featureless parallel bands, and extremely fine dimples.     

Mechanical behavior of cast single phase alloys solidified from undercooled melts

Several ingots (0.0254 m in diam × 0.10 m long) of nickel-30 wt pct copper, nickel-10 wt pct cobalt and iron-25 wt pct nickel were solidified with various undercoolings up to about 200 K, prior to nucleation of the solid. The materials were mechanically tested in the ascast condition. In nickel-30 wt pct copper and iron-25 wt pct nickel alloys the 0.2 pct offset yield strength, ductility and fatigue strength increased with undercooling. A linear relationship was established between 0.2 pct offset yield strength and the square root of secondary dendrite arm spacing in dendritic alloys (undercooled less than 170 K) or that of grain diameter in nondendritic alloys (undercooled more than 170 K). In iron-25 wt pct nickel limited testing indicated improvements in Charpy V-notch impact strength and in fracture toughness with undercooling. No improvement of tensile properties with undercooling was observed in nickel-10 wt pct cobalt, an alloy which solidified normally with very low microsegregation.