Relaxation mechanisms in Fe-Al-C alloys

The relaxation spectrum of Fe-Al alloys has been studied as a function of Al content and ordering reaction in Fe-Al. Three types of relaxation peaks are observed, with activation energies between 0.8 and 3 eV. Snoek-type relaxation is studied in Fe-(0 to 50 at. pct)Al and compared with the Snoek relaxation in pure iron (C in α-Fe), chromium (C in Cr), and nibium (O in Nb). The snoek-type relaxation peak in iron (at 314 K for 1 Hz) shifts to higher temperatures with increasing Al content in iron. Significant changes in the peak parameters occur when α-Fe is alloyed with Al, because of the ordering reaction in Fe-Al. Peculiarities of the carbon-atom distribution in ordered and disordered Fe-Al alloys are discussed using an atom-interaction model, in which the elastic interaction is supplemented by the chemical C-Al interaction. Two other peaks are observed only when a certain Al content is exceeded: a Zener peak for Fe-(>10 pct)Al and an X peak for Fe-(>26 pct)Al. Parameters of these peaks are discussed with respect to alloy structure. Three hypotheses are discussed for the X-peak mechanism with an activation energy about 1.7 eV. A map of relaxation peaks in the Fe-Al system is constructed. Dedicated to the memory of our colleague and friend Alexandr B. Lebedev.     

Transformation behavior of sintered porous NiTi alloys

TiH₂ powder is added as a reactant and pore-forming agent to produce porous NiTi shape-memory alloys (SMAs). The transformation behavior of porous NiTi alloys is investigated because it is relevant to the engineering and medical applications of SMAs. It is found that the transformation behavior of porous NiTi alloys is different from that of cast NiTi alloys. It is demonstrated for the first time, by in situ X-ray diffraction (XRD), that there is no R-phase transformation in porous NiTi alloys, and a broadened, two-peak phenomenon observed with a differential scanning calorimeter (DSC) is not associated with R-phase transformation. The characteristic transformation temperatures of porous NiTi alloys are independent of sintering temperature, sintering time, TiH₂ content, and the heating/cooling rate during thermal cycling between +123 and +423 K. Further, the latent heats of transformation are associated with the TiH₂ content and the sintering conditions.     

Strengthening mechanisms in solid solution aluminum alloys

A number of commercial and high-purity non-heat-treatable aluminum alloys are investigated in this work. It is found that both magnesium and manganese in solid solution give a nearly linear concentration dependence of the strength at a given strain for commercial alloys. This deviates from high-purity AlMg binary alloys, where a parabolic concentration dependence is found. Mn in solid solution is found to give a considerably higher strengthening effect per atom than Mg, both in terms of yield stress and initial work hardening rate. This strengthening effect is stronger comparing commercial grades to high-purity alloys. This enhanced strengthening is believed to be a synergy or clustering effect caused by interaction between Mn atoms and trace elements, probably silicon, in solid solution.     

The mechanisms of anneal hardening in Cu-Al alloys

The anneal hardening behavior of a Cu-16 at. pct. Al alloy was investigated by tensile testing, measurement of Young’s modulus and electrical resistivity, and electron microscopy. It was shown that three main stages of annealing processes can be distinguished. Stage 1 atT ≤ 150°C is characterized by vacancy annihilation and short-range diffusional processes giving rise to three successive, low maxima in yield and tensile strength. Substages in this temperature range are also evident from resistivity measurements. Stage 2 at 150 ≲ 275°C is essentially associated with solute segregation to dislocations giving rise to the main increase in flow stress. In Stage 3 at 725 ≤T ≤ 350°C recrystallization takes place.     

自蔓延高温合成TiNi多孔体合金

为了开发新型的生体材料，本文对自蔓延高温合成TiNi多孔体合金——人造骨材料进行了研究。以单质钛粉和镍粉作原料，在氩气保护条件下，采用预热点燃模式和热爆模式制得了不同形态的多孔TiNi形状记忆合金。采用SEM和XRD分析了样品的孔洞特征和相组成，测试了其力学性能，并研究了合成条件参数与样品表面形貌和孔隙状态之间的关系。结果表明，用SHS法制备TiNi多孔体合金是可行的。     

Creep deformation mechanisms in high-pressure die-cast magnesium-aluminum-base alloys

Creep of die-cast Mg alloys is described as an integral part of their plastic deformation behavior in terms of stress-strain-rate-strain relations. Creep tests yield information on yield stress, work hardening, maximum deformation resistance (minimum creep rate), and work softening. Testing in compression avoids influences by fracture. Data on the alloy AJ52 (5Al, 2Sr) in the temperature range between 135 °C and 190 °C are presented and compared to those for AZ91 and AS21. Die-cast Mg-Al alloys consist of fine grains with a grain boundary region containing intermetallic precipitates. Transmission electron microscopic observations indicate that basal glide is the dominant mechanism of deformation being supplemented by nonbasal glide and twinning to maintain compatiblity between the grains. The deformation resistance can be modeled with a composite approach assuming that the grain boundary region is relatively hard due to precipitation of intermetallic phases. The differences in long-term creep resistance at low stress are explained in terms of different strength and stability of precipitates in the different alloys. This article is based on a presentation made in the symposium entitled “Phase Transformations and Deformation in Magnesium Alloys,” which occurred during the Spring TMS meeting, March 14–17, 2004, in Charlotte, NC, under the auspices of the ASM-MSCTS Phase Transformations Committee.     

W-Mo-Ni-Fe重合金金属间化合物相析出机制研究

为了分析Mo元素含量对液相烧结W—Ni—Fe重合金显微组织的影响和探讨Mo元素含量与基地相、析出相等各相组成的关系，以及了解合金于炉冷后在固相与基地相接口析出的金属间化合物相之形成机制，本研究使用具有相同基地相组成但不同Mo元素含量(Mo的原子数分数为15％～59％)的试片，以SEM、EMPA观察显微组织的变化，并使用XRD对金属间化合物析出相进行结构分析与鉴定，最后再以DTA热分析仪器量测合金的相变化温度，以厘清金属间化合物相的析出机制。试验结果显示，当合金中含有高比例的Mo元素时，烧结后在基地相中固溶的Mo元素与W Mo元素总和将随之升高，除了造成基地相的凝固温度下降以外，也形成大量的(W，Mo)0.5-(Ni，Fe)0.5 x(z=0～0．04)金属间化合物析出相，而此金属间化合物相中，Mo元素所占的比例会随着添加的Mo元素比例增加而呈线性上升。热分析结果显示，金属间化合物相的析出温度大约是在1349～1355℃之间，而基地相的凝固温度则从1415℃降低到1336℃。根据这些现象可以了解，金属间化合物相的形成机制与合金成分中的MMo／(Mw MMo)比值有关，当比值高于0．66时，金属间化合物相的析出机制为偏晶反应(monotectic reaction)；当比值在0．5～0．66之间时，析出机制是属于共晶反应(eutectic reaction)；当比值在0．34～0．5之间时，析出机制可能是共析反应(eutectoid reaction)或者是包析反应(peritectoid reaction)；比值低于0．2时，则不会析出金属间化合物相。     

The mechanisms of grain refinement in dilute aluminum alloys

The contribution of the peritectic reaction in producing grain refinement in aluminum alloys has been studied in three binary aluminum systems. It appears that titanium has a unique effect compared with zirconium or chromium; there is evidence of a refinement mechanism associated with the peritectic reaction in Al?Ti alloys which was not observed in the other two systems. Additions of boron to Al?Ti and Al?Zr alloys emphasizes the difference in effect of these two transition elements as grain refiners. The significance of the present work is discussed with reference to conflicts apparent in earlier experimental data.     

Some data on the infiltration of porous iron-manganese alloys with copper and its alloys

Summary It is clearly shown that nonporous iron-manganese-copper alloys can be produced by infiltrating an iron-manganese matrix with copper or copper alloys.Additional data have been obtained on the effectiveness of copper, a copper-manganese master alloy, and brass as infiltrating materials; the feasibility of producing a nonporous iron-manganese alloy free from oxide inclusions is demonstrated.Data are presented on the formation of iron-manganese-copper alloys with different manganese contents during infiltration with copper and copper-manganese master alloys, under different infiltration conditions.The extent of phase interaction during infiltration and the condition of boundaries in alloys produced under different infiltration conditions have been investigated.     

Identification of dispersoids in Al-Mg alloys containing Mn

The submicron dispersoids in the ingot of AA 5083 have been identified. They precipitate during the soaking treatment before hot working. Strong evidence suggests that most of them have the composition of MnAl₄. A smaller number belong to E phase (Mg₃Cr₂Al₁₈) and CrAl₇. The presence of the dispersoids of MnAl₄ is in contradiction with the ternary phase diagram of Al-Mg-Mn. The particle possess the shape of a rod with a hexagonal cross section. The aspect ratio ranges from one to several tens. These dispersoids may be important to the hot ductility of AA 5000 series alloys.     

High strain fatigue fracture mechanisms in two phase alloys

Cyclic response and fatigue damage have been studied on precipitation-hardened Al-4 pet Cu alloy as a function of particle type and spacing by means of scanning and transmission electron microscopy. Specimens strain cycled in tension-compression under controlledplastic-strain tests formed intense intragranular slip bands by a disordering mechanism when the microstructures contained precipitates penetrable by dislocations. These bands were the sites of crack nucleation and Stage I propagation. With the addition of impenetrable particles, slip was dispersed homogeneously; consequently, crack nucleation and Stage I propagation shifted to the grain boundaries. The cyclic response results were then applied to Tomkins’ model of fatigue life prediction. It was found that his equations were a conservative estimate of the actual results.     

医用多孔钛及钛合金的研究进展

医用钛及钛合金材料由于能提高材料的骨引导作用,有利于骨组织的长入促进骨性结合而在临床上得到了越来越广泛的应用.文章从医用钛及钛合金材料的发展、成型方法及表面改性进行了概述.     

Identification of void shrinkage mechanisms

A method for identifying void shrinkage mechanism experimentally is presented. We treat the void shrinkage on the bonded interface during solid state diffusion bonding of similar metals. The voids are arranged at regular intervals. The experimental results were analysed by log(T/t_s) vs (1/T) and log t_s vs log P plots, where T is the absolute temperature, P the compressive stress, t_s is the time taken to attain the void shrinkage of a certain volume ΔV. We obtained t_s by measuring a certain growth of bonded area ΔS replaced by ΔV. Even if ΔS is adopted, we can identify the transition of the dominant mechanism by slopes of those plots. As a result, interface self-diffusion, volume self-diffusion and power law creep were experimentally identified as fundamental mechanisms which contribute to the void shrinkage process during diffusion bonding of metals.     

Solid particle erosion mechanisms in copper and two copper alloys

The amount and mechanisms of metal loss has been studied for oblique and normal impact on copper, 70/30 brass, and precipitation hardened Cu-2 pct Be, using steel ball bearings and velocities from 138 to 300 m/s. The weight losses are roughly the same for the three alloys though the mecahnisms of flow and fracture change from highly plastic for copper to pronounced adiabatic shear localization for the Cu-Be alloy. At these high velocities and ball sizes the deformation is adiabatic, thus localizing the flow and enhancing loss compared to a slow isothermal indentation. Also, inertial forces are large at the base of any extruded lip, thus playing a central role in removing metal. For normal impacts the principle weight loss occurs where the impacts partially overlap, and stems from the concentration of strain at local irregularities in the surface. Formerly a Graduate Student in the Department of Metallurgical Engineering, is now a Graduate Student in The Department of Ceramic Engineering at Ohio State University, Columbus, OH 43210.