Powder metallurgy approach for control of microstructure and properties in high strength aluminum alloys

High-strength products made from atomized Al-Zn-Mg-Cu-Co alloy powders have good combinations of strength, ductility, resistance to stress-corrosion cracking and fracture toughness. Powder Metallurgy (PJM) methods produce fine metallurgical structures and compositions which cannot be produced by Ingot Metallurgy (IJM) methods. Fine structures result from very rapid solidification and from the effect of fine dispersoids in restricting grain growth. Stress-corrosion cracking (SCC) performance is favored by grain morphology of PJM products. Co₂Al₉ particles in PJM products are 0.02 to 2.0 μm spheroids occurring frequently on grain boundaries where they may serve several functions in slowing SCC attack. Oxide particles are irregular shapes, 0.01 to 0.04 μm in size, occurring in clusters at grain boundaries and in grain bodies. Some of the oxide particles are magnesium oxide and alter the environment in a SCC crack to arrest attack. Porosity is not a significant factor in the structure of PJM products made by a vacuum compacting process. P/M wrought products have superior combinations of high strength and stress-corrosion cracking resistance compared to IJM 7075 and 7050 alloys. While equaling the fracture toughness of 7075 alloy, the PJM products at present have somewhat lower fracture toughness than 7050 alloy, due in part to a larger amount of second-phase particles in the form of Co₂Al₉ and oxide. This paper is based on an invited presentation made at a symposium on “Advances in the Physical Metallurgy of Aluminum Alloys” held at the Spring Meeting of TMS-IMD in Philadelphia, Pennsylvania, on May 29 to June 1, 1973. The symposium was co-sponsored by the Physical Metallurgy Committee and the Non-Ferrous Metals Committee of TMS-IMD.     

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     

Stress-corrosion cracking in equiaxed 7075

The environmental response of commercially produced high-strength Al alloys, such as 7075, depends strongly on the anisotropy of the grain structure. Minimum resistance to both stresscorrosion cracking (SCC) and hydrogen embrittlement is observed in the short transverse direction of the “pancake” grain structure in commercially produced alloys. It has not been established, however, exactly how the morphology of the grain structure mediates the SCC response or the SCC mechanism. Therefore, stress-corrosion testing of a high-purity 7075 Al alloy (low in Fe, Si, and Cr), having equiaxed grains, under tension (mode I) and torsion (mode III) loading in a solution of IN A1C1₃ has been performed. The SCC results in the two loading modes, including fractography, appeared to suggest that the predominant processes of SCC were hydrogen embrittlement in mode I and anodic dissolution in mode III, in agreement with prior work on a commercially produced 7075 alloy, but that severe corrosion during longer tests renders those results unsuitable for threshold determination in this very aggressive testing environment. Formerly with Carnegie Mellon University Formerly with Carnegie Mellon University     

Aging kinetics of aluminum alloy 7050

Precipitation hardening at room temperature (natural aging) and isothermal precipitation hardening at 422 to 455 K beyond the point of peak strength (overaging) were studied. Natural aging kinetics of the increase in strength and resistivity were comparable with those of alloy 7075. Alloy 7050, however, develops peak strength by aging at higher temperatures. This ability of 7050 to develop peak strength upon aging in the temperature region used to improve the resistance to stress-corrosion cracking and toughness of 7XXX alloys is attributed to the effect of Cu on increasing the temperature range of G.P zone stability.     

The effect of copper content and microstructure on the hydrogen embrittlement of AI-6Zn-2Mg alloys

Two commercially-processed Al-6Zn-2Mg alloys, 7050 and a “low copper” 7050, were tested for susceptibility to embrittlement by precharged hydrogen and by simultaneous cathodic charging and straining (SET procedure). Specimens were heat treated to underaged, peak-strength aged, and overaged conditions. In 7050, the peak strength and overaged conditions were not embrittled by hydrogen, though underaged material showed marked embrittlement. All microstructures tested for the low-copper alloy were embrittled. The results agree with the microstructural rationale established through earlier work on 7075 and 2124 aluminum alloys, particularly with respect to the susceptibility of underaged material to hydrogen. As in earlier work, the extent of dislocation transport of hydrogen, and local hydrogen accumulation at grain boundaries, evidently controlled the extent and degree of brittle fracture. These three important alloys can now be ranked in the order 7050, 2124, 7075 of increasing relative susceptibility to theonset of stress corrosion cracking.     

The transgranular SCC of a Mg-Al alloy: Crystallographic, fractographic and acoustic-emission studies

A study has been made of the propagation of transgranular stress-corrosion cracks in a Mg-7.5 wt pct Al alloy tested at room temperature in an aqueous NaCl-K₂CrO₄ solution; the studies were carried out on single cracks which initiated and propagated within single grains in coarse-grained bend specimens. The resulting fracture surfaces were flat, and two-surface analysis established their orientation to be 31–40. Specimens fractured at liquid-nitrogen temperature cleaved on 10•10, indicating that the 31•40 are not the normal cleavage planes in this material. The stress-corrosion fracture surfaces were cleavage-like in appearance, containing shallow steps which were matching and interlocking on opposite faces. Discrete acoustic signals were emitted during crack propagation, and these are considered to result from discontinuous crack advance. It is concluded that stress-corrosion cracking in this system occurs by discontinuous cleavage on (3140) planes. D. G. CHAKRAPANI, formerly a Research Assistant in the Department of Metallurgy and Mining Engineering, University of Illinois, Urbana, Ill. 61801.     

The transgranular SCC of a Mg-Al alloy: Crystallographic,fractographic and acoustic-emission studies

A study has been made of the propagation of transgranular stress-corrosion cracks in a Mg−7.5 wt pct Al alloy tested at room temperature in an aqueous NaCl-K₂CrO₄ solution; the studies were carried out on single cracks which initiated and propagated within single grains in coarse-grained bend specimens. The resulting fracture surfaces were flat, and two-surface analysis established their orientation to be {ie1155-01}. Specimens fractured at liquid-nitrogen temperature cleaved on {ie1155-02}, indicating that the {ie1155-03} are not the normal cleavage planes in this material. The stress-corrosion fracture surfaces were cleavage-like in appearance, containing shallow steps which were matching and interlocking on opposite faces. Discrete acoustic signals were emitted during crack propagation, and these are considered to result from discontinuous crack advance. It is concluded that stress-corrosion cracking in this system occurs by discontinuous cleavage on {ie1155-04} planes. Formerly a Research Assistant in the Department of Metallurgy and Mining Engineering, University of Illinois, Urbana, Ill. 61801.     

The effect of loading mode on the stress-corrosion cracking of aluminum alloy 5083

Recent studies have revealed that the mechanism of stress-corrosion cracking (SCC) of high-strength Al-Zn-Mg alloys involves both dissolution and hydrogen embrittlement (HE); moreover, under tensile-loading conditions, evidence exists that the hydrogen mechanism is dominant. In the present study, the role of HE in the SCC of Al-Mg alloys was investigated using commercial Al-4.4 wt pct Mg alloy, 5083. The susceptibility of this alloy to SCC in a saline environment was evaluated in Mode I (tension) and Mode III (torsion), using precracked fracture toughness specimens. The greater susceptibility found in Mode I indicates that HE is involved in SCC. As further evidence that HE is operating, susceptibility increased when As, a hydrogen recombination inhibitor, was added to the test solution under Mode I conditions. Issues related to the overall validity of the loading mode experiment are also addressed.     

Applied Stress Affecting the Environmentally Assisted Cracking

Stress corrosion cracking (SCC) is affected by the mode of applied stress, i.e., tension, compression, or torsion. The cracking is measured in terms of initiation time to nucleate a crack or time to failure. In a simple uniaxial loading under tension or compression, it is observed that the initiation time can vary in orders of magnitude depending on the alloy and the environment. Fracture can be intergranular or transgranular or mixed mode. Factors that affect SCC are solubility of the metal into surrounding chemical solution, and diffusion rate (like hydrogen into a tensile region) of an aggressive element into the metal and liquid metallic elements in the grain boundaries. Strain hardening exponent that affects the local internal stresses and their gradients can affect the diffusion kinetics. We examine two environments (Ga and 3.5 pct NaCl) for the same alloy 7075-T651, under constant uniaxial tension and compression load. These two cases provide us application to two different governing mechanisms namely liquid metal embrittlement (7075-Ga) and hydrogen-assisted cracking (7075-NaCl). We note that, in spite of the differences in their mechanisms, both systems show similar behavior in the applied K vs crack initiation time plots. One common theme among them is the transport mechanism of a solute element to a tensile-stress region to initiate fracture.     

Relationship between fracture toughness, fracutre path, and microstructure of 7050 aluminum alloy: Part II. Multiple micromechanisms-based fracture toughness model

A multiple micromechanisms-based model is developed to quantitatively relate the fracture toughness of partially recrystallized 7XXX aluminum alloys to their fracture surface morphology. The model is verified using the experimental data on partially recrystallized 7050 alloy reported in the companion article. It is then used to obtain a quantitative relationship between the fracture toughness and microstructural attributes. The model relates fracture toughness to microstructural parameters such as degree of recrystallization, grain size of recrystallized grains, thickness of recrystallized regions, total surface area of the constituent particles per unit volume, and microstructural anisotropy. The model predicts the changes in the fracture toughness with the specimen orientation.     

Retrogression and reaging and the role of dislocations in the stress corrosion of 7000-type aluminum alloys

The 7000-type aluminum alloys in the T6 temper are known to be highly susceptible to stress-corrosion cracking (SCC). Some years ago, a heat treatment known as retrogression and reaging (RR) was developed by one of the authors (B.C.), providing for enhanced stress-corrosion resistance without any sacrifice of yield or tensile strength in 7075 aluminum alloy. The idea behind the process was based on the suggestion that dislocations developed during quenching from the solution treatment were responsible for susceptibility to stress corrosion. In spite of considerable practical development of the RR process, the above basic hypothesis as to the role of dislocations has never been investigated. In the present work, the effect of the RR treatments on the dislocation structure of 7000-type aluminum alloys was studied using transmission electron microscopy (TEM). A clear relationship has been found between the presence of dislocations adjacent to grain boundaries and the susceptibility to stress corrosion of 7000-type aluminum alloys. The beneficial effect of the RR treatment on the SCC of 7000-type aluminum alloys in the T6 temper is believed to be due to the disappearance of the above dislocations as a result of RR treatment.     

The relation between microstructure and toughness in 7075 aluminum alloy

Electron microscopy, fractography, and notched tear tests have been used to investigate the effects of heat treatment upon the fracture behavior of aged 7075 aluminum alloy sheet. Toughness, as measured by crack propagation energy, decreases as the yield stress increases; the toughness of an overaged structure is inferior to that of an underaged structure at the same yield stress. The decrease of toughness with increased aging time is accompanied by a change in fracture mode from predominantly transgranular to intergranular. Transgranular fracture proceeds by dimple rupture and is facilitated by chromium-rich particles which are dispersed throughout the microstructure. Intergranular fracture proceeds by the fracture of grain boundary precipitate particles. The variation of fracture mode with aging time is attributed to a steady decrease of the intergranular fracture stress relative to the transgranular fracture stress, due to increasing grain boundary particle size. A possible explanation of this effect is discussed using the stress concentration due to colinear crack arrays as an analogy. The effects of quenching variations and two-step aging are discussed. It is shown that, in aged 7075, microstructural variables such as the width of precipitate-free zones and the nature of the matrix precipitate do not have a controlling effect on toughness.     

Effect of strain rate on stress corrosion cracking in duplex type 304 stainless steel weld metal

The influence of strain-rate on the stress-corrosion cracking properties of wholly austenitic Type 304 base metal and duplex austeno-ferritic Type 304 weld metal in boiling MgCl₂ was investigated using constant extension rate tensile testing techniques. Transgranular SCC in both base and weld metals is preferred at low strain-rates, while intergranular cracking in the base metal and interphase cracking along the austenite-ferrite interface in the weld metal are preferred at higher strain-rates. Promotion of the intergranular stress-corrosion cracking in the base metal and “interphase-interface” stresscorrosion cracking in the weld metal with increases in strain-rate may be mechanistically analogous. Stress-induced alterations in the grain or interphase boundary defect structure may make these regions preferentially susceptible to dissolution. W. A. BAESLACK III, Lt., USAF, formerly with Rensselaer Polytechnic Institute, Troy, New York     

The role of microstructure in hydrogen-assisted fracture of 7075 aluminum

Underaged, peak strength (T6), and overaged (T73) microstructures were studied in 7075 plate material. Hydrogen charged and uncharged tensile specimens of longitudinal orientation were tested between −196°C and room temperature. The results confirm a hydrogen embrittlement effect, manifested mainly in the temperature dependence of the reduction of area loss; a classical behavior of hydrogen embrittlement. The maximum embrittlement shifted to lower temperatures with further aging. The effect of hydrogen was largest for the underaged condition and smallest for the overaged, thus following the pattern found for the sensitivity to stress-corrosion cracking in high strength aluminum alloys. The fracture path was predominantly transgranular, with minor amounts of intergranular fracture. J. ALBRECHT, formerly with the Department of Metallurgy and Materials Science, Carnegie-Mellon University     

Metallurgical factors affecting fracture toughness of aluminum alloys

Crack extension in commercial aluminum alloys proceeds by the “ductile” or fibrous mode. The process involves the large, ~1 μm to ~10μm, Fe-, Si-, and Cu-bearing inclusions which break easily, and the growth of voids at the cracked particles. The linking-up of the voids is accomplished by the rupture of the intervening ligaments, and this is affected by the fine, ~0.01μm precipitate particles that strengthen the matrix. The ~0.1μm Cr-, Mn-, and Zr-rich intermediate particles are more resistant to cracking and may enter the process in the linking-up stage. The fracture toughness of aluminum alloys therefore depends on a) the extent of the heavily strained region ahead of the crack tip, which is a function of the yield strength arad modulus, b) the size of the ligaments which is related tof _c, the volume fraction of cracked particles, and c) the work of rupturing the ligaments. An approximate analysis predicts K_Ic varies asf _c-1/6, and this is in agreement with measurements on alloys with comparable yield strength levels. Studies in which the aging conditions are altered for the samef _cshow that the toughness decreases with increasing yield strength level. This degradation in toughness is related to the localization of plastic deformation. The tendency for localization is illustrated with the help of “plane strain” tension and bend specimens whose behavior is related to the toughness. Measurements of the strain distribution on the microscale show that slip is relatively uniformly distributed in a 7000-type alloy with low inclusion and particle content when the material is in the as-quenched and overaged conditions. In contrast the distribution is highly nonuniform in the peak aged condition where slip is concentrated in widely spaced superbands involving coarse slip bands with large offsets that crack prematurely. The connection between the tendency for slip localization and the fine precipitate particles which strengthen the matrix remains to be established. In overaged alloys grain boundary ruptures occur within the superbands. The amount of intergranular failure increases with grain size and is accompanied by a loss of fracture toughness. This paper is based on an invited presentation made at a symposium on “Advances in the Physical Metallurgy of Aluminum Alloys” held at the Spring Meetings of TMS-IMD in Philadelphia, Pennsylvania, on May 29 to June 1, 1973. The symposium was co-sponsored by the Physical Metallurgy Committee and the Non-Ferrous Metals Committee of TMS-IMD     

Fractographic lines in maraging steel—A link to fracture toughness

Lines of constant spacing are exhibited across the surfaces of fracture toughness specimens in an 18 Ni (200) maraging steel. The wavelength associated with this periodic lineage decreases, however, with decreasing fracture toughness as the steel is maraged for greater time to higher strength levels. The periodicity as well as the shape of the lines, which appear to trace successive positions of an advancing crack front, suggests that the crack advances by a repetitive discontinuous unit process such as postulated in the tensile ligament instability theory of fracture toughness. Calculations support interpretation of the wavelength as the process zone size of that theory. The lineage is found in several different types of fracture toughness specimens as well as in tensile specimens. Furthermore, such lineage has been detected for several alloys in addition to maraging steel, and in certain cases for cracking modes other than fast fracture,e.g., stress-corrosion cracking. However, for all cases where the periodic lineage appears, microvoid coalescence is the mode of ultimate separation. The lines are clearly distinguished from Wallner lines. Formerly National Research Council Postdoctoral Resident Research Associate in the Physical Metallurgy Branch This paper was presented at the 1971 TMS-AIME Fall Meeting in Detroit, Michigan.     

The effects of alloying elements on the susceptibility to stress-corrosion cracking of martensitic steels in salt water

The effects of a number of elements (C, Mn, Cr, Mo, Ni, Co, P, and S) on the stress-corrosion cracking (SCC) resistance in salt water of some quenched-and-tempered steels were investigated. Values of the threshold stress intensity for crack growth in salt water (K _Iscc) were measured using the cantilever beam test and precracked specimens. Values of the fracture toughness parameterK _Ix (an approximation ofK _Ic) were also determined. The steels were either Fe-C alloys to which alloying elements were added, or basically of AISI 4340-type composition in which alloying elements were varied. All steels in a series to show the effects of a given element were heat treated to the same yield strength, and generally the effects of an element were determined at two yield strength levels. The results show that only carbon and manganese are definitely harmful to SCC resistance.     

双级时效对7075合金应力腐蚀性能影响

双级时效处理虽能有效提高7075铝合金抗应力腐蚀开裂(SCC)性能,但同时会导致合金力学性能降低。为了同时提高7075铝合金的拉伸性能和抗SCC性能,并优化双级时效参数,对双级时效处理7075合金进行了正交试验。通过扫描电镜和透射电镜在慢应变速率实验中研究7075合金的SCC行为。结果发现,在130 ℃条件下保温4 h后,在170 ℃条件下保温8 h,合金抗拉强度、伸长率和应力腐蚀指数I_SSRT分别为488 MPa、10.8%和0.095。      

Brittle fracture of an Au/Ag alloy induced by a surface film

The film-induced cleavage model of stress-corrosion cracking (SCC) has been tested using an Ag-20 at. pct Au alloy in 1 M HClO₄ solution. Brittle cracks, both intergranular (IG) and transgranular (TG) in nature, were formed by high-speed loading of a thin foil covered with a dealloyed (nanoporous gold) layer. These cracks were found to propagate through the dealloyed layer and into the uncorroded bulk face-centered cubic (fcc) material for a distance of many microns. Hydrogen embrittlement (HE) can be excluded on thermodynamic grounds; thus, only film-induced cleavage can explain the observed decoupling of stress and corrosion in the fracture process.