Precipitation hardening

The topic of precipitation hardening is critically reviewed, emphasizing the influence of precipitates on the CRSS or yield strength of aged alloys. Recent progress in understanding the statistics of dislocation-precipitate interactions is highlighted. It is shown that Pythagorean superposition for strengthening by random mixtures of localized obstacles of different strengths is rigorously obeyed in the limit of very weak obstacles; this had been known previously as a result of computer simulation experiments. Some experimental data are discussed in light of this prediction. All of the currently viable mechanisms of precipitation hardening are reviewed. It is demonstrated that all versions of the theory of coherency hardening are woefully inadequate, while the theory of order hardening is capable of accurately predicting the contribution of γ′ precipitates to the CRSS of aged Ni-Al alloys. It is also convincingly shown that a new theory based on computer simulation experiments of the motion of dislocations through arrays of obstacles having a finite range of interaction cannot explain these same data, and is of doubtful validity in other instances for which its success has been proclaimed. A new theory of hardening by spinodal decomposition is proposed. It is based on the statistics of interaction between dislocations and diffuse attractive obstacles, and is shown to be in very good quantitative agreement with much of the limited data available. Some of the problems that remain to be addressed and solved are discussed. This paper is based on a presentation made at the symposium “50th Anniversary of the Introduction of Dislocations” held at the fall meeting of the TMS-AIME in Detroit, Michigan in October 1984 under the TMS-AIME Mechanical Metallurgy and Physical Metallurgy Committees.     

Anisotropy and Asymmetry of Yield in Magnesium Alloys at Room Temperature

Mechanical anisotropy and asymmetry are often pronounced in wrought magnesium alloys and are detrimental to formability and service performance. Single crystals of magnesium are highly anisotropic due to the large difference in critical resolved shear stress between the softest and hardest deformation modes. Polycrystalline magnesium alloys exhibit lower anisotropy, influenced by texture, solute level, and precipitates. In this work, a fundamental study of the effects of alloying, precipitate formation, and texture on the change in anisotropy and asymmetry from the pure magnesium single crystal case to polycrystalline alloys has been performed. It is demonstrated that much of the reduction in anisotropy and asymmetry arises from overall strengthening as solute, precipitates, and grain boundary effects are accounted for. Precipitates are predicted to be more effective than solute in reducing anisotropy and asymmetry, but shape and habit are critical since precipitates produce highly anisotropic strengthening. A small deviation from an ideal basal texture (15 deg spread) has a very strong effect in reducing anisotropy and asymmetry, similar in magnitude to the maximum effect produced by precipitation. Elasto-plastic modeling suggests that this is due to a contribution from basal slip to initial plastic deformation, even when global yield is not controlled by this mode.     

Cu-Precipitation Strengthening in Ultrahigh-Strength Carburizing Steels

Ultrahigh hardness levels greater than 700 VHN can be obtained in secondary hardening carburizing steels but depend on costly Co alloying additions to maximize hardness achieved through M₂C-type carbide precipitation strengthening. This study aims to incorporate nanometer-scale bcc Cu precipitates to both provide strength as well as catalyze M₂C nucleation in the absence of or with reduced Co. Cu additions of 1.0 and 3.7?wt pct were investigated, using a series of mechanistic models coupled with thermodynamic computational tools to derive final compositions. Thirty-pound experimental heats were cast of each designed alloy, samples of which were carburized and tempered to determine their hardness response. Characterization revealed the successful incorporation of Cu alloying additions into this family of steels, demonstrating a secondary hardening response even in the absence of Co. Matrix strength levels were close to those predicted by design models; however, all four alloys demonstrated a hardness deficit of approximately 200 VHN at the carburized surface, suggesting recalibration of the M₂C precipitation strengthening model may be required in these alloys.     

微合金高强度耐候钢的试验研究

在实验室试制了400、460MPa级耐候钢,结果表明,试验钢屈服强度分别达到450、550MPa,抗拉强度分别达到545、615MPa;400MPa级耐候钢的显微组织以铁素体为主,460MPa级的以粒状贝氏体为主;400MPa级的析出物主要是CuS2和TiN,主要强化机制是细晶强化、析出强化;460MPa级的析出物主要是CuS2和（NbTi）CN,其主要强化机制是细晶强化、析出强化及相变强化。采用电子背散射EBSD技术分析了其晶体学取向,其晶粒间取向主要是大角度晶界。     

Precipitation hardening of aluminum alloys

The author’s charge was to discuss recent trends in research and development on precipitation hardened aluminum alloys and to indicate where research is needed. This will be done for three areas: fatigue, properties of grain boundaries and interfaces, and stability of precipitates at elevated temperatures. Present strong precipitation hardened aluminum alloys do not have high endurance limits. One problem is that the small GP zones are cut by the dislocations giving rise to highly localized deformation which aids fatigue crack initiation. A duplex structure with relatively large uniformly spaced precipitates to give more homogeneous deformation plus small precipitates to give high yield strength is a promising approach. The structures of precipitation hardened aluminum base alloys are essentially controlled by the stabilities of the various precipitates and the interfacial energies. Precipitates with high interfacial energies tend to precipitate preferentially at grain boundaries giving embrittlement. Low interfacial energy means easy nucleation, a uniform precipitate distribution, and resistance to coarsening at elevated temperatures. For elevated temperature use, the precipitate must be stable at elevated temperatures. Precipitation hardened aluminum alloys do not have good elevated temperature properties because the hardening precipitates normally used, GP zones, are not stable at elevated temperatures. Thus a low interfacial energy, ductile precipitate, which is stable at elevated temperatures, is needed for aluminum. Possibilities for achieving such precipitates will be discussed.     

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.     

Creep strength of magnesium-based alloys

The high-temperature creep resistance of magnesium alloys was discussed, with special reference to Mg-Al and Mg-Y alloys. Mg-Al solid-solution alloys are superior to Al-Mg solid-solution alloys in terms of creep resistance. This is attributed to the high internal stress typical of an hcp structure having only two independent basal slip systems. Although magnesium has a smaller shear modulus than aluminum, the inherent creep resistance of Mg alloys is better than that of Al alloys. The creep resistance of Mg alloys is improved substantially by the addition of Y. Solid-solution hardening is the principal mechanism of the strengthening, but the details of the mechanism have not been elucidated yet. Forest dislocation hardening in concentrated alloys and dynamic precipitation in a Mg-2.4 pct Y alloy also contribute to the strengthening. An addition of a very small amount of Zn raises the dislocation density and significantly improves the creep resistance of Mg-Y alloys. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Creep strength of magnesium-based alloys

The high-temperature creep resistance of magnesium alloys was discussed, with special reference to Mg-Al and Mg-Y alloys. Mg-Al solid-solution alloys are superior to Al-Mg solid-solution alloys in terms of creep resistance. This is attributed to the high internal stress typical of an hcp structure having only two independent basal slip systems. Although magnesium has a smaller shear modulus than aluminum, the inherent creep resistance of Mg alloys is better than that of Al alloys. The creep resistance of Mg alloys is improved substantially by the addition of Y. Solid-solution hardening is the principal mechanism of the strengthening, but the details of the mechanism have not been elucidated yet. Forest dislocation hardening in concentrated alloys and dynamic precipitation in a Mg-2.4 pct Y alloy also contribute to the strengthening. An addition of a very small amount of Zn raises the dislocation density and significantly improves the creep resistance of Mg-Y alloys. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

The effect of solution heat treatment and quenching rates on mechanical properties and microstructures in AlSiMg foundry alloys

Four common AlSiMg foundry alloys have been solution heat treated at 813 K, quenched, and immediately aged at 423 K for up to 240 minutes. The mechanical properties are found to be related to the amount of Mg and Si in the alloys. A high strength is obtained after only 60 minutes of solution heat treatment, indicating that the solid solution is rapidly saturated on Mg and Si. The ductility is very much related to the amount of silicon present and the refinement of the silicon crystals within the eutectic areas, since silicon crystals are observed to crack when load is applied. Thus, a well-modified structure is the best way to obtain high ductility. Reduced quencing rates after solution heat treatment lead to a lower strength, since a lower number of hardening β′-Mg₂Si precipitates are formed. The ductility of alloys with 0.6 wt pct Mg is increased with a reduced quenching rate. A more ductile matrix corresponding to the lower amount of hardening precipitates can explain this. Alloys with 0.2 wt pct Mg remain relatively unchanged. A hypothesis that may explain this phenomenon is the precipitation of brittle silicon or formation of coarse Mg2Si within the dendrites.     

The effect of two-step aging on the quench sensitivity of an Al-5 Pct Zn-2 Pct Mg alloy with and without 0.1 Pct Cr

The effect of two-step aging on the quench sensitivity of an Al-5 pct Zn-2 pct Mg alloy with and without 0.1 pct Cr has been studied. Results show that the quench sensitivity effect can be eliminated in thin samples of these alloys by two-step aging if the slow cooling during quenching does not allow the precipitation process to proceed too far. Lack of achievement of full strength in the aged condition due to a slow quench rate can be attributed to 1) loss of vacancies during quenching and 2) formation of incoherent-type precipitates during quenching. The trend to lower strength due to the loss of vacancies can be reversed by two-step aging; however, if incoherent-type precipitates form, some strength potential of the alloy is permanently lost. A 0.1 pct Cr addition increases the quench sensitivity effect by accelerating incoherent-type precipitation during quenching. These incoherent precipitates, which appear in the form of bands within the grains and in the grain boundaries, lead to an increase in ductility. Formerly Research Assistant, M.I.T., Cambridge, Mass.